Determination of the Feldspats. — Wmchell. 
23 
As a family the feldspars grade, therefore, according to 
their alkaline base about as follows, from potash to lime:* 
Species. 
Bases. 
Oxygen, 
Potash. 
Soda. 
Lime. 
Ratio. 
Orthoclase 
dominant 
dominant 
traces 
traces 
traces 
traces 
traces 
traces 
accessory 
accessory 
% 
dominant 
I to i 
yi 
1:3:12 
1:3:12 
1:3:12 
1 :3 : 1 2 
1:3:10 to 
1:3:9 
1:3:8 
1:3:7 to 
1:3:6 
1:3:5 
1:3:4 
Microciine 
Anorthoclase 
Albite 
traces 
Oligoclase 
Andesine 
Labradorite 
Bytownite 
Anorthite 
^8 
1 to i 
r, 
dominant 
Thus grouped according to their varying acidity, they are 
found to vary in general, in a similar continuous order, in 
their physical and optical properties. 
Albite, oligoclase and andesine are not affected by acids, 
except by hydrofluoric acid; labradorite is attacked after long 
treatment, while anorthite is easily decomposed. The surface 
of the microscopic preparation, free from Canada balsam, is 
treated for several hours at a temperature not exceeding 40° 
(centigrade) in order to avoid melting the Canada balsam on 
the other side by which it is adherent to the glass slide; anor- 
thite leaves a skeleton of silica, while labradorite only shows 
a partial corrosion. The test can be made on a fine powder, 
and greater heat applied, even to boiling of the acid. The par- 
tial attack on labradorite can be made apparent by applying to 
the slide, after thorotigh washing in water, some aniline color 
(as malachite green) which will permanently color the gelatin- 
ous silica. 
Optic Cliarcicters. 
In the study of the feldspars it is frequently necessary to 
indicate the position of a right line situated in the plane 001 
or in 010. In order that such designation may be free from 
ambiguity it has been suggested by Max Schuster that the 
directions of such line shall be uniformly referred to the edge 
001 : GIG, with the crystal in the conventional position, i. e.. 
with the vertical axis perpendicular, whether in the plane 001 
*Compare: Cours de minerologie, De Lapparent; p. 403. 
