Ferrous-iron-bearing vermiculite-smectite series formed during alteration of chlorite to kaolinite, Otago Schist, New Zealand

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Abstract
Interlayered (µm scale) vermiculite-smectite minerals were found in drill-cores from altered Otago schist basement up to 2 m from the contact with the overlying sediment. Mineral compositions defined a series from Fe2+-rich trioctahedral vermiculite to Fe2+-bearing dioctahedral smectite; in these, tetrahedral substitution of Al for Si was unusually high (up to 3·3/8 in the formula unit). The minerals formed as intermediates during the alteration of chlorite to kaolinite under non-oxidizing, possibly acid conditions. The source of the altering solutions may have been overlying lignite seams.