Polytypism of Chlorite in Sedimentary Rocks

John B. Hayes
Denver Research Center, Marathon Oil Company, Littleton, Colo. 80122

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) IIb, the polytype of common metamorphic and igneous chlorites; (2) Ib(β = 90°): (3) Ib(β = 97°); (4) Ia. An additional stacking arrangement, Ibd, is defined herein as disordered chlorite which lacks an h0l 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-I crystallization sequence is proposed, from least to most stable: Ibd → Ib(β = 97°) → Ib(β = 90°). Conditions of low-grade metamorphism usually are necessary to cause conversion of Ib(β = 90°) to IIb, 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 IIb-chlorite-bearing metamorphic rocks, may yield essentially unweathered IIb chlorite to sites of deposition. Thus, clay-size IIb chlorite in unmetamorphosed sedimentary rocks can be interpreted as detrital. Caution is required, however, because IIb may be able to form authigenically at submetamorphic 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.

Clays and Clay Minerals; December 1970 v. 18; no. 5; p. 285-306; DOI: 10.1346/CCMN.1970.0180507
© 1970, The Clay Minerals Society
Clay Minerals Society (www.clays.org)