416 



MINERALOGY 



In decomposition through weathering it usually forms kaolin, 

 and under varied conditions forms zeolites, epidote, and quartz. 



Labradorite is a rock-forming soda-lime feldspar, dark gray to 

 nearly black in color, often showing a play of colors or iridescence. 

 In composition it is a member of the basic' end of the series of 



plagioclases. The relation 

 of its optical properties to 

 other members of the series 

 is shown in the table, page 



In occurrence it is asso- 

 ciated with the more basic 

 rocks, particularly the erup- 

 tive rocks. It is rarely as- 

 sociated with quartz or 

 orthoclase. In sections it 

 appears in characteristic 

 lath-shaped crystals, espe- 

 cially in the diabases. Since 

 the more basic feldspars are 



FIG. 465. -Diagram of the Pinacoidal Sec- the first to crystallize from 

 tion 010, showing the Extinction Angles of a magma, they often show 

 the Feldspar in Reference to the Basal zona l structure in which 

 Cleavage Cracks. , . 



the central portion is lab- 



radorite, each zone outwardly becoming less basic or calcic in 

 character. This change in composition, from basic to acid, may 

 also take place in passing from the exterior portions of a rock 

 mass to the interior portions. Labradorite is associated with 

 pyroxene, amphibole, and magnetite in those rocks in which the 

 ferromagnesian content is considerable. 



LEUCITE 



Leucite. Potassium aluminium metasilicate, KAl(Si0 3 ) 2 

 K 2 O = 21.5, A1 2 O 3 = 23.5, Si0 2 = 55; Pseudoisometric. Forms, 

 n(211), d(110), c(100); Twinning plane = 110; Cleavage, IK 

 imperfect ; Brittle ; Fracture, conchoidal ; H. = 5.5-6 ; G. = 2.45 

 -2.5 ; Color, white, gray ; Streak, white ; Luster, vitreous ; Trans- 

 parent to nearly opaque; = 1.509; w = 1.508; - = .001; 

 Optically (+). 



