Reaction progress in chloritic material: an electron microbeam study of the Taveyanne greywacke, Switzerland

J. metamorphic Geol., 1999, 17, 229–241

Reaction progress in chloritic material: an electron microbeam studyof the Taveyanne greywacke, SwitzerlandD. SCHMIDT,1 , * K. J . T . LIVI2 AND M. FREY 1

1Mineralogisch-Petrographisches Institute, University of Basel, 4056 Basel, Switzerland2Department of Earth and Planetary Sciences, The Johns Hopkins University, Baltimore, USA

ABSTRACT Chlorite and associated minerals from the volcanogenic Taveyanne metasediment of the western Helveticnappes, Switzerland, were investigated by electron microprobe (EMP) and transmission electronmicroscopy (TEM) in order to determine their textural and chemical evolution during low-temperaturemetamorphism. EMP analyses of chloritic material from sub-greenschist facies outcrops show a decreaseof Si and S(Ca, Na, K) with increasing metamorphic grade. A number of conclusions may be drawnfrom combined TEM images and analytical electron microscopy (AEM) data.1 In diagenetic-grade samples, chlorite crystals (observed maximum defect-free distance=80 nm) alwayscontain some 1 nm layers (with a maximum of 29% of all layers) and less frequently some 0.7 nmberthierine-like layers. With increasing grade, the amounts of 1 and 0.7 nm layers decrease, and mostchlorite from the epizone is structurally pure or contains less than 2% of 1 nm layers.2 A positive correlation was found between the amount of 1 nm layers and the Ca+K+Na content,indicating that the 1 nm layers are saponite.3 Observations and calculations suggest that the transformation reaction of saponite to chlorite takesplace by the replacement of the interlayer cations in saponite by brucite-like layers resulting in a localvolume decrease. In contrast, the destruction of berthierine has only minor influence on the local bulkvolume.These results confirm recent studies which show that the change in composition measured by EMP ofdiagenetic-grade chloritic material are mainly the result of mixtures of chlorite and saponite. The useof chlorite ‘geothermometry’ in such systems is greatly influenced by the presence of saponite and henceis not based on reaction equilibria, even though temperatures calculated in this study agree with tempera-tures derived from other methods. Therefore, chlorite evolution should be treated as a kinetically con-trolled grade indicator and developed as a qualitative scale similar to the illite crystallinity index.

Key words: chlorite ‘geothermometry’; Taveyanne metagreywacke; TEM–AEM analyses; very low-grademetamorphism

of those ‘geothermometers’ (Shau et al., 1990; HillierINTRODUCTION

& Velde, 1991; Jahren, 1991; Velde et al., 1991; Jahren& Aagaard, 1992; de Caritat et al., 1993; Jiang et al.,Chlorite is a common secondary constituent in very

low-to-medium grade metamorphic rocks. Several 1994; Essene & Peacor, 1995; Schmidt et al., 1997),mainly because of the possible contamination ofattempts have been made to use the wide compositional

variability of chlorite as an empirical geothermometer electron microprobe (EMP) analyses by the presenceof submicrometre-scale intergrowths of other sheet(Cathelineau & Nieva, 1985; Kranidiotis & MacLean,

1987; Cathelineau, 1988; Jowett, 1991; Zang & Fyfe, silicates. Shau et al. (1990) showed that the excess ofrelative AlVI to AlIV in chlorite is accompanied by a1995; Ruiz Cruz & Andreo, 1996). These authors

suggested that the non-stoichiometric behaviour of significant amount of Ca, Na and K. Their transmissionelectron microscopy (TEM) and X-ray diffractionchlorite is a sensitive indicator of the peak temperature

during low-grade metamorphism. Cathelineau & Nieva (XRD) studies revealed that the chlorite containedintercalated corrensite. Curtis et al. (1984), Hillier &(1985) and Cathelineau (1988) proposed that the

magnitude of the Si%R2+−2 exchange vector (%= Velde (1991) and Jiang et al. (1994) determined howmixtures between chlorite and other phases can leadvacancy) is temperature-dependent in authigenic

chlorite. to contamination of the chlorite analyses. Jiang et al.(1994) noted that the criterion of Foster (1962),Many authors have cautioned against the validitywhereby chlorite analyses with <0.5 wt% ofNa2O+K2O+CaO are regarded as pure, is not

*Present address: CRMC2-CNRS, Campus de Luminy, Case sufficient to exclude contamination with other phases.913, 13288 Marseille Cedex 9, France (e-mail: schmidt@crmc2.

On the basis of lattice fringe images from diageneticuniv-mrs.fr).

229© Blackwell Science Inc., 0263-4929/99/$14.00Journal of Metamorphic Geology, Volume 17, Number 2, 1999

230 D. SCHMIDT ET AL .

to low-grade metamorphic rocks, various authors et al., 1997). Schmidt et al. (1997) compared differentmetamorphic facies with illite crystallinity (IC) indicesshowed that the formation of chlorite is best under-

stood through the Ostwald step rule, i.e. chloritic of adjacent shales (Table 1), and correlated mineralogyand IC with temperatures derived from fluid inclusionmaterial undergoes a series of structural changes in

response to kinetic constraints rather than through data, stable isotope geothermometry and thermo-dynamic analysis of the Taveyanne greywacke samples.equilibrium reactions (e.g. Shau et al., 1990; Jiang et al.,

1994; see also Essene & Peacor, 1995). Estimated temperatures ranged between 210 and250 °C in rocks from the zeolite facies, 270–300 °C forIn a recent study, Schmidt et al. (1997) noted that

chlorite ‘geothermometry’ temperatures (Cathelineau, the prehnite–pumpellyite facies, and >300 °C for thepumpellyite–actinolite and greenschist facies.1988) in the volcanogenic andesitic Taveyanne

Sandstone are in agreement with those estimated by Two samples of each IC zone (FB108, EP3, EP29,EP23, Z1, GPB171) were made into doubly polishedfluid inclusion data, stable isotope geothermometry

and other thermodynamic equilibria. Schmidt et al. thin sections mounted with the heat-sensitive adhesiveCrystalbond 509 for EMP and TEM analyses. EMP(1997) concluded that their chlorite analyses contained

significant amounts of saponite because EMP analyses analyses were carried out with a JEOL 8600Superprobe equipped with four wavelength-dispersiveof chlorite exhibit a strong correlation between inter-

layer cations and apparent octahedral vacancies as spectrometers (WDS) and an energy-dispersive spec-trometer (EDS) at the University of Basel and Johnswell as with tetrahedral Al content. Backscattered

electron (BSE) imaging, however, did not indicate any Hopkins University. Quantitative analyses of chloritewere performed using wavelength-dispersive spec-contamination. In order to understand this contradic-

tion, we decided to carry out a more detailed analysis troscopy, an accelerating voltage of 15 kV, and a beamcurrent of 10 nA. In order to minimize evaporation ofof chlorite in samples from the Taveyanne Sandstone

using TEM methods. light elements (e.g. Na), the spot size was set to 6 mm.The following elements were measured: Si, Al, Ti, Fe,The first purpose of this study was to compare AEM

data with lattice fringe images of chloritic material Mg, Mn, Ca, Na, K. Total Fe was calculated as FeO.The data were reduced using a PROZA correctionfrom a range of sub-greenschist facies grades. This

identifies contamination phases in chlorite crystals and program at the University of Basel and the programCITZAF (Armstrong, 1989) at Johns Hopkinswill determine the variability of uncontaminated chlor-

ite compositions. With this knowledge, we can test the University. Electron-transparent foils were made fromfragments of petrographic thin sections using an argonhypothesis that chlorite can be used as a geothermo-

meter. The second purpose was to understand the ion mill, followed by coating lightly with carbon.Lattice fringe images were obtained from a Philipsreactions involving chlorite during diagenesis and very

low-grade metamorphism. Because chloritic materials 420ST microscope operated at 120 kV using a 50 mmobjective aperture. We carried out AEM work in TEMare important secondary minerals, detailed characteriz-

ation of the chemistry and textures will give important mode with an Oxford light-element EDS. Images werecollected so that the area of analysis could be correlatedclues for our understanding of the reaction progress

during diagenesis and metamorphism. Therefore, we with the lattice fringe image. This routine allowed usto compare the chemical composition and structuralpresent a quantitative reconstruction of prograde

mineral reactions that occurred during the growth of information for authigenic chloritic materials. Due tosmall crystal sizes, more than half of the analyses werechlorite in chloritic material of the Taveyanne

Sandstone. discarded because of possible overlap of the beamonto adjacent minerals. The spot size used wasapproximately 50 nm. Atomic wt% was calculated by

GEOLOGICAL SETTING AND METHODSthe Cliff–Lorimer thin-film ratio criteria (Cliff &Lorimer, 1975). The k factors relative to Si for Na, K,Chlorite data were obtained from the andesitic vol-

canoclastic Taveyanne metagreywacke of the western Ca, Mg, Fe, Al, Ti and Mn were empirically derivedfrom silicate standards.part of the Helvetic nappes (see Schmidt et al., 1997),

that is dated at 32 Myr old using the 40Ar/39Ar methodon amphibole (Fischer & Villa, 1990; Ruffini et al., RESULTS1995). The Taveyanne volcanism is thus linked withthe subduction/collision event of the Alpine orogenic

Petrography and EMP investigationsphase that was followed by regional metamorphism(Waibel, 1993). In this study area, Ramsay (1981) and Sixty-four samples of Taveyanne greywacke were

investigated by optical microscopy, of which tenDietrich & Casey (1989) noted an increase in metamor-phic grade with increase in strain from NW towards representative samples were selected for EMP analysis.

These samples generally contain the authigenic mineralSE. The metamorphism ranges from zeolite throughprehnite–pumpellyite to pumpellyite–actinolite and assemblage albite+chlorite+calcite+K-white mica+

quartz+titanite±laumontite±prehnite±pumpellyitelower greenschist facies (Coombs et al., 1976; Kisch,1980; Bussy & Epard, 1984; Burkhard, 1988; Schmidt ±actinolite and ±epidote. Authigenic chlorite is

REACTION PROGRESS IN CHLORITIC MATERIAL 231

Table 1. Comparison between mineral facies, metamorphic zones derived by illite crystallinity, expected temperature range (derivedby fluid inclusion investigations, stable isotope geothermometry and thermodynamic calculations) and chlorite geothermometry(modified after Schmidt et al., 1997).

No. Locality Facies Zone ICmeana (air-dried) [D2H±1s] Chl-T b[°C] Estimated T c [°C]

FB7 Gsteig ZEO Diagenesis 0.63±0.08 (3) 214±36 (27) ≤210–250

FB108 Diablerets ZEO Diagenesis 0.48±0.04 (6) 227±36 (23) 210–250

FB32 Taveyanne ZEO Diagenesis 0.42±0.03 (10) 242±30 (22) 210–250

EP3 Olden ZEO Diag./Anch. 0.41±0.01 (3) 245±38 (25) 210–250

EP29 Le Sex Prh–Pmp Anchizone 0.29±0.03 (3) 245±39 (23) 270–300

EP23 Chumme Prh–Pmp Anchizone 0.29±0.03 (6) 264±33 (27) 270–300

Z1 Bublet Pmp–Act Epizone 0.23±0.01 (2) 294±21 (36) 300–360

GPB147 Grachmatten Pmp–Act Epizone 0.24±0.03 (4) 303±25 (36) 300–360

GPB171 Wiler Pmp–Act Epizone 0.21 (1) 298±19 (33) 300–360

GPB174 Erschmatt GS Epizone 0.20±0.02 (3) 305±18 (37) >300–360

Temperatures derived by chlorite geothermometry are in agreement with the expected temperature range. Note, according to Kubler (1984), the limiting values for the low-grade and high-

grade boundary of the anchizone are 0.42° and 0.25° D2H Cu Ka, respectively. Values in parentheses are the number of measurements used for averaged values a Illite crystallinity after

Kubler (1984) b Chlorite geothermometry after Cathelineau (1988) c Estimated temperature range (Schmidt et al., 1997).

Fig. 2. Interlayer cation content of 289 chlorite analysesplotted against apparent octahedral vacancies. Hypotheticalmixtures of chlorite with other phases are shown as arrows. Ahypothetical mixture of chlorite and saponite fits well with

Fig. 1. In Fig 1, 289 EMP analyses of chlorites are plotted in a microprobe data. Chemical formulae: chloriteSi versus R2+oct diagram (R2+oct=Mg+Fe+Mn). R3+=trivalent Fe2.55Mg2.25Al1.2Si2.8Al1.2O10(OH)8; saponite (Sap)cations (p.f.u.); R2+oct=octahedral divalent cations (p.f.u.); %= Ca0.2Na0.1Mg3Si3.5Al0.5O10(OH)2 (modified after Schmidt et al.,vacancies (p.f.u.). Vacancies are calculated assuming: %= 1997).6-∑cationoct. Calculated trends of variations in analyses due tocontamination are shown by arrows for albite (Ab), calcite(Cc), K-feldspar (Kfs), muscovite (Ms), quartz (Qtz) andsaponite (Sap: Ca0.2Na0.1Mg3Si3.5Al0.5O10 (OH)2). The A plot of Si versus total octahedral divalent cationshypothetical chlorite composition for 289 EMP analyses of chlorite, considered from(Fe2.55Mg2.25Al1.2Si2.8Al1.2O10(OH)8) used to calculate these

BSE examination to be pure, reveals that mosttrends is similar to averaged EMP analyses of chlorites fromdiagenetic and anchizone-grade chlorite analyses scat-epizonal rocks of this study. In addition, the exchange vector

Si%R2+oct proposed in previous studies is shown (see text). ter within a wide range that indicates that thesechlorite analyses may be contaminated by other phases(Fig. 1). In Fig. 2, analyses of chlorite are plotted asubiquitous to all rocks from the Taveyanne greywacke.

Chlorite varies from green to brownish-green to 2Ca+Na+K cations versus apparent octahedral vac-ancies. Schematic trends of compositional deviation asabnormal blue-coloured specimens. In the zeolite and

prehnite–pumpellyite facies, chlorite replaces detrital compared to the hypothetical chlorite composition aredisplayed by arrows similar to Fig. 1. Figures 1 & 2micas as small intergrowths within stacks of muscovite,

and also rims detrital pyroxene and amphibole. In the reveal that the best correlation fit for diagenetic andmetamorphic chlorites for the Taveyanne Sandstone ismatrix, chlorite is also intergrown with authigenic

titanite, for which Bussy & Epard (1982) proposed a chlorite/saponite mixture. Analyses of apparentlypure chlorites from the diagenetic and anchizone gradethat both minerals are mainly replacements of volcanic

glass. In the pumpellyite–actinolite facies and lower indicate enrichment with Si, Na, K and Ca comparedto those of epizonal grade. In addition, the Altot of thegreenschist facies, chlorite is the most abundant

mineral. More detailed information is given in Schmidt diagenetic and anchizone-grade chlorites is lower withrespect to those of higher-grade chlorite. Selected EMPet al. (1997).


analyses of apparently pure chlorite are listed in analyses may be due to displaced surface atoms thatare mobilized during the ion-milling process fromTable 2.adjacent calcite, K-feldspar and albite grains (seePeacor, 1992). In support of this, TEM samples that

TEM investigationsconsist mainly of chlorite show, in general, an interlayercation content of ≤0.05 p.f.u.Crystals of apparently pure authigenic chlorite were

prepared for TEM examination in order to obtain AEM analyses of areas that contain mixtures of1 nm layers intercalated in chlorite show that moststructural and chemical information on the nanometre

scale. Based on lattice fringe images, three types of are smectite, probably saponite (Fig. 3). Note thathydrated saponite layers with thicknesses oflayers were observed that correspond to different

arrangements of tetrahedral sheets (T), octahedral 1.5–1.6 nm (Brindley, 1982) collapse in the vacuum ofthe TEM to 1 nm. In Fig. 3, the Ca, Na and Ksheets (O) and interlayer sites: (i) 1.4 nm layers

(TOT-O), (ii ) 0.9–1.0 nm layers (TOT), and (iii) 0.7 nm contents derived from AEM analyses were plotted asa function of the percentage of 1 nm layers perlayers (TO).

Combined selected-area electron diffraction (SAED) analysis area. Differentiation of smectite layers asopposed to mica layers was determined by thepatterns, lattice fringe images and AEM data of pure

1.4 nm layers show that these are in fact chlorite, for magnitude of the K content in AEM analyses. Weassumed that a significantly high KICa+Na contentwhich low yet significant amounts of Ca, Na and K

(S0.08 p.f.u.) were found. AEM analyses of adjacent in the AEM analyses indicates intercalation of chloritewith K-white mica. Less than 10% of all AEMquartz also revealed a significant amount of Ca, Na

and K cations (S(Ca, Na, K)average=0.03 p.f.u. based analyses show this feature and these analyses werenot considered in Fig. 3. A hypothetical mixture ofon 14 oxygens). EMP analyses, however, could not

detect these elements in quartz from the same thin chlorite/saponite is represented by an arrow thatintersects the vertical axis at Ca+Na+K=0.05 p.f.u.,section. We suggest that the alkali elements in AEM

Table 2. Selected EMP and AEM analyses of authigenic chloritic material (normalized to 14 oxygens p.f.u.).

Zone Diagenesis Anchizone Epizone

Sample FB108/68 EP3/43 EP3/47 EP23/65- EP29/78 Z1/70 GPB171/56 GPB171/61

#3 #7 #2 #2 #22 #9 #3 #6

EMP analyses

SiO2 31.24 27.41 29.81 28.94 29.36 26.63 26.59 26.47

Al2O3 17.32 18.79 16.99 17.75 17.04 20.22 20.13 20.58

TiO2 0.09 0.05 0.06 0.00 0.05 0.00 0.00 0.02

FeO 25.62 29.71 28.02 25.82 26.11 28.23 27.83 26.68

MgO 13.91 11.75 13.26 15.04 15.56 13.23 14.09 13.94

MnO 0.35 0.55 0.45 0.39 0.31 0.37 0.41 0.42

CaO 0.79 0.33 0.61 0.23 0.17 0.09 0.08 0.04

Na2O 0.09 0.07 0.10 0.01 0.01 0.03 0.04 0.00

K2O 0.06 n.a. n.a. 0.10 0.04 0.05 n.a. 0.01

Total 89.47 88.66 86.84 88.30 88.65 88.87 89.16 88.17

Si 3.20±0.02 2.92±0.01 3.11±0.02 3.03±0.02 3.06±0.02 2.81±0.01 2.79±0.01 2.79±0.01

Al(IV) 0.80±0.00 1.08±0.01 0.89±0.00 0.97±0.00 0.94±0.00 1.19±0.01 1.21±0.01 1.21±0.01

Al(VI) 1.29±0.01 1.28±0.01 1.20±0.01 1.21±0.01 1.15±0.01 1.32±0.01 1.28±0.01 1.35±0.01

Ti 0.01±0.00 0.01±0.01 0.00±0.00 0.00±0.00 0.00±0.00 0.00±0.00 0.00±0.00 0.00±0.00

Fe2+ 2.19±0.01 2.65±0.02 2.45±0.01 2.26±0.01 2.27±0.01 2.49±0.01 2.44±0.01 2.35±0.01

Mg 2.12±0.01 1.87±0.01 2.06±0.01 2.34±0.01 2.42±0.01 2.06±0.01 2.20±0.01 2.19±0.01

Mn 0.03±0.01 0.05±0.01 0.04±0.01 0.03±0.01 0.03±0.01 0.03±0.01 0.04±0.01 0.04±0.01

Ca 0.09±0.02 0.04±0.01 0.07±0.02 0.03±0.01 0.02±0.01 0.01±0.01 0.01±0.00 0.00±0.00

Na 0.02±0.01 0.01±0.01 0.02±0.01 0.00±0.00 0.00±0.00 0.01±0.00 0.01±0.00 0.00±0.00

K 0.01±0.00 n.a. n.a. 0.01±0.00 0.01±0.01 0.01±0.00 n.a. 0.00±0.00

AEM analyses

Si 3.48±0.20 2.88±0.12 3.06±0.20 2.96±0.15 3.01±0.18 2.90±0.09 2.79±0.17 2.98±0.24

Al(IV) 0.52±0.05 1.12±0.07 0.94±0.09 1.04±0.08 0.99±0.08 1.10±0.06 1.21±0.10 1.02±0.11

Al(VI) 1.47±0.12 1.32±0.08 1.25±0.11 1.36±0.10 1.44±0.12 1.35±0.07 1.20±0.10 1.24±0.13

Ti 0.01±0.00 0.00±0.01 0.04±0.01 0.00±0.00 0.00±0.00 0.01±0.00 0.01±0.00 0.01±0.00

Fe2+ 2.20±0.18 2.54±0.16 2.93±0.24 2.49±0.18 2.24±0.18 2.55±0.13 2.55±0.20 2.42±0.24

Mg 1.69±0.18 1.92±0.15 1.87±0.25 1.92±0.17 2.06±0.20 1.90±0.12 2.20±0.21 2.18±0.26

Mn 0.03±0.01 0.02±0.01 0.09±0.02 0.03±0.01 0.02±0.01 0.03±0.01 0.02±0.01 0.04±0.01

Ca 0.09±0.02 0.02±0.01 0.10±0.02 0.03±0.01 0.02±0.01 0.02±0.01 0.01±0.01 0.00±0.00

Na 0.00±0.00 0.01±0.01 0.00±0.00 0.00±0.00 0.00±0.00 0.00±0.00 0.00±0.00 0.00±0.00

K 0.03±0.01 0.02±0.01 0.04±0.01 0.03±0.01 0.00±0.00 0.02±0.01 0.02±0.01 0.01±0.01

1.0 nm layers [%] 18 3 9 2 0 1 0 0

0.7 nm. layers [%] 2 0 2 9 0 3 0 0

Because the Fe2+/Fe3+ ratio cannot be determined by EMP, all iron is considered to be divalent. To avoid contamination, only analyses that fulfil the criterion Ca+Na+K≤0.10 p.f.u.

were initially considered. Samples for EMP and AEM analyses were collected within 500 mm of each other in thin sections from the Taveyanne Sandstone. Note that the amount of smectite

layers increases with higher Si and S(Ca, Na, K) contents. More information concerning the calculation of the errors of EMP and AEM analyses is given in Appendix A.


Sandstone. The lattice fringe image contrast varieswithin small areas compared with higher grades,indicating variable orientation of the layers relative tothe incident electron beam (Fig. 4). In addition, somesamples (e.g. Fig. 4a, FB108) exhibit mottled texturesin fine-scale domains within the chloritic material. Thismottling is caused either by defects in the crystalstructure or by beam damage. Mottled textures wereobserved in samples from all metamorphic grades.

Chlorite mainly contains mixed layers of smectitewith less frequent occurrences of berthierine. Carefulexamination of our lattice fringe images reveals thatsmectite layers in the chlorite/smectite assemblage tendto have a Reichweite value of 1 as defined byJagodzinski (1949) (i.e. there is at least one chloritebetween each smectite layer). Chlorite/smectite layersalternate to build 2.4 nm layers (=corrensite) and canFig. 3. Ca, Na, K contents plotted versus frequency of 1 nm

layers/area. Data are obtained by combining HRTEM images be continuous up to 24 nm in thickness. AEM analysesand AEM analyses. Most AEM analyses can be explained by a obtained from the 1 nm/1.4 nm alternation sequencechlorite/saponite mixture. Chemical formulae for the in Fig. 4(a) show a significant amount of interlayercalculation of chlorite/saponite mixtures are given in Fig. 2.

cations (Ca, Na, K), indicating that smectite is present.Note that error bars are plotted for 2s.The inset of Fig. 4(a) exhibits a 1.0 nm layer thattransforms laterally to a 1.4 nm layer. This non-volume-preserved solution-mediated solid-state (SMSS)indicating that minor amounts of contamination may

influence the composition of chlorite during prep- reaction probably occurs by changing a collapsedsmectite layer into a 1.4 nm chlorite layer. SMSSaration, as mentioned above. Figure 3 implies that

most analyses can be interpreted as a mixture of processes are dominated by structural constraints, i.e.inheritance of parent material with topotactic relationschlorite and saponite.

The 0.7 nm layers are less common and therefore between parent and daughter structures.Some mineral reactions occur over longer distancestheir compositions could not be determined unequivo-

cally. AEM analyses of chloritic material containing and hence can only be explained by complete dissolu-tion, transport and precipitation (DTP) reactions. Asome 0.7 nm layers and to a lesser extent 1 nm layers,

however, show only few chemical differences compared common textural feature of such reactions within layerpackets are layer terminations. Consequently, the sur-to pure chlorite analyses (Table 2). The composition

of the 0.7 nm layers is therefore best described by the rounding layers are curved near those dislocations.Such dislocations occur particularly in samples frommineral berthierine and its structure is similar to

lizardite (TO). It is believed to be a metastable the diagenetic grade as shown in Fig. 4(b) (sample EP3).The change from two 0.7 nm layers into a 1.4 nmprecursor of chlorite (e.g. Ahn & Peacor, 1985; Cho &

Fawcett, 1986; Frey, 1987; Hillier, 1994). The composi- layer shown in Fig. 4(c) (sample EP3) is interpreted as aSMSS reaction that may be controlled by diffusion (seetion is close to chlorite as suggested by Lee & Peacor

(1983), Ahn & Peacor (1985), Livi (1994) and Baronnet, 1992), in which two 0.7 nm layers transformto a single chlorite layer. This reaction has been observedDalla Torre et al. (1996). In our study, berthierine-like

layers occur randomly intercalated in chlorite and less at all metamorphic grades in this study. This kind ofsolid-state transformation is approximately isochemicalfrequently as packets within chlorite, but can also be

locally ordered as 2.1 nm layers. assuming that the 0.7 nm layers are berthierine.Sixty-one lattice fringe images and AEM analysesSelected EMP and AEM analyses of chloritic

materials are given in Table 2. In order to compare were combined to investigate the occurrence of 0.7 nmberthierine and 1 nm smectite layers in chlorite packetsresults of both techniques, analyses were taken from

the same region within the thin section. When as a function of metamorphic grade. In Fig. 5, thefrequency of images with intercalated layers is plottedcomparing analytical results from EMP and AEM

analysis, chlorite from diagenetic and anchizonal rocks as a function of metamorphic grade. The area of theAEM analysis was estimated from the diameter of theoften gives different compositions for the same sample.

The two techniques, however, give mostly similar region damaged as a result of electron impact duringthe analysis. The total numbers of non-chlorite layersresults in samples from the epizone.were counted within each observed area and thepercentage of 0.7 nm and 1 nm layers is the number of

Samples from the diagenetic zoneeach layer/total number of layers. In authigenic chloritefrom diagenetic rocks (Fig. 5), chloritic material isIn diagenetic samples, small crystal sizes are character-

istic for chloritic materials in rocks from the Taveyanne composed of up to 29% smectite layers. Berthierine


a c

b

Fig. 4. Selected lattice fringe images of (001) planes ofdiagenetic chloritic material.(a) Intergrowth of chlorite and smectite in sample FB108.Local interstratification of 1.4 nm chlorite layers and 1 nmsmectite layers results in 2.4 nm corrensite layers. Assuming aprograde mineral reaction, the inset shows a 1 nm smectitelayer laterally transforming into a 1.4 nm chlorite layer.(b) Mixed-layering of chlorite, smectite and berthierine. Layerterminations are marked with white arrows.(c) Two 0.7 nm layers react to give one 1.4 nm layer. Thismineral reaction can be explained by a solution-mediatedsolid-state (SMSS) reaction with a minor change in rockvolume.

layers are also common but less frequent (<10%). decreasing proportions of layer terminations in thechloritic material indicates that the crystal defectLattice fringe images of chloritic material from dia-

genetic rocks show that all AEM analyses contain density decreases. We observed a general trend to adecrease in mixed layering. Chlorite crystals withsome 0.7 and 1 nm layers.mixed-layer phyllosilicates, however, were observed,including mixtures with K-white mica, but areas of

Samples from the anchizonepure chlorite were also present. The microtexture ofanchizonal chloritic materials may be best describedChloritic materials show, in general, an increase of the

crystal size as compared with diagenetic samples. The as mixtures of diagenetic and epizonal-grade chloritic


some mixed layers within chlorite (marked by blackarrows). Anchizonal-grade samples exhibit less frequentinterstratifications of 0.7 and 1 nm layers in chloritepackets (Fig. 5), and only one area indicated aconsiderable amount (23%) of 1 nm layers.

Chloritic material in anchizonal and higher-gradesamples commonly shows well-defined crystal inter-faces (Fig. 6b). Such boundaries occur between sub-parallel packets or at interfaces forming high-angleboundaries. Most images indicate that 1.4 nm layerstend to preserve their structure at high-angle grainboundaries between chlorite packets (Fig. 6b, sampleEP23). This contrasts with low-angle boundaries thatmay preserve other layered structures at their interfaces.

Samples from the epizone

Epizonal chloritic material is composed predominantlyof pure chlorite layers (Fig. 7a, sample GPB171),although some intercalated layers may occur (Fig. 7,sample GPB171). In general, the crystal sizes are largeand mainly free of intracrystalline layer terminations.The image contrast due to lattice distortions is ingeneral absent and fringes are straight in all investi-gated samples. This observation demonstrates thatepizonal-grade chlorite has the highest degree ofperfection compared with lower grades. The lowamount of contamination in epizonal chlorite issupported by our investigations by TEM which haveshown that chlorite is mainly pure or has <2% of1 nm layers (Fig. 5).

Layers of berthierine and saponite were oftenobserved at the interface of crystals with low-anglegrain boundaries. In Fig. 7(b), the angle between thetwo adjacent chlorite crystals is ≤1°. Images such asthis one suggest that low-angle boundaries maystabilize layers with structures other than chlorite.

Net transfer reactions and overall mineral reactions

EMP analyses and AEM data show that the composi-tions of the 0.7, 1 and 1.4 nm layers can be describedin terms of berthierine, saponite and chlorite, respect-ively. Following Thompson (1982), a set of linearly

Fig. 5. Frequency of 0.7 nm berthierine-like layers and 1 nm independent exchange vectors and additive compo-smectite layers are plotted versus proportion of layers per area nents have been selected to represent the chemicalinvestigated by combined HR TEM imaging and AEM analyses.

compositions of the minerals as shown in Table 3. TheWith increasing metamorphic grade, mixtures of chlorite, saponitechemical composition in the simplified system berthier-and berthierine transform into nearly pure chlorite.ine, saponite, chlorite and metamorphic fluid is

Table 3. Additive and exchange components for minerals inmaterials, implying that thermodynamic equilibriumchloritic material in rocks from the Taveyanne Sandstone.has not been attained.

TEM of sample EP23 shows areas having even Phase Additive component Exchange component

fringe contrast over larger areas than in diageneticClinochlore (Cln) Mg5Al2Si3O10(OH)8 Pl, Ed, Tk, Sdsamples (Fig. 6a), implying that anchizonal chloriticSaponite (Sap) Ca0.2Na0.1Mg3Al0.5Si3.5O10(OH)2 Pl, Ed, Tk, Sd

materials have fewer lattice distortions than diagenetic- Berthierine (Ber) Mg2.5Al1.0Si1.5O5 (OH)4 Pl, Ed, Tk, Sd

Quartz (Qtz) SiO2 –grade samples. In Fig. 6(a), a relatively large area ofFluid (W) H2O –

parallel lattice fringes is shown exhibiting nearlyPl=NaSiCa−1Al−1; Ed=NaAlSi−1%−1; Tk=SiMgAl−2; Sd=Al2%Mg−3.uninterrupted chlorite fringes. There are, however,


a b

Fig. 6. Selected lattice fringe images of (001) planes of anchizonal-grade chloritic material.(a) Some mixed layers (marked by black arrows) are intercalated in chlorite. In general, the image contrast is more constant ascompared to lower grades.(b) A high-angle grain boundary defined by the intersection of two chlorite packets. Chlorite layers tend to preserve their layerthicknesses at high-angle grain boundaries.

Fig. 7. Selected lattice fringeimages of (001) planes of epizonal-grade chlorite crystals.(a) The image contrast of chloriteis continuous and constant.Epizonal chlorite has the highestdegree of lattice perfectioncompared with lower grades.(b) A mixed-layering occurrence ata low-angle grain boundary. Low-angle boundaries stabilize layers

a b

with spacings less than chlorite.

accurately described by six components (cs): SiO2, rock system. Only the net-transfer reactions describethe crystallization and destruction of minerals andAl2O3, MgO, CaO, Na2O, H2O. In the terminology of

Thompson (1982), there are a total of 17 phase fluids during prograde metamorphism. Hence,components (cp) in the fluid–bulk system, for whichmineral reactions are completely described by 11 Table 4. Net-transfer reactions among minerals in sandstonesreactions (nr=cp−cs). These reactions may be divided from the Taveyanne greywacke (for abbreviations see Table 3).

into eight linearly independent exchange reactions (nex) k Net-transfer reactionand three linearly independent net-transfer reactions

(1) 4 Tk+3 Sd+Cln=4 W+7 Qtz(nnt) (Table 4). The exchange reactions alter the(2) 1/5 Pl+3/2 Tk+3/2 Sd+Sap=W+11/2 Qtz+3/10 Edcomposition of minerals but do not change the molar(3) 2 Tk+3/2 Sd+Ber=2 W+7/2 Qtz

amounts of phases present in the metamorphic fluid–


measurements of the progress of the net-transferDISCUSSION

reactions that occur during metamorphism yield overallmineral reactions that completely describe the abun-

Chlorite chemistry and ‘geothermometry’dance of phases as a function of metamorphic grade.The progress of the net-transfer reactions is determined Lattice fringe images demonstrate that apparently purefrom the chloritic material of samples between diag- chlorite from the lowest grade in the Taveyanneenetic and epizonal-grade rocks. Note that this simpli- Sandstone is composed mainly of three distinct layerfied approach assumes that the molar amount of types: berthierine (0.7 nm), smectite (1 nm) and chloritephases in chloritic material, as observed in this study, (1.4 nm). The maximum defect-free distance in termsis representative. Molar amounts per 1000 cm3 of of interlayering in diagenetic-grade samples is 80 nmminerals were determined by calculating the volume which was observed in lattice fringe images of chloriticof the minerals on the basis of layer frequency for each material in sample EP3.zone (Fig. 5). The volume of each phase was converted In contrast, AEM analyses of epizonal chloriticinto mole/volume using the molar volume data from material indicate that it consists of nearly pure 1.4 nmthe literature (Table 5). The progress f of the net- layers or is contaminated by only a few 0.7 and/ortransfer reaction k shown in Table 5 may be calculated 1 nm layers (≤2% 1 nm layers per area). We concludeby fk=nj,k(product)–nj,k(reactant) where nj,k is the number that the influence of other phases in EMP analyses ofof moles of additive jth mineral component of the kth chlorite can be neglected in epizonal-grade rocks fromnet-transfer reaction. The overall reaction the Taveyanne Sandstone. This is also corroborated

by Table 2 which shows that AEM analyses of pure0.09Pl+0.09Ber+0.43Sap+0.90Qtz+1.44Wchlorite from the epizone are, in general, in agreementwith EMP analyses. Furthermore, AEM analyses of=1.22Tk+0.75Sd+0.13Ed+0.51Chlchlorite from all metamorphic stages exhibit contami-

that formed chlorite from smectite and berthierine is nation of Ca, K and Na probably caused by displacedcalculated by the expression: surface atoms during the ion milling process. Reducing

the Ca, K, and Na content of chlorite by the sameS[Snj( j)]fk=0 amount of contamination present in adjacent quartz,

we suggest that our chlorite contains no more thanwhere nj is the stoichiometric coefficient of species j in0.05 S(Ca, Na, K) p.f.u.the kth net-transfer reaction and fk is the progress of

The question of whether or not pure chloritethe kth reaction. The change in rock volume as acontains compositional variations with grade isfunction of increasing metamorphic grade caused byclouded by the presence of contaminating phases andthe formation of chlorite may be determined by thethe inherent error in AEM data. EMP and AEM dataexpression:of anchizone-grade chlorite with low Ca, Na and Khave a greater spread than do epizone analyses.

DVr=∑kDV9 s,kfk However, there is a greater possibility for undetected

quartz contamination in EMP analyses of the finer-grained crystals of the anchizone than in epizonewhere DV9 s,k is the change in volume of the solid phases

calculated by the kth net-transfer reaction. The molar analyses. Therefore, although we cannot eliminate thepossibility that some variation with grade exists, wevolume of the exchange components were estimated

according to Ferry (1984) (Table 5). The total decrease cannot establish the extent of such variability.Since significant intercalation of smectite has anin local rock volume due to the chloritization of

berthierine and saponite is 1%. effect on chlorite compositions, especially Si and AlIV,

Table 5. Volume data and number of moles for minerals that are used for reaction progress calculations.

Phase Volume data [cm3/mol] Reference n[mol]: Diagenesis n[mol]: Anchizone n[mol]: Epizone Progress (f)

Cln 207.11 Anderson & Crerar (1993) 4.29 4.68 4.81 0.51

Ber 105.75 Anderson & Crerar (1993) 0.11 0.08 0.02 −0.09

Sap 223.69 Calculateda 0.44 0.10 0.01 −0.43

Qtz 22.69 Holland & Powell (1990) Calculated Calculated Calculated

Tk 2.59 Ferry (1984)b – – –

Sd 7.9 Holland & Powell (1990) – – –

Ed 9.54 Ferry (1984) – – –

Pl −0.54 Ferry (1984) – – –

Number of moles for minerals are determined per 1000 cm3. The change in volume due to the sudoitic exchange was calculated by the difference between the molar volume of clinochlore

and sudoite using volume data from Holland & Powell (1990) and the software program Thermocalc version 2.3. The molar volume for monoclinic saponite was calculated using

crystallographic data from Weiss et al. (1955): a=0.53 nm, b=0.92 nm, c=3.07 nm, b #97°. Note that this formula contains two layers of hydrated saponite. Calculated progress (f) of the

net-transfer reactions during chloritization of mixtures composed of berthierine, saponite and chlorite. a Al2%Mg−3: molar V °(Cln)—molar V ° (Sd) b Exchange vector is inverse to Ferry (1984).


the calibration of the chlorite ‘geothermometer’ of 3 Calculations of the change in volume were performedwith end-members.Cathelineau (1988) is probably influenced by contami-

nating smectite in the diagenetic and anchizone-grade 4 Table 6 shows that water and quartz are consumedduring the chloritization process. Additional waterchlorite crystals as suggested by Shau et al. (1990)

and Jiang et al. (1994). Thus, chlorite ‘geother- layers in the interlayer region are normally present inhydrated smectite due to its swelling behaviour. Thismometry’ measures the reduction of smectite in

chlorite. will alter the overall reaction, and it is reasonable toassume that water would be produced rather thanSeveral authors used the concept of reaction progress

measurements in clay minerals in order to monitor the consumed. We suppose that the fluids which aregenerated during chloritization of smectite, however,degree of deep diagenesis and very low-grade metamor-

phism in sedimentary rocks. For instance, the illite leave the rock shortly thereafter. Thus, the local changein rock volume was neglected in Table 6.crystallinity indices (see Frey, 1987), size and strain

measuring techniques based on XRD (e.g. Eberl et al., Two important conclusions can be inferred from thenet-transfer reaction (Table 6): (i) calculations clearly1990; Warr & Rice, 1994) or TEM techniques (e.g.

Merriman et al., 1990) are useful tools in order to demonstrate that chlorite in chloritic material fromthe Taveyanne greywacke grew mainly at the expensemonitor the stage of reaction progress. Essene &

Peacor (1995), however, pointed out that several of saponite, whereas the alteration of berthierine hasless impact on chlorite crystal growth, and (ii) thefactors such as time, fluid/rock ratio, starting materials,

deformation history and transformation mechanisms local volume decreases due to the chloritization ofchloritic material.may have an important influence on the chloritization

of the host rock. These authors believe that such The net-transfer reaction comprises both SMSS andDTP mechanisms that were recorded by the microtex-correlations must be treated with caution as they do

not generally represent true equilibration, and disparate tures. SMSS and DTP reactions leave little trace oftheir progress, however, and their relative importancevalues of one or more variables may result in very

different correlations of reaction progress and meta- to crystal growth cannot be measured accurately.Careful examination of our lattice fringe imagesmorphic grade. Nevertheless, many EMP studies have

shown that the progressive transformation of smectite suggests, however, that SMSS reactions in rocks fromthe Taveyanne Sandstone are common and may haveto chlorite via mixed-layer chlorite/smectite may be

applicable as a relative indicator of temperature in had an important influence on crystal growth ofchlorite. More information about solid-state andmetabasites (e.g. Cathelineau, 1988; Rahn et al., 1994;

Schmidt et al., 1997). However, we believe that the dissolution–precipitation reactions on the nanometrescale is given in Veblen (1992).progressive transformation of smectite to chlorite

cannot be used as a thermodynamically based geother-mometer for the reasons mentioned above. Like the

Smectite to chlorite transformation: the one layer modelillite crystallinity index, the smectite–chlorite trans-formation should be treated as a qualitative scale. A There are two simple SMSS reaction mechanisms for

forming a single layer of chlorite in smectite assumingmetamorphic grade indicator based on the Si contentof chlorite could map the relative increase of metamor- a one-layer model analogous to the chloritization of

biotite described by Veblen & Ferry (1983): (1) growthphism within a single lithology. The user of such ametamorphic indicator must, however, be aware that of a brucite-like layer into the interlayer region between

two TOT smectite layers, or (2) formation of a brucite-other factors may complicate interpretation of thedata, as mentioned above. like layer by removal of the tetrahedral sheets of one

TOT smectite layer. Both reaction mechanismsdecrease the local volume, but result in differentReaction progressvolume changes. Using the volume data of Table 5,the volume change due to mechanism 1 would be aThe net transfer in chlorite–berthierine–saponite mix-

tures between samples from diagenetic and epizonal- local decrease of 7.4%, compared with 53.7% formechanism 2 (see also Appendix B). We suggest thatgrade areas is determined by growth of chlorite at the

expense of saponite and berthierine (Table 6). There mechanism 1 is responsible for the chloritization ofsmectite in our samples as shown in Fig. 4(a). This isare, however, several factors that may affect the

calculations in our study. corroborated by several observations: the calculatedproportion of chlorite produced at the expense of1 The total reaction considers only the local mineral

assemblage and does not take into account other saponite in Table 6 is in better agreement with thismechanism; the change in rock volume is comparablemineral reactions which may also influence the abun-

dance of saponite and chlorite on a larger scale. to the calculated change (DV sap#DV total=−1.0%,Table 6); and mechanism 1 has also been observed by2 The number of lattice fringe images was restricted

and the size of the investigated area was small. Jiang & Peacor (1994).It is interesting to note that reaction mechanism 2Therefore, the molar content of minerals in the total

reaction may not be representative. depends on the occurrence of two adjacent smectite


layers which was rarely observed in our samples. chlorite: (i) chlorite replaced saponite by growth of abrucite-like layer into the interlayer region between twoBased on the data of Shau et al. (1990), Peacor (1992)

suggested that the normal prograde sequence from TOT smectite layers, and (ii) two berthierine layerstransformed into a chlorite layer probably by diffusionsmectite to chlorite should consist of saponite (smec-

tite)–saponite/corrensite–corrensite–corrensite/ of one T sheet. These reactions are in good agreementwith the calculated net transfer in our samples. Althoughchlorite–chlorite, whereas Beaufort et al. (1997) illus-

trated the smectite–corrensite–corrensite/chlorite– DTP reactions occur, it was not possible to evaluate theextent of this mechanism for the net-transfer reactionschlorite sequence. Since adjacent saponite-like layers

are rare in our study, the investigated sample series of in our sub-greenschist facies samples.Chlorite–berthierine–smectite mixtures decreased inprograde metamorphism from high diagenesis to

epizone represents the last stages of these sequences. volume during the formation of chlorite. In addition,quartz is consumed. Our data suggest that the localIn contrast, many authors report that in diagenetic

samples, smectite layer packets are common. In these change in rock volume is mainly caused by thebreakdown of saponite rather than by the destructioncases reaction mechanism 2 may be more important

for the saponite to corrensite transformation, although of berthierine. We suggest that the destruction ofberthierine may not be influenced by pressure to theit has not yet been reported.same order as for the breakdown of saponite.

Berthierine to chlorite transformationACKNOWLEDGEMENTS

The creation of one chlorite layer at the expense oftwo berthierine-like layers can be described by the We are most grateful for helpful comments by C. de

Capitani. This work was supported by Swiss Nationalreverse model of Baronnet (1992) in which Si and Alof the tetrahedral sheet of one berthierine layer diffuse Science Foundation Grant No. 20-43122.95/1 and the

United States NSF grant EAR-948090. The Johnsto an adjacent berthierine layer. The berthierine tochlorite transformation has little influence on the total Hopkins High-Resolution and Analytical Electron

Microbeam Facility was established by the NSF grantsnet-transfer reaction in the Taveyanne Sandstone dueto the small molar change of berthierine to chlorite as EAR-8300365 and EAR-8606864 and by a grant from

the Keck Foundation.metamorphism increases (Table 6). Furthermore, thedecrease in local rock volume is less significant forberthierine–chlorite than for saponite–chlorite trans-

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32, 12–17. The reaction mechanism 2 in which two saponites were replacedby one chlorite may be expressed by:Zang, W. & Fyfe, W. S., 1995. Chloritization of the hydrother-

mally altered bedrock at the Igarape Bahia gold deposit,2 saponite=chloriteCarajas, Brazil. Mineralium Deposita, 30, 30–38.

2 A0.2B0.1M3T4O10 (OH)2+10H2OReceived 11 August 1997; revision accepted 7 October 1998.

=[M3T4O10(OH)2+M3(OH)6]

APPENDIX A +4H4TO4+0.4A+0.2B

We assumed that the error (2s) of the EMP analyses was 0.5% for Because chemical analyses of pure saponite are not available, thethe major elements Si, Al, Fe and Mg but 30% for the minor local changes in volume of the solids were calculated using volumeelements Ti, Mn, Ca, Na, K. The errors (2s) for cations (cat) of the data of the end-members listed in Table 5. The local changes inAEM analyses were calculated as followed: volume, based on the one-layer models, were calculated by the

equation:2s=[√ctsSi/ctsSi+√ctscat/ctscat+(s(k)cat/Si/√ns)/kcat/Si]×2Xcat DV

V9 0Sap=

V9 0Sap−V9 0ChlV9 0Sap

whereby ctscat is the number of counts of the analysed cation ands(k)cat/Si is the standard deviation of the k value for the specific

Both mineral reactions in Appendix B that describe saponite (Si=cation/Si ratio. The number of spectra used for the k value3.5, Al=0.5) transforming into chlorite (Si=3, Al=2) include andetermination is given as ns and Xcat is the mol% of the analysedoverall uptake of Al and a release of Si. This is in agreement withcation on the basis of 14 oxygens.EMP analyses plotted in Fig. 1. Chlorite data show an enrichmentof Si in probably mixed-layered chloritic material in samples fromthe diagenetic and anchizonal-grade samples compared with epi-APPENDIX Bzonal-grade samples of probably nearly pure chlorites.

The reaction that creates one chlorite layer at the expense of one We assumed for the berthierine to chlorite transformation ansaponite layer (reaction mechanism 1) written analogous to Veblen isochemical process described simply by:& Ferry (1983) is as follows:

2 berthieriene=chloritesaponite=chlorite

The change in volume due to this transformation was calculatedA0.2B0.1M3T4O10 (OH)2+3M+3H2O+1.5O2 analogous to the above with volume data of the end-members

from Table 5.=[M3T4O10 (OH)2+M3(OH)6]+0.2A+0.1B

Documents

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