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Common or potash feldspar crystallizing in the monoclinic system and having two cleavages at right angles to each other. See Feldspar.

A feldspar closely related to orthoclase, but triclinic. It is chiefly a silicate of sodium, potassium, and aluminium. Sp. gr., 2.57 -- 2.60.

A transparent or translucent variety of common feldspar, or orthoclase, which often shows pearly opalescent reflections; -- called by lapidaries moonstone.

An igneous rock composed of orthoclase, elæolite, and sodalite.

A finegrained rock, flintlike in fracture, consisting essentially of orthoclase feldspar with occasional grains of quartz.

er; -- said of the eyes. "In his glassy eye." Byron. Glassy feldspar (Min.), a variety of orthoclase; sanidine.

A mineral of the feldspar group, like orthoclase or common feldspar in composition, but triclinic in form.

A kind of feldspar consisting of a laminated intertexture of albite and orthoclase, usually of different colors. -- Per*thit"ic, a.

, as in the chloride, sulphate, carbonate, or silicate, in the minerals sylvite, kainite, orthoclase, muscovite, etc. Atomic weight 39.0. Symbol K (Kalium).

A variety of orthoclase feldspar common in certain eruptive rocks, as trachyte; -- called also glassy feldspar.

A granular, crystalline, ingeous rock composed of orthoclase and hornblende, the latter often replaced or accompanied by pyroxene or mica. Syenite sometimes contains nephelite (elæolite) or leucite, and is then called nephelite (elæolite) syenite or leucite syenite.

ock,usually light gray in color and breaking with a rough surface. It consists chiefly of orthoclase feldspar with sometimes hornblende and mica.