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Petrological Significance of Illite Poly- Morphism in Paleozoic Sedimentary Rocks

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Petrological Significance of Illite Poly- Morphism in Paleozoic Sedimentary Rocks TIIE AMERICA\ MINERALOGIST, VOL. 48, NOVEMBER DECEMBER, 1963 PETROLOGICAL SIGNIFICANCE OF ILLITE POLY- MORPHISM IN PALEOZOIC SEDIMENTARY ROCKS Bnucr VnrnBl lwl JouN Hownn, Montana State Llnitersity, Missoula, Montana. Arsrnncr An r-ray diffraction investigation was made of the type and abundance of the poly- morphic form of illite in Paleozoic sedimentary rocks. Only the 1 Md and 2 M polymorphs were found to be present in appreciable amounts, with the 1 Md polymorph predominant in the (1 p size fraction. This large abundance of 1 Md polymorph illite in Paleozoic sediments indicates a predominant low-temperature origin for illite. Some data indicating relative mineral abundance changes with change in grain size in the sedimentary rocks investigated are also given. fNrnooucrron fn this study illite is defined as a minerai group found in sediments having a primarily dioctahedral muscovite-type structure with less than lo/s oI expandable layers interlayered in the structure. (The average door:10.05 A, much Iessthan 10/6 expandablematerial.) Although the illite thus defined probably contains a variety of minerals, it is difficutt to determine individual structures and compositions becauseof their sim- ilarity. SeeYoder and Eugster (1955)for a discussionof the problem. It is possible that true micas or pure muscovites are present but no con- clusive chemical-structural evidence has been presented yet to demon- strate such an occurrence.Thus the definition of illite used in this study is broad enoughto include all -10 A layeredstructure material found in sedimentary rocks. As a result, the term illite takes on a meaning similar to that of bauxite or limonite, i.e. a mineral group name. Illite generally representsover half of the clay minerals that are found in sedimentaryrocks (data in Weaver, 1959),and clay minerals are the most abundant group of silicatesin sedimentaryrocks as they are the main minerals in shales, which account for 75/6 of sedimentary rocks (Pettijohn, 1957).Clay mineralsare also found in most other sedimen- taryrocksinvarying amounts.Despite the fact that illite is very common, its mode (or modes)of origin is not known. One very important difficulty arisingin the investigationof the origin of illite is the structural similarity between illite and muscovite; both mineralsare dioctahedral micas. The problem of the origin of illite is simple if it is just fine-grained muscovite derived by the mechanical weatheringof high-graderocks as suggestedby Weaver (1959).However, many other kinds of dioctahedralmicas exist (Foster, 1956,1960). These I Present address: Geophysical Laboratory, Carnegie Institution of Washington, Washington, D. C. 1239 1240 B YEI,DE AND .I IIOWER micas differ in compositionand in their modesof origin (in particular in terms of temperature),but their grossstructures are similar. In nature, a mica of the compositionof muscoviteoccurs in igneousand metamorphic rocks; at the other extreme,another dioctahedral mica, glauconite,forms at earth surfaceconditions (Burst, 1958).Thus illite could representma- terial from a wide variety of environmentsof formation. The primary method of investigation used in the present study is r-ray diffraction. Identifications of the minerals present and in particular the structure of the illite weremade. The materialsused were from Paleozoic rocks of various agesand localities (Table 1). An attempt was made to include in the investigation samples of varying geographic locality, age, and tectonic environment so that a general conclusion might be drawn on the nature of the clay mineral assemblagesof Paleozoic sedimentary rocks. The type and abundance of the structural polymorphs of the illite (10 A material) were determined in order to provide a measure of the thermal history and hence indicate the origin(s) of the illitic material in sedimentaryrocks. Yoder and Eugster (1955)used synthetic and natural materials to determine the stability relations of the muscovite poly- morphs. (For a description of the polymorphs see Smith and Yoder (1956).)Their work resultedin the following relationships: 1 Md is a metastable polymorph preceding the stable polymorphs as the mineral grows. The 1 M polymorph is stable below the temperature range 200o-350o C. while the 2 M polymorph is more stable above this temperature range. In the laboratory 16" 1 lVtrl+1 M conversion is slow, and the 1 M*2 M conversion at 200"-3500 C is very slow Further work by Weaver (1958) shows that 2 M 10 A material will r,veatherto an expandable mineral with a 1 Md or disordered structure. Reconstitution by replacing potassium lost during weathering resulted in the structure'sreturning rapidiy to its 2 M structure. We have found by using molten LiNOa to removepotassium from muscovitesthat the 2 M structureis regainedwhen potassiumis re-introducedto the structure. It is thus concluded that the three naturally occurring polymorphs of muscoviterepresenting temperature stabilities would be 1Md, 1M, and 2M, with 1Md and lM polymorphs representing low temperatures (<200'-350" C.) of formation and the 2M polymorph representingthe high-temperaturematerial (>200o-350o C.). This is the basis for our petrological interpretation of the polymorphic form of illite in Paleozoic sedimentary rocks. As a matter of generalinterest the distribution of the mineralspresent in the Paleozoicsedimentary rocks was ascertainedin a grossmanner by analyzing differing particle size fractions. This was done in order to de- termine where the illite could be found and what relation this might have with respect to the polymorphic form of the material. ILLITE POLYMORPHISM 124l Cl,ay minerals: I, illite K, kaolinite C, chlorite (plus Al-serpentine?) S, Al-serpentine M, montmorillonoid K-S, kaolinite and Al-serpentine indistin- guishable M-L, mixed layered illite-montmorillonoid K?, kaolinite presence not determined Color: Bl, black dr, dark Gy, grey Br, brown Gr, green Numbers are designations used in the laboratory, e.g.: C-8, Cambrian, sample number eight. The symbols( and ) signify: (, less than 1 p fraction; ), greater than 1 p and some material of the less than 1 p fraction. Tasrn 1. ExprnrunNrer, Dara Clay No. Name and Locality Size (060) 2 Minerals M Cambri.an C3 Burgesssh.-B.C., Can. IS 1.500 50 Gy IS 1.502 45 C6 Gros Ventre-Wyo. IS 1.505 25 Gr IC 1.503 70 C8 Goodsir-B.C., Can. IC 1.502 70 Gr IC 1.505 70 C9 Lower Cambrian-B.C., Can 1 503 B1 1.503 c10 Trempelo lms.-Ill. I 1.503 25 I 1.503 15 c11 Eau Claire Ill. I 1.503 0 Gy-Gr I 1.502 0 c12 Wolsey-Mont. IC 1.504 0 Br ICK 1.504 0 c13 Flathead-Mont. I S-K 1.503 0 Br ICK 1.502 0 Ordoa,ician Ola Schenectady N. Y IC dr-Gr IC 15 02 Utica sh.-N. Y ICK? 10 B1 ICK? 1.505 5 o3 Sylvan sh.-Okla. I 1.510 0 Gr I 1.510 10 U5 Arbuckle lms.-Okla. I 1 509 20 Br M 1.503 15 o6 Simpson-Okla. CM 1.502 0 M I 500 0 07 Viola lms. Okla. 1.506 0 I.JUJ 1242 B. VELDE AND J. HOWER T*r-n 1-(continued) Clay 7o Name Size (060) Color No. and l,ocality Minerals 2M o8 Sylvan sh.-Okla. IC 1.503 0 Br I 1.506 50 o9 Kimmswick-Ill. I 0 I 1.505 q o10 Tulip Creek-Okla. IKS 1.508 10 BI IKS 1.509 20 o11 Maquoketa sh.-Ill. IC I. )UO Gy IC 100 o12 Mt. Hope-Ohio IC 1.505 BI IC 1.503 25 o13 Normanskill sh.-N. Y. IS 1.506 0 Gy IC 1.501 20 025 Normanskill sh.-N. Y. IC 1.502 0 BI IC 1.503 0 o27 Normanskiil sh.-N Y. IC 1.504 15 BI IC 1.504 35 Silurion, S1 Rochester sh.-N. Y. IS |.498 40 Gy IC 1.506 80 ss8l13 Clinton-Penna. IKC 1.505 A< Gy IC 1.504 35 S2 Camillus sh.-N. Y. IC 1.505 0 Gr-Gy IC 1.505 30 s2b Camillus sh.-N. Y. IC 1 504 Gr-Gy IC I .504 30 S3 Chittenango sh N. Y. IC 1.508 25 Gr-Gy IC 1.505 90 s11 St. Clair lms.-Ill. IM 1.501 40 IM 1.499 100 s12 Niagaran-Ill. I 1.501 0 I 1.503 65 s13 Sodus sh -N. Y. I 1.501 25 Gr-Bl I 1 500 40 s14 Rochester-N. Y. IC 1.504 5 IC 1.501 30 s15 Girardzau lms -Ill. IC 1.505 t.) IL 1.505 25 s16 Arisaig-Nova Scotia ICK? 1.501 20 IC 1.499 35 Detonian D58103 Marcellus-Penna. IC 5 BI IC 30 D5 Esopus sh.-N. Y. I K-S 15 BI ICK ILLITE POLYMORPI]ISM 1243 Trer.r l-(continued) Clay % Name and Locaiity Size (060) Color Minerals 2M D10 Bakoven sh.-N. Y. C 1.504 0 dr-Br C 1.501 50 D11 Hamilton N Y. (, 1.504 5 dr-Gr C | 502 50 D12 Ilamilton-N Y. 1.501 30 BI I 1.502 50 D19 Kiskatom redbeds-N. Y. 1 504 95 Red L 1 504 100 D20 Middlesex sh.-N. Y. S 1.504 10 BI (- 1.504 D2l Cardiff sh.-N. Y. CK? 1.501 0 BI CK? 1.502 D22 Moscow sh.-N. Y. I 0 BI C 1.501 35 D24 Mottville sh. N. Y. CK? 1.501 50 BI L 1.501 JU D25 Upper Chittenango-N Y. CK? 1.501 0 BI C r.502 30 D26 Geneseo-N. Y. CK? 1.501 5 BI CK 1.500 D27 Cardifi-N. Y. 1.505 40 BI (- 1.506 85 D28 Cardifi-N. Y. I .5UJ 40 BI (.
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