X-Ray Diffraction Techniques for Soil Mineral Identification

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Copyright 2007 Soil Science Society of America, 677 S. Segoe Road, Madison, WI 537, USA.Methods of Soil Analysis. Part 5. Mineralogical Methods. SSSA Book Series, no. 5.

Chapter 4

X-ray Diffraction Techniquesfor Soil Mineral

Identication

WILLIE HARRIS, Universit of Florida, Gainesville

NORMAN WHITE, ???

Soil mineralogy is determined routinely because of its strong inuence on soilbehavior, its use in soil classication, and its relevance to soil genetic process-es. Soils commonly contain primary minerals, which are formed from magmaand provide insight into characteristics such as parent material provenance, uni-formity, and weathering rates. Soils also contain secondary minerals, which areformed from weathering processes, and may have crystallographic characteristicsthat strongly inuence the physical and chemical properties of soils. For example,some phyllosilicates are termed expansible because they adsorb water and cationson interior surfaces within the crystal structure itself. Expansion of the interiorsurfaces, or interlayers, imparts high cation exchange capacity (CEC) relative tononexpansible phyllosilicates and most other secondary minerals. X-ray diffrac-tion (XRD) is the technique most heavily relied on in soil mineralogical analy-sis.

X-ray diffraction is a technique that provides detailed information about the atomicstructure of crystalline substances. It is a powerful tool in the identication of minerals inrocks and soils. The bulk of the clay fraction of many soils is crystalline, but clay particlesare too small for optical crystallographic methods to be applied. Therefore, XRD has long

been a mainstay in the identication of clay-sized minerals in soils. However, its useful-ness extends to coarser soil fractions as well. X-ray diffraction analysis can be conducted

on single crystals or powders. This chapter will be devoted to X-ray powder diffraction(Reynolds, 1989a), since that is the technique most applicable to soil mineralogy.

The objective of this chapter is to provide a detailed procedural reference for soilmineralogical determination by XRD. This chapter will emphasize well-established tech-niques, as well as some of the more recent innovative procedures. Alternative approacheswill be cited to the extent possible, along with their advantages and disadvantages. We areindebted to preceding works addressing XRD methodologies applicable to soils (Jackson,1956; Brown and Brindley, 1980; Whittig and Allardice, 1986).

Some familiarity of principles underlying XRD analysis is advisable before under-taking the procedures and interpretations. Many sources are available that address these

principles in detail (e.g., Klug and Alexander, 1974; Cullity, 1978). Other useful sources ofinformation about XRD theory and interpretation applied specically to powder methodsinclude Bish and Post (1989), Zevin and Kimmel (1995), and Moore and Reynolds (1997).The following section provides a brief introduction to some fundamental aspects of XRD.


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PRINcIPLES OF X-RAy DIFFRAcTION

Generation of X-ras

The part of the electromagnetic spectrum ranging in wavelength from approximately

103

to 10 nm is considered X-radiation, or X-rays. X-rays are produced by the rapid de-celeration of fast-moving electrons as they impinge on matter (Klug and Alexander, 1974).Production of X-rays for XRD analysis is accomplished using an X-ray tube consisting ofa lament electron source and a metal target (Fig. 41). The tubes are evacuated to mini-mize absorption of electrons accelerated from the lament (cathode) to the target (anode).Activation of the tube entails passing a current through the lament to establishing a cur-rent (e.g., 1030 mA) under high voltage (e.g., 3050 kV) between the lament and target.X-rays generated from the target during the rapid deceleration of electrons from the la-ment emerge from windows in the tube. The material comprising the window has a mini-mal tendency to absorb X-rays.

X-ray diffraction analysis uses monochromatic radiation. Intense X-radiation at a spe-cic wavelength can be produced when electrons from a source (e.g., tube lament) dis-lodge inner shell electrons from the atoms of the metal target. Instantaneous replacementof the dislodged electron by an electron from a specic lower energy shell results in thequantum release of a packet of energy corresponding to a specic wavelength and termedcharacteristic radiation (Fig. 42). Characteristic radiation is element specic and relatesto the atomic number of the element selected for the target in the X-ray tube. Hence, tubescan be selected based on the wavelength () that is most advantageous for the material be-ing analyzed by XRD (see "Selection of X-Ray Wavelength" below).

An X-ray tube also generates X-rays at other wavelengths in addition to the K radia-tion of its metal target. These include (i) a continuous spectrum across a broad range ofwavelengths and (ii) another signicant monochromatic intensity maximum arising fromK shell electrons being replaced by M shell electrons (K) (Fig. 42). Modern XRD instru-ments (X-ray diffractometers) are equipped with lters and/or monochromators that reducerelative amounts of unwanted radiation.

X-ra Diffration

X-ray diffraction occurs when X-rays are scattered by atoms arranged in an orderlyarray in crystals. The atoms serve as scattering centers (Moore and Reynolds, 1997), re-

emitting X-rays at the same wavelength as the incident radiation in all directions (coherentscattering). The orderly arrangement of atoms results in the scattered X-rays within thecrystal being (i) in phase in specic directions dictated by symmetry and atomic spacingsand (ii) out of phase in all other directions (Fig. 43). The X-rays that are in phase con-structively interfere and emerge as intense beams (diffracted beams) from the crystal, while

Fig. 41. Schematic crosssection of a sealed-off la-ment X-ray tube (aftercullit,1978). High voltage ac-

celerates electrons from thelament to the metal target,where X-rays are generatedfrom their rapid deceleration.The X-rays exiting one of thewindows are collimated andfocused in X-ray diffractionanalysis.


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3 X-ray diffraction

those that are out of phase destructively interfere and hence have minimal emergence. Thissystematic combination of constructive and destructive interference arising from the pe-riodicity of atoms in crystals is X-ray diffraction. Detailed information about the internal

symmetry and arrangement of atoms in crystals can be gained from XRD. However, this

chapter will emphasize only those aspects that are most pertinent to the identication ofnaturally occurring minerals in soils.

A simplifying way to intuitively comprehend the relatively complex phenomenon ofXRD is to envision regularly spaced planes of atoms in mineral structures (Fig. 43). Thedistance between a given set of planes is termed d-spacing. The d-spacing, although on ascale of Angstroms, can be determined quite accurately using XRD. The principles under-lying this determination are elegantly expressed by the Bragg equation:

n = 2d sin [1]

Fig. 42. (A) Schematic of an atom, depictingelectron shells and the energy transitions for K,K, and L characteristic radiation. K arisesfrom the replacement of K-shell electrons byelectrons from the L shell; K, by replacement ofK-shell electrons by M-shell electrons, and L,by replacement of L-shell electrons by M-shellelectrons. (B) Generalized depiction of an X-rayspectrum, showing peaks in intensity at wave-lengths (energy levels) corresponding to char-acteristic radiation. The highest-energy (shortestwavelength) characteristic radiation shown is K.Peaks marked K

and K

2, which are seldom

resolved in XRD data, arise from contribution ofelectrons from two sublevels in the L shell.

Fig. 43. Schematic representa-tion of XRD by regularly spacedplanes of atoms in a crystal. Theta() is the angle that the beammakes with the atomic planes;2 is the angle that the diffractedbeam deviates from the primarybeam; d is the distance betweenequivalent atomic planes in thecrystal (d-spacing); and is wave-length of the radiation. Note thatwhen DE + EF = n, where n isan integer, the diffracted beamsfrom each plane of atoms would be

in phase, leading to constructiveinterference which accounts forXRD. In effect, when that conditionis met, an XRD peak is observed.The Bragg equation can be usedto calculate d-spacing from the 2angle at which the diffraction peakoccurs.


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where n is an integer, is wavelength of the radiation, d is d-spacing, and is the anglebetween the planes and the incident X-ray beam. This equation expresses the condition fordiffraction, in effect, that for a given d-spacing and wavelength of radiation diffraction willoccur at a unique angle between the beam and the set of planes in question.

The factor in the Bragg equation of interest to mineralogists is the d-spacing, whichcan be determined in XRD analysis by xing and measuring the angle where a peak inX-ray intensity occurs. Such measurements can be made for a single crystal, for a mineral

in powder form, or for a mixture of minerals in powder form. Information gained fromdiffraction angles and relative peak intensities for pure minerals can be used to establishstructural details of those minerals. X-ray diffraction can also be used to identify the miner-

als present in a mixture, such as the soil clay fraction.

X-Ra Powder Diffration Analsis

Obtaining useful information from XRD requires the ability to control and/or measureangular relations between incident and diffracted radiation. Two types of instruments havebeen used to perform X-ray powder diffraction analysis: the XRD powder camera and theX-ray diffractometer. The powder camera approach entails recording diffraction maximacones on cylindrically mounted photographic lm surrounding the specimen. The dif-fractometer records the intensity of the diffracted beam electronically at precise angles asthe specimen is scanned over an angular range (Fig. 44). Modern diffractometers have anumber of advantages over the powder camera and are the more commonly used instru-ments in soil mineralogy. Therefore, the diffractometer approach will be emphasized in thischapter, but the d-spacing and intensity data obtained from either type of instrument are

interpreted the same way.

SAMPLE PREPARATION FOR X-RAy DIFFRAcTION ANALySIS

Sample preparation is an important aspect of soil mineralogical analysis by XRD.Preparatory procedures must be judiciously selected according to the objectives of the ana-lyst. Accurate assessment of soil mineralogy requires a thorough understanding of these

procedures and their consequences for data interpretation.

The simplest preparation procedure is to grind the soil as is into a powder neenough to mount in the focal plane of the diffractometer. However, the minerals occurringin soil clay fractions are often very different in nature from minerals that dominate the

coarser fractions. Grinding the whole soil makes it difcult to detect and identify the clay-sized minerals due to dilution and other confounding effects of the coarser components.This is a disadvantage because the clay-sized minerals often have a disproportionate effecton chemical and physical properties of soils. Thus, if the objective is an effective character-ization of all minerals, it is best to analyze individual particle size fractions separately.

Some preparatory procedures are necessary for denitive identication of expansi-ble phyllosilicates. The actual spacing between basal (001) atomic planes in expansiblephyllosilicates varies with temperature, relative humidity, and exchangeable cations inthe interlayer region of these minerals. Therefore, comparison of the variable d-spacings

(d001) must be made under standardized conditions (e.g., temperature, cation saturation) toenable identication based on differential responses to these conditions.

The following sections provide explanations and detailed procedures pertaining tosample preparation.


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Mounting Samples for X-Ra DiffrationLow Salt Samples

Priniples

The way that a sample is mounted can have major effects on the resulting data (Harwardand Theisen, 1962; Harward et al., 1962; Gibbs, 1965; Bish and Reynolds, 1989). It is im-

portant to be aware of these effects as well as their underlying causes. Some of the criticalfactors that should be accounted for are degree of preferred orientation of crystals, accurate

positioning of the planar sample surface within the instrument, differential settling (formounts prepared from particles in suspension), and innite thickness (i.e., thick enoughthat no increase in diffraction intensity occurs with further thickening).

One fundamental consideration is that diffraction is only detected for the atomic crys-tal planes that are coplanar with the focal plane of the diffractometer (see Nature of X-Ray Diffraction Data). This means that crystal planes would be evenly represented onlyif there is random orientation of crystals in the sample (Thompson et al., 1972). Randomorientation occurs naturally for anhedral, equidimensional crystals. However, there is atendency for crystals with a dominant crystal face (e.g., platy crystal habit) or elongatedmorphology to be oriented with plates or long axes parallel to the surface of the mount. Thedegree of this preferred orientation increases in proportion to the pressure applied in estab-lishing the planar surface of the powder. Thus, the applied pressure must be minimal if anear-random orientation is sought. The distinction between random and preferred orienta-tion is moot if all minerals in the mount are anhedral and equidimensional. See ImportantConsiderations in Mount Selection below for a discussion of advantages of preferred vs.random orientation.

It is critical that the planar upper surface of the powder be precisely positioned in thefocal plane of the instrument to obtain accurate d-spacing. An offset of that position is pos-sible for cavity mounts, which require that the powder be packed into recessed cavities. Forsuch mounts it is necessary that the powder be level with the upper surface of the mount.

Some mounting techniques entail the deposition of particles in suspension onto thesurface of the mount (suspension mounts). That means that particles undergo settling until

the liquid phase evaporates or is removed by suction. Differential settling based on dif-ferences in particle density, shape, etc. can occur during this time, resulting in a stratica-tion which biases the resulting XRD data (Gibbs, 1965). In effect, the particles that settleslowest are concentrated at the top of the cake and hence are over-represented relativeto their mass fraction. Differential sedimentation is most pronounced when (i) the liquid

phase is left to evaporate (e.g., sedimentation on glass slide), and (ii) there is a wide range

Fig. 44. Schematic representation of the components of anX-ray diffractometer.


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in particle size and/or density. The effect can be reduced by rapidly removing the liquidphase using suction (e.g., through use of a lter of ceramic tile) or by reducing the rangein particle sizes.

Innite thickness of the sample is desirable for a number of reasons (Bish and

Reynolds, 1989; Moore and Reynolds, 1997). For one thing, it enables greater reproduc-ibility, because at innite thickness, sample thickness no longer affects relative diffractionintensity. Also, innite thickness ensures that the mounting medium itself will have noinuence on the XRD results. The energy of the beam at innite thickness is effectivelyconsumed by diffraction and absorption phenomena (Moore and Reynolds, 1997). Failureto have innite thickness results in greater attenuation of peaks at higher 2 angles than atlower angles because the depth of beam penetration is less for the latter. For example, thedepth of vertical penetration for a beam capable of 200-m total penetration would be 3.5m at 2 2 and 100 m at 60 2. Calculation of innite thickness can be difcult becauseit is dependent on such factors as beam intensity, packing density, and X-ray absorption

characteristics of the components in the mixture. However, it can be veried for the ma-terials being analyzed by adding incremental amounts of sample material until high angleintensities show no further increase. Also, if the underlying mount generates a diffraction

peak itself at a reasonably high angle, then the disappearance of that peak with incrementaladditions would verify innite thickness.

Partile-Size considerations

Sample particle size must be ne enough to (i) obtain adequate statistical represen-tation of the components and their various diffracting crystal planes and (ii) avoid dif-fraction-related artifacts (Bish and Reynolds, 1989). This means that sand-sized particlesrequire grinding, as by ball milling, mortar and pestle, or blender. Samples with platy orbrous minerals are most effectively ground using bladed devices such as blenders. It isgenerally recommended that a cooling liquid, such as acetone or alcohol, be used in grind-

ing to avoid sample degradation from heat. A summary of grinding methods was providedby Bish (1994). He recommended that particles be ground to 0.01 mm for quantitative as-sessments.

Important considerations in Mount Seletion

It is necessary to know whether preferred or random orientation best serves the ob-

jectives of the analysis. Mounting approaches can be selected to minimize or maximizepreferred orientation. Random orientation is desirable if the objective is to obtain infor-mation about all crystal planes, while preferred orientation is advantageous if the primaryobjective is to detect and identify platy minerals such as phyllosilicates. In the latter case,however, the preferred orientation effect must be accounted for in the interpretation of thedata.

A randomly oriented powder is the appropriate choice in cases where informationabout crystal planes oblique to the dominant cleavage face is required. Such cases include(i) XRD characterization of a mineral, (ii) verifying the identity of a mineral using powderdiffraction reference data, and (iii) discriminating between related mineral species that

produce similar XRD patterns in oriented mounts, a common purpose in soil mineralogicalanalysis. For example, the b crystallographic axis dimension, which can be used to differ-entiate dioctahedral from trioctahedral phyllosilicates, can only be measured if a sufcientnumber of crystals are oriented with the dominant cleavage face oblique or perpendicularto the focal plane of the goniometer.

Intuitively, random orientation may appear to be a pure and unbiased approach to


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sample mounting. However, it is often advantageous to preferentially orient certain crystalfaces, as in the case of identifying expansible phyllosilicates. Furthermore, random orien-tation is difcult (if not impossible) to achieve for samples containing platy or elongatedcrystals. The analyst faces the dilemma of creating a planar surface for a powder while not

applying enough force to orient plates, laths, or tubes parallel to the plane. Even the slight-est pressure can induce preferred orientation of some samples. This sensitivity to pressureas applied to the surface of the powder makes it difcult to attain acceptable precision. Ineffect, relative X-ray intensities may vary signicantly between different mounts of thesame sample. It is generally much more difcult to achieve good precision with randomorientation than with preferred orientation for samples with platy minerals because theseminerals naturally orient parallel to the packing plane.

Suspension mounts (see below) prepared using a porous mounting medium (e.g., un-glazed ceramic tile, lter) enable convenient cation saturation treatments and provide pre-ferred orientation, both of which are advantageous for identication of expansible phyllo-

silicates. Suspension mounts should generally be avoided for silt-sized material (includingground sand) due to differential settling of material during the drying period. However,some silt and sand fractions may contain expansible phyllosilicates whose identicationwould be made easier by using a suspension mount. A separate suspension mount could bemade, in addition to a dry mount for the purpose of identifying the phyllosilicates.

Achieving rapid dewatering under suction (to minimize differential settling) is dif-cult for some samples (e.g., ne clay may clog pores). Effective orientation can be accom-

plished without differential sedimentation by preparing the mount from a paste.

Mount Storage

Identication of expansible phyllosilicates requires that conditions of relative humid-ity (RH) be controlled, as will be elaborated on later. Therefore, it is necessary to store thesamples under conditions that maintain the targeted humidity, such as a desiccator, untilthey are scanned on the X-ray diffractometer. (See Relative Humidity Control underMineral Identication from XRD Data).

Dr Mounting Tehniques

Random Powder Mount. The dry powder material is mounted in such a way as tominimize the force required to form a planar surface of the powder that is to be positioned

in the diffractometer. A common technique is to loosely drop the material into the cavity ofthe mount from the side, that is, such that the plane to be mounted is in a vertical positionas the mount is prepared (Fig. 45). This is accomplished by machining the sample holderto have a shallow rectangular cavity with one open side as positioned horizontally. A glassslide with an etched (frosted) surface toward the powder is used (avoiding a smooth contactsurface, which might induce orientation) to retain the powder in the mount as it is beingloaded edge-wise and vertically. Binder clips can be used to hold the glass slide in placeand to provide a stand to hold the mount in a vertical position. Once the cavity is lledwith the powder, the mount is positioned horizontally and the glass slide removed by lift -ing it straight up to avoid inducing preferred orientation. Another method for attaining ran-

dom orientation involves the use of an air brush to spray dry samples (Hughes and Bohor,1970; Hillier, 1999). This method, which produces a powder of spherical aggregates, re-

portedly minimizes preferred orientation and yields XRD data that are highly reproducible.However, the heat required by the method can dehydrate expansible phyllosilicates andthereby hinder their identication.


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Oriented Powder Mount.

This mounting technique can be used with any cavity mount. The sample is packedinto the cavity of the mount using a at surface, such as a spatula blade or glass slide, withuniform and sufcient force to promote the preferred orientation of platy and elongatedcrystals. Reasonably good precision can be attained if care is taken in applying uniform,rm pressure in packing the mount with each preparation.

Wet Mounting Tehniques (For


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surface tension, then allow it to dry. Cation saturations should be made before thematerial is pipetted onto the slide. Drying can be accelerated (to lessen differentialsedimentation) by placing the slide in an oven at slightly elevated temperature (e.g.,40C), but oven drying can exacerbate peeling. Care must be taken that the suspension be

free of any salt that may have been previously added to the suspension (e.g., in inducingocculation to reduce water volume). The optimal concentration of the suspension is a

balance between having sufcient material on the slide for effective XRD assessmentand not having so much as to result in the development of cracks upon drying. A rstapproximation of sufcient material is about 30 mg cm2.

Ceramic Tile Mount.Suspension mounts can also be prepared by depositing thesuspension onto a porous (unglazed) ceramic tile and applying suction to acceleratedewatering and hence lessen the extent of differential sedimentation. Occasionally, claysaturated with monovalent cations disperses sufciently that some of the material passes

through the tile pores, but this is not usually a serious problem. Dewatering by suctionpermits the addition and rinsing of salt solutions to the sample in the process of cationsaturation. Also, heat treatments (explained in a subsequent section) can be performedwhile the sample is on the tile. Different laboratories have devised various means to applysuction to ceramic tiles (e.g., Rich, 1969). The common denominators are a rectangularslot that forms an elastic seal around the edge of the tile, a means of bracing the tile in a

level position, and a means of applying suction to the underside of the tile (Fig. 45). Aneffective sealing material is commercial silicon sealant, which dries to a soft but durableand elastic solid. The tile is pressed into a tightly tting rectangular cavity formed by thesealant. The top of the tile is positioned to extend above the sealant, such that suspensions

can be retained on the tile by surface tension. The ceramic tile XRD mount is prepared asfollows, assuming the area of the tile to be about 10 cm2 (deviations should be adjusted

proportionately for the mass of material specied):

1. Place the tile in the suction apparatus and apply suction.

2. Wet the slide with a few milliliters of deionized water and allow it to pull through.

3. Pipette about 8 to 10 mL of suspension at a particulate solids concentration of ap-proximately 30 mg L1 onto the tile, being careful that it stays on the tile via surfacetension.

4. Continue suction until the excess water is removed, leaving the material to be ana-lyzed plated on the tile.

5. Rinse the material by washing several times (totaling about 25 mL) with deionizedwater to ensure that all salts are removed, or selected salt solutions can be appliedif controlled cation saturation is pursued. If salt solutions are used to saturate theexchange complex with a cation, wash the slide (25 mL) to remove the excess salt.

6. Tiles can be stored in a desiccator when preparation is complete, which permitscontrol of relative humidity and prevents dust from contaminating the surface.

0.45-m Membrane Filter Mount. This mounting approach (Drever, 1973; McAlisterand Smith, 1995) can be used as an alternative to the ceramic tile approach and has thesame advantages that come with dewatering by suction. Also, lters and lter suctiondevices are easily purchased from vendors of scientic supplies, in contrast to unglazedceramic tiles and the customized suction devices required for their use. The use of ltersinvolves transferring the material from the lter to a rigid support such as a glass slide,which requires practice and skill. Be careful to watch for ne clay material being pulled


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through the pores of the lter. The lter mount is prepared as follows, assuming the areaof the lter surface to be about 10 cm2 (deviations should be adjusted proportionately forthe mass of material specied):

1. Place lter on its suction manifold mount, and apply suction.

2. Apply a few milliliters of deionized water to wet the lter and allow the water to bepulled through the lter.

3. Pipette about 8 to 10 mL of suspension at a particulate solids concentration of ap-proximately 30 mg L1 onto the lter and allow it to drain under suction.

4. Rinse the material with about 25 mL of deionized water to ensure that all salts areremoved, or selected salt solutions can be applied and rinsed if controlled cationsaturation is pursued.

5. Remove lter carefully with tweezers and place on a tissue or other absorbing sur-face, sample side up.

6. Allow the lter to dry for 5 to 10 min.

7. The sample must now be transferred from the lter to a glass slide. Pick up the lterby the edge with tweezers, and carefully place it, centered on top of the glass slide,sample side down. Alternatively, the glass slide can be positioned on top of the ltercake and a tissue can be used to turn over the slide and lter.

8. Gently rub the bottom of the lter to ensure good contact of the sample with theglass slide.

9. Using a glass rod, crease the lter along the edges of the glass slide by pressingdown at an angle and sliding the rod along both edge contacts between lter and

slide.

10. Gently peel the lter off the slide longitudinally. This usually results in the sampleadhering to the slide.

11. Slides can be stored in a desiccator when preparation is complete, which permitscontrol of relative humidity and prevents dust from contaminating the surface.

Oriented Paste (Smear) Mount.This mounting approach entails spreading thesample as a paste onto a at surface (Thiesen and Harward, 1962; Gibbs, 1965). Asmear mount eliminates the risk of differential sedimentation, but lacks some of the

advantages of suspension mounts. For example, cation saturation cannot be performeddirectly on the smear mount. A smear mount can be prepared as follows:

1. Work the sample into the consistency of a paste (putty-like), moist enough to haveplasticity but dry enough not to be excessively sticky.

2. Place an aliquot of the paste on the edge of a glass slide.

3. Using a spatula, spread the paste all the way across the slide in one motion. Thepaste should be spread such that the surface is smooth and at, which sometimesrequires practice.

Mounting HighSoluble Salt SamplesSamples in arid regions can contain appreciable concentrations of soluble salts. These

salts are important minerals and should be preserved in the process of mineralogical analy-sis. Preservation of salts precludes all washing procedures and requires dry mounting ap-

proaches, such as random or oriented powder mounts as described above, for XRD analy-sis. Soils with high salt concentrations can be analyzed on a whole-soil basis or after me-


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X-ray diffraction

chanical sieving to obtain a ner particle size fraction (e.g.,


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Reagents

1MMgCl2

1MKClDeionized water

Materials

0.45-m membrane lters or unglazed ceramic tilesSuction ltering apparatus

Proedure

1. Prepare two 0.45-m membrane-lter mounts or ceramic-tile mounts per sample,as described under Oriented Suspension Mounts. The cation designation Mg orK should be included in the mount labels. Use a heat-resistant labeling marker ifheat treatments are to be conducted.

2. Apply 25 mL of the 1MMgCl2 or KCl to the Mg and K mounts, respectively,or about 25 mL per 300 mg of clay on the slide. This can be applied all at once if thelter apparatus accommodates the whole volume, or in smaller (e.g., 5 mL) aliquotsas may be necessary in the case of ceramic tiles.

3. Apply 25 mL of deionized water as a rinse.

Glerol Solvation

Rationale

Cation saturation alone is not usually sufcient for condent distinction between

common expansible phyllosilicates. Saturation with Mg sometimes produces similar d-spacings (about 14 ) for these minerals. Glycerol [C

3H

5(OH)

3] is used diagnostically in

conjunction with Mg saturation because of its tendency to solvate (due to its polar nature)the interlayer regions in bilayer conguration for smectites and as monolayers for vermicu-lites (Walker, 1950; Brindley, 1966). The resistance of vermiculites to solvation by glycerolarises from its higher charge density and hence cation occupancy. The layers of vermiculiteare mutually attracted to cations, and this force of attraction is sufcient to deter glycerolincursion. The lower layer charge of smectites allow the solvated interlayers to expand to ad-spacing of about 18 , thereby distinguishing these minerals from vermiculites.

A number of polar organic intercalates other than glycerol can form interlayer com-plexes with smectite (MacEwan and Wilson, 1980; Whittig and Allardice, 1986), includingalcohols, ethers, amines, and polyamines. However, glycerol has several advantages for

phyllosilicate identication, including good peak separation between solvated and non-solvated phyllosilicates, less variability of the d-spacing as a function of layer charge, andhigh stability of the complex (Whittig and Allardice, 1986). Therefore glycerol is specied

below for routine analyses. However, ethylene-glycol is also used extensively for solva-tion, and is more prone than glycerol to form a bilayer in smectite with high charge oftetrahedral origin (beidellite).

comments

Previous publications addressing soil mineralogical methods (e.g., Jackson, 1956;Whittig and Allardice, 1986) have presented procedures of glycerol solvation that entailnumerous washing and rinsing steps using a centrifuge. There is some uncertainty in these

procedures as to an adequate but not excessive amount of glycerol to be applied. One wayto avoid this uncertainty is simply to apply the glycerol directly to the mounted Mg-satu-


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3 X-ray diffraction

rated clay using either 0.45-m membrane lter mounts or ceramic tile mounts while theyare moist. Adding to air-dried sample surface may result in incomplete penetration into thesample.

Reagents30% Glycerol [C3H

5(OH)

3]/water mixture

Deionized water

Materials

0.45-m membrane lter or unglazed ceramic tilesSuction ltering apparatus

Proedure for cerami Tile Mounts

1. Apply a sufcient quantity of 30% glycerol to cover the Mg-saturated clay plated on

top of the tile by surface tension (see Oriented Suspension Mounts and CationSaturation), and allow excess to pull through.

2. Remove tile from suction apparatus and allow to air dry for a few hours, being care-ful to protect it from dust.

3. Conduct X-ray scan, or store in glycerol desiccator until scanning is performed.

Proedure for 0.45-m Membrane Filter Mounts

The membrane lter mount differs from the ceramic tile in that for the former the clayends up on a glass slide rather than on the original medium (i.e., lter). The glycerol ap -

plication is best made after the transfer from the lter to the slide has been performed. Thiscan be done by spraying a 10 to 20% glycerolwater solution directly on the Mg-saturatedclay plated on the slide using a very ne misting device, covering the surface completely

but lightly. It is best to then allow the slides to equilibrate in a glycerol desiccator for atleast 24 h.

X-RAy EXAMINATION OF SAMPLES

X-ra Diffratometers

Presumably most readers will have access to and some familiarity with an X-ray dif-

fractometer. Alignment and optimization involve instrument-specic procedures and willnot be discussed here, but a brief summary of some basic aspects of diffractometers mostpertinent to soil mineralogical analysis will be presented. There are a number of sourc-es where more detailed information about diffractometers can be found (e.g., Klug andAlexander, 1974; Cullity, 1978; Moore and Reynolds, 1997).

An X-ray diffractometer is a sophisticated system designed to generate X-rays, opti-cally focus them onto a sample, scan with precise angular control, and monitor the relativeintensity of the diffracted beam as a function of 2 (the angle is conventionally expressedas 2 rather than , since 2 is the actual angular deviation from the incident beam). Themain components are a generator to produce high voltage, a tube to produce X-rays, a

beam collimator, a sample mounting stage, a detector, a monochromator, and a goniometer(Fig. 44). The goniometer controls the angular relationship between the incident beam,sample surface, and detector. For routine analysis, the goniometer maintains an equal angle

between the focal plane and both the incident beam and the path to the detector that re-ceives the diffracted beam. This means that diffraction from crystal atomic planes that arecoplanar with the focal plane will be registered as a peak in intensity by the detector at the


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2 angle that "solves" the Bragg equation for the d-spacing of that particular set of atomicplanes.

The sample stage is positioned in the instrument such that the sample is exposed to theincident radiation within a focal plane in the center of what is termed thegoniometer circle,

while the radiation source and detector are positioned on the circle itself. The goniometercontrols the angle that the incident radiation makes with the focal plane, as well as the rela-tive position of the detector. Effective control of the X-ray beam requires some opticalcollimation and focusing. The beam is collimated as it emerges from the X-ray tube, such

that only X-rays parallel to the goniometer circle are incident to the sample. There is focus-ing of the beam, in that it diverges slightly from the collimator to the sample, and convergesto the detector (Fig. 44).

Monochromatic radiation is necessary for XRD analysis using a powder diffrac-tometer. The X-ray tube emits intense characteristic (K) radiation from its metal target,

but some white- and K radiation is emitted as well. Modern X-ray diffractometers are

equipped with devices in the beam path that serve to reduce intensities along the spectrumother than the wavelength of choice. One such device is a metal lter, with a precipitousX-ray absorption maximum (absorption "edge") that results in selective absorption of Kradiation generated from the target of the X-ray tube. The metal used for the lter must

be selected based on the K wavelength of the target metal. A more effective device forthe purpose of achieving a high degree of wavelength purity is the crystal monochroma-tor. This is a crystal positioned in the beam path between the sample and diffracted beamdetector such that it diffracts only K radiation into the detector.

Modern diffractometers are computer controlled and collect data digitally. They have

a stepping motor, which enables the operator to select the step width (fraction of a degree2 at which counts are collected) and dwell time (the time interval for each step). Peak/background ratios can be increased by using longer dwell times. Digital data have a numberof advantages over analog strip chart data recordings, standard for older diffractometers.Data collected digitally can be conveniently manipulated graphically and mathematically.The area under diffraction peaks, an index of relative intensity, can be rapidly calculatedusing computer programs or spreadsheets. Also, the diffraction data can be modeled, aswill be discussed in a subsequent section.

choie of X-Ra Wavelength

Shorter-wavelength (higher energy) radiation has the advantages of lower attenuationin air and less absorption by the sample. Longer-wavelength radiation, however, produces

better resolution of peak maxima (a larger 2 interval between adjacent peaks, as can beveried by applying the Bragg equation), which is particularly advantageous for the larged-spacings of expansible phyllosilicates. The most commonly used radiation for soil min-eralogical analysis is CuK ( = 1.54 ) because it is a reasonably good tradeoff withrespect to energy and peak separation. MoK ( = 0.71 ), FeK ( = 1.94 ), and CrK( = 2.28 ) are higher- and lower-energy alternatives.

Safet: An Important consideration

X-ray diffraction analysis requires high voltage to generate and detect potentiallyharmful high-energy radiation. Hence, not only is there the hazard of X-ray exposure, butof contact with high voltage as well. Careless, ill-informed operation of XRD equipmentcan result in serious injury or death. It is imperative that all persons who are permittedaccess to the equipment be thoroughly trained in safe operating procedures and aware ofthe potential risks. Fortunately, modern X-ray diffractometers have a number of safety


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5 X-ray diffraction

features that minimize the risk of exposure to X-rays. These include lead-lined cabinets orother enclosures and safety switches that close the shutter to the X-ray tube when the X-ray chamber is accessed. It is best, however, not to become complacent about safety evenwhen using modern fail safe equipment. For example, it is a good idea to routinely check

the beam path using an appropriate X-ray detection instrument even when the shutter lightindicates that it is closed.

conduting Sans

The X-ray tube is energized by establishing a voltage potential and current ow be-tween the lament and the target (commonly Cu). The magnitude of the current and volt-age is established by optimization procedures for the diffractometer. The sample mount is

braced into the focal plane of the goniometer by means of a mechanical device providedfor the sample stage. It is important to verify at this time that the sample surface is not pro-truding above or recessed below the focal plane. Displacement from the plane will resultin inaccurate d-spacing determinations. Routine operation for diffraction analysis involvesa scan from lower to higher 2 angle. The selection of scanning range is based on expec-tations regarding likely components. The starting 2 angle is commonly set at 2, whichis sufciently low to detect peaks of large d-spacings characteristics of expansible phyl-losilicates. It is generally a good idea to have at least one scan from 2 to 60 to cover thetotal range where peaks of common soil minerals are likely to occur. Additional scans fordifferent saturation-, solvation-, or heat treatments need only cover the range necessary to

get the needed discriminatory information.

Nature of X-Ra Diffration DataThe results of XRD using a powder diffractometer are generally plotted as 2 angle

on thex axis vs. X-ray intensity as measured by the detector on they axis (Fig. 46). Theintensity is usually expressed as counts per second, although it is most accurately a relativemeasurement affected by various conditions, including the current and voltage at which theX-ray tube is operating and the counter efciency. Relative intensity is measured as rela-tive areas under diffraction peaks. The relative intensity of a diffraction peak produced bya given set of atomic planes in a crystal, assuming all atomic planes are equally represented(random orientation), is dictated by the composition and arrangement of atoms in the unitcell (i.e., the smallest atomic unit displaying the symmetry of the crystal). In effect, ele-

ments of the crystal differ in the way that they scatter and absorb X-rays, and their positionsin the unit cell affects the degree to which X-rays emerge in phase for a given direction.Thus, relative intensity does not provide a simple 1:1 index of the mass fraction of mineralsin a mixture; calibrating relative intensities to mass fractions is one of numerous challengesto quantication by XRD, as will be discussed below.

It is important to understand that a diffractometer only monitors diffraction for crystalplanes that are coplanar with the focal plane. This does not mean that diffraction isnt oc-curring for crystal planes that are otherwise oriented, but simply that the detector is posi-tioned uniquely (angle of incidence = angle of diffraction) to solve the Bragg angle forthe focal plane. It follows that the greater the number of crystals of a given mineral with a

particular crystal face oriented in the focal plane, the greater the diffraction intensity willbe for that face. Hence, preferred orientation of a given crystal plane (i.e., nonrandomorientation) is a major factor to be accounted for in the interpretation of X-ray powderdiffraction data. Mounting procedures that enhance or minimize preferred orientation aredescribed in the section entitled Mounting Samples for Diffraction.


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6 Harris & White

MINERAL IDENTIFIcATION FROM X-RAy DIFFRAcTION DATA

Mineral identication is based on d-spacings and relative peak intensities. All min-erals generate multiple diffraction peaks. Identication is much simpler if only one min-eral is present in the sample, but even then it is not necessarily a matter of certainty andmay require corroborating data (e.g., elemental or thermal analysis). Mixtures of miner-als can produce complex XRD patterns that present a challenge in mineral identication.However, several factors mitigate the complexity somewhat for soils. Most soils containonly a few minerals, and these minerals tend to segregate into particle size fractions, whichare normally analyzed separately to further reduce complexity. Also, the minerals that oc-cur frequently in soils constitute only a small fraction of the >40,000 that have been identi-ed. Generally, the analyst quickly acquires a familiarity with minerals likely to be found

in soils from specic environments and parent materials.

Relative Humidit control

In our discussion of XRD mounts, we pointed out that it is important to store samplesunder specic RH before analysis because expansible phyllosilicate d-spacings are depen-

Fig. 46. Sequences of X-ray diffraction pat-terns for soil clays (2.00.2 m) scanned afterspecied cation-saturation (Mg and K), glycerol(Gly) solvation, and heat treatments. Peaks arelabeled by d-spacings (). (A) The clay fromthe Orangeburg series (Ap horizon, Georgia)shows a 4- peak that is minimally affected bycation saturation and shifts and broadens withheat treatment to a peak at 2 . This behaviortypies hydroxy-interlayered vermiculite as itoccurs in the coastal plain of the southeasternUSA. There is a small peak that persists at 4 at 500C, which is due to a small amount ofchlorite. Also present are kaolinite (7. and3.57 ), gibbsite (4.5 ), and quartz (4.26and 3.34 ). Note that peaks for gibbsite andkaolinite disappear at 300 and 500C, respec-tively, due to dehydroxylation. (B) Clay fromthe Sharkey soil (Ap, Louisiana) shows peaks

at and 4 that have mainly shifted to 0 at 300C. Note the increase in intensity of the0-, 5- (second-order), and 3.34- (third-order)peaks with heat. This behavior suggests thatboth smectite and vermiculite are present. Asmall peak (not labeled) intermediate between4 and 0 may be attributable to some resis-tance to collapse of these expansible phyllosili-cates. The small 4- peak at 500C indicatesthat some of the 4- peak is attributable tochlorite. Also present are mica (illite) (0, 5,and 3.34 ), kaolinite (7. and 3.57 ), andquartz (4.26 and 3.34 ). Note that at 25C the0- peak is completely attributable to mica,but is enhanced by the collapse of expansiblephyllosilicates with increasing temperature.Also, quartz, mica, smectite, and vermiculiteall contribute to the 3.34- peak at 500C,whereas only mica and quartz contribute to itat 25C. (These soils were analyzed as partof USDA Regional Project S-207. Special ac-knowledgment is due Dr. Ben Hayek at AuburnUniversity, who conducted the XRD analyses.)


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7 X-ray diffraction

dent on RH. Also, some minerals (e.g., halloysite) are prone to irreversible dehydrationand d-spacing change upon air-drying; hence, it is advisable to maintain the soil (and clayfraction obtained from it) in a eld-moist state if such minerals are expected. The mineralidentication approaches given below specify treatments that inuence RH. Thus, it is nec-

essary to ensure that the appropriate conditions apply. This can be accomplished by usingdesiccators with atmospheres controlled with various substances. For example, a desiccantsuch as P

2O

5or anhydrous CaSO

4can be used for low humidity, water for high humidity,

and a saturated solution of Mg(NO3)

2(RH 54%) for a stable intermediate RH.

General Method

1. A reference of mineral powder diffraction data is needed for identication of un-known minerals. Table 41 contains limited diffraction data for some minerals thatcommonly occur in soils. However, it is useful to have a comprehensive reference

such as the Mineral Powder Diffraction Files compiled by the Joint Committee onPowder Diffraction Standards (JCPDS) and published by the International Centrefor Diffraction Data.

2. Determine relative peak areas (i.e., relative integrated intensities) and calculate thed-spacings from the 2 peak maxima by means of the Bragg equation. Most diffrac-tion systems are equipped with software that automatically calculates peak areas andd-spacings, but this can also be performed using a spreadsheet.

3. Start by identifying the peaks for the minerals that you know to be present. Forexample, almost all soils contain quartz. Soils described as calcareous probably con-tain calcite, but may also contain dolomite. Mark these peaks, but remember that

these peaks may be shared with other minerals in the sample.4. Begin with the d-spacing for the highest intensity peak not already identied. Refer

to Table 41 in this chapter or other reference source to determine prospective min-erals with a maximum intensity peak matching that d-spacing. Then check to see ifother highrelative intensity peaks for the prospective mineral are also present. If so,then tentatively mark these peaks with a designation for that mineral.

5. Find the most intense peak not explained by the rst mineral tentatively identied.Then follow the same procedure as described in Step 3 to tentatively identify asecond mineral, then a third, etc., until all peaks are accounted for. Often two ormore minerals can have common or proximal peaks, and this must be taken into

account in assessing relative peak intensities. The intensities are additive, such thatcommon peaks would be more intense than if only one mineral were generatingthem. Relative peak intensities reported for reference diffraction les are a guide asto what to expect, but will not necessarily correspond perfectly to your results evenif there is only one mineral contributing to the peak. Many factors can inuencerelative intensity, with a particularly important one being tendency for preferredorientation. Different specimens of the same mineral can differ in crystal habit andhence in the tendency to orient preferentially.

6. Heat treatments can be used to support the identication of common soil mineralsthat undergo thermal decomposition or crystallographic shifts in response to dehy-

dration. Normally, routine mineralogical analysis of the soil clay fraction includesheat treatments (e.g., 25, 110, 300, and 550C) to establish the diagnostic criteria forphyllosilicate identication. However, the identication nonphyllosilicate minerals,such as gibbsite, goethite, and gypsum, can also be corroborated when their peaksdisappear at temperatures corresponding to their thermal decomposition (Table 42,Fig. 46).


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Harris & White

7. Phyllosilicates cannot usually be identied without interpretations from special di-agnostic treatments (cation saturations, glycerol solvation, heat treatments). Theseinterpretations are described in the next section.

8. Soils high in soluble salts pose a challenge for XRD mineral identication becausethey may contain several salt minerals with overlapping peaks, resulting in over-

printing of important accessory mineral peaks by peaks of more abundant miner-als. Complementary approaches such as electron microscopy in conjunction withelemental microanalysis may prove necessary to obtain a thorough mineralogicalassessment of soils high in salts.

Methods for Disriminating Among Phllosiliates

Phyllosilicates minerals are distinguished from one another mainly by the XRD peaksgenerated from atomic planes parallel to their dominant cleavage face (001). Multiple

peaks for a given mineral may be generated from these planes over a typical 2 scan be-cause constructive interference can occur when the path difference is an integral number

of wavelengths (consider the n of the Bragg equation). These peaks are commonly referredto as rst-order or d001, second-order or d002, etc. They are also sometimes termedbasal refections. They appear evenly spaced on an XRD pattern, which is a tip off forthe experienced eye that phyllosilicates are present. Intensities (peak areas) of differentorder peaks for a mineral commonly vary due to specic structural and compositional ef-fects of the unit cell on the diffracted beam. The d-spacing of higher-order peaks calculate

Table 41.Major d-spacings for some minerals that occur in soils. Spacings given for vermiculite andmontmorillonite are approximate values for Mg saturation; d-spacings for these and other expansiblephyllosilicates can vary with changes in relative humidity even for homoionic conditions. Italics desig-nates peaks of greatest intensity. It is useful to have a comprehensive reference such as the MineralPowder Diffraction Files compiled by the Joint Committee on Powder Diffraction Standards (JCPDS) andpublished by the International Centre for Diffraction Data.

Minerals Major d-spacings

Kaolinite 7.15, 3.57, 2.38Halloysite 10.710.0 (hydrated), 7.6, 4.4, 3.4Muscovite 10.0, 5.0, 3.33Biotite 10.4, 3.32Vermiculite 14.4, 7.18, 4.79, 3.60Chlorite 14.3, 7.18, 4.79, 3.59, 2.87, 2.39Montmorillonite 18.0, 9.0, 4.49Quartz 3.34, 4.26, 1.82

Dolomite 2.88, 2.19, 1.80Calcite 3.04, 2.29, 2.10Aragonite 3.40, 1.98, 3.27Anatase 3.51, 1.89, 2.38Rutile 3.26, 1.69, 2.49Gypsum 7.56, 3.06, 4.27Feldspar 3.183.24Amphiboles 8.408.48Gibbsite 4.85, 4.37, 2.39Goethite 4.18, 2.45, 2.70Hematite 2.69, 2.59, 1.69Ilmenite 2.74, 1.72, 2.54

For Mg saturation and glycerol solvation. Subject to variation with relative humidity, and fromspecimen to specimen.


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9 X-ray diffraction

to be equal to the d001 spacing divided by the order (i.e., d002 = d001/2). It is importantto recognize and account for these peaks, so they can be allocated to the mineral and not

be mistaken for other minerals. Also, sometimes they occur at the same position as majorpeaks for other common soil minerals, and their effect must be accounted for in explainingthe relative intensities of peaks. For example, the third-order peak for micas (d003; or fortwo-layer polytypes, d006) (3.33 ) is not easily resolvable from the most intense peak forquartz (3.34 ).

Expansible phyllosilicates have variable d00L spacings that relate to cation satu-ration and relative humidity, while nonexpansible species are insensitive to these vari-

ables. Therefore, nonexpansible species can be distinguished from expansible speciesby their xed d-spacings in XRD patterns that have undergone different treatments, andfrom each other (in most cases) by d-spacings and relative intensities of different ordered

peaks. Distinctions between expansible phyllosilicates require the application of diagnos-tic criteria based on response to cation saturations, glycerol solvation, and heat treatments.Different species of expansible phyllosilicates respond differently to these diagnostic con-ditions. Based on these general characteristics of phyllosilicates, a system for distinguish-ing phyllosilicate species is given below.

Begin by conducting XRD scans for Mg-saturated, glycerol-solvated samples from2 to 60 2; and for K-saturated samples, from 2 to 32 2 at room temperature and afterheating for 4 h at 110, 300, and 550C. The longer scan for the Mg-glycerol treatment is forthe purpose of detecting some soil nonphyllosilicate minerals whose largest peaks occurabove 32. Alternatively, the long scan can be run for the K-saturated sample.

Mark all peaks by the d-spacing value and refer to Table 41. The table can serve as aquick reference and general guide except for cases where there is signicant interstratica-

Table 42. Selected diagnostic d-spacings () of common soil minerals at specied conditions of cationsaturation, glycerol solvation, and heat treatment. See also Table 4 for additional XRD reference datafor soil minerals.

Mineral

Diagnostic d-spacings

MgMg +

glycerol K, 25C K, 110C K, 300C K, 550C

Smectites 1415 1518 1014 1012 10 10

Hydroxy-interlayeredsmectite

1415 1718 1114 1214 1112 1011

Vermiculites 14 14 1012 1011 10 10

Hydroxy-interlayerdvermiculite

14 14 14 1314 1113 1012

Chlorites 14, 7 14, 7 14, 7 14, 7 14, 7 14, 7

Dioctahedral mica 10 10 10 10 10 10

Trioctahedral mica 10.3 10.3 10.3 10.3 10.3 10.3

Halloysite 10 10 10 7.2 7.2 no peak

Kaolinite 7.2 7.2 7.2 7.2 7.2 no peak

Gibbsite 4.86 4.86 4.86 4.86 no peak no peak

Goethite 4.18 4.18 4.18 4.18 no peak no peak

Gypsum 7.56 7.56 7.56 7.56 3.50 3.50

Smectites (e.g., montmorillonite, beidellite, and montmorillonite) require further analyses forspecies-level distinction. See Methods for Discriminating Among Phyllosilicates section.


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20 Harris & White

tion. Any mineral for which all the conditions along its row are met is likely to be present,although there are exceptions that cant all be captured in a simple table. The steps below

provide some elaboration and qualications.

1. Does the Mg-glycerol XRD pattern show a peak at approximately 18 ? If so, doesthis peak shift to a lower d-spacing with K saturation, and collapse to 10 by 300C?If both of these conditions are met then a smectite mineral is likely present. Low-charge smectites with interlayer charge arising mainly from the octahedral sheet(e.g., montmorillonite, hectorite) generally have d-spacings 18 . Higher-chargesmectites, particularly those with signicant layer charge arising from the tetrahe-dral sheet (e.g., beidellite, saponite, nontronite) may show less expansion (1718) and a tendency to collapse more readily (Malik and Douglas, 1987). However,distinction between smectite species generally requires special procedures. A simpledistinction reported by Borchardt (1989) for octahedrally vs. tetrahedrally charged

smectites is that the former tend to expand to 18 when exposed to glycerol vaporonly, while the latter require exposure to liquid glycerol. Other approaches involvingLi saturation and heat treatment exploit the tendency for Li to enter vacant sites inthe octahedral layer of montmorillonite, thereby neutralizing layer charge and elimi-nating interlayer expansion (Greene-Kelly, 1952, 1953; Lim and Jackson, 1986).Random interstratication of smectite with other phyllosilicates can also produced-spacings intermediate between 14 and 18 . Generally, interstratication should

be suspected if the peak is very broad. Further explanation of interstratication willbe given below.

2. A broad peak in the range of 14 to 15 that undergoes only partial collapse (i.e.,

does not collapse to a d-spacing of 10 at 300C) is generally interpreted to be hy-droxy-interlayered smectite (i.e., it contains appreciable nonexchangeable hydroxy-Al polymers) (Barnhisel and Bertsch, 1989).

3. Does the Mg-glycerol XRD pattern show a peak at approximately 14 ? If so, doesthis peak shift to a lower d-spacing with K saturation, and collapse to 10 by 300C?This behavior is indicative of vermiculite.

4. A 14- mineral is generally interpreted to be hydroxy-interlayered vermiculite if itshows little change with K saturation at room temperature and collapses to a d-spac-ing >10 at 300C (Barnhisel and Bertsch, 1989) (Fig. 46).

5. Does the Mg-glycerol XRD pattern show a peak at approximately 14 that doesnot change with K saturation or heat treatment at least up to 300C and usually upto 550C? If so, this mineral is most likely a chlorite. In some cases, chlorite can

be present at too low a concentration to be observed at treatments before the 550Cheat treatment but is observed in this treatment because of the appearance due tointensication of the peak near but usually slightly lower than 14 .

6. Does the Mg-glycerol XRD pattern show a peak at approximately 10 that shiftsto approximately 7.2 after heating to 110C and disappears at 550C? If so, thismineral is halloysite. Alternatively, if the kaolinite peak is very broad, is there a peakthat rises sharply near 4.2 and decreases at a much slower rate over the next 3 to4? This may be the sign of dehydrated halloysite.

7. Does the Mg-glycerol XRD pattern show a peak at approximately 10 that does notchange with any other heat or saturation treatment? If so, the mineral is most likelymica.

8. Does the Mg-glycerol XRD pattern show a peak at approximately 7.2 that disap-pears with 550C heat treatment? If so, the mineral is most likely kaolinite.


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2 X-ray diffraction

Interstratifcation (Mixed Layering) in Phyllosilicates

Phyllosilicate crystals sometimes occur as randomly or regularly alternating layers oftwo or more phyllosilicates (MacEwan and Ruiz-Amil, 1975; Reynolds, 1980), rather thanas crystallographically discrete phases. This condition is referred to as interstratication or

mixed layering. It results in a series of XRD peaks that differ from those that would haveappeared if the interstratied minerals had occurred as independent phases. Some samplescontain the same two minerals in both interstratied form and discrete phases. As a furthercomplication, mixtures of elementary clay particles can apparently produce the same effectas interstratication on XRD patterns (Nadeau et al., 1984).

Interstratication is one of the major challenges to XRD mineral identication andquantication. It is essential to be able to recognize and interpret the effects of interstrati -cation. Computer models that aid in detecting and quantifying the extent of interstratca-tion are described below in Computer Diffraction Modeling.

The simplest form of interstratication to interpret from XRD data is regular inter-stratication, which in the ideal case is a perfect alternating pattern of layers of the type(MSMSMS, etc.). Regular interstratication is common in soils where primary phyllosili-cates (derived from crystalline rocks) such as mica or chlorite are being altered to second-ary expansible phyllosilicates. For example, the weathering of chlorite from mac meta-morphic rocks can involve a stage of regularly interstratied chlorite-vermiculite (Johnson,1964). Two primary phyllosilicates (e.g., chlorite and mica) can also occur as a regularlyinterstratied phase. Regular interstratication results in a d001 that is the sum of the d001spacings for the two phases, since the two phases together rather than individually estab-lish the repeating periodic pattern of the unit cell (Table 43). Thus, the lowest angle peak

(d001) for a regularly interstratied chlorite-mica would be 14 + 10, or 24 . Remember,though, that the intensity of peaks can vary according to structural and composition vari -ables, and the 24- peak is sometimes weak and hard to see on what is commonly a steep

baseline at low angles. Often the second-order peak is more intense.

Response to saturation and heat treatments for regularly interstratied phyllosilicatesmust be interpreted in light of both interstratied layer types. For example, regularly in-terstratied mica-vermiculite would have a second-order peak with Mg saturation at 25Cat about 12 . This peak would collapse to 10 with K saturation after heating to 110C.Regularly interstratied mica-chlorite, on the other hand, would have a second-order 12- peak that would not change with heat treatment due to the nonexpansible nature of the

interstratied phases.Randomly interstratied minerals are affected by heat treatments and cation satura-

tions in much the same manner as the minerals that form the interstratication, but thelocations of the peak are more complex. For example, if the d-spacing for a peak froman interstratied mineral is higher in the Mg glycerol pattern when compared with theMg-saturated XRD pattern, then the interstratication includes smectite. If there is no dif-ference between these two patterns, but there is a change when the mineral is K-saturatedand heated to 110C, then the mineral contains vermiculite. These effects are especiallyimportant when there are low amounts of one mineral interstratied with another. For ex -ample, if 10% smectite is interstatied in 90% kaolinite or chlorite, the Mg-saturated XRD

pattern will appear to contain only those minerals because the smectite d001 is the sameas that of chlorite and the d002 is the same as the d001 of kaolinite. When the sample isglycerol solvated, however, the sample peaks will broaden due to the deviation of the d00Lof the smectite from that of the other minerals, allowing the detection of the smectite in -terstratication.


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22 Harris & White

Interpretation of randomly interstratied phases is challenging, and one of the majorconfounding factors in assessments of phyllosilicates by XRD. This is because the dif-fraction effects result from packets of varying amounts of pure endmember minerals. Thelocation of peaks is the result of a weighting of the effects of diffraction for each mineral.

Where the peaks for both minerals are close, a sharp peak results. Where the peaks for theminerals greatly differ, no peak or a broad peak may be formed (for a detailed explanationsee the method of Mering as described in Sawhney, 1989). Another indication of randominterstratication is irrational higher order peaks (basal reections). Computer modeling

programs can be helpful in accounting for random interstratication, and in determiningproportions of the phases, as explained in the section Computer Diffraction Modelingbelow.

crstallite Size

Often the size of diffracting crystallites is important in describing the activity of amineral in soils, because surface area increases with decreasing particle size. The Scherrerequation (Klug and Alexander, 1974) gives a rst approximation of the value for this fac-tor:

L = (K)/( cos ) [2]

whereL is the mean crystallographic dimension in the same units as the wavelength ,Kis a constant near unity, is the width of a peak at half height in radians of 2 correctedfor instrumental factors, and is the location of the peak. The Scherrer equation may un-derestimate particle thickness, however, because it does not account for effects of atomic

substitutions. Another caveat is that it is actually crystal domain thickness rather than directparticle thickness that is calculated; it is common for soil mineral particles to contain nu-merous (smaller) crystal domains.

QUANTIFIcATION: LIMITATIONS AND APPROAcHES

Limitations

We begin with limitations to stress the fact that in most cases soil mineral quanti-cation is semiquantitative at best. Accurate mineral quantication by XRD can only beachieved under ideal conditions that include the following:

Mineral phases are discrete and well crystalline, which rarely applies for soils.

The degree of preferred orientation for each phase can be controlled or accountedfor.

Mounting artifacts like differential sedimentation are avoided.

Relatively few phases are present.

Deviation from these ideal conditions can result in considerable interlaboratory disparity inmineral proportions reported for the same samples (Ottner et al., 2000).

Quantication requires accurate calibration of the relative integrated XRD intensities

with relative weight fractions of phases present. A major impediment to attaining quanti-tative results for soils is the compositional and structural heterogeneity of soil minerals,which results in variability in peak intensity even within a given mineral species. Solidsolution series such as with feldspars also add complexity to XRD interpretations, as com-

pounded by specimen to specimen variability in such critical properties as crystal habitand tendency for preferred orientation. Phyllosilicate quantication is further complicated


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23 X-ray diffraction

Table 43.Major d-spacings for some minerals that occur in soils, listed for each mineral in the com-monly observed order of decreasing XRD peak intensity. Spacings given for expansible phyllosilicatesare approximate values for Mg saturation at room temperature; d-spacings for these minerals can varywith changes in relative humidity even for homoionic conditions. This is a partial listing of soil mineralsand d-spacings; it is recommended that analysts consult a comprehensive reference such as the MineralPowder Diffraction Files compiled by the Joint Committee on Powder Diffraction Standards (JCPDS) andpublished by the International Centre for Diffraction Data.

Mineral groups Minerals Major d-spacings

Amphiboles hornblende 8.52, 3.16, 2.73

riebeckite 8.40, 3.12, 2.73

tremolite 8.38, 3.12, 2.71Carbonates aragonite 3.40, 1.98, 3.27

calcite 3.03, 1.87, 3.85

dolomite 2.88, 2.19, 1.80Chlorites chamosite 7.05, 3.53, 2.52, 14.1

clinochlore 3.54, 7.07, 4.72, 14.1Expansible phyllosilicates montmorillonite 18.0, 9.0, 4.49

vermiculite 14.4, 7.18, 4.79, 3.60Feldspars albite 3.19, 4.03, 3.21

anorthite 3.20, 3.18, 4.04

microcline 3.24, 3.29, 4.22

orthoclase 3.31, 3.77, 4.22Kaolins halloysite 10.710.0 (hydrated), 7.6, 4.4, 3.4

kaolinite 7.17, 3.58,Micas biotite 10.1, 3.37, 2.06

muscovite 10.1, 3.36, 5.04Oxides, hydroxides anatase 3.51, 1.89, 2.38

gibbsite 4.85, 4.37, 2.39

goethite 4.18, 2.45, 2.70

hematite 2.69, 2.59, 1.69

ilmenite 2.74, 2.52, 1.72

quartz 3.34, 4.26, 1.82

rutile 3.26, 1.69, 2.49Phosphates uorapatite 2.80, 2.70, 2.77

strengite 4.38, 5.50, 3.11

variscite 4.29, 5.39, 4.83wavellite 8.67, 8.42, 3.22

Pyroxenes augite 2.99, 3.23, 2.95

enstatite 3.18, 2.88, 2.54Serpentines antigorite 7.29, 2.53, 3.61

chrysotile 7.31, 3.65, 4.57Sulfates epsomite 4.21, 5.35, 2.68

gypsum 7.56, 3.06, 4.27

jarosite 3.08, 3.11, 5.09Suldes marcasite 2.69, 3.43, 1.75

pyrite 1.63, 2.71, 2.43Zeolites and related minerals analcime 3.43, 5.60, 2.93

clinoptilolite 3.97, 8.99, 3.91

heulandite 3.92, 2.96, 8.85

palygorskite 10.4, 4.47, 4.26

sepiolite 12.1, 2.56, 4.31


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24 Harris & White

by varying degrees of interstratication and interlayer occupancy with nonexchangeablemetal polymers. Hydroxy-interlayered minerals, which are common secondary phyllosili-cates in highly weathered soils, do not occur in pure deposits and hence have not been wellcharacterized for their chemical and crystallographic properties. Furthermore, the nature

and abundance of interlayers as well as characteristics of the 2:1 components probably dis-play appreciable geographic variability.

Chapters are already available that address mineral quantication techniques usingXRD in relative detail (e.g., Brindley, 1980; Reynolds, 1989b; Snyder and Bish, 1989;Bish, 1994; Hughes et al., 1994). Also, some approaches to mineral quantication use othermethods (e.g., chemical composition, cation exchange capacity, thermal analysis) to com-

plement XRD (e.g., Jackson, 1969; Karathanasis and Hajek, 1982). Such sources shouldbe consulted if quantication is the major goal of the analyst. We will cover XRD quanti-cation in only an introductory way in this section, addressing basic principles and sum-marizing two commonly used techniques. A subsequent section, Computer Diffraction

Modeling, addresses modeling applications to aid in quantifying phyllosilicate mixturesfor oriented samples.

Theoretial considerations

The integrated intensity (I) of a given XRD peak is controlled by a variety of in-strumental, mounting, and sample-related factors, including weight fraction of the phasegenerating the peak. Absolute measurements ofIare impractical and unnecessary for the

purposes of quantication, thanks to the following relationship,

Ia

/Ib

=Kab

Wa

/Wb

[3]

whereIa

andIb

are relative intensities of selected peaks for minerals a and b, respec-tively, and W

aand W

bare weight fractions of minerals a and b relative to each other in the

sample. Noncrystallographic variables cancel out in the derivation of this equation. Thecalibration constant,K

ab, a key to mineral quantication by XRD, is also termed the min-

eral intensity factor(MIF) (Reynolds, 1989b). TheKab

(or MIF) can be determined empiri-cally by mixing standards of two minerals at a known weight fraction and conducting anXRD scan, in which case it can be calculated as the only unknown in the above equation.However, there is no guarantee that the standard will accurately match the species as itoccurs in the sample with respect to diffraction characteristics. Alternatively, it can be cal-

culated from the scattering factor as determined using XRD modeling computer programs(Reynolds, 1989b). The latter approach is faster and eliminates the necessity of havingmultiple standards on hand. Also, it can enable an accounting for compositional (e.g., Fevs. Mg) variations by iteratively specifying the elemental proportions until relative inten-sities of specic basal reections (for phyllosilicates) of the modeled pattern match theactual pattern.

Equation [3] is the basis for several approaches to mineral quantication by XRD.Two approaches, the intensity ratio method and the internal standard method, are explainedin the next sections. Another approach involving full-pattern tting (the Rietveld method;Rietveld, 1969), originally developed for crystal structure renement, has been adapted forquantitative XRD analysis (Bish and Howard, 1988; Smith, 1989; Bish, 1994). Full-pat-tern tting for mineral quantication is most advantageous for mixtures of well-orderedminerals, but it has been applied successfully to soils and geologic materials (e.g., Jackmanet al., 1997; Aylmore and Walker, 1998; Gualtieri, 2000; Jones et al., 2000; Planon andDrits, 2000; Altomare et al., 2001; Srodon et al., 2001). The full-pattern approach involves


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computer modeling, which is addressed in a later section of this chapter.

Intensit Ratio Method

An important caveat is that this method and the one that follows (internal standard

method) are based on theoretically sound principles, but they are unlikely to yield absolutequantitative results for soil minerals due to the factors summarized under Limitations,above. Nevertheless, these approaches do provide objective means of comparing relativeamounts of minerals within limited contexts (e.g., changes with depth, landscape). Usuallyit would be best to report data as an index (e.g., relative peak area index) rather than as

percentage.

Equation [3] can be rearranged as follows:

Wa

= (1/Kab

)(Ia/I

b)W

b[4]

Of course, 1/Kab is still a constant, which we could labelKba. Hence,W

a=K

ba(I

a/I

b)W

b[5]

We can represent a mixture of several minerals in terms of the weight fractions of theminerals, assuming all components are crystalline and detectable, and therefore the sum ofweight fractions is one:

Wa

+ Wb

+ Wc

= 1 [6]

We can determine the calibration constantsKca

andKcb

, say, by mixing known weights ofminerals a and c and minerals b and c and determining the values ofI

a/I

bandI

b/I

c(alterna-

tively, the constants could be calculated from computer XRD modeling software). In effect,we would determine the calibration constant for all minerals in the mixture in relation toone selected mineral in the mixture, in this case mineral c:

Kca

(Ia/I

c)W

c+K

cb(I

b/I

c)W

c+ W

c= 1 [7]

We can then solve forWc, the only unknown variable in the equation. Knowing W

callows

us to calculate Wa

and Wb

from the expressions representing them in the equation.

The intensity ratio method has a theoretical basis, is objective, and can be done withhigh precision. However, it fails to provide a check for accuracy since it forces the cal-

culated weight fractions to total one. This is a disadvantage, because accuracy is not con-strained to follow precision, and hinges on the degree to which the standard minerals matchtheir counterparts in the sample with respect to diffraction intensities (Bish, 1994). Theinternal standard method, explained next, does not require the constraint that the calculatedweight fractions total one.

Internal Standard Method

Equation [5] is also the basis for the internal standard method. However, Eq. [6] be -comes:

Wa + Wb + Wc = ? [8]Also, all the expressions representing weight fractions on the left side of Eq. [4] can

be calculated directly. This is because the mineral c would be added in known amounts(W

c) to the sample, hence the term internal standard. Favorable characteristics of an inter-

nal standard mineral are that (i) it has intense diffraction peaks, (ii) it doesnt occur in the


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26 Harris & White

sample, (iii) its strongest peak does not overlap with peaks of the minerals in the sample,and (iv) it occurs in the same 2 range as the peaks of the minerals being assessed in thesample. Corundum is a mineral that generally meets these characteristics and is a fre-quently used internal standard. Pyrophyllite is sometimes used as an internal standard for

samples containing appreciable phyllosilicate components because its orientation proper-ties and peak position are similar to other phyllosilicates.

cOMPUTER DIFFRAcTION MODELING

There are many programs that simulate XRD data. Some require detailed knowledgeof the crystal structure, including atom locations, symmetry, unit cell size, and types andamounts of substitution in the structure. These programs can give valuable information forinterpreting diffraction patterns, but often require information that is not available to the re-searcher. (Also, most methods are designed to simulate random powder diffraction patterns

rather than oriented patterns.) The principle reason for modeling XRD patterns is that itallows you to interpret at least qualitatively differences that are occurring between patterns.For example, models of a set of XRD patterns for a series of depths in a prole may allowyou to explain the types of changes in the mineralogy that accompany weathering, such asincreases in a mineral relative to others or decreases in mineral crystallinity.

Any program expected to simulate observed XRD patterns with any accuracy requiresthe correction for diffractometer parameters, including the radiation source, slit sizes, andthe presence of a theta compensating slit (device that maintains constant area of incident

beam exposure on sample). Proper use requires that these be preset to values that are usedby the diffractometer producing the observed patterns so that the calculated patterns are

consistent with those observed. Failure to set these parameters correctly can produce a spe-cious match with the observed patterns.

In general, there are two types of computer programs available for use in soil mineral-ogy: those that yield specic data about individual minerals in samples and those that can

be used to model entire diffraction patterns to quantify the minerals in the sample.

Mineral charaterization Programs

Several programs have been written to help in the characterization of individual min-erals in clay fractions. The most well known program is NEWMOD (Reynolds, 1985), andless well known but potentially useful are the MUDMASTER and GALOPER packages(Eberl et al. (1996, 2000).

When a sample is examined by XRD, the resulting peaks are averages resulting frombillions of individual crystals of different sizes and with different amounts of strain. TheMUDMASTER and GALOPER packages are written to aid in the characterization of crys-tallite size distributions and strain on the XRD peaks in the sample. Both programs assumethat the mineral examined is not interstratied. MUDMASTER (Eberl et al., 1996) uses

prole analysis of peaks to determine the crystallite size in the crystal direction corre-sponding to the peak. Proper use of the program requires high-quality XRD data collectedat relatively close steps (0.02 2) to calculate the crystallite size distribution. GALOPER

(Eberl et al., 2000) is a companion program to MUDMASTER that calculates crystal sizedistributions (CSDs) that result from crystal growth in open and closed systems using sev-eral growth mechanisms. The program also calculates the effects that two types of weather-ing reaction should have on crystal size. When used together, the programs have potentialfor interpreting the origin of a mineral in a soil.

NEWMOD (Reynolds, 1985) is probably the most useful computer program available


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for interpreting oriented XRD patterns for clay samples. The program calculates one-di-mensional (oriented) XRD patterns for pure phyllosilicates and for interstratications ofany two phyllosilicates. Once these minerals are individually simulated, the simulated pat-terns may be combined to simulate a mixture of minerals. It is not possible to model XRD

patterns for samples with nonphyllosilicates or the palygorskite-sepiolite group using thisprogram. NEWMOD allows for the calculation of effects of isomorphous substitution,ordering of interstratication, and particle size on the minerals present in a sample. Theresults are singularly useful for the estimation of isomorphous substitution within those

phyllosilicates with a high degree of substitution. A small amount of Fe substitution, forexample, in most 2:1 phyllosilicates may result in orders of magnitude changes in totalXRD intensity. This substitution can be estimated with NEWMOD using the changes in therelative intensities of the 00L peaks observed in the XRD pattern. NEWMOD is probablythe best available program for the interpretation of patterns resulting from interstratica-tions of one mineral with another. Calculation of particle size on the diffraction peaks of

phyllosilicates using peak broadening and shifts is also possible, and the program allowscustom size distributions such as those determined from MUDMASTER and GALOPER.Interpretation of differences in hydration in smectite XRD patterns is also possible usingthe program. A subprogram within NEWMOD can also be used for mineral quanticationusing simulated patterns if the only nonphyllosilicate present is quartz (which although notincluded in the program is used as an intensity standard and can be added as a data le).

Mineral Quantifcation Programs

Mineral quantication programs can vary from relatively simple to extremely detailed

and complex. In general, the programs use methods described earlier in our discussion ofquantication. The more easily applied programs, such as FULLPAT (Chipera and Bish,2003) and ROCKJOCK (Eberl, 2003), use reference patterns. More rigorous Rietveld pro-grams require more detailed information about the minerals in the sample.

The most common method, used in the programs FULLPAT (Chipera and Bish, 2003)and ROCKJOCK (Eberl, 2003), involve the use of patterns of reference standards, prefer-ably run on the same XRD unit under the same conditions as the sample. Both methodsinvolve the use of internal standards both in the reference standards and in the samples.Both programs are also written to work as Microsoft Excel spreadsheets. The mathematicsof the quantication is that covered in the Internal Standard Method section above. The

use of the internal standards has the advantage that it does not require the total to equal100%. The disadvantage is that the reference standards need to have similar characteristicsas the minerals in study samples. Application of these methods requires a consistent inter-nal standard. Therefore, anyone using this method is advised to obtain a large supply of themineral used as the internal standard to prevent problems resulting from small differences

between batches.

Another approach involving full-pattern tting is the Rietveld method (Rietveld,1969), which was originally developed for crystal structure renement and was adaptedfor quantitative XRD analysis (Bish, 1994). This approach uses no reference patterns butcalculates the pattern based on the crystal structures of the minerals present in the samples.

This method requires much more knowledge about the minerals present in the sample andat least some knowledge of crystallography. The use of the method has been simpliedsomewhat by use of modern graphical user interface, such as the EXPGUI package for theGeneral Structure Analysis System (GSAS) program (Toby, 2001), but it is still beyond thecapabilities of all but dedicated mineralogists. The programs work well for quantication


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2 Harris & White

of well-ordered minerals, but can be used with soils and geologic materials. The biggestlimitation is that a known crystal structure or a close approximation to the structure is re -quired. On the other hand, the programs can be used to rene a crystal structure if you cancome up with an approximation, even if it is in a mixture.

computer Modeling of Oriented X-Ra Diffration Patterns

The rst step in proper use of any XRD simulation program is to identify the mineralsin the sample examined. This requires interpretation of the observed patterns. It is impos -sible to model observed data without knowing what minerals are present in the sample.Examine all patterns carefully. Know how standard minerals react when treated in the samemanner as the samples. Do not forget to look at all the fractions even if there is especialemphasis on one fraction because a mineral may be more easily identied in another frac-tion (see Towe, 1974, for an example of the importance of this practice).

Although any of the programs listed earlier could be used to model oriented XRDpatterns, only NEWMOD is designed specically for that purpose. The programs that usereference patterns could be utilized if the reference patterns were also from oriented sam-

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X-Ray Diffraction Techniques for Soil Mineral Identification