Chemical composition: KAlSi, 08, (K, 0 = 16.9%, A1, 03 = 18.4% and SiO, = 64.7%); in some varieties Na replaces K to certain extent.

Physical properties:

(i) Form: Commonly prismatic, elongate or flat. It also occurs in massive, lamellar and granular forms.

(ii) Colour: White, gray and flesh-red.

ADVERTISEMENTS:

(iii) Lustre: Vitreous to pearly.

(iv) Streak: White.

(v) Hardness: 6.

(vi) Cleavage: Perfect 2 sets aligned at 90°.

ADVERTISEMENTS:

(vii) Fracture: Conchoidal to uneven.

(viii) Specific gravity: 2.56.

(ix) Diaphaneity: Transparent to translucent.

Use: Orthoclase is used in the manufacture of porcelain and pottery, for the production of glazes on earthenware, sanitary ware, enameled brick etc. It is also used in manufacture of opalescent glass, as a binder for abrasives. Moonstone, an opalescent and colourless variety of orthoclase is used as gemstone.