SILICATES, T1TANATI 



111 



t he \\rll-known gridiron structure, recognized in sections and charac- 

 tt -fistic <>f microcline, serving as an easy means of distinguishing 

 mirrncliiir from orthoclase. 



Microcline is associated with orthoclase in the older igneous 

 rooks, asuranitrs and syenites, but it is not associated with sanidine 

 in the more recent felsites and lavas. 



Amazon stone is the name given to a green variety of microcline, 

 beautiful specimens of which are obtained near Pikes Peak, Colo- 

 rado ; in fact most microcline is very apt to be light green in color, 

 the cause of which is not known. 



ALBITE 



Albite. Sodium aluminium trisilicate, NaAlSi 3 O 8 ; NaaO = 

 11.8, A1 2 O 3 = 19.5, SiO 2 = 68.7; Triclinic; Type, Centrosym- 

 motric; &:b:c = .633:1:558; a = 94 3' ; p = 116 29'; 

 \ = 88 9'; 100 A 010 = 90 3'; 100*001 = 63 35' ; =010 A 001 

 = 86 24'; Common forms, c(001), b (010), m(110), M(110), 

 x (101) ; Twinning, albite, Carlsbad, Baveno, and Manebach laws 

 common, also pericline twins ; Cleavage, basal perfect, brachypina- 

 coidal less eo, and m imperfect ; Brittle ; Fracture, uneven ; H. = 

 6-6.5 ; G. = 2.6-2.65 ; Color, white, gray to reddish ; Streak, white ; 

 Luster, vitreous; Transparent to opaque ; a = 1.525; P = 1.530; 

 y = 1.531 ; a - -y = .006; Optically (+) ; 2 V = 77 39'. 



B.B. Fuses a little easier than orthoclase, insoluble in acids, 

 and will not yield a potassium flame, at least not a strong one. 



General description. In habit albite is tabular parallel to the 

 brachypinacoid rather than elongated, though both habits occur. 

 ( 'rystals are combinations of the base, brachypinacoid, and the two 

 unit prisms ; other forms are not common. Well-formed crystals 

 occur in cavities and veins in granites, syenites, or gneisses, espe- 

 cially the more acid varieties, where they are associated with 

 such minerals as topaz, beryl, tourmaline, chrysoberyl, and other 

 rare species. In color albite is more apt to be white than is ortho- 

 clase, and the two good cleavages are not at 90 but at 86. Twin- 

 ning striations caused by twinning after the albite law are charac- 

 teristic and nearly always appear on the basal cleavage. In 

 sections albite is colorless, with very low relief, and is similar in many 

 respects to orthoclase, with which it may be intimately intergrown 

 in laminae parallel to the macropinacoid (perthite), or in concen- 



