- UNISILICATES. 259 
If the base is in the dioxide state, the formula becomes 
RO,Si, an example of which occurs in zircon, whose for- 
mula is Zr 0,51. 
There are several natural groups of species among the 
Unisilicates. 
GROUP. STATE OF BASES. CRYSTALLIZATION. 
1. Chrysolite group, protoxide, Trimetric. 
2. Willemite group, protoxide, Hexagonal. 
3. Garnet group, protoxide and odincnie 
sesquioxide, 
4. Zircon group, dioxide, Dimetric. 
5. Idocrase and Sca-  protox. and ses-} Were 
polite groups, quiox. j ; 
; ; T1imetric ; plane angle 
6. Mica group, Be and ses- q giles ilo. aioe 
4 ‘ ceous. 
7. Feldspar group, protox. and ses-) Monoclinic or triclinic, 
quiox. If I nearly 120°. 
In the Seapolite, Mica and Feldspar groups part of the 
species contain an alkaline metal in the basic portion, and 
such kinds have generally an excess of silica. Among the 
feldspars, the species containing only calcium as the protox: 
ide base, is a true Unisiheate. In the others, there is an 
excess directly proportional to the increase of the soda, as 
explained beyond. 
Chrysolite.—Olivine. 
Trimetric. In rectangular prisms having cleavage par- 
allel with 7-7. Usually in imbedded grains of an olive- 
ereen color, looking like green bottle-glass. Also yellow- 
ish grecn. Transparent to translucent. H.=6-7. G.=3:3 
-35. Looks much like glass in the fracture, except in its 
having cleavage. 
Co omposition. (Mg, Fe), O,Si=, for a common variety, 
Silica 41°39, magnesia 50:90, iron protoxide 7%: 71=100. 
The amount of iron is variable. B.B. whitens but is in- 
fusible. With borax forms a yellow bead owing to the iron 
present. Decomposed by hydrochloric acid, and the solu- 
tion gelatinizes when evaporated. AHyalosiderite is a very 
ferruginous variety which fuses B.B. 
Diff. Distinguished from green quartz by its occurring 
