OCTAHEDRAL OCCUPANCY AND THE CHEMICAL COMPOSITION OF DIAGENETIC (LOW-TEMPERATURE) CHLORITES

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1 Clay Minerals (1991) 26, This and the following five papers were presented at a joint meeting of the Metamorphic Studies and Clay Minerals Groups of the Mineralogical Society under the auspices of IGCP 294 entitled "Phyllosilicates as indicators of very low-grade metamorphism and diagenesis" held from 4-6 July 1990 at the University of Manchester. OCTAHEDRAL OCCUPANCY AND THE CHEMICAL COMPOSITION OF DIAGENETIC (LOW-TEMPERATURE) CHLORITES S. HILLIER ANO B. VELDE Ecole Normale Sup(rieure, D(partement de G~ologie, 24, rue Lhomond, Paris Cedex 05, France (Received 3 August 1990; revised 24 October 1990) A B S T R A C T: The chemical composition of about 500 diagenetic chlorites, determined by electron microprobe, has been studied in six different sedimentary sequences spanning conditions from early diagenesis to low-grade metamorphism, in the temperature range ~ The range of Fe/(Fe + Mg) is almost complete and is positively correlated with AI. Five sequences show the same compositional variation. In each, the most siliceous chlorites have the lowest R 2+, substantially more octahedral than tetrahedral AI, and the lowest octahedral totals. Conversely, the least siliceous have the highest R 2+, nearly equal octahedral and tetrahedral A1, and octahedral totals close to that for an ideal trioctahedral mineral. A dioctahedral substitution Si[]R2 2 (where [] represents a vacant octahedral site) which decreases with temperature, describes this variation. Low octahedral totals are, however, induced by the method of calculation and need not indicate vacancies; for published wet chemical analyses of metamorphic chlorites they may simply indicate oxidation of Fe. lntergrown dioctahedral phyllosilicates may partly account for apparent vacancies in diagenetic chlorites. Nevertheless, the correlation of composition with temperature and similarities to the temperaturerelated evolution of synthetic chlorites, suggest that diagenetic chlorites are compositionally distinct from, but metastable with respect to, fully trioctahedral metamorphic chlorites. Temperature-related trends are modified by bulk composition, complicating their potential use for low-temperature geothermometry. Chemical analyses of diagenetic chlorites appear to show that, compared to metamorphic chlorites, they are more siliceous, have less Fe + Mg, and lower octahedral cation totals (Curtis et al., 1984, 1985). The low octahedral totals may be due to vacant octahedral sites, indicating that low-temperature chlorites have a dioctahedral component, or isomorphous substitution. For example, Boles & Franks (1979) noted that diagenetic chlorites from Wilcox Formation sandstones had compositions between di-trioctahedral chlorites (total octahedral cations = 10) and trioctahedral chlorites (total octahedral cations = 12). For metamorphic chlorites, octahedral vacancies are, generally, not that important (Foster, 1962; Velde, 1973; Laird, 1988). Therefore, during the passage from diagenesis to metamorphism, octahedral occupancy appears to increase until eventually chlorites become fully trioctahedral. Indeed, it has recently been proposed that (along with tetrahedral A13+) octahedral occupancy is dependent mainly upon temperature, and sensitive enough to be used as a geothermometer of broad applicability (Cathelineau & Nieva, 1985; Cathelineau, 1988). Exactly what the low octahedral totals represent, however, is not really clear, mainly The Mineralogical Society

2 150 S. Hillier and B. Velde because of problems with the methods of chemical analysis, and with the assumptions upon which the calculated structural formulae are based. With microprobe methods of analysis, both the anion content of the mineral, and the oxidation state of Fe are not determined. Therefore, structural formulae are generally calculated by normalizing cations to an ideal number of valences (oxygen equivalents), and assuming all Fe to be Fe 2+, although sometimes a small proportion of Fe 3+ is also assumed. Recently, Laird (1988) pointed out that, in the chlorite structural formula, octahedral occupancy is the parameter most dependent on the nature of the assumptions. Recalculation of analyses which represent mixtures of chlorite with, for example, small amounts of smectite or illite, will also produce a low octahedral total if expressed as a chlorite formula (Curtis et al., 1984). Minor contamination may be hard to detect and difficult to avoid, given that in the diagenetic zone, phyllosilicates, including chlorite, are often fine grained and intimately intergrown (Ahn & Peacor, 1985). Thus, apart from dioctahedral substitution, low octahedral totals may also arise because of contamination or non-ideal stoichiometry. The present paper is a study of the chemical composition of chlorites formed at relatively low temperatures in diagenetic and very low-grade metamorphic rocks, with particular emphasis on some of the questions concerning octahedral occupancy. The approach adopted was to compare the chemical composition and diagenetic/metamorphic trends for several different sets of chlorite analyses, taken from a variety of diagenetic and very lowgrade metamorphic sequences. In all, six different sedimentary sequences have been examined, which together span conditions from early diagenesis to low-grade metamorphism, with temperatures ranging from ~ They also cover a wide range of bulk rock compositions, as least as far as sedimentary rocks are concerned. In addition, data on the chemical composition of metamorphic chlorites, compiled by Foster (1962) have been re-examined for the purpose of comparison. CALCULATION OF STRUCTURAL FORMULAE AND CHOICE OF ANALYSES For each of the studies described below, all chlorite analyses were obtained by electron microprobe under conditions similar to those recommended for clay minerals by Velde (1984); all data were obtained at 15 kv, but current and counting times varied. All chlorite analyses were re-calculated as chlorite structural formulae on the basis of 28 oxygen equivalents (56 negative charges) i.e. assuming an ideal anion framework of O20(OH)16. All Fe was assumed to be Fe 2+. Some support for this commonly made assumption, or at least that Fe 3+ is only likely to be present in small amounts, will be discussed later, following comparison of the trends shown by wet chemical analyses for metamorphic chlorites (Foster, 1962) with those of the diagenetic chlorites presented here. It should also be noted that this assumption also results in higher octahedral totals in the ideal structural formula than if some proportion of the Fe is assumed to be Fe 3+. A significant content of Na, K, or Ca in a "chlorite" analysis suggests that it is contaminated. To try to minimize this problem, only analyses with <0.5 wt% total Na20 + CaO + K20 were selected. This limit was considered to be more than sufficiently restrictive, as even analyses of chlorite made by analytical transmission electron microscopy, where the electron beam excites a very much smaller volume than by electron microprobe, commonly show totals of Na + K + Ca equivalent to significantly more than 0.5 wt% total of the oxides (cf. Curtis et al., 1985).

3 Composition of diagenetic chlorites 151 SOURCES OF ANALYSES AND GEOLOGICAL SETTINGS The chlorite analyses come from six sedimentary sequences, with a diverse range of ages and geological settings. The first two sets were taken from work by Medhioub (1987), also reported on by Velde & Medhioub (1988). One is from Eocene to Upper Cretaceous rocks from a borehole in southwest Texas, and the other from Upper Cretaceous rocks from a borehole in the Niger Delta. The Texas analyses are all from sandstones from six different depths (663, 1164, 2163, 2345, 2703 and 2950 m). In total, 29 individual chlorite analyses passed selection. The geothermal gradient in the area is close to 28~ and the samples span a temperature interval of ~ the clay mineral assemblage is typical for Gulf Coast sequences. For the Niger Delta sequence, 96 analyses were selected from samples from eight different depths: at 1618, 1769 and 1770 m, the analyses are from both shales and sandstones; at 1774 and 1783 m, from silty sandstones only; and at 1996, 2173, and 2453 m, from shales and feldspathic sandstones. The present-day geothermal gradient is -55~ but vitrinite reflectance data indicate that, at some time, the palaeogeothermal gradient was higher, probably close to 100~ (Velde et al., 1986). The clay mineralogy consists of illite-smectite (<15% expandable layers) and chlorite, with kaolinite present in only the shallowest sample. The third set consists of 37 analyses made by the first author from eight outcrop samples of lacustrine shales from the Middle Devonian of the Orcadian Basin, northern Scotland. Vitrinite reflectance data for these samples range from 0.7% to 6.1% (mean reflectance in oil) indicating palaeotemperatures from ~ (Hillier, 1989). The large temperature range results from the regionally variable effects of burial, and contact metamorphism by a large igneous intrusion of probable Late Devonian age, believed to underlie much of the Caithness area of the basin (Hillier, 1989). Associated clay minerals include illite and illitesmectite, and some of the lowest maturity samples contain kaolinite. Corrensite is also common in the sequence, but was not detected by XRD in the samples analysed, although the chlorites themselves may have formed from corrensite (Hillier, 1989). Both the fourth and fifth series of analyses are of samples from Lower Palaeozoic and Proterozoic rocks from the Massif Armorican, France. The first of these is from the work of Paradis (1981) on Early Ordovician to Early Devonian age rocks from the Central Armorican Zone SE of Brest. A total of 81 analyses from 9 different samples were selected. Two samples are from metamorphic zone A, four samples from metamorphic zone B, and three samples from metamorphic zone C, as described by Paradis et al. (1983). Chlorite occurs with pyrophyllite in both zones A and B, and with chloritoid in zone C, the mineral assemblages indicating that these are very aluminous rocks~ and that palaeotemperatures probably range upwards of 270~ (Paradis et al., 1983). The other Massif Armorican series is from the study by Beaufort (1986) of the Brioverien (Late Proterozoic) metasediments that surround the massive sulphide deposit near Rouez. The rocks are described as pelites and meta-greywackes, metamorphosed under greenschist facies conditions in the Hercynian orogeny, producing a clay mineral assemblage that is essentially illite plus chlorite. No more precise data on palaeotemperature are available. A total of 158 analyses were selected from 49 different samples. The last set are from samples of the Precambrian Belt supergroup, Glacier Park, Montana (N. El Moutaouakkil ENS, unpublished data). The supergroup consists of various alternating carbonate and siliciclastic formations representing deposition in a variety of shallow marine and inter-tidal environments. A total of 84 analyses were selected from five

4 152 S. Hillier and B. Velde Fe Fe 9 BrBst 9 Texas Niger 9 Rouez Mg AI Mg FIG. 1. Chemical composition of the six series of chlorites as represented oll an AI-Fe-Mg diagram. AI different samples. Maxwell & Hower (1967) showed that the rocks of the Belt supergroup have been subjected to conditions varying from high-grade diagenesis to low-grade metamorphism and, based on oxygen isotopic data, Eslinger & Savin (1973) concluded that palaeotemperatures in this region ranged from ~ These six sets of chlorite analyses will be referred to as the Texas, Niger, Orcadian, Brest, Rouez and Montana sets, respectively. The total number of analyses is 485 and all are available on request. Chemical composition RESULTS Most diagenetic chlorites reported in the literature are Fe-rich, and although the majority of the chlorites considered here are similarly so, there is an almost complete range between Fe- and Mg-rich varieties (Fig. 1, Table 1). Variation within each series is, however, relatively restricted so that each forms a distinct group on the A1-Fe-Mg plot of Fig. 1, with Fe contents decreasing in the order Brest > Rouez, Niger, Texas > Orcadian > Montana. For metamorphic chlorites, A1 is almost always in the range 30-40% in the A1-Fe-Mg plot (Velde, 1985). By comparison, many of the diagenetic chlorites, especially the Fe-rich species, are relatively richer in AI and poorer in (Fe + Mg). Nevertheless, about half of the analyses plot within the range shown by metamorphic chlorites and notably these include all of the Mg-rich examples. Thus, there is a correlation between Fe/(Fe + Mg) and A1, the most Fe-rich chlorites being the most aluminous, and the most Mg-rich chlorites the least aluminous. In Fig. 2 each series of analyses has been plotted in the trioctahedral half of the vector representation of chlorite compositions as presented by Wiewiora & Weiss (1990). This is a useful plot to show the relationships between Si, R 2+, A1, and octahedral occupancy; Si and R 2+ are represented by the orthogonal axes, and total A1 and octahedral occupancy are shown by isolines (contours). Metamorphic chlorites tend to plot along, or close to, the line of full octahedral occupancy, between the compositions corresponding to non-aluminous

5 Composition of diagenetic chlorites oo d~ -d 9 F r~ 4 o 0 4 o o ~%~, "~ oo zo~ o,,6 ~o~ d r~ o~ gf g ~

6 154 S. Hillier and B. Velde serpentine and amesite, but restricted to AI contents from formula positions, judging from analyses reported by Foster (1962). Positions along this line (and any parallel one) correspond to variation in the amount of Tschermak substitution, A12R2+_ISi_ ]. Considered together, Si contents of the diagenetic chlorites vary between -4.6 and 7-00 formula positions (Fig. 2). For metamorphic chlorites, Foster (1962) found a similar range of Si content between and 6-5. However, although the overall range of Si content is comparable, there are distinct differences if the Si content is considered in relation to the Fe/(Fe + Mg) ratio and octahedral occupancy. The majority of the relatively Si-rich (> 5-4 formula positions) metamorphic chlorites detailed by Foster (1962) are Mg-rich, the Fe-rich varieties being almost exclusively Si-poor. The diagenetic chlorites follow this pattern in that both the Mg-rich Montana and Orcadian series are relatively siliceous, and the very Fe-rich Brest series relatively poor in silica, but it also appears that Fe-rich diagenetic varieties can have relatively siliceous compositions when in addition they have low octahedral totals. The most important feature, however, is that in five of the six series, which together extend across the complete range of Fe/(Fe + Mg), the compositional trends are essentially the same. For each, the most siliceous compositions are characterized by the lowest R 2+ ' / '//'~. //'~ Serpentlne/~ //'/ /// = ///" Si 6 ~.,"/"~0 /' 9 Texao // AI=8,/ { '9 R //'//// ////// /////// 0%~<~ '/ /,, ~,' Ames lte, 0 Nlger R2+ //"//"" /////// ///,//'/" Si S R FI6.2. Chlorite compositions plotted in the vector representation of Wiewiora & Weiss (1990) with Si and R 2+ as orthogonal axes, and total AP + and octahedral occupancy shown by isolines. 1t2+ 2.

7 Composition of diagenetic chlorites 155 totals and by the lowest octahedral occupancy. The only difference is in the position of each series which appears to be related to the relative content of Fe, Mg and A1, the Mg-rich Al-poor series plotting towards the top diagram, and the Fe-rich Al-rich series towards the bottom (Fig. 2). The only exception to this trend is the Orcadian series which presents a more dispersed distribution reflecting the variable A1 content and the variable Fe/(Fe + Mg) ratio of chlorites from this group. Octahedral occupancy With few exceptions, the octahedral occupancy of chlorites from all six sequences is deficient, i.e. substantially less than the ideal total of 12 cations for a fully trioctahedral chlorite (Fig. 2). The examples of structural formulae given in Table 1 were chosen to cover approximately the range of octahedral occupany shown in each series. Overall, octahedral totals <11.5 are common, and some series include numerous examples with totals significantly <11 (Fig. 2). Furthermore, each of the series shows a relatively wide range of occupancy, the most restricted ranges being shown by the Mg-rich, Al-poor chlorites that characterize the Montana and the Orcadian sequences. For chlorites from the Niger, Orcadian, Rouez and Montana sequences, the ranges extend up to near full occupancy. In contrast, for all chlorites from both the Texas and the Brest sequences octahedral occupancy is significantly deficient, the maximum occupancies found being and 11.76, respectively. It is also interesting to note that one or two of the chlorites from the very aluminuous Brest sequence have octahedral occupancies close to that of the di-trioctahedral chlorite sudoite, but unlike previously reported analyses of sudoite in which Mg is the dominant bivalent cation (Newman & Brown, 1987), these examples are almost exclusively dominated by Fe. In all samples, the low octahedral totals result from the necessity to assign more AP + to the octahedral sheet than to the tetrahedral sheet. For metamorphic chlorites, octahedral A1 is usually slightly less than tetrahedral A1 (Foster, 1962). In contrast, in all of the diagenetic series, octahedral A1 is, almost without exception, greater than tetrahedral A1 (Fig. 3). This excess of A1 in the octahedral sites is due to the relatively high Si/AI ratio of most of the diagenetic chlorites compared to metamorphic chlorites of similar alumina content. This is evident in Fig. 2 where, with the exception of the Orcadian set, compositional variation trends are parallel or sub-parallel to lines of constant A1. Each of the series is characterized by a relatively constant AI content, but by variable contents of Si and total R 2+. Chlorites which plot close to the line of full occupancy between serpentine and amesite are similar in composition to metamorphic chlorites. Chlorites with low occupanies are more siliceous and contain less total (Fe + Mg). Temperature/compostion relationships The relationship between temperature and composition was examined for the Texas, Niger, Orcadian and Brest sequences. For the Texas sequence, present-day temperatures were estimated from the sample depth and geothermal gradient; for the Niger sequence temperatures were estimated assuming a palaeogeothermal gradient of --100~ (Velde et al., 1986); for the Orcadian sequence temperatures were derived from vitrinite reflectance data using the relationship of Barker & Pawlewicz (1986); for the Brest sequence a temperature of 270~ was assumed for pyrophyllite bearing samples (e.g. Frey, 1987), and 300~ assumed for the higher grade samples which contain chloritoid. In using

8 156 S. Hillier and B. Velde Brest Niger AI(VI) 3 I I I AI(IV) AI(VI) 3 Orcadian 4 0 # / AI(IV) Rouez Foster (1962) AI(VI) 3 Montana g ~ AI(VI) AI('V) Al(IV) FIG. 3. Octahedral AI vs. tetrahedral AI for the diagenetic chlorites and for analyses of metamorphic chlorites compiled by Foster (1962). estimated maximum temperatures it is assumed that chlorite formation occurred at maximum temperature. This seems a reasonable assumption as chlorite formation during diagenesis tends to be favoured as temperature increases. Although all the temperatures are rough estimates, the samples cover a very wide temperature range and should be in more or less the correct relative order with respect to maximum temperature, as indicated by increasing depth or vitrinite reflectance.

9 Composition of diagenetic chlorites 157 The relationship between temperature and octahedral occupancy for the four series is shown in Fig. 4, along with the data of Jahren & Aagaard (1989) for chlorites from some North Sea sandstones, and the regression line of Cathelineau & Nieva (1985) for hydrothermal chlorites. For each sample the data are shown as the mean composition and the compositional range. A general increase in octahedral occupancy with increasing temperature is seen, but the different series seem to show different trends, and in individual samples and at any temperature it is clear that the range of compositions is large. For the data from the present study, the correlation coefficient for the best-fit line through the mean compositions is The Orcadian chlorites, which are relatively rich in Mg and Si and poor in A1, all have relatively high octahedral occupancies, all those from temperatures <250~ plotting well above the regression line of Cathelineau & Nieva (1985). Compositions from the Texas series generally show the lowest occupancies with means close to those that would be predicted for this temperature range from the relationship for hydrothermal chlorites. However, the ranges of composition are rather large. The Niger series appears to be similar to the data of Jahren & Aagaard (1989), plotting in a zone parallel to, but below, the hydrothermal chlorite line. By far the most distinct series are the chlorites from Brest. Temperatures near 270~ are indicated by the presence of pyrophyllite in these samples, yet many chlorites from this very aluminous sequence have abundant vacant octahedral sites, the majority plotting well below the line given by Cathelineau & Nieva (1985). The relationship between tetrahedral A1 and temperature is shown in Fig. 5, together with the regression line calculated for hydrothermal chlorites by Cathelineau (1988). Again there is a general relationship showing that tetrahedral A1 increases with temperature. In li Sum (VI) 11.oo XI" AO D N B A J i i i i i i i FL6.4. Relationship between octahedral occupancy and temperature, with data represented as mean composition and compositional range. Also shown is the regression line calculated by Cathelineau & Nieva (1985) for hydrothermal chlorites, and the data of Jahren & Aagaard (1989) for chlorites from some North Sea sandstones. T = Texas, O = Orcadian, N = Niger, B = Brest, J = Jahren & Aagaard (1989). Toc

10 158 S. Hillier and B. Velde AI(W) ~ I A o DN 0 B A J i i i i i f 50 1 O T~ FIG. 5. Relationship between tetrahedral AI content and temperature, with data shown as mean composition and compositional range. Also shown is the regression line calculated by Cathelineau (1988) for hydrothermal chlorites, and the data of Jahren & Aagaard (1989) for chlorites from some North Sea sandstones. T = Texas, O = Orcadian, N = Niger, B = Brest, J = Jahren & Aagaard (1989). fact, this relationship is better defined than the relationship of occupancy to temperature, the best-fit line through the mean compositions [AI(IV) = (T~ ] having a correlation coefficient of Nevertheless, the ranges per sample are considerable, and notably almost every analysis plots above the relationship found for hydrothermal chlorites by Cathelineau (1988). It is also notable that the Brest data are much less discordant, although there are examples with higher values for tetrahedral A1 than in any other series. Compositional variation DISCUSSION The six series of diagenetic chlorites span an almost complete range of Fe- to Mg-rich varieties indicating that, like metamorphic chlorites, substitution between Fe and Mg is probably complete. Using XRD to determine chemical composition, both Bailey & Brown (1962) and Shirozu (1960) noted a general trend for increasing tetrahedral A1 to be accompanied by increasing octahedral Fe in metamorphic IIb chlorites. As compositions derived from XRD assume an equal distribution of A1 between tetrahedral and octahedral sites, this same relationship between A1 and Fe/(Fe + Mg) is evident for the diagenetic chlorites (Fig. 1). Individually, however, each of the series show more limited Fe for Mg substitution or variation in A1 content, the most significant compositional variation being the Si and total R 2+ contents. In the vector representation used in Fig. 2, this compositional variation is described by the substitution vector SiAIVI[]AI TM_ 1R 2+_z (where U represents a vacant octahedral site). This is a coupled substitution involving both octahedral and

11 Composition of diagenetic chlorites 159 tetrahedral sheets. Total A1 remains constant so that the substitution may be concisely written as Si[]R2_2. For five of the series studied the dominant compositional variation can be described by this dioctahedral substitution. It also describes the bulk of the temperature related compositional variation seen in hydrothermal chlorites (Cathelineau & Nieva, 1985; Cathelineau, 1988). Together, the Si-rich diagenetic chlorites occupy the central part of the region of Si-rich chlorites synthesized by Velde (1973) in the chemical system MgO-A1203-SiO2-H20 (MASH). Fransolet & Schreyer (1984) suggested that the synthetic Si-rich chlorites might be regarded as a solid solution between trioctahedral and dioctahedral chlorites according to the substitution AI2Mg_3, and other authors have referred to this dioctahedral substitution (Laird, 1988). However, the compositional trends for the natural Si-rich diagenetic chlorites indicate that if such a solid solution occurs the substitution is not AI2Mg_ 3 but Si[]R2_2. Octahedral occupancy In support of the occurrence and the importance of octahedral vacancies in chlorites, reference is often made to the classic study of metamorphic chlorites by Foster (1962). Nevertheless, regardless of how the structural formula is calculated, most metamorphic chlorites usually show little indication of vacant octahedral sites, especially those from the highest metamorphic grades (Laird, 1988). In fact, vacancies are also slight, <0.2 per 12, in most of the chlorites studied by Foster (1962), although examples with higher vacancies were not uncommon. Foster (1962) found that occupancy was deficient by an amount approximately equal to half the excess octahedral R 3+ (i.e. octahedral R 3+ in excess of tetrahedral R3+), and concluded that the excess R 3+ replaces R 2+ in the ratio of 2 : 3. Clearly, this is not the same relationship found for the diagenetic chlorites in which, in terms of a substitution, excess R 3 + replaces R 2+ in the ratio of 1 : 2, the resulting charge imbalance being compensated by a coupled substitution in the tetrahedral sheets of Si 4+ for R 3+ in the ratio of 1:1. Both types of substitution, however, satisfy the relationship between octahedral occupancy and excess octahedral R 3+ as found by Foster (1962). The reason is that this relationship is inherent in the method of calculating a structural formula on the basis of an ideal number of valences. Cation proportions are calculated by normalizing total positive charge to balance the ideal total of 56 negative charges. Hence, if the total positive charge is increased, the number of cations will be correspondingly decreased by the process of normalization. It is therefore of interest to try to establish why the octahedral totals of many of the chlorites examined by Foster are low, the dual purpose being to emphasize that from ideal structural formulae alone it is not possible to conclude with certainty that there are vacant octahedral sites. In most of the chlorites studied by Foster (1962), tetrahedral AP + is greater than octahedral AP + (Fig. 3) but less than the total octahedral R 3+ due to the additional presence of Fe 3+. The fact that octahedral vacancies in the calculated formulae of the majority of Foster's chlorites result from excess R 3+ due to Fe 3+ is evident from the relationship between occupancy and total Fe 3+ content per formula unit shown in Fig. 6. The line on this figure is the relationship between octahedral occupancy and excess octahedral R 3+ found by Foster (1962). Points which plot along and above this line represent chlorites in which vacancies are due entirely to the presence of Fe 3+. Those points above the line represent chlorites in which, beside excess Fe 3+, some Fe 3+ appears

12 160 S. Hillier and B. Velde necessary to balance the net negative charge arising from substitution of AP + in the tetrahedral sheets. The distance above the line indicates the amount of Fe 3+ necessary for charge balance. For those analyses which plot below the line, excess R 3+ in the octahedral sheet is partly due to Fe 3+ and partly due to AP +. The important point is that the vast majority of analyses plot close to, or above, the line indicating that the octahedral occupancies are directly related to the Fe 3+ content; only in a few instances is excess R 3+ dominantly due to A13+ (as it is for the diagenetic chlorites). If all those examples in which there is any contribution to excess R 3+ by A13+ (all points below the line) are discarded, and the remaining analyses (110 out of 154) plotted in the Si versus R 2+ diagram, it is apparent that most examples with >0.5 formula positions occupied by Fe 3+ are Si-poor varieties which occupy a zone running parallel to the R 2+ axis (Fig. 7). Those with the highest Fe 3+ content have the lowest R 2+ contents and the lowest octahedral occupancies. The trend of the analyses parallel to the R 2+ axis suggests that this replacement only affects the composition of the octahedral sheets. Furthermore, its restricted location at Si-poor compositions indicates that it is related to the region occupied by Fe-rich chlorites, most of which are Si-poor, but not associated with the Mg-rich chlorites which tend to be more siliceous. This trend and its location are precisely what would be expected if the excess Fe 3+ present is due to secondary oxidation according to the reaction: 2Fe(OH) = 2FeO(OH) + H20 in which case, the structural formulae should be calculated on the basis of an anion framework that contains more oxygen and less hydroxyl. Unfortunately, on the basis of the structural formulae alone, it is not possible to distinguish oxidation from a dioctahedral replacement of the type Fe3+2Fe2+_3. But, given that most metamorphic chlorites show near full octahedral occupancy, and considering the lack of evidence for replacements of the Fe3 + ~ ,,, -,-..,.,, ~,, Octahedral occupancy F16.6. Relationship between octahedral occupany and the amount of Fe 3+ (formula positions) for chlorite analyses from Foster (1962). The line is the relationship between occupancy and excess octahedral R 3+ as found by Foster (1962).

13 Composition of diagenetic chlorites! i61 ///'//.///// / /./'' / / ~ /' Si // / ~' ( Fe3+ ~ / '~,~ ~/L~/"~ ~ / / / mesite ( i i ~ i i i R2+ FI6.7. Si vs. R2+, showing chlorite analyses from Foster (t962) in which tetrahedral AI is greater than octahedral AI, divided into categories by Fe 3+ content. type R3+2R2+ 3, oxidation appears to be the more probable alternative. This is not to say that Fe 34 is not originally present in chlorites in any amount, indeed it appears from Fig. 6 that in many samples, small amounts are required for structural balance. Large amounts (>0.5 formula positions ~5wt% Fe~O3), however, that result in significantly low octahedral totals, suggest that oxidation has occurred. Foster (1962) concluded that Fe 3+, even if in excess of that required for structural balance, should be regarded as a normal constituent of chlorite, but she based this conclusion largely on the relationship between octahedral occupancy and excess octahedral R 3+, which as discussed above is induced by the calculation procedure. The earlier suggestion by Hey (1954) that any chlorite with more than 4% Fe203 should be regarded as oxidized, although arbitrary, is probably a reasonable assumption. In older classification schemes, oxidized chlorites represent the type of leptochlorites that can be reduced to orthochlorite composition (see discussion in Hey, 1954). In contrast, the significantly low octahedral total of diagenetic chlorites cannot be explained as a consequence of oxidation, because all Fe was assumed to be Fe z+. This assumption minimizes total positive charge so that the number of cations normalized to the ideal anionic charge is maximized. If some of the Fe present is Fe 3+, allowance for this would decrease the octahedral totals further, totals which are already low due to excess octahedral A1. As a result of the often considerable excess octahedral A1, the diagenetic chlorites represent the type of leptochlorites that cannot be reduced to orthochlorite composition. The favoured explanation offered by Curtis et al. (1985) to account for the Si-rich compositions and low octahedral totals of diagenetic chlorites from reservoir sandstones was minor interstratification with residual smectite layers (residual in the sense that the diagenetic chlorites might have developed from a precursor swelling chlorite). Curtis et al. (1985) also noted, however, that such a replacement sequence is not compatible with the delicate morphology of the chlorites which indicates growth from solution. In the present

14 162 S. Hillier and B. Velde study we tried to avoid the potential problem of contamination by rejecting any "chlorite" analysis with >0.5 wt% total Na20 + K20 + CaO. Even so, small quantities of these oxides are invariably present such that minor "contamination" appears to be unavoidable. Furthermore, any analyses contaminated with small amounts of quartz, kaolinite, or pyrophyllite would, of course, not be excluded. The potential effect of various contaminants is shown in Fig. 8. Two hypothetical chlorite compositions with full occupancy have been modified by the addition of 10% kaolinite, quartz, and talc. Contamination with -10% of any of the common types of dioctahedral phyllosilicates would lower the octahedral total to near 11.5 and shift compositions to points lying somewhere between the effect shown for kaolinite and quartz. Hence, contamination could conceivably produce compositional trends like those shown in Fig. 2. Recent TEM studies of argillaceous rocks show that there is a progressive decrease in fine scale intergrowth of phyliosilicates accompanied by an increased segregation into larger coherent packets with increasing diagenetic grade (Ahn & Peacor, 1985; Lee et al., 1985). Consequently, with the electron microprobe it may be difficult to obtain pure analyses of phyllosilicates from argillaceous diagenetic rocks. This ought to be much less of a problem for analyses of diagenetic chlorite made by analytical TEM, and for analyses made by either TEM or microprobe on the well crystallised chlorites that occur in sandstones. Yet analyses of chlorite from sandstones made by both techniques consistently show Si-rich R2+-poor compositions and low octahedral totals (Boles & Franks, 1979; Curtis et al., 1985; Ahn & Peacor, 1985). The octahedral totals of analyses made by analytical TEM are frequently as low as 11, which if due to "contamination" would indicate (from Fig. 8) about 15-20% of intergrown dioctahedral phyllosilicates. Shau et al. (1990) have recently argued that most if not all "anomalous" chlorite compositions and temperature related compositional trends are a result of interstratification with other phyllosilicates. Many of the examples cited are from altered basic igneous -- //////// ////Q/ /~///. //////// s, 6 is.e / ////,~ //// T= Talc -- 1 I/ i i Fl6.8. Potential effect of contamination by 10% of kaolinite, quartz, and talc on two hypothetical chlorite compositions. R2+

15 Composition of diagenetic chlorites 163 rocks, or similar, where mixed-layered chloritic minerals are known to be very common from studies based on XRD. In contrast, from XRD patterns, most of the low-temperature chlorites found in sedimentary rocks do not appear to be interstratified (e.g. the Tuscaloosa chlorite reported by Curtis et al., 1984), so that it is not so obvious that a similar explanation applies. If the compositional variation for the sedimentary diagenetic chlorites is simply due to intergrown smectite and/or illite, then the small, but significant, amounts of alkali elements present (2Ca + Na + K) and the occupancy of the octahedral sites should be correlated. For the five series which show the same compositional trend on Fig. 2, the correlation coefficients range from indicating that the correlation is poor. Notably, the Orcadian series, which does not show the same trend, has a correlation coefficient of 0.65, significantly better than any of the other series. Like the other series, the diagenetic trend of the Orcadian chlorites shows a decrease in Si content, but unlike the others this is accompanied by an increase in A1 (note that the Orcadian analyses given in Table 1 are arranged in order of increasing diagenetic grade). Although data are limited, a decrease in Si and an increase in AI content appear to be characteristic of the saponite-corrensitechlorite conversion sequence (Inoue et al., 1984; Bettison & Schiffman, 1988). As stated earlier, the Orcadian chlorites probably formed from a corrensite precursor (Hillier, 1989) so it is probable that the different diagenetic trend, and the significantly better correlation between alkalis and octahedral occupancy, does in this one example reflect contamination by small amounts of residual corrensite layers which are progressively eliminated with increasing grade. Since trioctahedral smectites such as saponite tend to be relatively Al-poor, the effect of minor contamination with corrensite, with its saponite-like component, would be similar to the effect of contamination of chlorite with talc as shown in Fig. 8; this is confirmed in Fig. 2. Though there may be some contribution from contamination to the general trend observed in the other series, there are a number of points which suggest that, in large part, the compositional variation indicates a real difference in the crystal chemistry of sedimentary diagenetic chlorites compared to the chlorites found in their metamorphosed equivalents. The first point is that the compositional variation appears to be a function of temperature (Cathelineau & Nieva, 1985; Cathelineau, 1988; Jahren & Aagaard, 1989). The correlation of octahedral occupancy with temperature found in the present study is rather more variable than that found by Cathelineau & Nieva (1985), but the correlation of tetrahedral AI, reflecting the Si content, is reasonably good. It is difficult to explain these correlations as simply due to contamination; to do so requires a good coincidental relationship to temperature. The second point is that similar Si-rich chlorites were synthesized in the MASH system by Velde (1973). The range of compositions for the diagenetic chlorites towards the Si pole coincides almost exactly with that found by Velde (1973) for the maximum extent of solid-solution for the synthetic 14/~ chlorites (Fig. 9). The only difference is that the natural examples are more restricted in terms of the range of AI content. The restricted field occupied by the natural chlorite compositions corresponds to the region where the most marked effect of increasing temperature in the synthetic system is to decrease the Si content and increase the content of Mg (Fig. 9). To the right of this region the dominant effect of increasing temperature is to increase the A1 content. Notably, this is the region into which many Orcadian chlorites fall, and the region where analyses of corrensite would tend to plot. Both this correspondence to the synthetic system and the relationship of composition to

16 164 S. Hillier and B. Velde s,/ si/ % R2 AI Mg I J, AI FIG, 9. Left: comparison of the range of diagenetic chlorite compositions with the maximum extent of solid-solution in the MASH system after Velde (1973). A = chlorites from Texas, Niger, Brest, Rouez and Montana series, B = Orcadian chlorites, C = maximum extent of solid-solution in the experimental system. Right: effect of temperature on the extent of solid-solution in the MASH system after Velde (1973). temperature suggest that sedimentary diagenetic chlorites are indeed compositionally distinct. Although we have no data on the polytypism of the chlorites in the series studied, it is tempting to speculate that the compositional variation is related to the change in chlorite polytypism from type I to type II with increasing temperature in sedimentary rocks as proposed by Hayes (1970). In this respect, it is notable that the analytical TEM analyses of chlorites from sandstones presented by Curtis et al. (1985) were all of the Ib (13 = 90 ~ polytype. As discussed above, the compositions of the diagenetic chlorites, and the compositional trends observed during progressive diagenesis, can be explained in terms of a dioctahedral substitution of the type Si~R2_z. Nevertheless, it has been shown that the vacancies in the ideal structural formulae are inherent in the method of calculation, and, therefore, may not necessarily exist. A possible alternative is non-ideal stoichiometry of the anion framework. For example, Yau et al. (1988) suggested that excess positive charge (due to excess octahedral Al), in low-temperature chlorites from the Salton Sea geothermal field, may be compensated by the substitution of oxygen into the hydroxyl site. They calculated structural formulae by normalizing to the ideal number of cations, and hydroxyl and oxygen were calculated for charge balance. With or without octahedral vacancies, however, analyses of diagenetic chlorites, from the present study and obtained independently by both by analytical TEM (Curtis et al., 1985; Ahn & Peacor, 1985) and electron microprobe (Boles & Franks, 1979), show them to be more siliceous and to have lower total R 2+. Chlorite geothermometry Cathelineau & Nieva (1985) and Cathelineau (1988) calibrated the tetrahedral A1 content and the octahedral occupancy of hydrothermal chlorites with temperature and suggested that these parameters can be used for geothermometry. The apparent decrease in solidsolution with increasing temperature, however, suggests that the low-temperature chlorites are metastable with respect to fully trioctahedral chlorites, presumably of the IIb polytype,

17 Composition of diagenetic chlorites 165 which lie on the amesite-serpentine binary join. Temperature clearly has a strong effect on composition, but other kinetic factors should not be ignored, and in this respect perhaps it is significant that the tetrahedral A1 content of all the relatively old sedimentary chlorites is significantly greater than for the chlorites from presently active hydrothermal systems (Fig. 5). Furthermore, in the present study it has been assumed that the chlorites are "prograde" and that their compositions will be related to the maximum temperature as recorded by such indicators as vitrinite reflectance. It is, however, possible that "retrograde" alteration or the formation of new chlorite and/or overgrowths on existing grains may have occurred at temperatures less than maximum. Indeed, such phenomena may account for the wide spread of compositions found in many samples. The data of the present study also show that the compositions of low-temperature chlorites are dependent on bulk composition. The most aluminous series (Brest) have the most aluminious chlorites, and conversely the least aluminous carbonate-rich series (Montana, Orcadian) contain the least aluminous chlorites. The high A1 content of chlorites from the relatively high-temperature (270~ Brest series results in numerous examples with high tetrahedral A1, but with very low octahedral occupancies (<11). Many of the values for occupancy are comparable to those found for the lowest temperature (40-100~ series from the Texas Gulf coast. The better correlation of tetrahedral A1 with temperature suggests that it is less influenced by bulk composition, perhaps because it is determined by two substitutions which together can counteract, to some extent, the influence of chemical controls. The Brest sequence is also notable because, although it is one of the highest temperature examples, there are no chlorites with full octahedral occupancy. Experiments in the MASH system, and the distribution of natural mineral compositions, suggest that the solid-solution series for metamorphic chlorites is broken between 45% Mg, 30% A1, and 25% Si (i.e. R2+gAI6Si 5 on Fig. 2) and pure amesite (Velde, 1973). Thus, a speculative explanation for the absence of fully trioctahedral chlorites in the aluminous Brest sequence is that the decrease in the dioctahedral substitution with increasing temperature shifts the compositions towards this solid-solution gap, and at some point they become unstable. The other series tend to converge on to a small range of fully trioctahedral compositions corresponding to A1 contents of between 30 and 25% (cation) as found by Velde & Rumble (1977) for chlorites from quartz- and white mica-bearing assemblages in metamorphic rocks. The potential effects of bulk composition and kinetics obviously complicate attempts to use the compositions of low-temperature chlorites for geothermometry. Clearly, such methods are likely only to apply to appropriate bulk compositions, comparable to that on which the calibration with temperature was made. Nevertheless, many sedimentary series probably fall into a rather limited compositional field (most diagenetic chlorites from sandstones being relatively Fe-rich and aluminous) such that an appropriate calibration of tetrahedral A1 with temperature, or simply comparison of this parameter to determine the relative order of a sample suite, might be usefully employed. CONCLUSIONS Data on the chemical composition of almost 500 diagenetic and low-grade metamorphic chlorites obtained by electron microprobe have been examined in relation to compositional variation, and trends related to diagenetic grade. In accordance with more limited data

18 166 S. Hillier and B. Velde from previous studies, the diagenetic chlorites tend to be more siliceous, have lower total (Fe + Mg), and lower octahedral occupancy, compared to metamorphic chlorites with similar alumina contents. A general model is proposed whereby during the passage from diagenesis to metamorphism, chlorites become progressively less siliceous, richer in (Fe + Mg), and octahedral occupancy increases. As they become less siliceous, tetrahedral A1 increases at the expense of octahedral A1 so that total A1 is more or less conserved. The general relationship between octahedral occupancy and temperature of formation, together with the Si-rich R2+-poor compositions of diagenetic chlorites, can be described in terms of a dioctahedral substitution Sil]R2_2, which decreases with increasing temperature. In contrast, there are no obvious diagenetic trends for Fe/(Fe + Mg) which, along with AI content, is probably determined by the chemical environment. The relationship of natural compositions to temperature, and the occurrence of the same temperature-related trends in the experimental MASH system, suggest that the observed compositional variation is due to a real difference in the crystal chemistry of lowtemperature sedimentary chlorites, compared to those from metamorphic rocks. This could be due to dioctahedral substitution, as described, but from the ideal structural formulae alone it is not possible to be certain that there are vacant octahedral sites. The apparent decrease in solid-solution with increasing temperature indicates that the sedimentary diagenetic chlorites are probably metastable with respect to fully trioctahedral lib chlorites, as found in their metamorphic equivalents. Besides the observed relationship to temperature, the compositional variation may therefore be significantly affected by other kinetic factors, and is certainly not independent of bulk composition, all these factors complicating its potential use for geothermometry in diagenetic and low-grade metamorphic rocks. Finally, although we have presented various arguments to the contrary, we cannot completely rule out that the data reflect a variable mixture of "contaminants", and it will be necessary to carry out further detailed studies with the TEM to determine precisely what has been analysed, before we can be certain whether or not the chemical compositions of low-temperature diagenetic chlorites are truly any different from chlorites in metamorphic rocks. ACKNOWLEDGMENTS S.H. gratefully acknowledges a post-doctoral fellowship from The Royal Society, European Science Exchange Program, funded by Elf Aquitaine. Discussions with Trevor Clayton helped to shape several of the ideas presented and the manuscript was improved by the comments of Thomas Theye and two anonymous referees. REFERENCES AnN H.J. & PEACOR D.R. (1985) Transmission electron microscopic study of chlorite in Gulf Coast argillaceous sediments. Clays Clay Miner. 33, BAILEY S.W. & BROWN B.E. (1962) Chlorite polytypism: I. Regular and semi-random one layer structures. Am. Miner. 47, BARKER C.E. & PAWLEWICZ M.J. (1986) The correlation of vitrinite reflectance with maximum temperature in humic organic matter. Pp in: Lecture Notes in Earth Sciences 5. Palaeogeothermics (G. Bunterbath & L. Stenga, editors). Springer-Verlag, Berlin. BEAUFORT D. (1986) Defenition des dquilibres chlorite-mica blanc clans la metamorphisme et la metasomatimse: ~tude des metasediments encaisant l'amas sulfure de Rouez. Th6se, Univ. Poitiers, France. BETrISON L.A. & SCHIFFMANN P. (1988) Compositional and structural variations of phyllosilicates from the Point Sal ophiolite, California. Am. Miner. 73,

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