NEW YORK STATE MUSEUM 



Orthoclase (potash feldspar) KAlSi,O s 



Orthoclase is a silicate of aluminium and potassium. Part of 

 the potassium is often replaced by sodium giving rise to a variety 

 known as soda-orthoclase. 



The crystals of orthoclase are monoclinic, a type of frequent 

 occurrence being that shown in fig. 204. Types of prismatic 



Fig. 204 



Orthoclase 



habit, often orthorhombic in aspect from the equal development 

 of the basal pinacoid and positive hemiorthodome (fig. 205), are 

 often found in the transparent variety called adularia. 



Twin crystals occur quite frequently and are ordinarily of three 

 types, the Carlsbad, the Baveno and the Manebach type. 1 The 

 cleavage of orthoclase takes place in two directions parallel to 

 the basal and clinopinacoid and at an angle which is close to 

 90°. Gleavable masses are quite common. Also compact non- 

 cleavable masses resembling flint. 



The luster of orthoclase is vitreous or pearly and the color is 

 commonly flesh-red, yellowish, white or colorless; more rarely 

 gray or green. 



Orthoclase abounds in igneous rocks and constitutes an 

 important element in granite, gneiss and syenite and in the form 

 of sanidine is common in the volcanic rocks rhyolite, trachyte 

 and phonolite. It is quarried in Maine, Connecticut, Massachu- 

 setts and Pennsylvania and at Bedford and Fort Ann N. Y. 



Orthoclase is used in the manufacture of porcelain and china, 

 as a constituent of the body of the ware and also to produce the 

 glaze. 



1 These forms of twinning are illustrated by specimens and models in 

 the collection of the New York state museum. 



