FELDSPAR GROUP. 213 
tained as follows. Adding together the anorthite and albite 
formulas, we have CaNa,Al, O,,8i;; then dividing by 2, the 
formulas becomes $Ca$Na,Al0,,51,, which expresses the 
composition of andesite. With 3 parts of the Ca unisili- 
cate, and 1 of the Na, tersilicate, the composition is that 
of labradorite. So it is for other combinations, that is for 
other species between anorthite and albite in composition. 
The quantivalent ratio for the R, Al, Si, in the several 
species of the group, is as follows: V means friclinic in 
crystallization, and LV monoclinic. 
“SYSTEM OF SYSTEM OF 
RATIO, CRYSYTAILIZA- RATIO. CRYSTALLIZA- 
TION. TION. 
Anorthite, 1:3:4 Vv: Oligoclase, 1:3:9 Vi 
Labradorite, 1:3:6 V, Albite, Loa Sie V. 
Hyalophane, 1:3:8 iy; Microcline, 1:3:12 VE. 
Andesite, Lac 8 Ve Orthoclase, 1:3 : 12 IV. 
These are the normal ratios; but there is some variation 
from them in the analyses, part of which is variation in 
actual composition, and part a result of interlamination or 
mixture of two feldspars together. ‘Thus, orthoclase occurs 
mixed with microcline, albite, or oligoclase. But while such 
mixtures account for the soda found in some analyses of 
orthoclase, it does not for that in all, since soda does occur 
in many specimens of pure orthoclase, replacing part of the 
potash. It is the same with the triclinic feldspar micro- 
cline, which has the composition of orthoclase, and may 
have the alkali portion all potash or part soda, one analysis 
of typical microcline giving only 0°48 of soda. It is, hence, 
not safe to calculate the percentage of orthoclase present 
in a feldspar from the percentage of potash. Moreover, 
potash is present in much albite. 
The above ratios show that anorthite has for the ratio 
between R+R8 and Si, 4: 4, or 1:1, as in true unisilicates ; 
while in albite and orthoclase, the same ratio is 4:12 or 
1:3, that of a tersilicate, as above stated. 
C. In physical characters: the hardness being between 6 
