Abstract
ABSTRACT Illite, chlorite, and an iron-rich septechlorite (chamosite) were identified in the Clinton ironstones and associated rocks of New York State. The compositions of these clay minerals were semi-quantitatively determined by X-ray methods and a single wet analysis. The composition of the illite is: (H3O)2(SiAl)8(Al3.5Fe0.5)O20(OH)4; the chlorite is (MgAl)2.5(Fe+2Fe+33.5(Si2.65Al1.35)O10(OH)8; the chamosite is (Al0.81Fe+21.70Mg0.36Mn0.04Ti0.01) (Si1.36Al0.64)O5(OH)4 but iron and magnesium are inferred to vary to Fe1.24Mg0.82. Texture, composition, and stratigraphy indicate that the illite is detrital, chamosite is a primary, syngenetic precipitate and chlorite is diagenetic. An increase in the Fe/Mg ratio of diagenetic minerals may be related to a relative increase in the ferrous iron activity of interstitial solutions during digenesis. This change in composition of the solution could result from the extraction of magnesium during dolomitization and/or an increase in the reducing potential of the environment.
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