SILICATES, TITANATES, ETC. 



41f> 



OLIGOCLASE 



< 'ry-tals of oligoclase arc not common, but when well developed 

 they arc of (lie same general habit as albite. It is usually massive 

 or cleavable. Twinning after the albite law is the cotninon form, 

 producing st nations on the base. Pericline twins are also developed 



Fi... H).S. Diagram of the Extinc- 

 tion Angles of the Feldspars on 

 the Base in Reference to the Pin- 

 acoidal Cleavage 010. 



-80 



FIG. 464. Diagram of the Pinacoidal 

 Section 010, showing the Inclination 

 of the Pericline Twinning Plane in Ref- 

 erence to the Basal Cleavage Cracks. 



polysynthetically, producing striations on the brachypinacoid ; the 

 inclination of these striations to the basal cleavage cracks or to the 

 edge c/b varies with the composition of the specimen, and is of great 

 ai.<tance in locating the position of the crystal in the series of 

 plaidoclases; that is, in roughly determining its composition. 

 Fig. 463 is a diagram showing how this angle is measured in a plus 

 or minus direction and also giving its value for each member of 

 the series. 



Oligoclase being a member of the acid end of the series is associated 

 with orthoclase and albite in the granites, syenites, diorites, and 

 their porphyritic equivalents, where the individual crystals usually 

 contain large quantities of glass as inclusions. 



