Abstract

Tiny chlorite crystals that appear to be localized on fault surfaces of mudstones were discovered in Lower Devonian (Helderberg) rocks of the central Hudson Valley, New York. Both the Ib and IIb chlorite polytypes and 2M1 trioctahedral mica were identified using X-ray powder film patterns. Indirect methods indicate that the chlorite composition is approximately (Fe3Al1.2-1.4R)6(A11.2-1.4Si)4O10(OH)8 where R signifies undetermined octahedral cations. Abundant inclusions of chlorite and a small amount of titaniferous material occur in the chlorite crystals; no 1:1 phvllosilicate mineral wras detected as either an interstratified or single phase. Single crystal study of an apparent IIb chlorite shows that the layer sequence is irregular and distorted from that of the ideal IIb polytype. The mineral assemblage of Ib chlorite, distorted IIb chlorite, and 2Mi trioctahedral mica apparently formed by local isochemical metamorphism in a low-to-intermediate temperature range. Crystal growth was initiated by faulting and continued after fault movement ceased.

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