208 HAROLD L. ALLING 
The exact nature of the isomeric equivalences among the feldspars is not 
clear; they may be due to the structure of the salts independently of the acids 
they represent, or to isomerism among the acids themselves. 
Barbier sought to find distinct chemical differences between the 
two and suggested that the minerals may be distinguished by the 
fact that orthoclase often contains traces of lithium and rubidium 
while microcline does not seem to carry them. Boeke? pointed 
out that the two salts may have a selective solubility for these 
rarer alkalies, or that these control the magnitude of the twinning. 
But such a distinction is not fundamental as is shown by Vernadsky? 
when he found an unquestioned specimen of microcline from the 
Ilmen Mountains that contained rubidium to the extent of 3.13 
per cent of Rb,O. 
It is generally stated by those who believe that there is no 
physical-chemical difference between orthoclase and microcline 
that the specific gravities of the two are identical. It is true that 
they closely approximate each other, as would be expected in the 
case of minerals so closely allied, but the slight differences are 
significant when definitely charted after careful analysis as is 
shown by the diagram, Figure 7, on page 231. Let us defer final 
conclusions on this argument till we come to its more detailed 
examination. 
Another difference between the two minerals is the varying 
values of the indices of refraction. Weinschenk has stated it very 
clearly when he calls attention to the fact that orthoclase has lower 
indices of refraction in all crystallographic directions than anortho- 
clase, while those of microcline are a little lower in one direction 
and a trifle higher in others.4 
Still another difference indicating a fundamental distinction 
between orthoclase and microcline is the thermal constants of 
these minerals as shown in Table I.° 
tF. W. Clarke, ‘Constitution of the Natural Silicates,” U.S. Geol. Surv. Bull. 
558, P. 35. 
: ioe E. Boeke, Grundlagen der Physikalisch-Chemischen Petrographie (1915), 
‘ : Vernadsky, Bull. Soc. Min., Vol. XXXVI (1914), p. 258. 
4 Weinschenk-Clark, Petrographic Methods (1912), p. 330. 
5D. M. Liddell, Metallurgists’ and Chemists’ Handbook, second ed., 1918, pp. 207-8; 
Joseph W. Richards, Metallurgical Calculations, 1918, p. 140. 
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