Prehnite Veins in Triassic Diabase, Coopersburg, Pennsylvania
Veins, principally of prehnite, occur in diabase near Coopersburg, Pennsylvania. Early formed vein minerals are pyroxene and albite (An < 5) in which most of the crystals are choked with fluid inclusions. Commonly, the albite has inclusion-free rims of the same composition as the cores. Later minerals in the sequence are an amphibole followed by chlorite and prehnite. Minor quartz and calcite occur with the prehnite.

Diabase near the veins shows three zones of alteration. Adjacent to the vein is a 2 to 3 cm zone of amphibole with minor quartz and prehnite. The second zone is principally chlorite. In the third zone the texture of the diabase is preserved, but hypersthene is altered to chlorite and the plagioclase is choked with inclusions. In fresh rock, plagioclase lacks inclusions. It is suggested that the inclusion-choked albite crystallized from a hydrothermal liquid, whereas the inclusion-free rims formed after a change in state of the liquid to a vapor.