L-N'ISILICATES. 257 



Color deep red to cinnamon color ; also brown, black, 

 green, emerald-green, white. Transparent to opaque . Lus- 

 tre vitreous. Brittle. H. =6*5-7 "5. G. =3*1— 4-3. 



1. 



Composition and Varieties. The general formula for the 

 species is (R 3 R) 2 I2 Si 3 ; in which R may be calcium, mag- 

 nesium, iron, manganese, and R may be aluminum, iron, 

 chromium. The varieties owe their differences to the pro- 

 portions of these elements, or the substitution of one for 

 another. Most garnets fuse easily to a brown or black glass ; 

 but the fusibility varies with the constituents, and chrome- 

 garnet is infusible. They are not decomposed by hydro- 

 chloric acid ; but if first ignited, then pulverized and treated 

 with acid, they are decomposed, and the solution usually 

 gelatinizes Avhen evaporated. 



There are three series among the varieties : one, that of 

 alumina-garnet, in which the sesquioxide base is chiefly 

 aluminum ; the second, that of iron-garnet, in which the 

 sesquioxide base is chiefly iron instead of aluminum ; and 

 third, chrome-garnet, in which it is chromium. 



I. Alumixa-Garxet. 



Almandite (Almandine). An iron alumina-garnet, Fe 3 

 A10 12 Si 3 = Silica 36*1, alumina 20 - 6, iron protoxide 43*3 = 

 100. It occurs of various shades of red from ruby-red and 

 hyacinth-red, to columbine-red and brownish red. When 

 transparent it is called precious garnet ; and, if not so, 

 common garnet. 



Grossularite (including Cinnamon Stone). A lime alu- 

 mina-garnet, Ca 3 A10 1 oSi 3 = Silica 40*1, alumina 22*7, lime 

 37*2=100, but often with some iron protoxide in place of 

 part of the lime. The name Grossularite was given to a 

 pale-green garnet, in allusion to the color, from the Latin 

 name for gooseberry. Cinnamon Stone includes the cinna- 

 mon-colored variety. 



