Polytypism of Chlorite in Sedimentary Rocks

Abstract

Abstract Four chlorite polytypes of Bailey and Brown (1962) have been identified by X-ray diffraction in clay-size chlorites of soils, sediments, and sedimentary rocks: (1) II b , the polytype of common metamorphic and igneous chlorites; (2) Ib(ß = 90°); (3) Ib(ß = 97°); (4) I a . An additional stacking arrangement. I ba , is defined herein as disordered chlorite which lacks an h 0 l diffraction band in the 2.4–2.5 Å region. Most type-I chlorites are authigenic, as demonstrated by thin-section petrography. Type-I chlorites form during diagenesis, or less commonly during halmyrolysis, at temperatures and pressures less than those of low-grade metamorphism. A type-1 crystallization sequence is proposed, from least to most stable: I ba → I b ( ß = 97°) → Ib(ß = 90°). Conditions of low-grade metamorphism usually are necessary to cause conversion of I ba ( ß = 90°) to II b , the most stable and common polytype. Chemical composition has little or no influence upon polytype relative stabilities; temperature is much more important. Sediment source areas with high relief, abundant rainfall, cold climate, and which contain II b -chlorite-bearing metamorphic rocks, may yield essentially unweathered II b chlorite to sites of deposition. Thus, clay-size II b chlorite in unmetamorphosed sedimentary rocks can be interpreted as detrital. Caution is required, however, because II b may be able to form authigenically at submetamor-phic temperatures, because it is the most stable polytype. Petrographic evidence is useful in such cases. Chlorite polytypism as a geothermometer can be applied to several geologic problems: (1) the authigenic versus detrital origin of clay minerals in sedimentary rocks, particularly in graywacke matrix; (2) the recognition of diagenetic facies or gradients, areally and stratigraphically, within given geologic provinces; (3) the detection of hydrothermal and incipient metamorphic effects. Chlorite polytypism merits general application as an interpretive tool.