Tin-: PRINCIPAL ROCK-FORMING MINIMAL- r,i3 



Color, white; I nt erfercnce colors fir-t OP NT gray to 

 yellow; p. 478. 



Orthoclase; Monodinic ; n = 1.52;^ a = .000. 



Plates and lath-shaped crystalline outlines, Drains, and 

 irregular masses; Cleavages, 001 and 010 well de- 

 veloped; Relief low, surface smooth; Colorless, 

 transparent or cloudy through decomposition ; Carls- 

 bad twinning common; Interference colors low grays 

 of the first order; Optically negative ( ); p. 403. 



Microcline; Triclinic ; n = 1.53; "Y a = .007. 



Like orthoclase but twinning very common, showing 

 the characteristic gridiron structure ; Optically nega- 

 tive (-); p. 409. 



Leucite; Large crystals will appear doubly refracting, 

 see p. 610. Colorless amphiboles and andalusite 

 may not show pleochroism. See p. 416. 



Kaolinite; Monoclinic ; n = 1.54; -y a = .008. 



White scales, leafy or fibrous aggregates; Relief very 

 low ; Interference colors gray first order ; a secondary 

 or decomposition product; Optically ( ); p. 501. 



lolite; Orthorhombic ; n = 1.54; y a. = .009. 



Short prisms, grains, and irregular masses; Cleavage 

 not developed ; Colorless ; Relief very flat, much like 

 quartz; Pleochroism not marked in thin sections, 

 except in the surrounding halos; Interference colors 

 first order grays to yellow; Optically negative ( ); 

 p. 440. 



Enstatite ; Orthorhombic; n=1.66; -Y-a=.009. 

 Crystalline outlines, prismatic, fibrous, or in rounded 

 grains ; Inclusions often symmetrically arranged ; 

 Cleavage prismatic at 92; Interference colors high 

 first order about the same as quartz ; Optically (+) ; 

 p. 421. 

 &. Double refraction greater than that of quartz. 



Zeolites ; some of the zeolites possess a double refrac- 

 tion as high as .012; p. 612. 



Gypsum; Monoclinic; n = 1.524; \ a = .010. 



Granular, tablets, and fibrous aggregates ; Cleavage well 

 developed with abundant cracks and often twinned ; 

 Colorless; Interference colors first order yellow to 

 orange; Optically positive (+); p. 536. 



