Detcrinhiatio)! of tlic Feldspars. — / \ iiuJicll. 
Trace 
001 
ng&ng 
Tim 
Fig. 23.— Projection of tlie principal indices of Ortlioclase (u'^ ), Auorthoclase (n^ ) 
and of Microcline (n"' ) on the plane perpendicular to the edge 001 : 010. 
one of the principal diagnostics. Its position is best expressed, 
summarily, by projections on the plane perpendicular to the 
edge ooi : oio. Figure 23 shows the comparative positions 
of the optic plane in orthoclase, auorthoclase and microcline, 
as given by Michel-Levy and Lacroix.* It appears that in 
th^e potash feldspars the axial plane is nearly parallel to the 
base 001, and consequently agrees substantially with the direc- 
tion of the easy cleavage. It appears also that the axis of 
least elasticity (%, c) is nearly or quite perpendicular to the 
face 010, the axis of greatest elasticity (//p, a ) lies in the plane 
GIG and nearly parallel to the base, varying in microcHne 5° 
into the acute angle, and the axis of mean elasticity perpen- 
dicular to GGi, except in microcline, in which it varies from 
3° to 5° from perpendicularity. 
In a few sanidines (a glassy form of orthoclase peculiar to 
lavasof Mesozoicor later date), the axial plane is perpendicular 
to the easy cleavage, in which case the axes //^ and ;/,„ mu- 
tually change places. Such orthoclase is distinguished by 
Michel-Levy as orthose defornie. 
In the soda-line feldspars the position of the optic plane is 
*Mineraux des Roches, p. 192. 
