MINERAL COMPOSITION OF THE AVERAGE SHALE

Transcription

1 MINERAL COMPOSITION OF THE AVERAGE SHALE By D. H. YAAtON Department of Geology, The Hebrew University, Jerusalem. [Received 7th October, 1961] ABSTRACT Mineralogical compositions have been calculated from a number of average analyses of shales and clays using the scheme of Imbrie and Poldervaart (1959). The average shale comprises 59 per cent. clay minerals, 20 per cent. quartz and chert, 8 per cent. felspars, 7 per cent. carbonates, 3 per cent. iron oxide minerals, 1 per cent. organic matter and 2 per cent. other minerals. Illite predominates among the clay minerals. INTRODUCTION The usual method of estimating the average chemical compos;tion of argillaceous rocks is to calculate the mean of selected analytical data. Quantitative mineralogical analyses of clays and shales are still difficult to obtain, and the mean chemical composition has, therefore, been made to serve as basis for estimates of the average mineralogical composition of such sediments. The general impression from the few estimates so far made is that clay minerals compose less than half of the average shale. Pettijohn (1957) quotes the estimates of Leith and Mead (1915), according to which the average shale is composed of 89 clays, 89 quartz, and ~ felspars, carbonates, Fe-minerals and others, and those of Clarke (1924), which gave 25 per cent. clay minerals, 22 per cent. quartz and 30 per cent. felspars; the felspar figure of Clarke was considered too high since some of the potash may have been present in sericite. Another estimate is that of Krynine (1948) who mentions only briefly (p. 154) that shales are a mechanical mixture consisting of approximately 50 per cent. silt (mainly quartz), 35 per cent. clay and 15 per cent. authigenic minerals and cement--a conclusion which appears to be derived from the estimates of Leith and Mead (1915). Modern mineralogical analyses of fine grained rocks by X-ray and other methods supply much more detailed information on the actual mineralogical composition of such rocks than when the earlier estimates of mineralogical composition were made, even though reliable quantitative results are still relatively rare. At the same time the average chemical composition of the various clay minerals have become better known, and norms for the calculation of the mineralogical composition from chemical data can thus be improved. Recently, Imbrie and Poldervaart (1959) described a method suitable for the routine calculation of the mineralogical composition of sedimentary rocks from chemical analyses, which takes into account 31

2 32 D.H. YAALON a suite of 14 common sedimentary minerals, including four clay minerals. They claim that an agreement within _+5 per cent. is obtained for the major constituents or groups when compared with actual mineralogical analyses. In the present paper the scheme of Imbrie and Poldervaart (1959) is applied to a number of analyses which were calculated to represent the average composition of shales or clays of certain environments. For most of the analyses the scheme was found to be applicable without change, but it had to be slightly modified in certain instances, as will be shown below. It is suggested that the results of the calculation (Table 1) give a much better picture of the average mineralogical composition of shales than estimates made hitherto. An independent calculation was made on the basis of the estimates of Krynine (1948) of the average mineralogical composition of all sediments (p. 133), of sandstone (p. 151), and of the relative abundance of sandstones, shales and limestones (p. 156), from which the average mineralogical composition of shales can be deduced. This and other previous estimates are compared with the results of the new calculations (Table 2). CALCULATION PROCEDURE In the scheme used the appropriate equivalent quantities of the elements or oxides are first assigned to the minor constituents (gypsum, pyrite, apatite, ruffle), including all Na20 to albite. CO2 is apportioned first to calcite and the rest to dolomite. The remaining MgO and all the K20 are assigned to micas (illite, sericite) and to chlorite, together with the appropriate amounts of Fe203, A1203, and SiO2. The remaining A1203 is computed as montmorillonite, again with the appropriate quantities of the other oxides. Remaining Fe203 is computed as hematite and uncombined SiO2 as chert or quartz. Obviously, such a general scheme cannot consider all the minerals commonly occurring in sediments, nor can it be applicable without modification under all circumstances. In the analyses of tropical clays, it was found necessary to calculate A1203 that remains after deduction of the amount required for micas as kaolinite instead of as montmorillonite. Since kaolinite, next to illite and montmorillonite, is the most commonly occurring clay mineral, this modification will be required in calculation of analyses of strongly weathered clays in which kaolinite is usually abundant or dominant. In certain Russian analyses the amount of CaO exceeded the equivalent amount of CO2; in such instances, the excess CaO was calculated as anorthite. This seems justified, as clay minerals contain Ca 2+ only as an adsorbed ion, and the occurrence of a large amount of sphene seems unlikely. Separate calculations were made by assigning sulphur either to gypsum or to pyrite, but because of the small amount of sulphur present this did not make any difference in the grouping; where the oxidation states of sulphur or iron are givenin the analyses no choice has to be made.

3 THE AVERAGE SHALE 33 TABLE 1--Mineral composition of average shales and clays (in weight per cent.) Components Clay minerals... Quartz and chert... Felspars... Carbonates... Fe-oxides... Others I * * "1 2 2 l--average shale, composite of 78 samples (Clarke, 1924). 2--Clays of th~ Russian Platform; average of 252 composites made up from 6804 samples (Vinogradov and Ronov, 1956). 3--Sarne analyses as No. 2, but weighted by Green (1959) to give equal weight to geological time divisions. 4--Marine and lacustrine clays of the Russian platform; 305 composites arnples made up from 7932 specimens (Ronov and Khlebnikova, 1957). Includes all the samples considered under 2 and Average composition of over 10,000 individual complete and partial analyses of all types of clays of the Russian platform (Ronov and Khlebnikova, 1957). Kaolinite calculated among clay minerals. 6--Shales from 7 American states; adjusted mean of 226 samples, some only partially analysed (McKelvey, 1960). Contains 4 per cent. organic matter. 7--Clays from 7 American states; adjusted mean of 712 samples, some only partially analysed (McKelvey, 1960). Part of carbonates calculated as ankerite. 8~Precambrian lutites; weighted average of 36 analyses (Nanz, 1953). FeO calculated as magnetite. 9--Marine clays of the Russian platform ; average of 1550 individual samples, partially or fully analysed (Ronov and Khlebnikova, 1957). Lower Palaeozoic samples weakly represented. 16--Continental clays of cold and temperate climates of the Russian platform; average of 1570 individual samples, partially or fully analysed (Ronov and Khlebnikova, 1957) Clays of tropical zones of the Russian platform; average of 6660 individual samples, partially or fully analysed (Ronov and Khlebnikova, 1957). Kaolinite calculated among clay minerals. *Carbonates calculated from CaO, as CO~ not listed separately. TABLE 2---Comparison of previous and present estimates of the mineral composition of average shales and clays (in weight per cent.) Components Previous estimates Leith and Mead Clarke (1915) (1924) Derived from Krynine (1948) Present estimates Range* Proposed general average? Clay minerals... Quartz and chert Felspars... Carbonates... Fe-oxides... Others " ' *Only analyses 1 to 7 of Table 1 are considered, tmedian value, weighted to add to 100 per cent.

4 34 D.H. YAALON The norm assigns iron first to pyrite, then to clay minerals and the remaining Fe203 to hematite. In the analysis of the average Precambrian shale (No. 8) it was preferred to assign FeO to magnetite, together with equivalent amounts of Fe203, and only the remaining Fe203 was taken into account for illite and montmorillonite. Unfortunately, in certain analyses of the Russian platform clays no distinction is made between CO2 and loss on ignition. In such cases calcite was calculated on the basis of CaO (after appropriate deductions for apatite and gypsum). A check made by adding COz and HzO + required for clay (which combined make up the loss on ignition) gave reasonable values. In the present tabulation the calculated minerals are combined into six representative groups, which is likely to eliminate several uncertainties and enhance the overall accuracy of the calculation. Though difficult to check by any other means, the accuracy of the estimates is probably similar to that of single analyses and true within 9 5 per cent. of the calculated values. Uncertainties which are inherent in the calculation of the average chemical composition are of course also found in the subsequent mineralogical transformation. As the frequency distribution of the minor constituents appears to be log-normal (Ronov and Khlebnikova, 1957; McKelvey, 1960), their average abundance is somewhat overestimated when calculated, as is usually done, as the arithmetic mean. DISCUSSION The results of the present calculations differ considerably tu those made previously, especially in the estimate of the amount of clay minerals, which are shown to comprise over 50 per cent. of the average shale and up to 70 per cent. of certain groups of clays. On the other hand, the amount of felspars is much lower than previously estimated, and quartz or chert is by far the second most important mineral group in clays and shales. There is a considerable difference in the average chemical composition of clays according to Clarke (1924) and of those of the Russian platform. The Russian clays are richer in CaO and contain less NazO and SiO2, which results in the estimated content of carbonates being higher and that of felspar and quartz lower than those calculated from the values of Clarke (1924). The independent calculation derived from the data of Krynine (1948) (Table 2) comes close to the calculation based on those of Clarke (1924) (No. 1, Table 1), but differs considerably from the new average estimate (Table 2). Clay Minerals. The scheme of Imbrie and Poldervaart (1959) makes allowance only for the micas, montmorillonite and chlorite. The fact that it does not make allowance for kaolinite has already

5 THE AVERAGE SHALE 35 been mentioned. X-ray evidence indicates that kaolinite is generally more abundant than chlorite, although on the whole it takes third place in abun&mce after the micas and smectites. In strongly weathered clays kaolinite becomes the dominant clay mineral and due allowance has to be made for this in the calculations, especially for the tropical days. Because of the assignment of all K20 to the mica group, illite appears in these calculations as the most common clay mineral. This is in excellent agreement with the conclusions drawn by some authors from X-ray results (Weaver, 1959). Various authors have noted and discussed the higher K20 content of ancient shales (Nanz, 1953; Vinogradov and Ronov, 1956) which is in agreement with mineralogical evidence showing that the character of clay sediments becomes increasingly monomineralic with increasing age, with illite becoming predominant. Quartz and chert. Uncombined silica is clearly revealed by the calculations as the mineral group second in abundance. The most common value is close to 20 per cent. Imbrie and Poldervaart (1959) assign the uncombined silica to chert, but there is a great deal of mineralogical evidence that detrital quartz is actually more abundant than secondary silica in shales, and the group has therefore been called quartz + chert. Krynine (1948) made an estimate of the proportions of detrital quartz and secondary chalcedony in total sediments and in sandstones, and on this basis the proportion of quartz to chalcedony (chert) in shales and clays would be 3 to 1, or 15 per cent. quartz and 5 per cent. chert for the average shale. Felspars. As the estimate of felspar is based on Na20 alone, and disregards the other cations (K, Ca), this value is likely to be subject to the greatest uncertainty in the calculation and is possibly systematically too low. The disregard of the K- and Ca-felspars is partially compensated for by the fact that not all Na20 is present in felspars. A recent statistical evaluation of the average Na content in shales by Burns (1960) gave 0.81 per cent. Na20 as median, while the arithmetic average was 1.24 per cent., indicating a distinct skew positive distribution. The frequency distribution curves drawn by Ronov and Khlebnikova (1957) show clearly a log-normal distribution for Na20 in clays, which indicates that the geometric mean, failing between the median and arithmetic mean, might be the most suitable value for use in such calculations. If either of the above mean values were used for the computation of the felspar content, this would amount to 7 per cent. and 10 per cent. respectively, which falls within the range of the calculated values in Table 1. The low estimate of 4 per cent. in the tropical clays and up to 16 per cent. in continental clays appears in good agreement with general experience. Carbonates. According to the various estimates the variation in carbonate content in the average shales is rather large, although

6 36 D.H. YAALON the estimate is most commonly around 6-7 per cent. As has been noted previously the average clays of the Russian platform appear much more calcareous than the estimate based on Clarke's values, presumably due to the inclusion of a larger number of analyses of transitional rocks. For the three environmental groups of clays the values are much lower. Not surprisingly the Precambrian slates also are low in carbonates. Some indication that the amount of dolomite increases in relation to calcite in ancient rocks (Vinogradov and Ronov, 1956) was also evident according to these computations. 1run oxides. The highest average value of iron-oxide minerals in clays according to these calculations is about 6 per cent., while certain groups of clays do not allow for any free Fe203. The norm makes allowance for hematite only, but magnetite is probably even more abundant and has been taken into account where the oxidation state of iron is given in the analysis. Other minerals. This last group includes a varied set of minerals --pyrite, apatite, gypsum, rutile--which, each separately, are present in average clays and shales in amounts less than 1 per cent. Included in this group is also organic matter, which, according to the present calculations, generally amounts to close on 1 per cent., but reaches quite high values in certain groups of clays, such as 4 per cent. in the average American shale (No. 6, Table 1). CONCLUSIONS The calculation scheme of Imbrie and Poldervaart (1959) seems well suited to the calculation of the mineralogical composition of shales and clays based on average chemical analyses, although it requires minor modifications in certain instances. The calculated mineral compositions of the average shale vary somewhat depending on the source of analyses used. The data in the last column of Table 2 are proposed for general use, and are considered to represent the weighted mean of these estimates. Thus the average shale contains nearly 60 per cent. clay minerals, or about 3 times more than uncombined silica, and the total amount of felspars, carbonates and other minerals approximates to 20 per cent. REFERENCES BtSRNS, J. R., Prof. Pap. U.S. geol. Surv., No. 400-B, p CLAaKE, F. W., Bull. U.S. geol. Surv., 700, p. 29. GREEN, J., Bull. geol. Sue. Amer., 70, IMBR1E, J., and POLDERVAART, m., J. sediment. PetroL, 29, 588. KRYNINE, P. D., J. Geol., 56, 130. LEITH, C. K., and MEAD, W. J., Metamorphic Geology. Holt, New York, pp. 60, 316. MCKELVEY, V. E. (editor), Prof Pap. U.S. geol Surv., No. 400-A, p. 63. NANZ, R. H., J. GeoL, 61, 51. PETTIJOHN, F. J., Sedimentary rocks. Harper, New York, p RONOV, A. B., and KHLEBNIKOVA, Z. V., Geokhimiya, p VINOGRADOV, A. P., and RONOV, A. B., Geokhimiya, No. 2, p. 3. WEAVER, C. E., Clays and Clay Minerals (A. Swineford, editor). Pergamon Press, London, p. 154.