206 HAROLD L. ALLING 
THE POTASSIUM COMPONENT 
Orthoclase.—This mineral (dp6os, straight, and kas, angle), 
KAISi,Os, is usually regarded as a salt of trisilicic acid, H,$1,Os. 
In regard to its chemical composition zm nature and its distinction 
from microcline we shall have more to say. 
Microcline.—The name (uxpds, small, and xXivew, to incline) 
has reference to the fact that the angle (89°30’) between the two 
perfect cleavages differs but little from a right angle. Although 
the chemical formula of microcline is given as identical with that 
of orthoclase the petrographer usually has little difficulty in recog- 
nizing and separating it from orthoclase by the multiple pericline 
or ‘‘gridiron”’ twinning of the former as revealed by the microscope. 
Orthoclase either does not twin in this manner or does so on such 
an extremely fine scale as to be submicroscopic. Whether there is 
any physical or chemical difference between the two potash feld- 
spars has been a matter of debate for some time. Investigators 
are divided between the two general theories; one group maintains 
that they are one and the same mineral, and differ from one another 
only in the magnitude of the twinning. On the contrary, another 
group maintains that there is a fundamental difference between 
the two minerals which although of identical composition possess 
different chemical structures. 
Dimorphism of the Potassium Component.—‘‘It appears highly 
probable that if the cross twinning and interpenetration of micro- 
cline become so minute as to be invisible under the microscope the 
crystals would be indistinguishable from those of orthoclase, and 
would, in fact, possess all the properties of that mineral. Many 
authors regard orthoclase as pseudo-symmetric; if this be so, all 
the feldspars may be in reality anorthic [triclinic].””* 
According to E. Mallard and A. Michel-Lévy it seems highly probable 
that orthoclase and microcline are not dimorphous, but identical, since they: 
proved that the optical behavior of orthoclase would be a necessary conse- 
quence of an intimate multiple twinning of microcline lamellae after the albite 
and pericline law.? 
«H. A. Miers, Mineralogy, p. 460. 
2 Rosenbusch-Iddings, Microscopic Physiography of the, Rock-Forming Minerals, 
Pp. 320. 
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