Prehnite

Prehnite, a hydrous silicate of alumina and lime, the alumina usually partly replaced by ferric oxide. It is a widely diffused mineral, and, although first discovered at the Cape of Good Hope, has been found in great beauty in some places on the continent of Europe and in Scotland. Prehnite exhibits a great variety of forms, being found in crystals in fan-shaped and cockscomb-like groups, granular, reniform, fibrous, &c. It is sometimes colourless, but more generally greenish, and sometimes yellowish. It occurs, as a product of the alteration of various silicates, in veins and cavities in crystalline igneous rocks, such as diorite, porphyrite, &c. Less commonly it is met with under similar conditions in granitoid and schistose rocks, and occasionally in lodes associated with copper.