SUBSILICATES. 289 



Composition. H 2 Bc»Al 10 Si 2 = Silica 41-20, alumina 35 -22, 

 glucina 17'39, water 6*19=100. B.B. fuses with much dif- 

 ficulty to a white enamel ; not acted on by acids. 



Biff. The cleavage of this glassy mineral is, like that of 

 topaz, very perfect, but is not basal. The cleavage distin- 

 guishes it from tourmaline and beryl. 



Obs. Occurs in the Ural, and with topaz in Brazil. 



The crystals of this mineral are elegant gems of them- 

 selves, but they are seldom cut for jewelry on account of 

 their brittleness. 



Datolite.— Datholite. Humboldtite. 



Monoclinic. Crystals without distinct cleavage ; small 

 and glassy. Also botryoidal, 

 with a columnar structure, 

 and then called botryolite. 

 Color white, occasionally gray- 

 ish, greenish, yellowish, or red- 

 dish. Translucent. H. = 5-5*5. 

 G.=2*8-3. 



C Composition. H 2 Ca 2 B 2 O 10 Si 2 

 "Silica 37 '5, boron trioxide 

 21 *9, lime 35*0, water 5'6= 

 100. Botryolite contains twice 

 the proportion of water. B.B. 



becomes opaque, intumesces and melts easily to a glassy 

 globule coloring the flame green. Decomposed by hydro- 

 chloric acid ; the solution gelatinizes on evaporation. 



Biff. Its glassy complex crystallizations, without cleavage, 

 are unlike any other mineral that gelatinizes with acid. 

 It is distinguished from minerals that it resembles also by 

 tingeing the blowpipe-flame green. 



Obs. Occurs in cavities in trap rocks and gneiss. Found 

 in Scotland ; at Andreasburg ; at Baveno ; Toggiana ; also 

 Bergen Hill, in N". J.; in Connecticut, at Soaring Brook, 

 14 miles from New Haven ; and near Hartford, Berlin, Mid- 

 dle-field Falls, Conn. ; also in great abundance at Eagle 

 Harbor in the copper region, Lake Superior, and on Isle 

 Soyale ; also near Santa Clara, Cal. 



Homilite. A black silicate of iron and calcium, resembling gadoli- 

 nite, but affording from 15 to 18 per cent, of boracic acid with 32 of 

 silica ; formula R a B 2 O ]0 Si 2 . From Brevig, Norway. 



