SILK 'ATIX TITA NATES, ETC. 463 



When the constituent oxides in the required proportion are fused, 

 sillimanite i> tWined; ju>t \vh;it are the conditions required to 

 produce the two less stable forms, cyanite and andalusite, is not as 

 yet exactly known. 



BASIC ORTHOSILICATES 

 DATOLITE 



Datolite. A borosilicate of calcium, Ca(BOH)SiO 4 ; CaO = 

 35.0, B 2 O 3 = 21.8, Si0 2 = 37.6, H 2 O = 5.6; Monoclinic; 

 Type, Digonal Equatorial ; ft : b : c = .6344 : 1 : 1.2657 ; p = 89 

 51'=100 A 001; 100 A 110 = 3224 / ; 001 A 101 = 63 16'; 001 A 011 

 = 51 41'; Common forms, c(001), a (100), m (101), m, (Oil), 

 n (111), b (010), g (012), x (102), other forms also numerous ; Cleav- 

 age, none; Brittle; Fracture, uneven; H. = 5-5.5; G. = 2.9-3; 

 Color, white, pale green, or yellow ; Streak, white ; Luster, vitreous ; 

 Transparent to translucent; a = 1.624; p = 1.652; -y = 1.669; 

 y - a = .045; Optically ( - ) ; Axial plane = 010; Bx A 6 = 1- 

 4 in the acute angle p. 



B.B. Fuses easily with intumescence, and in the forceps alone 

 yields a green flame (boron). Gelatinizes with HC1. In the closed 

 tube yields water. 



General description. Crystals are short, prismatic, or equidi- 

 mensional combinations of a, m, c, n, x, m x , and m, all of which are 

 very bright, with a high luster, except x, which is dull. A massive 

 form, very much like porcelain on fracture surfaces, occurs in the 

 Lake Superior copper regions. 



Datolite is a secondary mineral deposited from solutions in the 

 cavities, cracks, and veins, especially of basalts and gabbros, where 

 it is associated with calcite, zeolites, prehnite, and quartz. It is a 

 common mineral in the quarries opened in the traps of the New 

 England and Middle states. Fine specimens have been obtained 

 at Bergen Hill and Patterson, New Jersey ; at Westfield, Massa- 

 chusetts, and at New Haven, Connecticut. 



EPIDOTE GROUP 



The epidote group is made up of basic orthosilicates of the general 

 formula R" 2 R'" 2 (AlOH)(SiO 4 )3, in which R" is Ca, Mn, or Fe, and 

 R'" is Al, Fe, Ce, or Mn. Some epidotes are orthorhombic, but 



