Analytical electron microscopy in mineralogy; exsolved phases in pyroxenes

Ultramicroscopy 8 (1982) 109-120 109 North-Holland Publishing Company

ANALYTICAL ELECTRON MICROSCOPY IN MINERALOGY; EXSOLVED PHASES IN PYROXENES

Gordon L. Nord, Jr.

959 National Center U.S. Geological Survey

Reston, VA 22092

Analytical scanning transmission electron microscopy has been successfully used to characterize the structure and composition of lamellar exsolution products in pyroxenes. At operating voltages of 100 and 200 keY, microanalytical techniques of x-ray energy analysis, convergent-beam electron diffraction, and lattice imaging have been used to chemically and structurally characterize exsolution lamellae only a few unit cells wide. Quantitative x-ray energy analysis using ratios of peak intensities has been adopted for the U.S. Geological Survey AEM in order to study the compositions of exsolved phases and changes in compositional profiles as a function of time and temperature. The quantitative analysis procedure involves i) removal of instrument- induced background, 2) reduction of contamination, and 3) measurement of correction factors obtained from a wide range of standard compositions. The peak-ratio technique requires that the specimen thickness at the point of analysis be thin enough to make absorption corrections unnecessary (i.e., to satisfy the "thin-foil criteria"). In pyroxenes, the calculated "maximum thicknesses" range from 130 to 1400 nm for the ratios Mg/Si, Fe/Si, and ca/si; these "maximum thicknesses" have been contoured in pyroxene composition space as a guide during analysis. Analytical spatial resolutions of 50-100 nm have been achieved in AEM at 200 keV from the composition-profile studies, and analytical reproducibility in AEM from homogeneous pyroxene standards

is ± 1.5 mol% endmember.

I. INTRODUCTION

For the last fifteen years transmission electron microscopy (TEM) has been revealing many

new complexities in the crystal structures and compositions of minerals formed in a variety of geologic environments [1] . Understanding

these complexities and the thermodynamic and kinetic parameters which control their formation is a necessary step in the application of mineral systems to geological problems [2]. High resolution and analytical transmission electron microscopy (HRTEM and AEM) studies of minerals are now extending conventional TEM work, spec- ifically by high resolution imaging, diffraction and X-ray energy analysis.

Of these three high-resolution techniques, lattice-fringe imaging has been used the most often in mineralogy particularly for minerals with large unit cells [4]. The use of multi- beam images of crystal structures has been used for some complex structure types but only rarely have the experimental images been compared to calculated ones [5,6]. Microdiffrac- tion, rocking-beam and convergent-beam electron diffraction techniques have not yet been util- ized in mineralogical studies. Analytical electron microscopy in mineralogy is exclusive- ly done by X-ray energy or wavelength disper- sion and only in a few cases have any attempts been made to produce high quality reproducible analyses.

Table I SOME PRESENT USES OF ANALYTICAL ELECTRON MICROSCOPY IN MINERALOGY

I. Characterization of Fine-grained Phases a. Asbestos [7,8,9,10]

b. Cosmic Dust [ii] c. Fine-grained uranium ore [12] d. Alteration products [13,14] e. Phases in gas-rich meteorites [3,15] f. Fine-grained phases in quickly cooled

lunar igneous rocks [16]

g. Inclusions in minerals and glasses [17]

I I . Characterization of Complex Structures a. Nonstoichiometry

[18,19] Pyrrhotite Fe7S 8- FeS

[20] Al-rich Mullite - 3A1203"2SIO 2

b. Biopyriboles Intergrowths of Biotite-pyroxene- amphibole [6]

c. Modulated Structures Intermediate plagioclase

(Na,Ca)All.sSi2.508 [21]

III. Characterization of Transformation-Induced Microstructures

a. Exsolution in pyroxenes [22,23,24,25]

b. Exsolution in NiFe meteorites [40] c. Mechanisms of ~rowth in pyroxenes,

amphiboles [2ul

d. Compositions of exsolution products and construction of subsolidus phase dia- grams (pyroxene-plagioclase) [27'28'29]

e. Mechanisms of hydration reactions in biopyriboles [6,30]

110 G.L. Nord, Jl~ Analytical electrotl microscopy #l nziHeralogy

Three principle uses of HRTEM and AEM in min-

eralogy are set out in Table i, which cites

references to particular research topics. Of

particular importance, especially in light of

the work done on respirable particulates, is

the characterization of fine-grained phases.

Perhaps the most difficult particulate studies

are those of cosmic dusts (Fig. l) and the fine-

grained phases in meteorites. The importance

of these fine-grained phases, however, is very

large in relation to their size because they

are thought to be the most primitive materials

in the solar system, containing noble gases of

presolar origin. HRTEM has also contributed

greatly to understanding complex mineral struc-

tures such as the biopyriboles; a group of

minerals belonging to a polysomatic series of

sheet, single-chain and multi-chain silicates.

Studies such as these have increased our under-

standing of the reaction relationships between

rock-forming silicates. HRTEM and AEM techi-

ques have also greatly expanded our under-

standing of the importan t feldspar, pyroxene,

and amphibole minerals. The use of high-resol-

ution techniques is necessary because diffusion

rates in these minerals are very slow and even

at geologic time scales, transformation-induced

microstructures can still be only a few nano-

meters in size. In particular, AEM analyses

of microstructures in these minerals can lead

to a greater understanding of their subsolidus

phase relations; the nature of the subsolidus

phases in various silicate systems has not been

easily accessible by other experimental techni-

ques. A necessary condition for increased con-

trol on the subsolidus phase boundaries in

these systems is high quality microchemical

analyses; the remainder of this paper addresses

this problem.

2. X-RAY ENERGY DISPERSIVE ANALYSIS IN THE AEM

The advantage of microanalysis in a trans-

mission electron microscope is spatial resolu-

tion. Modern transmission electron microscopes

with scanning capability (STEM) are able to

focus a 60 to 200 keV electron beam to less

than i0 nm in diameter at the specimen surface.

It is a straightforward and easy process to

obtain an X-ray energy spectra from a thin

specimen, however there are a number of prob-

lems which make quantitative analysis of thin

films difficult. Some problems associated with

quantitative analysis are the occurrence of

spurious X-rays and electrons, low count rates,

and specimen contamination. The importance of

these problems vary with each instrument but

must be dealt with before accurate analyses

can be obtained [31].

The U.S. Geological Survey instrument is a

JEOL 200 keV TEM retrofitted with scanning

capability and a TracorNorthern 2000 X-ray en-

ergy dispersive system. Because the instrument

was not specially designed for analytical work

several modifications were made to reduce the

"systems background". One example of "systems

background" is a high energy X-ray flux derived

from the apertures of the microscope condenser

system. To detect this f]ux a test specimen

was used which emits both low energy and high

energy characteristic lines. The test specimen

was a ZnAI204 spinel with X-ray emmission lines

at 1.009 keV (Zn L~), 1.487 keV (AI K~), and

8.638 keV (Zn K~).

Figure 1 : Euhedral clinoenstatite laths in

samples of interplanetary [39] dust collected

by high altitude aircraft (micrograph courtesy of Phil Fraundorf).

Figure 2 : "In-hole" X-ray energy spectra,

0-10.24 keV, collected for 120 seconds in a

JEOL-200B operating at 150 keV. The electron

beam was situated at a position 1 Dm off the

thin edge of a ZnAI204 specimen. Spectra A

was taken without the tantalum aperture in

place whereas spectra B was taken with a 130 ~m

tantalum aperture placed above the objective

pole piece.

G.L. Nord, Jr. / Analytical electron microscopy in mineralogy 1 11

An "in-hole" spectrum was obtained (Fig. 2a)

by placing the beam at a position 1 micrometer

off the edge of the test specimen. This spec-

trum represents X-ray emission from the sample by uncollimated X-ray or electron radiation.

A significant Zn K~ peak was observed but no

Zn Lu or A1 K~ peaks were present. The ab-

sence of the low energy lines indicates that

the uncollimated radiation is mainly X-rays,

This is because cross-sections for fluorescence by X-ray photons favors excitation of

the higher energy line, Zn L~ whereas excita-

tion by electrons favors the lower energy lines Zn L~ and A1 Ks. The X-ray flux was eliminated

by the incorporation of a 1.0 mm thick tantalum,

130 ~m aperture above the objective pole piece; the "in-hole" spectrum (Fig. 2b) shows no

characteristic peaks of A1 or Zn. We have also

found that we can increase the peak/background

ratio of A1 Ks significantly by operating

the instrument at 100 keV instead of 200 keV

(Fig. 3). The source of this high background

appears to be electron-tailing and scattering from the objective pole piece [31].

using the ratio of the integrated intensities

of a particular characteristic peak to a single

reference peak, after background subtraction. This method was first used in AEM by Cliff and

Lorimer at the University of Manchester and has has been generally adopted for most mineralog-

ical applications. [32,33] The ratio method re-

lies on the fact that absorption and fluorescence corrections are not necessary when deal-

ing with samples that are very thin. The ratio

of integrated peak intensities therefore is

directly related to the ratio of compositions

by a correction factor K:

C B KAB I B

For studies on rock-forming silicates the re-

ference element B is silicon. The correction

factor KAB has been calculated for various elemental ratios by Goldstein et al. [34] and

varies with accelerating voltage but is inde-

pendent of concentration. Calculation of the factor involves a consideration of ionization

cross-sections and fluorescent yields as well

as the absorption of the X-rays by the detector window and detector materials but does not

consider the effects of sample-detector geo-

metry. Therefore, in order to use the ratio technique, K factors must be measured for each

elemental ratio of interest from well char-

acterized standards.

Figure 3 : X-ray energy spectra, 0-10.24 keV collected in a JEOL-2DOB operating at 200 keV

(Spectra A) and i00 keV (Spectra B). The

test specimen was ZnAI204, a natural spinel; a small amount of Fe is also present. The vert-

ical scales for both spectra are identical,

the 200 keY spectra has a higher background than the 100 keV spectra.

After eliminating instrumental artifacts the next problem is to develop a method of ob-

taining high quality, reliable analyses from

an analytical transmission electron microscope. We have adopted the very simple technique of

1.6"

1.4-

1.2. Cmg

1.0. Csi

0.8-

0.6-

0.4.

0.2.

0

/ol ivine

/-~- enstat i te

,/~gornet

diopside glass

0 0.2 0.4 0.6 0.8 1.0 1.2

Img / I si

Figure 4 : Determination of K correction factor

for the ratio Mg/Si. The mineral standards

are well characterized microprobe standards.

l 12 G.L. Nord, Jr. /Analy t ica l electron microscopy in mineralogy

An example of the measurement of K_ is - ng:Sl

shown in Figure 4 which relates intensity ra-

tios measured on standard specimens in the

AEM at 200 keV to their mass concentration

ratios. The slope of the line is KMg:S i and

has a value of 1.63 = 0.05. Note that the

intensity ratios are also a function of the

method by which the spectral data is reduced

to peak areas and the method used for back-

ground subtraction. In addition, the intensi-

ties are a function of the thickness of the

area analyzed and analyses must be obtained

from areas thin enough so that absorption and

fluorescence corrections can be neglected. It

follows therefore that the value of K factors

are unique to each instrument and the data

reduction method. Factors measured at both

100 and 200 keV in the JEOL 200B at the U.S.

Geol. Survey are shown in Figure 5. They are always slightly greater than the calculated

factors.

Of particular importance in doing AEM analysis

is to remain faithful to the thin film concept. If the thickness is too great, absorption be-

comes increasingly important, especially for

the light elements. Figure 6 illustrates the

effect of increasing thickness on the intensity

of aluminum and zinc from a natural spinel,

gahnite, ZnAI204. Absorption becomes important at thicknesses greater than 300 nm and there-

fore corrections must be made. In order to

4500-

4000-

3500

5000

25O0

52000 o

L_)

1500

I000

5OO

0

~o p

S Z zn

6

/

/ 2bo 3oo ' 550 600 Thickness (nm)

Figure 6 : Integrated peak intensities for

A1 K~ and Zn K~ versus thickness on gahnite,

ZnAI204, in the JEOL-200B operating at 100 kV.

Thicknesses were determined by measuring the

distance between top and bottom contamination

spots.

1.8

1.7

1.6

1.5

1.4

,~ 1.5

1.2 ~ I.I "~ 1.0

0.8

O.

0

o I 0 0 k e Y

• " 2 0 0 k e Y

N M A S P S C K C T V C M F C N r 13 e o i , I , ° I ,, ,, l I I I I = I I =I I

i 5 6 7 8 keV

Figure 5 : K-factors determined at both I00 and 200 keV for ratios pertinent to pyroxene analyses. Calculated factors are shown for comparison.

G.L. Nord, Jr. / Analytical electron microscopy in mineralogy 1 13

make absorption corrections, the thickness of the sample must be known accurately. A variety

of methods for measuring sample thickness have

been proposed, the easiest method is the use

of contamination spots as a surface marker and

then tilting the sample through a known angle

to obtain the thickness. Measured thicknesses

obtained by this method are subject to errors

of 10% or greater. If, however, the analysis can be obtained from areas of a thickness less

than that for which corrections are necessary,

a great source of analytical error can be eli-

minated. This approach is known as the "thin

film criterion."

Goldstein et al. [34] proposed that for any

set of two elements, A and B, considered in the

ratio method, the absolute value of the expres-

sion

(XB - XA) " P " t/2

should be less than 0.1 or an absorption cor-

rection is necessary. In this relationship t

is thickness, p is density and

B

XB = ~/9)specimen " Csc

where #/9)B is the absorption coeffic- speclmen

ient of element B in the specimen and ~ is

the angle between the specimen plane and the

line-of-sight of the detector in the plane con-

taining both the incident electron beam and the detector line-of-sight. Thus the "maximum

thickness" for analysis without calculating an

absorption correction is

B A tma x = 0.2/lp/P)spec - p/P)spec I • Csc ~ • p

The value of tma x has been calculated for the

pyroxene mineral system (see appendix for an

example of a "maximum thickness" calculation).

The composition of pyroxenes can be represented

by the major endmember components MgSiO 3 (en-

statite or En), CaSiO 3 (wollastonite or Wo) and

FeSiO 3 (ferrosilite or Fs). The ratios of

interest therefore, are Mg/Si, Ca/Si and Fe/Si.

Because pyroxene minerals do not occur with

CaSiO 3 contents greater than 50%, the composi-

tions are usually represented by a quadilateral

where the upper corners are represented by

(CaMg)SiO 3 (Diopside) and (CaFe)SiO 3 (Hedenber-

gite). Figures 7a,b,c show the values of tma x

on the pyroxene quadrilateral for each ratio.

The compilation of the minimum values of tma x

for all ratios (Fig. 7d) indicates that analy-

ses must be made in areas that are <1300-3000 A

thick in order to eliminate absorption effects.

Quantitative analysis in AEM is also dependent

upon knowing the stoichiometry of the sample,

especially in silicate systems where oxygen can

not be measured directly by EDA. For pyroxenes where all elements in the mineral except oxygen

can be detected by an X-ray energy spectrometer

the analysis is straightforward. The normal-

ized concentration of an element A, in the

group of elements detected, is given by the

expression

CA/Csi

= CA/Csi + CB/Csi + ... + Cn/Csi CA

These concentrations can then be converted to

oxide weight percents. For the case of hydrated

(Co,Mg)Si 05

Mg/si /

------~Q,Fe)Si05

\

MoSiO 3 FeSiO 5

Fe/S; %0 °0

%5oO

MgSiO 3 b o°@@ @ @

c d

FeSiO 3

Figure 7 : "Maximum thicknesses" of pyroxenes for which absorption corrections

can be ignored. The thicknesses in Angstroms for the ratios Mg/Si(a), Ca/Si(b), Fe/Si(c) are compiled in (d) as a guide for analysis.

114 G.L. Nord Jr ' Analytical electron microscopy in mineralogy

minerals, such as the amphiboles and mlcas, an

assumption must be made about the OH- content

based on other information.

The reproducibility of analyses in pyroxene is

quite good if the thin-film criterion is care-

fully followed. Figure 8 shows a cluster of

AEM analyses from a homogeneous pyroxene, pre-

viously analyzed by careful electron microprobe

analysis (EMPA) [35]. The average of the AEM

analyses is within the range found by micro-

probe analysis and the deviation is no greater

than .~1.5 mole% endmember. This deviation re-

flects the lower counting statistics in thin

-film analysis where integrated peak areas com-

monly contain only a tenth of the counts in

comparable microprobe analysis.

D! ~ ....

o EN FS~ / o v

9 0 8 0 70

Figure 8 : Distribution of AEM analyses (open circles) of an orthopyroxene from Emali, Kenya.

The small parallepiped encloses the range of

EMPA analyses on the same sample.

3. TYPES OF STUDIES IN THE PYROXENE MINERAL

SYSTEM WHERE AEM IS NECESSARY

Pyroxenes are common rock-forming minerals that

occur in terrestrial and lunar rocks, meteor-

ites and cosmic dust. Their relatively simple

chemistry and subsolidus phase relations make

transformation-induced microstructures ideal

recorders of their high-temperature history as

well as their subsequent rates of cooling. A

quantitative description of these subsolidus

reactions can lead to a great deal of informa-

tion about the evolution of planetary crusts

as well as the earliest formed solar material

in meteorites and cosmic dust.

The pyroxene subsolidus is dominated by a large

solvus separating calcium-rich phases from the

calcium-poor phases (Fig. 9). The exsolution

microstructures generated in pyroxenes due to

the presence of this solvus are well described

in the mineralogical literature [30'41] • The

size of these exsolved phases range from 10 ° to

105 nm and although the larger sizes can be

studied by the electron microprobe the smaller sizes can only be studied by AEM. We must know

the composition and structure of these lamellae in order to understand the pyroxene subsolidus.

eel\. Aug

]000

° oo

70011 1 , , , ,

WooEn52 Fs48 Wo50Fn34Fs16

Figure 9 : Pseudobinary of pyroxene subsolidus

along tieline shown in Figure 10. The natural

orthopyroxene-augite pyroxene couple used in

the annealing experiments has a bulk composi-

tion shown by the open triangle. The absence

of compositional change shown by the AEM analy-

ses after annealing suggests that the solvus

limb on the augite-side is steeper than pre-

viously thought (OPX = orthopyroxene, Pig =

pigeonite, Aug = augite).

Another consideration is the rate of approach

to equilibrium obtained by the two-phase as-

semblages formed in pyroxenes with bulk com-

positions that lie within the solvus. This

rate can be estimated by the extent and shape

of the compositional profile at the two phase

interface. The high spatial resolution of AEM

is ideal for this type of study.

Huebner and Nord [36] attempted to use AEM to

study pyroxene exsolution products in samples

from the Moore County meteorite. The 100 ~m

width of the exsolved phases provided a natural

diffusion couple and therefore by heating the

couple at temperatures higher up on the solvus,

an increased solubility for the adjacent phase

occurs and the compositional profile should be

modified. Such experiments can be used to

determine diffusion coefficients. The composition of the host orthopyroxene and exsolved

phase augite are shown in Figure 10; both EMPA

and AEM analyses are plotted. A traverse across

the interface at 200 keV indicates a spatial

resolution of ~I00 nm; the profile is relative-

ly flat up to the interface (Fig. 11). A pro-

file taken across a similar interface, heated

at 975°C for 7 days, showed no change. Huebner

and Nord interpreted the results as, I) the

solvus limbs are in fact very steep and no

change would have been expected, 2)the reaction

G.L. Nord, Jr. / A nalytical electron microscopy in mineralogy 1 15

°,/ \.d

+'%o Orthopyroxene

En Moore County Meteorite Fs

Figure 10 : Compositions of natural pyroxene diffusion couple determined both by electron microprobe analysis (EMPA) (open squares) and by analytical electron microscopy (AEM) (open circles).

20 ! 20

P

o o

io~ io.

5- 5.

SO

50

#0

30

20

lO

Moore County Meteorite SiO 2 I

/ M g O / ° ~ - - ~ o o /

Orthopyroxene / Augite

Figure ii : Quantitative AEM analyses across orthopyroxene-augite interface from a pyroxene crystal taken from the Moore County Meteorite. JEOL-200B operating at 200 keY.

is controlled by an immobile interface, unlike- ly at this temperature or, 3) diffusion rates are very low at the annealing temperature. A maximum diffusion rate for the experiment can be determined from the AEM data using the relationship

C - C I = erf x C o- C I 2 (Dr)

where C o is the starting composition (Wo42.5 for the augite lamellae), C I is the expected final composition (Wo35 from the pseudobinary in Fig. 9), C is the observed composition (Wo41 1.5 mol % less than Co) , t = 60.48 x 104sec (7 days) and x is the analytical resolution (I000~). Thus the maximum diffusion coefficient (D) is 6.6 x 10-17cm2/sec.

A few compositional profiles have been measured in pyroxenes from other slowly cooled rocks. Issacs and Peacor [37] recently measured a particularly interesting profile in augites from

high temperature metamorphic rocks from the

Adirondacks of Upper New York State. The augites (AUG) contained lamellae parallel to (100) of the low-calcium orthorhombic phase, orthopyroxene (OPX). In addition, the OPX lamellae were found to have marginal zones,

I00-150~ wide of the low calcium monoclinic phase, clinohypersthene (CHP)(Fig. 12); clinohypersthene is equivalent to pigeonite.

Figure 12 : (i00) Lattice fringe image of the interface between orthopyroxene lamellae (opx)

and augite host (aug). A narrow zone of low- calcium monoclinic pyroxene (CHP) is present between the host and lamellae (micrograph courtesy of A. Issacs[37].

0.4-

0 . 3 -

XCa Xsi

0.2-

0.1-

°o °

e o •

• o • • • e l

AUGITE

6 560

~•o • I100} . OPX

o i l ° • •

16oo lsbo A

Figure 13 : Compositional profile of Ca/Si across the interface shown in Figure 12. The traverse is a composite that superimposes four traverses (i00 ~ between points) consisting of of XCa/Xsi ratios from a total of 54 points.

1 16 G.L. Nord, Jr. /Analytical electron microscopy #7 mineralogy

Boundaries of the phases are sharp, as shown

by lattice fringe imagingoOf the (100)otype

reflections (d100 OPX=I8A; d100 CHP=9A; d200 Aug=4.5~, 100 is absent in Augite). AEM an-

alyses across the (100) interfaces (Fig. 13) show a concentration profile with higher Ca

concentration in the augite immediately adja-

cent to the CHP phase. Issacs and Peacor sug- gest that the low calcium monoclinic phase is

a necessary precursor to the equilibrium low

calcium orthohombic phase and thus effectively lowers the nucleation barrier. The presence

of the high calcium concentration at the growth

interface suggests that growth of this pre-

cursor phase is diffusion controlled.

Another important aspect of understanding the

growth of exsolution microstructures in pyro-

xenes is the structure of the interface. Inter-

face structure and chemistry may determine its

mobility and, therefore, the growth kinetics of

exsolved phases. Figure 14 shows an 80 nm wide

lamella of high calcium monoclinic augite in a host of low calcium orthorhombic orthopyroxene.

The specimen is from Emali, Kenya. The inter-

face between the monoclinic lamellae and ortho-

rhombic host contains a set of nearly periodic

interface dislocations. These dislocations are

seen directly in an a'b* orientation n-beam

high-resolution image (Figs. 15, 16) and appear

as bright areas on the (100) interface. This

bright area is formed because of the absence

of atoms with electron scattering potential at

the dislocation core. The 4.4 ~ fringes are

easily seen in Figure 15 crossing perpendicular

to the interface. Two 020 fringes from the

orthopyroxene side are terminated at the dis-

location core indicating a [010] Burgers

vector for the dislocation, reflecting the lar-

ger b cell edge of the augite. Figure 16

shows more clearly an unusual extension from

the core area into the augite side of the interface. This extension is one pyroxene

cell edge wide, ~9~, lies on (010) of augite,

and is characterized by several bright dots on one edge. This image contrast is similar to

that of HREM n-beam images of narrow amphibole lamellae, the double chain hydrated silicate

with b = 18~[ 6] . Amphibole lamellae have been

found to form along (010) of pyroxene as either alteration products or perhaps exsolution pro-

ducts[30]. The extension, is tentatively in-

interpreted as a single chain nucleus for the subsequent growth of a double chain amphibole. The open core area of the dislocation acts as

a conduit for hydration.

Prior to the development of microscopes with

small probe-forming capability, the only way to

determine the reciprocal lattice of such narrow

lamellae as that in Figure 14 was by selected-

area electron diffraction (SAED). The limit of SAED is approximately 500 nm at 200 kV, several

times the size of the lamellae under investiga-

tion. However with a focussed probe, conver ~ gent beam diffraction (CBD) patterns can be

Figure 14 : Transmission electron micrograph

of an 80 nm wide augite lamellae in an orthopyroxene host from Emali, Kenya. Black con-

tamination spots are visible in the lamella, as

well as strain contrast from a set of disloca-

tions at the lamella-host interface.

Figure 15 : n-beam structural image of opx-aug

interface showing 4.4 A (020) lattice fringes

and a [010] interface dislocation. Two (020)

fringes from the opx terminate at the disloca-

tion. Image taken on a Phillips 400T at the

National Bureau of Standards courtesy of Nancy Tighe.

Figure 16 : n-beam structural image of opx-aug

interface at a slightly different defocus con-

dition showing an extended defect parallel to (010) of augite at the interface dislocation.

The defect is suggested to be structurally

similar to amphibole; a double chain hydrated silicate.

G.L. Nord, Jr. /A nalytical electron microscopy in mineralogy 1 17

Figure 17 : Convergent-beam diffraction pat-

terns from host orthopyroxene (A) and augite

lamella (B).

formed where the convergence angle is essenti-

ally determined by the diameter of the second condenser aperture [38]. Figure 17 shows CBD

patterns for both the orthopyroxene host (A)

and augite (B) lamella. The absence of the

h + k odd spots in the CBD from the augite

lamella reflects the difference in the space groups, Pbca for orthopyroxene and C2/c for

augite. The CBD pattern in Figure 17B was

obtained from a lamella 80 nm in width.

An analytical spatial resolution of 90 nm for X-ray energy analysis was demonstrated in the

JEOL 200B on the same sample. One requirement

of the analysis was a count rate high enough

to give reproducible results. Analysis of suc-

cessively larger lamellae indicated that little

or no X-ray contribution from the host was

obtained only when the lamella width exceeded 90 nm (Fig. 18). Analytical spatial resolution

is of course dependent on the geometric limita-

tions of the situation; a detector at a higher take-off angle, smaller beam size, and lower

contamination rate would improve the resolution.

AEM and HRTEM are presently contributing im-

portant insights into the evolution of exsolution microstructures in clinopyroxenes. Coupled

with careful quantitative work and an experi-

mental mineralogy program, AEM presently has

a potential to more accurately define the sub-

solidus pyroxene system; evaluate the role of

interfaces to growth; relate the structure of

defect sites to nucleation processes; and de-

termine the value of diffusion coefficients at

high analytical spatial resolutions.

In a more general sense AEM and HRTEM will face

additional challenges in mineralogy. Some of

these are indicated in Table II below. Although

the list is not meant to reflect all possible

challenges, it does emphasize two lines of discovery: one for technological advances and

another toward further understanding of our

cosmic origins.

TABLE II : CHALLENGES FOR AEM IN MINERALOGY

I. Characterization of fine phases

a. Primitive cosmic material - cosmic dust

and meteorites b. Environmental material -respirable dust

II. Complex crystal structures -- defect sites for trace elements, trace-element

geochemistry and radioactive-waste disposal

III. Transformation-Induced Microstructures

a. Determination of subsolidus phase

relations in mineral systems

b. Reaction mechanism paths - nucleation

sites and transition phases

c. Kinetics of reactions - especially at

low temperatures (i.e0, water-rock

interactions)

formation of ore deposits

d. Determination of diffusion coefficients

in silicates

D~90n m Emali, Opx "k~d

/ / / ~ • EMPA o ATEM

/°° En Fs

Figure 18 : Quantitative ATEM analyses of augite

lamellae in Emali orthopyroxene host. EMPA

analyses of host are compared to ATEM analyses.

The ATEM analysis trend toward the (Ca,Mg)SiO 3 apex reflects attempts at analyzing successive-

ly larger augite lamellae. No contribution

from the host occurred at a lamella widths > 90 nm (100 keV on JEOL-200B).

ACKNOWLEDGEMENTS

I would like to thank G. Wandless for her lab

assistance; M. Ross and J. McGee for critical reviews of the manuscript; and P. Fraundorf and

A. M. Issacs for permission to publish figures

from their works.

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APPENDIX Calculation of "Maximum Thickness"

Consider a pyroxene from lunar sample 77115-34

for which the electron probe composition has

been determined [16] .

Mass Concentration (C)

wt% oxides wt% element

MgO 19.23 0 43.43

AI203 0.55 Mg 11.54 SiO 2 55.58 A1 0.29

CaO 5.05 Si 26.12

TiO 2 0.80 Ca 3.59 FeO 18.76 Ti 0.48

99.97 Fe 14.63

99.97

The mass absorption coefficient for X-rays

emitted by each element in the specimen is

calculated by the expression

em em C ~/P)spec. = ~ ~/P)i i

em where ~/P)i is the mass absorption coef-

ficient for the emitter (em) in the absorber

(i) and C i is the mass concentration (wt%) of

of the absorber in the specimen.

am ~/P)i Ci (cm2/g)

absorber

O M@ A1 Si Ca Ti Fe Spec.

Mg 948 53 2 209 96 17 895 2219

A1 586 503 1 131 60 ii 561 1854

Si 376 325 10 86 39 7 365 1209

Ca 45 40 1 139 5 1 47 278

Ti 26 23 1 79 28 1 27 184

Fe 10 9 0 30 ii 2 10 72

Thin film criteria for Mg/Si ~ =28 °

p =3.4g/cc

Si Mg . t = 0"2/l~/P)spec - ~/P)specl Csc ~ p max

0.2/I1209 cm2/g - 2219 cm2/gl(2.13)(3.4g/cm 3 )

= 2.73 x 10-5cm = 2730 A for Mg/Si

Similarly 4280 A for AI/Si

2960 A for Ca/Si

2760 A for Ti/Si 2420 A for Fe/Si

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Analytical electron microscopy in mineralogy; exsolved phases in pyroxenes