Greisen

Greisen is a highly altered granitic rock or pegmatite. Greisen is formed by autogenic alteration of a granite and is a class of endoskarn.

Greisens appear as highly altered rocks, partly coarse, crystalline granite, partly vuggy with miarolitic cavities, disseminated halide minerals such as fluorite, and occasionally metallic oxide and sulfide ore minerals, borate minerals (tourmaline) and accessory phases such as sphene, beryl, topaz, etcetera.