Determination of the Feldspars. — Winehell. 37 
vertical thread of the nicol — not with the horizontal — and the 
rotation from that position towards the right, necessary to 
bring the cleavage or the albite made into agreement with the 
same thread, is the angle of extinction desired. 
Extinctions which range from 55° upward to 88° indicate 
the bisectrix «p Those which occur between 48° and 3° 
indicate % perpendicular to the section. 
{(T) Sectiofis Perpendicular to the Axis of Mean Elasticity, )i^^. 
M. de Federov has studied the extinctions in sections per- 
pendicular to the axis )im. They present the highest colors 
between crossed nicols, and a somewhat characteristic figure in 
convergent light. 
As shown by the general epures they vary, from albite to 
andesine inclusive, from +2° to — 2. They are not, therefore, 
sufficiently characteristic to separate the acid andesines from 
the albites. The basic feldspars, on the contrary, ranging 
from • — 2° (Andesine, Abs Ans) to — 10° for labradorite, and 
to — 36° for anorthite, are susceptible of distinction in these 
sections. It appears, therefore, that very diverse methods suc- 
ceed in this basic series and generally fail in the acid series. 
Still the sections of maximum birefringence are capable of 
rendering service, especially in the absence of twinning. 
(i') Sections Perpendicular to the Optic Axes. 
M. de Federov has also given the extinction angles on the 
optic axes in simple crystals (i'); and to these Michel-Levy 
has added those of twinned crystals, both those of the Carls- 
bad type and those of the albite. The numerals (i) and (i') 
are made to represent the two individuals of the albite twinned 
crystal, and (2) and (2') the two adjacent albite individuals of a 
Carlsbad-twinned crystal. It is evident that the parts (i) and 
(i') belong to one or the other of the parts (2), (2'). The 
following table gives the extinctions on the axis (A). Col- 
umns (2) and (2') and column (i) are added by Michel-Levy. 
The former shows the extinctions on the individuals (2) and 
(2'), and the latter the angle of the plane of the optic axes with 
the trace of the cleavage 010, which can easily be obtained 
from the figure in convergent light. 
