Saussurite

Saussurite (named after Saussure), a dull, opaque, white, gray, or greenish compact mineral, of indeterminate or variable composition, which has resulted from the alteration of felspar, as in the rock called Gabbro (q.v.). Examined microscopically saussurite is seen to consist of a compound aggregate of minute fibres and grains of zoisite and other minerals.