Weathering of the New Albany Shale, Kentucky, USA: I. Weathering zones defined by mineralogy and major-element composition

Applied Geochemistry 24 (2009) 1549–1564

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Applied Geochemistry

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Weathering of the New Albany Shale, Kentucky, USA: I. Weathering zonesdefined by mineralogy and major-element composition

Michele L.W. Tuttle *, George N. BreitUS Geological Survey, MS 964D, Box 25046, Denver, CO 80215, United States

a r t i c l e i n f o

Article history:Available online 3 May 2009

0883-2927/$ - see front matter Published by Elsevierdoi:10.1016/j.apgeochem.2009.04.021

* Corresponding author.E-mail address: [email protected] (M.L.W. Tuttle).

a b s t r a c t

Comprehensive understanding of chemical and mineralogical changes induced by weathering is valuableinformation when considering the supply of nutrients and toxic elements from rocks. Here minerals thatrelease and fix major elements during progressive weathering of a bed of Devonian New Albany Shale ineastern Kentucky are documented. Samples were collected from unweathered core (parent shale) andacross an outcrop excavated into a hillside 40 year prior to sampling. Quantitative X-ray diffraction min-eralogical data record progressive shale alteration across the outcrop. Mineral compositional changesreflect subtle alteration processes such as incongruent dissolution and cation exchange. Altered primaryminerals include K-feldspars, plagioclase, calcite, pyrite, and chlorite. Secondary minerals include jaro-site, gypsum, goethite, amorphous Fe(III) oxides and Fe(II)-Al sulfate salt (efflorescence). The mineralogyin weathered shale defines four weathered intervals on the outcrop—Zones A–C and soil. Alteration of theweakly weathered shale (Zone A) is attributed to the 40-a exposure of the shale. In this zone, pyrite oxi-dization produces acid that dissolves calcite and attacks chlorite, forming gypsum, jarosite, and minorefflorescent salt. The pre-excavation, active weathering front (Zone B) is where complete pyrite oxidationand alteration of feldspar and organic matter result in increased permeability. Acidic weathering solu-tions seep through the permeable shale and evaporate on the surface forming abundant efflorescent salt,jarosite and minor goethite. Intensely weathered shale (Zone C) is depleted in feldspars, chlorite, gypsum,jarosite and efflorescent salts, but has retained much of its primary quartz, illite and illite–smectite. Goe-thite and amorphous FE(III) oxides increase due to hydrolysis of jarosite. Enhanced permeability in thiszone is due to a 14% loss of the original mass in parent shale. Denudation rates suggest that characteris-tics of Zone C were acquired over 1 Ma. Compositional differences between soil and Zone C are largelyattributed to illuvial processes, formation of additional Fe(III) oxides and incorporation of modern organicmatter.

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1. Introduction

Continental weathering impacts water quality, ecosystem sus-tainability, agriculture, global climate change and human healthby redistributing nutrients and toxicants bound in unweatheredrock into the environment. Chemical weathering of rock occurs be-cause minerals alter to approach equilibrium with near surfaceconditions (Birkeland, 1974; White, 2003). The extent of disequi-librium is dependent on the formation conditions of the minerals(Goldich, 1938). Sedimentary rocks, especially those containingdetrital minerals inherited from previous weathering cycles, aregenerally less susceptible to alteration in weathering regimes thanthose formed at high temperature and pressures in the Earth’s inte-rior. Exceptions to the generalized stability of these sedimentaryrocks are those deposited in strongly reducing environments thatcontain organic matter, pyrite, and other sulfide minerals (Chigira

Ltd.

and Oyama, 1999). Under surficial weathering conditions, sulfideminerals oxidize and produce low pH fluids (Jambor et al., 2000).In the absence of buffering minerals such as carbonates, low pHsolutions enhance chemical weathering through hydrolysis of claysand feldspars with concomitant leaching of alkali elements(Chigira and Oyama, 1999; Stillings and Brantley, 1995; Stillingset al., 1995; Hamer et al., 2003). Secondary phases form whensolutions become oversaturated from reaction with other minerals,evaporation and mixing with ground and surface water (Jamboret al., 2000).

Many field and laboratory studies have been conducted onacidic drainage from sedimentary rocks, including those focusedon weathering of black shale and associated sandstone (Sullivanand Yelton, 1988; Puura and Neretnieks, 2000; Preda and Cox,2001; Carlson and Whitford, 2002; Strawn et al., 2002; Schieber,2004; Hammarstrom et al., 2005; Joeckel et al., 2005; Elswick,2006, plus those cited in Section 2 below). Common themes inthese studies are the oxidation of pyrite, alteration of mineralsby the acidic weathering solutions, transfer of weathering products

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into the broader environment, and formation of secondary mineralphases. This and companion studies (Tuttle et al., 2001, 2003,2009) describe these processes during weathering of the DevonianNew Albany Shale in eastern Kentucky.

The New Albany Shale and laterally time-equivalent units suchas the Antrim Shale, Chattanooga Shale, and Ohio Shale underliethousands of km2 of the eastern and central USA. The shale wasdeposited very slowly in relatively deep (>200 m water depth), an-oxic ocean basins (Potter et al., 1980). Although these conditionsgenerally favor lateral homogeneity of detrital particles in a depo-sitional unit, variations in the abundance of authigenic phases suchas sulfides result from microenvironments within any single bed.The trace-element enrichment characteristic of New Albany Shale(Leventhal and Hosterman, 1982; Ripley et al., 1990; Tuttle et al.,2003) is directly related to preservation of organic matter and for-mation of sulfide minerals in the anoxic sediment.

Experimental studies have shown that the New Albany Shaleand Ohio Shale generate acidic solutions when exposed to aeratedaqueous solutions. Solutes in the weathering solution originatefrom sulfide-mineral and organic-matter oxidation and silicate-mineral dissolution (Shirav and Robl, 1993; Robl, 1994). In naturalsettings, weathering solutions may discharge into local streams orat the ground surface where efflorescent salts form upon evapora-tion. The composition of these salts can be used as a proxy for thecomposition of weathering solutions (Jambor et al., 2000). Duringrainfall events, the efflorescent salts dissolve, producing acidic, me-tal-rich solutions (Bayless and Olyphant, 1993; Jambor et al., 2000).The fate of elements released by this process depends on the efflo-rescent-salt:rainfall ratio, the chemical evolution of the solution asit reacts with geologic media, and characteristics of ground andsurface water with which the solution mixes.

Compositional changes of minerals during acidic weathering offine-grained sedimentary rocks such as the New Albany Shale aretypically subtle and hard to quantify. In this paper, major-elementconcentrations and quantitative mineral data in the parent shaleprovide a reference for documenting changes in mineral composi-tion across the New Albany Shale weathering profile. Changes inmineralogy, mineral composition, and major-element compositionof the shale are explained by incongruent and congruent mineraldissolution, secondary mineral formation, cation exchange and lossand gain of amorphous oxide phases. Four distinct weatheringzones across the outcrop are resolved and characterized by themagnitude of these quantified changes.

2. Study site

Clay City, Kentucky is a popular location for New Albany Shaleweathering studies (Petsch et al., 2000, 2001a,b; Jaffe et al., 2002;Kolowith and Berner, 2002; Wildman et al., 2004). This portion ofKentucky has a humid, subtropical climate (McKnight and Hess,2000) and receives an average precipitation of about 45 cm/a. Thearea is unglaciated with rugged knobs and hills dissected by mod-erate- to high-gradient streams, forested, and on the western edgeof the Western Allegheny Plateau (EPA’s Knobs-lower Scioto Dis-sected Plateau Ecoregion; Woods et al., 2002). The Plateau is partof the Appalachian Plateau geomorphic province. Average erosionrates for the Appalachian Plateau in nearby West Virginia are 7–10 m/Ma, with rates of 4–11 m/Ma on hilltops (Clifton, 2005).New Albany Shale soils around Clay City are mapped as Jessie-town-Muse-Rohan complex, are on 20–45% slopes, and are influ-enced by the movement of shale and materials down slope(Hayes, 1993). They are shallow (average distance to bedrock is77 cm), well drained, predominately silt, and acidic (pH < 4).

The site for this study is a road cut just west of Clay City (de-scribed in Pryor et al., 1981; Figs. 1 and 2), which is ideal for study

because the New Albany Shale beds are horizontal and well ex-posed in a series of 40-a old road cuts. The horizontal beds of thisshale intersect the long-term weathering profile (Fig. 2), allowingcollection of weakly to intensely weathered shale along one depo-sitional unit. This sample strategy minimizes variation in parentshale, a key concern in weathering studies (Marsan et al., 1988;Reheis, 1990). A second, 40-a weathering profile into the outcropresults from the excavation of the road cut and provides an oppor-tunity to investigate short-term weathering processes in variablyweathered shale that is exposed ‘suddenly’ to surface conditions.Composition of efflorescent salts on the outcrop is a chemicalproxy for solutions generated during weathering of the NewAlbany Shale. Lastly, core from a shallow drill hole 8 km from theClay City outcrop provides unweathered (parent) New AlbanyShale from the same stratigraphic sequence as sampled on theoutcrop.

3. Methods

3.1. Sampling protocol

Samples of the Huron Member, New Albany Shale, were col-lected along a continuous depositional unit extending 20 m fromsoil on the road-cut edge towards the road-cut center (road-cutlocation: latitude 37�52.74000N and longitude 83�57.07170W)(Fig. 2A). Sampling protocol (detailed in Tuttle et al., 2003) includedcollection of two shale samples at each sample location—surfaceshale characterized by a yellowish weathering rind on exposed sur-faces (referred to as exposed shale) and a dark brown shale (re-ferred to as unexposed shale) collected 5 cm into the outcropdirectly behind the exposed sample (Fig. 2B). Four soil sampleswere collected 0.5 m above and 0.5 m below the projected extentof the sampled shale unit. Although the soil has likely moved downslope (Hayes, 1993), it was derived from the Huron Member, whichis the only unit underlying the hill slope. Four samples of efflores-cent salt were collected across an outcrop interval with slow waterseepage (referred to as the salt accumulation zone, SAZ; Fig. 2C).

The Kentucky Geological Survey’s Well Sample and Core Libraryin Lexington, KY provided shallow core material from the drill holeclosest to the road-cut location. The drill hole, designated D-8, isless than 8 km from the road cut and is located within the blockbounded by the following coordinates: 37�480 and 37�490N latitudeand 83�540 and 83�550W longitude. Eight Huron Member sampleswere collected (three contained massive nodule pyrite). Only datafrom the five samples without nodules are discussed in this paper.Efflorescent salt formed from oxidation of pyrite during storage ofthe core was also collected.

All road-cut and core samples were split into two equivalentfractions. One was ground to <150 lm and used for all chemicaland mineralogical analyses. The other was kept intact for scanningelectron microscopy and laser ablation-inductively coupled plasmaanalysis of pyrite (data from these analyses are discussed in Tuttleet al., 2009).

3.2. Chemical analyses

The ground sample was further split into two additional frac-tions. One was heated to 750 �C for 2 h. The ash was digested usinga 4-acid digestion (HCl, HNO3, HClO4, HF) or by sintering with Limetaborate (Taggart, 2002), and the major-element compositionwas determined using inductively coupled plasma-atomic emis-sion spectrometry (ICP–AES) (Bullock et al., 2002). The second frac-tion was analyzed by a combustion-infrared absorption techniquefor total S. A combustion-thermal conductivity cell technique wasused to determine total C and, after treatment of the sample with

Fig. 1. Stratigraphic column of New Albany Shale at the study site showing the approximate horizon sampled. B, Boyle Dolomite; D, Duffin Unit; H, Huron Member; F, FoerstiaZone; T, Threelick Bed; C, Cleveland Member (photograph modified from Pryor et al., 1981). Inset map shows map of Kentucky with approximate location of Clay City.

M.L.W. Tuttle, G.N. Breit / Applied Geochemistry 24 (2009) 1549–1564 1551

acid to remove carbonate, organic C. Carbonate C concentrationwas then determined by difference. All results are reported on awhole-rock basis. One blank; three standard reference materials(SRMs), including at least one certified SRM from the NationalInstitute of Standards and Technology (NIST); and one digestionduplicate were included in each batch of 40 samples for eachmethod. Major oxides, C species, and total S showed a recovery be-tween 90% and 110% and a relative standard deviation (RSD) of lessthan 10%.

The second fraction also was used to determine S species. Sul-fur in acid-volatile sulfide (S2�), disulfide (S2�

2 ), and SO4 was sep-arated and gravimetrically quantified using an analytical schememodified from Tuttle et al. (1986). The mono- and disulfides pro-duced H2S when treated with an acidified chromous chloride-acidsolution under an inert atmosphere. The H2S was collected asAg2S. The SO4 was dissolved during the HCL extraction and pre-

cipitated as BaSO4. Results are reported on a whole-rock basis.Precision was determined by calculating the relative standarddeviation (RSD) of duplicate samples (one out of every 10 analy-ses) and was considered acceptable if the RSD was no greaterthan 10%. Accuracy was determined by analyzing standards com-prised of known amounts of pure sulfide and sulfate minerals andwas considered acceptable if recovery was in the range of 90–110%.

Efflorescent salts collected on the outcrop contained small frag-ments of shale. To remove the shale, the samples were dissolved indeionized water (1:500 ratio by weight). Solids (generally bits ofshale) were removed by filtration, weighed, and their mass sub-tracted from the total sample weight. The solution was acidifiedwith ultra-pure HNO3 and submitted for analyses by inductivelycoupled plasma-mass spectrometry (ICP–MS) (Lamothe et al.,2002). Results are reported on a dry-weight basis. One certified

Fig. 2. (A) Photo of transect sampled along the New Albany Shale road cut. White dots show 7 of the 11 shale sample locations (additional four locations are off the left side ofthe photo) and four soil sample locations; (B) exposed shale with weathering rind (lighter shade) and unexposed shale 10 cm behind surface (darker shade); and (C)efflorescent salt on the face of the road cut.


NIST standard was included in the analyses and indicates a recov-ery between 90% and 110%. Salt analyses were not duplicated.

3.3. Mineralogical analyses

Major mineral constituents of the unheated fraction of shaleand soil were identified by powder X-ray diffraction (XRD) ofside-packed mounts using the scan conditions described below.The ROCKJOCK program of Eberl (2003) quantified the mineraland organic-matter content. The program optimizes a whole-pat-tern fit using previously scanned mineral standards. Samples wereground in a McCrone (Use of trade names is for descriptive pur-poses only and does not imply endorsement by the US Govern-ment) mill to 5 lm, along with an internal standard of ZnO. ASiemens D500 using Cu–Ka radiation and a monochrometer col-lected data from 5� to 65� 2H at 0.02� steps and a count time of2 s per step. The lower limit of determination varies with crystal-linity and long-range order of the minerals and is considered tobe 0.5 wt% for the phases considered. The accuracy of the methodevaluated with synthetic mineral mixtures is estimated to be with-in 10%. Precision for major minerals (>15 wt%), and organic matter(peat as standard) is 10% and 25% for minor minerals (<15 wt%).

Data labeled as quartz include well-crystalline quartz plus less-ordered silica phases consistent with recrystallized chert. Data la-

beled as illite refer to well-ordered 2M1 illite and that labeled asillite–smectite refer to interstratified illite and smectite. Pyrite con-centration calculated from Sdisulfide data correlates with, but ismuch more sensitive than, pyrite(XRD) and will be used in all subse-quent discussions of pyrite abundance. The presence of amorphousFe(III) oxides (including oxyhydroxides) that are poorly resolved byXRD was suspected from the Fe chemistry and estimated by thedifference between the amount of Fe measured by chemical anal-ysis and the amount of Fe contained in other phases measuredby XRD. Estimates of efflorescent salt abundance by XRD werenot possible due to lack of standards, although qualitative datawere collected and are discussed further in Tuttle et al. (2009). To-tal efflorescent salt content was calculated from the SO4 concentra-tion and a mineral stoichiometry estimated in Section 4.3. The XRDpeaks from these salts did interfere with quantification of chloriteand organic matter in exposed shale in the SAZ, resulting in anoverestimation of the content of these phases in three samples.

4. Results

4.1. X-ray diffraction data

Weight percent of mineral and organic phases in selected samplesis listed in Table 1 and plotted in Fig. 3A and B. Phase concentrations

Table 1Concentrations of minerals and organic matter in parent Huron shale in core, exposed shale on road cut (‘ex’ preceded by meters from outcrop edge), unexposed shale on road cut(‘unex’ preceded by meters from outcrop edge), and soil on road-cut edge (0 m).

Fieldnumber

Meter/exposure

Quartz(wt%)

K-feldspar(wt%)

Plagioclase(wt%)

Calcite(wt%)

Gypsum(wt%)

Jarosite(wt%)

Goethite(wt%)

Illite(wt%)

Illite-smect(wt%)

Chlorite(wt%)

Organicmatter(XRD)(wt%)

Pyrite(wt%)

Efflorescent.salt (wt%)a

Fe2O3

(am)(wt%)a

KY-12 0-soil 36 2.1 <0.5 <0.5 <0.5 <0.5 2.8 26 27 2.6 3.2 <0.5 <0.5 4.9KY-15A 2.0-ex 47 3.8 <0.5 <0.5 <0.5 1.7 1.4 23 17 1.7 3.9 <0.5 <0.5 2.6KY-15B 2.0-unex 49 3.4 <0.5 <0.5 <0.5 1.4 1.9 23 16 1.9 3.8 <0.5 <0.5 3.1KY-10A 2.6-ex 48 3.1 <0.5 <0.5 <0.5 1.8 0.8 23 19 1.4 2.8 <0.5 <0.5 2.2KY-10B 2.6-unex 47 3.4 <0.5 <0.5 <0.5 1.8 1.4 22 18 1.7 4.5 <0.5 <0.5 0.8KY-16A 3.1-ex 47 2.8 <0.5 <0.5 <0.5 1.0 1.3 23 18 1.8 5.0 <0.5 <0.5 2.7KY-16B 3.1-unex 47 3.9 <0.5 <0.5 <0.5 1.8 1.3 23 17 1.7 3.8 <0.5 <0.5 2.6KY-17A 3.7-ex 45 3.6 <0.5 <0.5 <0.5 2.5 1.6 23 18 0.9 4.7 <0.5 <0.5 3.3KY-18A 4.4-ex 43 3.3 0.7 <0.5 <0.5 4.3 0.8 21 18 1.5 7.0 <0.5 <0.5 1.9KY-19A 5.8 exp 38 2.6 1.0 <0.5 <0.5 7.7 <0.5 18 18 1.9 13 <0.5 0.6 1.6KY-19B 5.8-unex 42 2.9 0.9 <0.5 <0.5 10 <0.5 20 14 1.2 7.8 <0.5 <0.5 0.8KY-20A 6.6-ex 37 2.9 1.0 <0.5 <0.5 9.9 <0.5 15 14 –b 9.0b <0.5 4.0 <0.5KY-24A 8.1-ex 35 2.8 1.4 <0.5 1.2 5.6 <0.5 11 16 –b 6.0b <0.5 11 <0.5KY-24B 8.1-unex 43 5.1 1.4 <0.5 0.5 6.1 <0.5 18 18 1.4 5.6 <0.5 <0.5 1.5KY-25A 10.6-ex 28 3.1 0.8 <0.5 2.3 3.4 <0.5 12 13 –b 7.0b 1.0 9.3 1.3KY-26A 12.8-ex 41 3.6 1.5 <0.5 0.8 4.7 <0.5 16 19 1.4 10 2.8 <0.5 1.6KY-26B 12.8-

unex41 3.5 2.0 0.8 1.7 2.8 <0.5 16 16 2.2 9.8 5.0 2.7 2.6

KY-28A 16.1-ex 41 3.7 1.2 0.5 <0.5 6.6 0.7 19 18 1.8 6.4 1.2 <0.5 1.1KY-28B 16.1-

unex41 3.4 1.8 <0.5 0.8 4.7 0.8 18 22 1.7 5.0a 1.9 2.7 1.5

KY-60 Core 43 5.1 1.4 2.4 <0.5 <0.5 <0.5 16 16 6.0 9.4 6.5 <0.5 <0.5

a Concentrations estimated using chemical results (see Section 3.3).b XRD interference from efflorescent salts. Organic-matter concentration adjusted with Corg concentrations; no data available to correct chlorite concentrations.


sum to between 100% and 106%, with a median of 103%. The XRDorganic-matter content (Table 1) was compared to that calculatedfrom Corganic contents (Table 2). [The organic matter in the New Al-bany shale is Type II (Petsch et al., 2000). Organic-C concentrations(Corg) are converted to organic-matter concentrations by using aType II factor of 1.2 (Tissot and Welte, 1984)]. Excluding theexposed samples in the SAZ that contain large quantities of efflo-rescent salts, the XRD and chemical data correlate very well(r = 0.95).

In parent and weathered shale, primary minerals include quartz,illite, illite–smectite, chlorite, K-feldspar (microcline and ortho-clase), plagioclase (mainly oligoclase), pyrite and calcite. Thechanges in abundance of some primary minerals across the profileare large (Fig. 3A), but comparison of exposed and unexposed shalefrom the same locations did not detect systematic differencesexcept for chlorite and organic matter in samples affected by efflo-rescent-salt interference. From parent to weathered shale on theedge of the outcrop (unweathered to 2.0-ex in Table 1 and onFig. 3A), increasing minerals include quartz (43–47 wt%) and illite(16–23 wt%). Illite–smectite is essentially constant (16–17 wt%).Minerals that decrease include chlorite (6.0–1.7 wt%), K-feldspar(5.1–3.8 wt%), plagioclase (1.4–<0.5 wt%), pyrite (6.5–<0.5 wt%),calcite (2.4–<0.5 wt%), and XRD organic matter (9.4–3.9 wt%).

Crystalline phases produced solely by weathering processes(undetected in parent shale and referred to as secondary minerals)include gypsum, jarosite, goethite, amorphous Fe(III) oxides, andthe soluble Fe/Al efflorescent salts (Table 1 and Fig. 3B). Theformation of all secondary phases is tied to the release of Fe, SO4

and acid during pyrite oxidation. Goethite and amorphous Fe(III)oxide increase in shale at the edge of the outcrop as jarositedecreases (correlation coefficient for the proportion of Fe in goe-thite + amorphous Fe(III) oxide versus the proportion in jarositeis �0.98). Unexposed shale contains slightly less jarosite than thecorresponding exposed shale. Goethite has low and generally sim-ilar concentrations (<2 wt%) in exposed and unexposed shale.Amorphous Fe(III) oxide concentration is initially low in exposedshale relative to unexposed shale; however, this relationship re-

verses as weathering progresses. Gypsum in detectable concentra-tions (0.5–2.3 wt%) only occurs in slightly weathered and SAZsamples. Small amounts of efflorescent salt (3 wt%) occur in unex-posed samples from shale nearest the center of the outcrop. Efflo-rescent salt concentrations are highest in exposed samples in theSAZ (up to 11 wt%).

4.2. Chemical data

Results of all chemical analyses are listed in Tuttle et al. (2003).The major-element chemical results discussed in this paper aresummarized in Table 2 and plotted in Fig. 4 (organic matter inFig. 3A). Summed, the major oxides and organic-matter concentra-tions range from 90% to 109% with a median of 97%. The loss ofmineral phases discussed above is consistent with the chemicaldata. Calcium, principally bound in calcite in the parent rock andgypsum as a secondary phase, is depleted in the intensely weath-ered shale near the outcrop edge to concentrations <0.1 wt% CaO.Similarly Na2O decreases from 0.4 to 0.2 wt%, consistent with theobserved mass loss of plagioclase. Iron that resides in pyrite andchlorite in parent shale and in secondary phases (jarosite, goethite,efflorescent salt and amorphous oxides) in weathered shale dem-onstrates large spatial variability (4–9 wt% as Fe2O3). Aluminaand SiO2 reside in multiple mineral phases (quartz, clays, feldspars,and, in the case of Al2O3, in efflorescent salts). The Al2O3 concentra-tion essentially is constant from the unweathered shale to theweathered shale on the outcrop edge (13–14 wt%). Silica, on theother hand, increases from 53 to 66 wt% across the same transect.Concentrations of K2O, residing in feldspars, clays and jarosite, re-main relatively unchanged along the profile (approximately 4 wt%)except for lower values in exposed shale in the SAZ. Magnesium(primarily in clays) decreases (1.4–1.0 wt% MgO).

4.3. Mineral chemistry

The consistency of the chemical composition of minerals inshale was tested by examining the ratio of measured concentra-

Fig. 3. Concentrations of (A) primary mineral and organic matter and (B) secondary minerals, in parent Huron Member shale from the unweathered core, unexposed shale,exposed shale, and soil (0 m) plotted along the weathering profile. The shaded area delineates the salt accumulation zone (SAZ). XRD organic-matter data are corrected forinterferences.


tions to expected concentrations calculated from mineral abun-dances (Table 1) and mineral stoichiometry (Table 3) (see Section5). Minerals such as quartz, plagioclase, K-feldspar, pyrite, calcite,jarosite, goethite and amorphous Fe(III) oxide were assigned gen-erally accepted compositions. Illite (XRD results consistent with a2M1 illite) was assigned a composition approximating a hydro-muscovite similar to formulas presented in Newman (1987; Table1.22), in which the octahedral sites are filled with Al. Chlorite dif-fraction characteristics are consistent with an Fe-rich chlorite sim-ilar to ripidolite as defined in Foster (1962). Iron not accounted forby pyrite was assigned to chlorite, and Mg, Al and SiO2 were addedin proportions compatible with Foster’s ripidolite. The K, Na, Al,

Mg, Fe and SiO2 remaining were then used to calculate the compo-sition of a generalized illite–smectite. The resulting stoichiometricformula had a charge balance within 5%, was within the limits ofknown compositions, and compatible with the New Albany Shaledepositional and diagenetic history.

A general stoichiometry of efflorescent salt was approximatedbased on the composition of water-soluble extracts. On average,the Al and Fe concentrations are 1:1 on a molar basis (Tuttleet al., 2009). A generalized stoichiometry for Fe–Al sulfate salt(Table 3) is used to reflect the 1:1 metal relationship and is consis-tent with salts identified with XRD. The stoichiometry does not in-clude waters of hydration.

Fig. 3 (continued)


5. Discussion

5.1. Assessment of weathering effects on mineralogy and chemistry

As shale weathers to soil, mineral and major-element abun-dances reflect gains, losses, and changes in rock density. Changesin chemical composition of individual minerals occur, but may goundetected with XRD if the changes do not affect the mineralstructure (e.g. cation exchange and incongruent dissolution). Inmany weathering studies, major-element concentrations, invariantmineral stoichiometry, and normative calculations are used toquantify minerals across a weathering profile. Because mineraland element concentrations are not independent variables in thisapproach, it is impossible to track the compositional changes inminerals as the rock weathers to soil. In addition, density changesin the weathering rock result in an increase or decrease in mea-sured concentrations even in the absence of loss or gain of the ele-ment or mineral. Several transformation techniques minimize theimpact of undetected mineral compositional and density changeson the analytical data presented in Figs. 3 and 4.

5.1.1. Evolution of mineral composition during weatheringQuantitative mineralogical data for the New Albany Shale

profile (Table 1) were measured by XRD and the stoichiometryof minerals in parent shale (Table 3) independently determined.Therefore, the major-element data in Table 2 can be used to

track changes in mineral composition. First, a calculated oxideconcentration in each sample is derived from the mineral abun-dance and parent-shale mineral stoichiometry ([oxide]c). Next,the measured oxide concentration ([oxide]m) is compared to thiscalculated oxide concentration (Fig. 5). As density affects [oxi-de]m and [oxide]c equally, the use of these ratios eliminatesthe impact of this variable. If [oxide]m/[oxide]c is one, no compo-sitional change has occurred during weathering. Deviation of theratio from one is attributed to compositional changes not de-tected by XRD.

Ratios in parent shale from the core are near one for all ele-ments (Fig. 5), indicating that the quantitative mineralogy andmineral stoichiometry adequately account for the measured con-centrations of elements prior to weathering. Ratios for SiO2,Al2O3 and MgO change little across the weathering profile. In con-trast, Na2O and K2O are depleted relative to the concentrations pre-dicted by the abundance of the mineral phases. Feldspars, illite andillite–smectite host these elements whose low ratios may repre-sent incongruent dissolution and change in the interlayer cationabundances in response to the acidic weathering conditions (seeSection 5.2). Ratios for CaO initially are >1, especially in exposedshale, suggesting an undetected secondary phase. In shale at theoutcrop edge, ratios fall as CaO concentrations approach the ana-lytical detection level. Ratios for S and Fe are not shown in Fig. 5because efflorescent salt and amorphous Fe(III) oxide contentswere estimated from chemical data.

Table 2Concentrations of major elements, calculated organic matter (OM = Corganic � 1.2), and elements Zr and Nb in parent Huron shale in core (average of KY-57, -58, -59, -60, -64),exposed shale on outcrop (‘ex’ preceded by meters from outcrop edge), unexposed shale on outcrop (‘unex’ preceded by meters from outcrop edge), and soil on the road-cut edge(0 m).

Field No. Meter/exposure

Na20(wt%)

Al2O3

(wt%)CaO(wt%)

Fe2O3

(wt%)K2O(wt%)

MgO(wt%)

Stotal

(wt%)SiO2

(wt%)TiO2

(wt%)OM(wt%)

Zr(mg/kg)

Nb(mg/kg)

01-KY-12 0-soil 0.16 17 0.02 8.2 4.2 1.3 0.15 60 0.81 1.8 130 1101-KY-14 0-soil 0.16 18 0.01 9.0 4.6 1.4 0.08 58 0.81 1.2 135 1101-KY-13 0-soil 0.18 14 0.01 6.0 3.7 0.90 0.09 65 0.93 2.0 150 1201KY-11 0-soil 0.18 14 0.08 8.2 3.7 0.97 0.33 59 0.82 4.0 130 1001-KY-15-A 2.0-ex 0.22 14 0.01 5.3 3.9 0.99 0.47 66 0.79 4.7 135 1001-KY-15-B 2.0-unex 0.22 13 0.01 6.3 3.5 0.90 0.46 65 0.76 4.4 120 1101KY-10-A 2.6-ex 0.18 13 0.01 4.3 3.8 0.97 0.39 67 0.78 4.1 125 8.901-KY-10-B 2.6-unex 0.22 13 0.04 3.8 3.6 0.97 0.62 67 0.77 5.4 115 1301-KY-16-A 3.1-ex 0.18 13 0.01 5.0 3.7 0.90 0.33 65 0.77 47 125 1101-KY-16-B 3.1-unex 0.20 13 0.01 5.3 3.7 0.90 0.50 65 0.76 4.6 125 1001KY-17-A 3.7-ex 0.22 13 0.01 6.2 3.6 0.93 0.59 63 0.76 4.7 135 1101KY-17-B 3.7-unex 0.26 13 0.01 4.3 3.7 0.90 0.66 65 0.75 5.2 130 1201KY-18-A 4.4-ex 0.29 13 0.01 5.5 3.8 0.99 1.1 64 0.75 5.5 150 1201KY-18-B 4.4-unex 0.33 13 0.01 4.9 3.7 0.90 1.3 64 0.74 6.9 130 1201KY-19-A 5.8 exp 0.34 11 0.01 6.5 2.9 0.74 1.9 52 0.65 9.0 120 1001KY-19-B 5.8-unex 0.35 11 0.03 6.3 3.2 0.80 1.9 61 0.71 7.2 135 1101KY-20-A 6.6-ex 0.34 11 0.07 6.9 3.1 0.93 3.2 53 0.66 7.5 100 1001KY-20-B 6.6-unex 0.32 11 0.10 6.7 3.7 0.89 2.9 55 0.66 5.9 110 9.301KY-24-A 8.1-ex 0.27 10 0.53 7.2 3.0 1.0 4.4 46 0.57 4.6 120 8.601KY-24-B 8.1-unex 0.33 12 0.23 5.5 4.0 0.85 1.6 60 0.72 6.2 150 1001KY-25-A 10.6-ex 0.27 11 0.86 7.5 3.2 0.95 4.1 49 0.59 5.6 125 8.501KY-26-A 12.8-ex 0.36 12 0.38 6.2 3.9 0.85 2.7 60 0.71 8.1 160 1101KY-26-B 12.8-unex 0.34 11 1.4 8.7 3.5 1.0 4.5 56 0.68 9.1 165 1001KY-28-A 16.1-ex 0.34 12 0.37 6.4 3.8 0.86 1.7 57 0.67 5.9 120 1001KY-28-B 16.1-unex 0.35 11 0.22 7.7 3.5 0.83 3.3 53 0.69 5.0 170 9.4Huron Member Core 0.42 13 1.2 6.1 3.9 1.4 2.7 53 0.69 8.5 150 11


5.1.2. Absolute changes in chemical and mineral concentrationsThe use of index phases to normalize chemistry of weathered

material is well established. This approach differentiates apparentconcentration change (change due to gain or loss of another con-stituent or change in bulk density of the sample due to compactionor dilation) from absolute concentration change (mass loss or gainof an element of interest in an original volume of weathering rock)(Goldich, 1938; Marshall and Haseman, 1942; Jackson and Sher-man, 1953; Esson, 1983; Driese et al., 2000). These index phases(commonly minerals containing Ti, Zr, and rare earth elements;Amundson, 2003) are thought to be immobile during weathering,especially in shale where they have already undergone at leastone weathering cycle prior to deposition. Quartz has been usedin cases where density sorting of Ti and Zr minerals is suspected,particularly in soils (White, 2003). Recent studies (Gardner,1980; Brimhall and Dietrich, 1987; Brimhall et al., 1988, 1991;Stiles et al., 2003) have shown that, under certain conditions, com-mon index phases such as Ti and Zr minerals are mobile and atmo-spheric Zr can contaminate soils.

Concentrations of TiO2 across the New Albany weathering pro-file show that TiO2 is relatively constant in shale near the center ofthe outcrop and in the SAZ. In shale close to the outcrop edge, con-centrations steadily increase by 14% (Fig. 4). This increase is consis-tent with the net mass loss of weathering products estimated fromunstable constituents in the shale (CaO and CO2 from calcite disso-lution, SO4 from pyrite oxidation, and CO2 from organic-matteroxidation) (Fig. 3). The good agreement between the magnitudeof weathering products lost and TiO2 enrichment supports theassumption that Ti is immobile. For completeness, the same anal-ysis was performed to test the utility of Zr and Nb as index ele-ments. Unlike TiO2, the concentrations of Zr and Nb changeirregularly across the weathering profile. The large concentrationvariations of these two elements suggest that the minerals con-taining them were originally heterogeneously distributed or a ma-

jor fraction of the elements were redistributed by the acidicweathering conditions.

The mass transfer coefficient s (derived by Brimhall andcoworkers and summarized in Brimhall et al., 1991; Amundson,2003) calculates the fractional mass gain or loss of an elementusing concentrations of that element and the index element inthe weathered sample and parent material. Because the s valueis calculated as a ratio of chemical data, it does not require densitydata (Amundson, 2003):

s ¼ ððCj;w=Cj;pÞ=ðCi;w=Ci;pÞÞ � 1; ð1Þ

where Cj,w and Cj,p are the concentrations of phase j in weatheredand parent material, respectively; and Ci,w and Ci,p are the concen-trations of the index phase (TiO2 in this instance) in weatheredand parent material, respectively. A s value of �1 indicates thatthe phase has been completely lost from the shale, a value of zeroindicates no loss, and a value greater than zero indicates a gain(one indicates 100% gain). Traditionally, s calculations assess themass changes of chemical elements; however, because there arequantitative mineral data, s calculations have been applied to min-eral phases as well.

5.2. Silicate and aluminosilicate weathering in the New Albany Shale

The [oxide]m/[oxide]c values near one for SiO2 and Al2O3 acrossthe weathering profile indicate that the measured phases (quartz,illite, illite–smectite, chlorite, K-feldspar, plagioclase, and, wherepresent, efflorescent salts) adequately account for the SiO2 andAl2O3 measured in each sample. The s values for these two oxides(Fig. 6A) indicate that SiO2 and most Al2O3 are largely conserved inthe weathering shale. The slight decrease in Al2O3 (s = 0.9) is con-sistent with a small portion of the Al being released during weath-ering of feldspars and chlorite and incorporated into efflorescentsalts. The slight increase in SiO2 (s = 1.1) may be due to amorphous

Fig. 4. Major-element chemistry in parent Huron Member shale from the unweathered core (average of five samples), unexposed shale, exposed shale, and soil (0 m) from theroad cut plotted along the weathering profile. The shaded area delineates the salt accumulation zone (SAZ).


Table 3Stoichiometry of minerals and wt% of major elements in mineral formula (most tabulated as oxides). See text for derivation of formulas.

Mineral Formula Al2O3

(wt%)CaO(wt%)

Fe2O3

(wt%)K2O(wt%)

MgO(wt%)

Na2O(wt%)

SiO2

(wt%)S(wt%)

Quartz SiO2 100K-feldspar KAlSi3O8 19 17 64Plagioclase Na0.8Ca0.2Al1.2Si2.8O8 23 4 9 64Illite K0.94(Al1.98)(Si3.1,Al0.90)O10(OH)2 36 11 45Illite–smectite K0.52Na0.25(Mg0.53Al1.6)(Si3.6,Al0.4)O10(OH)2 26 6 4 3 56Chlorite (Mg3.8Fe6.8Al2)(Al3Si5O20)(OH)16 19 41 9 24Calcite CaCO3 56Gypsum CaSO4�H2O 32 19Pyrite FeS2 67 53Jarosite KFe3

3+(SO4)2(OH)6 47 10 13Goethite Fe3+OOH 90Iron hydroxide Fe3+(OH)3(amorphous) 74Efflorescent salt

(dehydrated)(Fe2+SO4)2�Al2(SO4)3 17 24 25

Fig. 5. Concentrations of major elements measured in samples compared to concentration calculated from quantitative mineralogy in samples and mineral stoichiometry([oxide]m/[oxide]c) plotted along the weathering profile.


SiO2 from the alteration of aluminosilicates, but there is no data tosupport this hypothesis.

Quartz, illite–smectite, and illite collectively account for 70% ofthe mineral mass in the parent shale and 85% of the mass in weath-ered shale near the outcrop edge. The s values for quartz approxi-

mate one across the outcrop, with no significant differencebetween corresponding exposed and unexposed samples(Fig. 6B). The constant s value in the shale suggests that the lossof quartz in the soil likely is unrelated to chemical weathering,but instead is attributed to illuviation (sorting) that occurs during

Fig. 6. s Values plotted along the weathering profile for (A) major elements (as oxides) and (B) primary minerals and organic matter. Shaded area delineates the saltaccumulation zone (SAZ).


soil development along steep slopes. Although s values for illite–smecite show more scatter than those for quartz, no systematic de-crease or increase of the mineral was observed (Fig. 6B). The massof illite–smectite in the soil is enriched by 40% compared to theparent shale. This increase is attributed to the illuvial processesproposed for the quartz decrease.

The s values for illite increase by 30% in the shale near the out-crop edge, an increase substantially greater than expected by themass loss of reactive phases. Illite is not expected to form underthe acidic weathering conditions (von Reichenbach and Rich,1975); however, SiO2 and Al2O3 balance calculations support the

increase (Fig. 5). This apparent inconsistency is attributed to uncer-tainty in the diffraction properties of illite in the acid weatheringenvironment. The approach used to quantify mineral abundancedid not consider vermiculite, a high-charged clay that is a likelyweathering product of chlorite (Walker, 1975). Acidic weatheringof chlorite can produce vermiculite with small amounts of K orhydronium occupying interlayer sites that contract on drying. Sucha vermiculite would have diffraction properties that resemblethose for illite. The contribution of weathered chlorite to the illiteabundance, however, is likely to be less than 4% and should haveaffected weathered samples across the outcrop similarly (Fig. 3).

Fig. 6 (continued)


Further resolution of the unexplained enrichment of illite likelywill require more detailed study of the clay components of thesample beyond that possible with the ROCKJOCK XRD method.

Chlorite is a minor clay phase (6 wt% in the parent shale) whoseabundance decreases quickly on the outcrop. Outcrop shale sam-ples contain about 25–30% of the amount measured in the parentshale. The loss of chlorite is equal between the exposed and unex-posed shale. There is a slight enrichment of chlorite in the soil, pos-sibly related to illuvial processes.

During weathering of the parent shale, about 40% of the K-feld-spar and all of the plagioclase are removed (Fig. 6B). An additional30% of the K-feldspar is lost during soil formation. Forty-five per-

cent of the Na remains in the shale near the outcrop edge althoughplagioclase was not detected. The remaining Na likely is con-tained in exchange sites of the illite–smectite even though the[Na2O]m:[Na2O]c value indicates that Na is preferentially depletedfrom the illite–smectite due to cation exchange.

5.3. Sulfide weathering in the New Albany Shale

The New Albany Shale contains significant amounts of pyriteand has limited acid-neutralizing capacity in the form of carbon-ates and reactive silicates. The result is acidic weathering that pre-vails across the outcrop and controls much of the mineral


alteration and secondary mineral formation. In certain intervals, asubtle increase in pH can result in profound mineral alteration (e.g.hydrolysis of jarosite to form Fe(III) oxides). Along with acidity, SO4

and Fe(II) are generated during pyrite oxidation (equations of pyr-ite oxidation are summarized in Tuttle et al., 2009). Sulfate formssecondary minerals, but many of these are relatively soluble inwater and eventually are eluted from the weathering profile. TheFe(II) released during pyrite oxidation also forms soluble phases,but once oxidized to Fe(III), it is relatively immobile (Schwertmannand Taylor, 1989). The distribution of Fe in primary and secondaryminerals across the weathering profile is shown diagrammaticallyin Fig. 7. Except for the efflorescent salts that form on the exposedshale in the SAZ, Fe distribution in exposed shale is similar to thatin the underlying unexposed shale (Fig. 7).

Pyrite contains about 65% of the Fe in the parent New AlbanyShale with the remainder apportioned to chlorite. Iron and SO4 lostfrom pyrite and Fe from chlorite forms jarosite, Fe oxides, gypsum,and minor efflorescent salt in the shale interval nearest the centerof the outcrop. This interval contains up to 45% more Fe than theparent shale even though pyrite and chlorite concentrations de-crease (Fig. 6B). The secondary Fe minerals form in abundancesgreater than expected for in situ weathering of the two primaryminerals. Addition of Fe via seepage is unlikely due to the low per-meability in this interval (Wildman et al., 2004). The excess Fe inthis zone suggests that the parent shale in this interval originallycontained a larger amount of pyrite than the average used forunweathered shale from the core (massive pyrite up to 27 wt%

Fig. 7. The proportion of total Fe in each mineral plotted along the weatheringprofile of unexposed and exposed shale. Shaded area delineates the salt accumu-lation zone (SAZ).

was measured in some Huron core samples that were not includedin the average). Jarosite in shale samples nearest the center of theoutcrop and the SAZ accounts for between 25% and 70% of the Fe. Inthe weathered shale near the outcrop edge, jarosite is apparentlyconverted to goethite and poorly crystalline Fe(III) oxide. This con-version is likely driven by a small increase in pH that results fromthe hydrolysis of feldspars that are depleted in this interval(Fig. 6B). Amorphous Fe(III) oxides and goethite host 70% of theFe in the intensely weathered shale and 87% in the soil. In all, upto 40% of the original Fe has been lost from the shale during weath-ering (Fig. 6A). In the soil, s is zero, indicating a 40% gain in Fe dur-ing pedogenesis to concentrations similar to those in the parentshale. This gain from shale at the edge of the outcrop to soil canbe explained by accumulation of additional amorphous Fe(III) oxi-des to concentrations near 5 wt%. Although speculative, the addi-tional oxides may reflect transport of aqueous Fe(II) inweathering solutions to the soil where aerated conditions favoroxidation and formation of Fe oxides.

5.4. Organic-matter weathering

Buffering of organic-matter oxidation by pyrite occurs in theshale near the center of the outcrop as pyrite reacts as an electrondonor much faster than organic matter (Petsch et al., 2000). Organ-ic-matter concentrations decrease by about 20% through the SAZ(Fig. 6B) and 50% in the shale at the outcrop edge. Loss in exposedshale is very similar to that in unexposed shale, indicating that theoxidation of the organic matter is more affected by long-termweathering processes than excavation of the road cut. Wildmanet al. (2004) attribute porosity and permeability in the shale to or-ganic-matter oxidation. In the soil, s values indicate that 60–80% ofthe ancient organic matter has been lost (Fig. 6B). These values area minimum as modern organic litter in the soil contributes to theoverall organic-matter concentration.

6. Weathering zones in the New Albany Shale

Mineralogical and chemical changes in the New Albany Shale,as recognized by mineral and chemical abundances, oxide-concen-tration ratios, and s values, define four distinct weathering zonesacross the Clay City outcrop. The zones include the weakly weath-ered shale; the pre-excavation, active weathering front; the inten-sely weathered shale; and soil (Fig. 8). Bounds for each zone areassumed to be subparallel with the ground surface of the hillside;however, their precise location beyond the sampled transect wasnot verified.

The weakly weathered shale (Zone A) is located from 11 to morethan 16 m laterally into the hill from the natural ground surface(Fig. 8). The mineral composition of Zone A is most similar to theunweathered shale in core. Pyrite remains detectable, althoughsamples exposed on the excavation surface in this interval contain10–35% less pyrite than unexposed shale 10 cm behind the outcropsurface. Loss of pyrite is attributed to oxidation with the resultingacid solution dissolving most of the calcite to produce smallamounts of gypsum. Consistent with the acidic and oxic conditionsgenerated in the exposed shale (Brown, 1971), jarosite generally ismore abundant in exposed than unexposed shale. The small accu-mulations of efflorescent salts detected in the unexposed shale areconsidered the product of in situ oxidation of pyrite rather thanevaporation of discharging weathering solutions as observed inZone B. Mass loss in this zone has been limited as evident in theshale’s competence and low permeability. The minimal alterationof the New Albany Shale exposed in this zone reflects the limitedweathering that has occurred subsequent to the excavation ofthe road cut 40 a ago.

Fig. 8. Position and characteristics of defined weathering zones in the New Albany Shale at Clay City.


Zone B encompasses the interval bounded by the weaklyweathered shale in Zone A and the intensely weathered shale inZone C and includes the SAZ. Designated the pre-excavation, activeweathering front, this zone is located 5.8–11 m into the hill fromthe outcrop edge (Fig. 8). Within Zone B, pyrite oxidation is nearlycomplete, carbonates are absent, and feldspars and organic matterdecrease. Increased permeability related to the mass loss frommineral and organic-matter removal facilitates discharge of theacidic groundwater produced as shallow water moves throughintervals of active pyrite oxidation. Upon evaporation of the weath-ering solutions, secondary phases such as efflorescent sulfate saltsand jarosite form on the outcrop surface. Compositional differencesbetween the exposed shale and unexposed shale are attributed tothese secondary phases.

The intensely weathered shale that defines Zone C is locatedfrom the natural edge of the outcrop to 5.8 m into the hill(Fig. 8). This interval hosts steep gradients of mineral and chemicalalteration. The composition of the unexposed shale in this zone isvery similar to the exposed shale, indicating that weathering sub-sequent to excavation has had little effect on shale composition.The content of goethite and amorphous Fe(III) oxide increase inthis zone, consistent with a slight increase in pH of 3–4 that favorshydrolysis of jarosite and complete oxidation of dissolved Fe. Theshale has undergone a mass loss of up to 14% from mineral disso-lution and organic-matter oxidation. The result is less induratedshale with relatively high permeability. Acidic weathering solu-tions have infiltrated through this zone over long periods of time,resulting in removal of nearly all but the most resistant silicatephases (quartz, illite, and illite–smectite). Using the average ero-sion rates for the Appalachian Plateau (7–11 m/Ma; Clifton, 2005)and assuming steady state advance of the weathering front tomatch the denudation rate, the upper portion of shale in Zone C(�6 m) is the product of over 1 Ma of weathering. This simplisticapproach ignores dramatic climatic change and concomitant

changes in hydrology; nonetheless, it suggests a slow rate of ad-vance of the weathering front through the New Albany Shale inKentucky.

Correlating the soil with the shale transect is complicated be-cause of downward creep of soil on steep slopes (Hayes, 1993).Although the soil sampled may not correlate exactly with the shalebed sampled on the outcrop, it is derived from the Huron Member.Three processes that affect the major-element and mineral compo-sition of the soil are: (1) illuviation that concentrates clays and de-pletes quartz and K-feldspar, (2) additional accumulation ofgoethite, and (3) transport of weathering solutions to the soiland formation of amorphous Fe-oxide and efflorescent salt.

7. Conclusions

In a humid, subtropical climate, the first reaction in the weath-ering process of black shale is oxidation of pyrite. In rocks such asthe New Albany Shale, carbonate-mineral content is insufficient tobuffer all the acid produced by this reaction. The excess acid slowlyalters aluminosilicates (incongruent dissolution of feldspars andcation exchange in clays). The Fe and SO4 released from pyrite,Ca from calcite, and the Al2O3 and K from aluminosilicates formefflorescent sulfate salts, gypsum and jarosite. Once the pyrite isconsumed, the excess acid is partially neutralized by slow reactionwith aluminosilicates. As pH increases, jarosite hydrolyzes, form-ing goethite and (or) amorphous Fe oxides. At this stage of weath-ering, organic matter begins to oxidize. The delay in oxidation oforganic matter is due to pyrite being a faster electron donor thanorganic matter. Iron oxides and minor efflorescent salts form inthe soil, likely due to discharge and partial oxidation of Fe-richweathering solutions at the surface. Quartz, illite, and illite–smec-tite are sorted in surface soil by illuvial processes as the soil creepsdown the steep hillslope.


The four distinct weathering zones recognized in the New Al-bany Shale reflect mineralogical and chemical changes resultingfrom the weathering processes. Characterizing the minerals andchemistry of these zones, and, subsequently being able to linkthem to specific weathering reactions, provides a framework thatcan be applied to other studies of weathering pyritic black shale.One such study investigates the mobility, transport, and fate oftrace-metals released during weathering of the New Albany Shaleat the study site (Tuttle et al., 2009).

Acknowledgements

The authors acknowledge the following: Cyrus Berry (US Geo-logical Survey), for his sulfur speciation analyses and John Bullock(US Geological Survey) for chemical analyses. We wish to thank theKentucky Geological Survey for allowing us to sample the core ofparent New Albany Shale, and to Jim Hower at the University ofKentucky Center for Applied Energy Research for providing infor-mation on the New Albany Shale outcrops. The authors wish tothank Lisa Stillings (US Geological Survey) and an anonymous re-viewer for their comments and improvement of this manuscript.

References

Amundson, R., 2003. Soil formation. In: Drever, J.I., Holland, H.D., Turekian, K.K.,Exec. (Eds.), Treatise on Geochemistry, vol. 5. Elsevier, New York, pp. 1–35.

Bayless, E.R., Olyphant, G.A., 1993. Acid-generating salts and their relationship tothe chemistry of groundwater and storm runoff at an abandoned mine site insouthwestern Indiana. J. Contam. Hydrol. 12, 313–328.

Birkeland, P.W., 1974. Pedology, Weathering, and Geomorphological Research.Oxford University Press, New York.

Brimhall, G.H., Dietrich, W.E., 1987. Constitutive mass balance relations betweenchemical composition, volume, density, porosity, and strain in metasomatichydrochemical systems: results on weathering and pedogenesis. Geochim.Cosmochim. Acta 51, 567–587.

Brimhall, G.H., Lewis, C.J., Ague, J.J., Dietrich, W.E., Hampel, J., Teague, T., Rix, P.,1988. Metal enrichment in bauxite by deposition of chemically-mature eoliandust. Nature 333, 819–824.

Brimhall, G.H., Lewis, C.J., Ford, C., Bratt, J., Taylor, G., Warin, O., 1991. Quantitativegeochemical approach to pedogenesis: importance of parent materialreduction, volumetric expansion and eolian influx in laterization. Geoderma51, 51–91.

Brown, J.B., 1971. Jarosite-goethite stabilities at 25 �C, 1 atm. Mineral. Deposita 6,245–256.

Bullock, J.H., Jr., Cathcart, J.D., Betterton, W.J., 2002. Analytical methods utilized bythe United States Geological Survey for the analysis of coal and coal combustionby-products. US Geol. Surv. Open-File Rep. 02-389.

Carlson, E.H., Whitford, J., 2002. Occurrence and mineralogy of efflorescence in LateDevonian black shales, north-central Ohio. Geol. Soc. Am. Abstr. Prog. 34, 351.

Chigira, M., Oyama, T., 1999. Mechanism and effect of chemical weathering ofsedimentary rocks. Engin. Geol. 55, 3–14.

Clifton, T., 2005. Erosion rate of the Appalachian plateau in the vicinity of the NewRiver Gorge, West Virginia. Geol. Soc. Am. Abstr. Prog. 37, 18.

Driese, D.G., Mora, C.I., Stiles, C.A., Joeckel, R.M., Nordt, L.C., 2000. Mass-balancereconstruction of a modern Vertisol: implications for interpreting thegeochemistry and burial alteration of paleo-Vertisols. Geoderma 95, 179–204.

Eberl, D.D., 2003. A user’s guide to ROCKJOCK—A program for determiningquantitative mineralogy from powder x-ray diffraction data. US Geol. Surv.Open-File Rep. 2003-78.

Elswick, E.R., 2006. Partitioning of metals between soil and dissolved phases,Heneryville bed, New Albany Shale. Geol. Soc. Am. Abstr. Prog. 38, 486.

Esson, J., 1983. Geochemistry of a nickeliferous laterite profile Liberdade Brazil. In:Wilson, R.C.L. (Ed.), Residual Deposits: Surface Related Weathering Processesand Materials. Geol. Soc., vol. 11. Special Publications, London, pp. 91–99.

Foster, M.D., 1962. Interpretation of the composition and a classification of thechlorites. US Geol. Surv. Prof. Paper 441-A.

Gardner, L.R., 1980. Mobilization of Al and Ti during weathering—isovolumetricgeochemical evidence. Chem. Geol. 30, 151–165.

Goldich, S.S., 1938. A study in rock weathering. J. Geol. 46, 17–58.Hamer, M., Graham, R., Amrhein, C., Bozhilov, K.N., 2003. Dissolution of ripidolite

(Mg, Fe-chlorite) in organic and inorganic acid solution. Soil Sci. Soc. Am. J. 7,654–661.

Hammarstrom, J.M., Brady, K., Cavotta, C.A., 2005. Mineralogical controls on acid-rock drainage at Skytop, Center Co., PA. Geol. Soc. Am., Northeastern Section,40th. Ann. Meeting Abstr. Prog. 37, 64.

Hayes, R.A., 1993. Soil Survey of Powell and Wolfe Counties, Kentucky. US Dept. ofAgriculture, Soil Conservation Service.

Jackson, M.L., Sherman, G.D., 1953. Chemical weathering of minerals in soils. Adv.Agron. 5, 219–318.

Jaffe, L.A., Peucker-Ehrenbrink, B., Petsch, S.T., 2002. Mobility of rhenium, platinumgroup elements and organic carbon during black shale weathering. Earth Planet.Sci. Lett. 198, 339–353.

Jambor, J.L., Nordstrom, D.K., Alpers, C.N., 2000. Metal-sulfate salts from sulfidemineral oxidation. In: Alpers, C.N., Jambor, J.L., Nordstrom, D.K. (Eds.), SulfateMinerals—Crystallography, Geochemistry, and Environmental SignificanceReviews in Mineralogy and Geochemistry, vol. 40. Mineral. Soc. Am.,Washington DC, pp. 305–393.

Joeckel, R.M., Clement, B.J.A., Bates, L.R.V., 2005. Sulfate-mineral crusts from pyriteweathering and acid-rock drainage in the Dakota Formation and GranerosShale, Jefferson County Nebraska. Chem. Geol. 215, 433–452.

Kolowith, L.C., Berner, R.A., 2002. Weathering of phosphorus in black shales. GlobalBiogeochem. Cycles 16, 1140–1147.

Lamothe, P.J., Meier, A.L., Wilson, S.A., 2002. The determination of forty-fourelements in aqueous samples by inductively coupled plasma-massspectrometry. In: Taggart, J.E., Jr. (Ed.), Analytical methods for chemicalanalysis of geologic and other materials, US Geological Survey. US Geol. Surv.Open-File Rep. 02-223, 1–11 (Chapter H).

Leventhal, J.S., Hosterman, J.W., 1982. Chemical and mineralogical analysis ofDevonian black shale samples from Martin County, Kentucky; Caroll andWashington Counties, Ohio; Wise County, Virginia; and Overton County,Tennessee, USA. Chem. Geol. 37, 239–264.

Marsan, F.A., Bain, D.C., Duthie, D.M.L., 1988. Parent material uniformity anddegree of weathering in a soil chronosequence, northwestern Italy. Catena15, 507–517.

Marshall, C.E., Haseman, J.F., 1942. The quantitative evaluation of soil formation anddevelopment by heavy mineral studies: a Grundy silt loam profile. Soil Sci. Soc.Am. Proc. 7, 448–453.

McKnight, T.L., Hess, D., 2000. Climate Zones and Types: The ‘‘Köppen System”.Physical Geography: A Landscape Appreciation. Prentice Hall, Upper SaddleRiver, NJ.

Newman, A.C.D., 1987. Chemistry of Clay and Clay Minerals. Wiley and Sons, NewYork.

Petsch, S.T., Berner, R.A., Eglinton, T.I., 2000. A field study of the chemicalweathering of ancient sedimentary organic matter. Org. Geochem. 31, 475–487.

Petsch, S.T., Eglinton, T.I., Edwards, K.J., 2001a. 14C-Dead living biomass: evidencefor microbial assimilation of ancient organic carbon during shale weathering.Science 292, 1127–1131.

Petsch, S.T., Smernik, R.J., Eglinton, T.I., Oades, J.M., 2001b. A solid state13C-NMRstudy of kerogen degradation during black shale weathering. Geochim.Cosmochim. Acta 65, 1867–1882.

Potter, P.E., Maynard, J.B., Pryor, W.A., 1980. Final report of special geological,geochemical, and petrological studies of the Devonian Shales in the AppalachianBasin. Final Report prepared for US Dept. of Energy Eastern Gas Shales Project,Contract DE-AC21-76MCO5201.

Preda, M., Cox, M.E., 2001. Trace metals in acid sediments and waters,Pimpama catchment, southeast Queensland, Australia. Environ. Geol. 40,755–768.

Pryor, W.A., Fisk. H.N., Kepferle, R.C., Kiefer, J., 1981. Road Log. In: EnergyResources of Devonian-Mississippian shales of Eastern Kentucky. An.l FieldConf. Geol. Soc. Kentucky, April 9–11, 1981. Kentucky Geological Survey,Lexington.

Puura, A., Neretnieks, I., 2000. Atmospheric oxidation of the pyritic waste rock inMaardu, Estonia, 2: an assessment of aluminosilicate buffering potential.Environ. Geol. 39, 560–566.

Reheis, M.C., 1990. Influence of climate and eolian dust on the major-elementchemistry and clay mineralogy of soils in the northern Bighorn Basin, USA.Catena 17, 219–248.

Ripley, E.M., Shaffer, N.R., Gilstrap, M.S., 1990. Distribution and geochemicalcharacteristics of metal enrichment in the New Albany Shale (Devonian–Mississippian), Indiana. Econ. Geol. 85, 1790–1807.

Robl, T.L., 1994. Iron sulfide oxidation—impact on chemistry of leachates fromnatural and pyrolyzed organic-rich shales. In: Alpers, C.N., Blowes, D.W. (Eds.),Environmental Geochemistry of Sulfide Oxidation. Am. Chem. Soc., Symp. Ser.550, pp. 574–592.

Schieber, J., 2004. On the origin and significance of pyrite beds in Devonian blackshales from the Eastern US. Geol. Soc. Am. Abstr. Prog. 36, 372.

Schwertmann, U., Taylor, R.M., 1989. Iron oxides. In: Dixon, J.B., Weed, S.B. (Eds),Minerals in the Soil Environment, 2nd ed. Soil Sci. Soc. Am., Madison Wisconsin,pp. 379–438.

Shirav, M., Robl, T.L., 1993. Laboratory simulation of natural leaching processes ofeastern USA oil shale. Environ. Geol. 22, 88–94.

Stiles, C.A., Mora, C.I., Driese, S.G., 2003. Pedogenic processes and domainboundaries in a vertisol climosequence: evidence from titanium andzirconium distribution and morphology. Geoderma 116, 279–299.

Stillings, L.L., Brantley, S.L., 1995. Feldspar dissolution at 25� C and pH 3: reactionstoichiometery and the effect of cations. Geochim. Cosmochim. Acta 59, 1483–1493.

Stillings, L.L., Brantley, S.L., Machesky, M.L., 1995. Proton adsorption at an adulariafeldspar surface. Geochim. Cosmochim. Acta 59, 1473–1482.

Strawn, D., Doner, H., Zavarin, M., McHugo, S., 2002. Microscale investigation intothe geochemistry of arsenic, selenium, and iron in soil developed in pyritic shalematerials. Geoderma 108, 237–257.

Sullivan, P.J., Yelton, J.L., 1988. An evaluation of trace-element release associatedwith acid mine drainage. Environ. Geol. 12, 181–186.


Taggart, J.E., Jr. (Ed.), 2002. Analytical methods for chemical analysis of geologic andother materials, US Geological Survey. US Geol. Surv. Open-File Rep. 02-0233.

Tissot, B.P., Welte, D.H., 1984. Petroleum Formation and Occurrence. Springer-Verlag, New York.

Tuttle, M.L.W., Breit, G.N., Goldhaber, M.B., 2003. Geochemical data from NewAlbany shale, Kentucky: a study in metal mobility during weathering of blackshales. US Geol. Surv. Open-File Rep. 03-207.

Tuttle, M.L.W., Breit, G.N., Goldhaber, M.B., 2009. Weathering of the New Albanyshale, Kentucky: II. Redistribution of minor and trace elements. Appl. Geochem.24 (5), 1565–1578.

Tuttle, M.L.W., Goldhaber, M.B., Breit, G.N., 2001. Mobility of metals fromweathered black shale: the role of salt efflorescences. Geol. Soc. Am. Abstr.Prog. 33, 191.

Tuttle, M.L., Goldhaber, M.B., Williamson, D.L., 1986. An analytical scheme fordetermining forms of sulphur in oil shales and associated rocks. Talanta 33,953–961.

von Reichenbach, H.G., Rich, C.I., 1975. Fine grained micas in soils. In: SoilComponents. In: Gieseking, J.E. (Ed.), . Inorganic Components, vol. 2. Springer-Verlag, New York, pp. 59–96.

Walker, G.F., 1975. Vermiculites. In: Soil Components. In: Gieseking, J.E. (Ed.), .Inorganic Components, vol. 2. Springer-Verlag, New York, pp. 155–190.

White, A. F., 2003. Natural weathering rates of silicate minerals. In: Drever, J. I. (Ed.),Treatise on Geochemistry, vol. 5. Holland, H.D., Turekian, K.K., Exec. (Eds.),Elsevier, New York, pp. 133–168.

Wildman, R.A., Berner, R.B., Petsch, S.T., Bolton, E.W., Eckert, J.O., Mok, U.,Evans, J.B., 2004. The weathering of sedimentary organic matter as acontrol on atmospheric O2: I. Analysis of black shale. Am. J. Sci. 304, 234–249.

Woods, A.J., Omernik, J.M., Martin, W.H., Pond, G.J., Andrews, W.M., Call, S.M,Comstock, J.A., Taylor, D.D., 2002. Ecoregions of Kentucky (color poster withmap, descriptive text, summary tables, and photographs): Reston, VA., US Geol.Surv. (map scale 1:1,000,000).

Documents

Weathering of the New Albany Shale, Kentucky, USA: I. Weathering zones defined by mineralogy and major-element composition