TJNISILICATES. 



255 



If the base is in the dioxide state, the formula becomes 

 R 4 Si, an example of which occurs in zircon, whose for- 

 mula is Zr 4 Si. 



There are several natural groups of species among the 

 Unisilicates. 



GROUP. 



1. Chrysolite group, 



2. Willemite group, 



3. Garnet group, 



4 Zircon group, 



5. Idocrase and Sea- 



polite groups, 



6. Mica group, 



7. Feldspar group, 



STATE OF BASES. 



protoxide, 

 protoxide, 

 protoxide and) 



sesquioxide, \ 

 dioxide, 

 protox. and ses-) 



quiox. ) 



protox. and ses-/ 

 quiox . ( 



protox. and ses-)_ 

 quiox. ) 



CRYSTALLIZATION. 



Trimetric. 

 Hexagonal. 



Isometric. 



Dimetric. 



Dimetric. 



Tiimetric ; plane angle 

 of base, 120° ; mica- 

 ceous. 



Monoclinic or triclinic, 

 I A /nearly 120°. 



In the Scapolite, 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 Unisilicate. 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 i-i. Usually in imbedded grains of an olive- 

 green color, looking like green bottle-glass. Also yellow- 

 ish green. Transparent to translucent. ll. = 6-7. Gr.=:3'3 

 -3*5. Looks much like glass in the fracture, except in its 

 having cleavage. 



Composition. (Mg, Fe) 2 4 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. Hyalosiderite is a very 

 ferruginous variety which fuses B.B. 



Diff. Distinguished from green quartz by its occurring 



