SILICATES, TITAN ATKS, KTC. 



STAUROLITE 



477 



Staurolite. HFeAl 6 Si..>Oj3 ; ( '(imposition variable; Ortho- 

 rtiomhir; Type, Didigonal Equatorial j a:b:6 = .4734 : 1 : .0828 ; 

 100 A 110 = 25 20' ; 001 A 101 =55 16'; 001 A 011 = 34 19' ; Com- 

 mon forms, c(001), b (010), m(110), r(011); Twinning plane, 

 x (032), also z (232), both common; y (130) ; rare; Cleavage, b 

 distinct, m in traces; Brittle; Fracture, uneven; H. = 7-7.5; 

 G. = 3.65-3.75; Color, shades of dark brown to nearly black ; 

 Streak, white to yellowish ; Luster, vitreous to resinous ; Trans- 

 lucent to opaque; a ='1.736; p = 1.741; y = 1.746; Optically 

 ( + ) ; y - a = .010 ; Axial plane = 100 ; Bx a = c ; 2 V = 88 

 46'. 



B.B. Infusible ; when containing much manganese fuses at 4. 

 With borax and S. Ph. yields an iron reaction. Insoluble in acids. 



General* description. In crystalline habit, short prismatic, 

 combinations of the unit prism, macrodome, and the basal and 

 brachypinacoids. The surface is often dull from alterations. 



Fia. 500. Staurolite. Windham, Maine. 



Interpenetrating twins in which the composition face is 032, and 

 the vertical axes of the two individuals are at right angles, are 

 characteristic, and it is from this twinning that the name Staurolite, 

 " cross-shaped," is derived, Fig. 297. Twins in which the vertical 



