Fins. 1 lf-,,.[IrNv:, lqTfl THE PETROLOGY OF SECONDARY GYPSUM ROCKS: A REVIEW ~

Secondary gypsum rocks, formed from anhydrite by the action of ground waters and/or surface weather-ing and unmodified by tectonic influences, are divided into two main petrographic groups, porphyroblastic and alabastrine secondary gypsum. The two groups though often occurring together, are thought o be essen-tially distinct. Porphyroblastic gypsum is the first to form and sometimes dates from early diagenesis. Alabastrine.gypsum includes a wide wtriety of related textures. Hydration fabrics vary from indefinite grains with irregular extinction to more perfectly defined granoblastic gypsum. Reerystallizatior ~ of hydra-tion gypsum leads to unequigranular granoblastic gypsum, in unstrained and perfectly oriented grains. Prophvroblastie gypsum may also recrystallizc in conjunction with the alabastrine variety. In most cases of gypsifieation there is no evidence for any volnme increase or associated crushing, and there may often be a net volume reduction due to solution. Excess amounts of certain trace-elements, notably strontium and boron, are released from some anhydrite rocks during hydration and occur in the form of such minerals as celestite and probertite. Three reaction mechanisms appear to be possible, all of which may occur naturally to a greater or lesser extent. Late stage displacive gypsum veins possibly form from the excess sulfate removed in solu-tion during gypsification, or alternatively may precipitate from sulfate-rich eonnate waters