Deterniinatioti of the Feldspars. — Wincluil. 29 
Methods of Determination. 
{a) Extinction on the Base and Brachypinacoid. 
Schuster and Mallard* established the relations existing 
between the extinction angles on the base and the brachypina- 
coid, and the changing acidity of the plagioclases. The preva- 
lence of favorable cleavages renders it a simple matter to 
obtain plates parallel to these faces. For purposes of deter- 
mination it is usually necessary only to make a coarse powder 
from one of the crystals, from which may be selected such 
cleavage fragments as affords these two directions. These 
may be distinguished not only by the difference in the facility 
of the cleavage, but also by the different interference figures 
given in convergent light. Cleavage pieces parallel to 001 will 
appear larger and more abundant than those parallel to 010. 
They will be apt to show some trace of the albite striations, 
and they will never show a bisectrix, but instead will exhibit 
the indefinite extinction characteristic of -'/„,• Anorthite 
comes nearest to exhibiting a bisectrix in a basal section. On 
the contrary cleavage fragments parallel to 010 are likely to 
have two straight parallel edges, caused by the easy basal 
cleavage. These, however, should not be confounded with the 
basal fragments bounded by the prismatic cleavages. At the 
same time if the fragments be parallel to 010 they invariably 
reveal a bisectrix n^ , either perpendicular or somewhat in- 
clined to the axis of the microscope when examined in con- 
vergent light. To this statement labradorite (from Abi Ani to 
Abi Ans) may be considered an exception, inasmuch as the 
inclination of the axis ( n^, ) is so great that the point at which 
it pierces the plane of the section is outside the field of the 
microscope. It may still be distinguished from a basal section 
which gives the same black bar by the application of the 
quartz of sensitive tint, which shows the lowering of color 
characteristic of the bisectrix n^ although not so decidedly 
as when the axis is exactly perpendicular. In a similar man- 
ner the black bar seen in labradorite in a basal section may 
be shown to be associated with n-^, . The basal section of 
*Uber die optische Orientirung der Plagioclase. Min. u. Petro- 
graph. Mitt., Tschermak, 1880, III, 117-284. 
Sur risomorphisme des feldspaths tricliniques. Bui. Soc. Min., 
France, 1881, IV, p. 103. 
