Secondary gypsum rocks, formed from anhydrite by the action of ground waters and/or surface weathering 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 to be essentially distinct. Porphyroblastic gypsum is the first to form and sometimes dates from early diagenesis. Alabastrine gypsum includes a wide variety of related textures. Hydration fabrics vary from indefinite grains with irregular extinction to more perfectly defined granoblastic gypsum. Recrystallization of hydration gypsum leads to unequigranular granoblastic gypsum, in unstrained and perfectly oriented grains. Prophyroblastic gypsum may also recrystallize in conjunction with the alabastrine variety. In most cases of gypsification there is no evidence for any volume 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 solution during gypsification, or alternatively may precipitate from sulfate-rich connate waters.