274 DESCRIPTIONS OF MINERALS. 



and 7; the specific gravity, between 2*44 and 2*75 ; lustre 

 vitreous, but often pearly on the face of perfect cleavage ; 

 and each species transparent to subtranslucent. 



II. Acidic and Basic Feldspars. 



Oligoclase, albite, and orthoclase are called acidic feld- 

 spars, because of the large amount of the acidic element, 

 silicon, in their constitution, analyses giving GO to 70 per 

 cent, of silica ; and labradorite and anorthite are called 

 basic feldspars, the amount of silica being 42 to 55 per 

 cent. Correspondingly, eruptive, and metamorphic rocks 

 in which either of the acidic feldspars is a prominent con- 

 stituent — for example, granite, gneiss, trachyte, true por- 

 phyry — are called acidic rocks ; while those rocks in which 

 basic feldspars are constituents — like doleryte, and a large 

 part of eruptive rocks — are called basic rocks. 



III. Distinctions of the Triclinic Feldspars. 



The triclinic feldspars are distinguished from the mono- 

 clinic (e. g. orthoclase) by the occurrence of very fine stria- 

 tions on the cleavage surface, sometimes too fine to be seen 

 without a good pocket-lens. These striations are due to 

 multiple twinning parallel to the other cleavage face, as ex- 

 plained on page 57. They are rarely absent from triclinic 

 feldspar crystals. They are best brought out by transmitted 

 polarized light, in which a transverse section of the crys- 

 tal is seen banded with spectrum colors, each band corre- 

 sponding to one plate of the twin structure. 



The triclinic feldspars, andesite excepted, may be dis- 

 tinguished from one another by an optical method when 

 the cleavage direction can be made out. For this purpose 

 a plate is prepared parallel to the plane of easiest cleavage. 

 In such a plate the multiple twinning is parallel to the other 

 cleavage plane, or the shorter diagonal. When the plate is 

 placed on the stage of a polariscope, between crossed Nicol- 

 prisms, as the stage is revolved, the adjoining bands of color 

 become dark alternately, and the angle through which the 



