THE MINERALOGRAPHY OF THE FELDSPARS 211 
otherwise solidify, thereby making possible their crystalline devel- 
opment.’’? 
: While it seems as if the matter cannot be definitely settled 
by laboratory work unless fluxes or high pressures are employed, 
yet the writer is of the opinion that microcline is an isomer of 
orthoclase. This implies that above the transformation tempera- 
ture orthoclase is the stable mineral, while below it microcline is 
the normal form. If orthoclase passes this point on cooling with- 
out inverting, then the mineral exists in a metastable condition. 
THE SODIUM COMPONENT 
Albite——Albite (albus, Latin for white) is the soda feldspar, 
NaAlSi,Os, which crystallizes in the triclinic or anorthic system.. 
Structurally it can be represented by the same general formulas 
given above by substituting Na for K. 
Barbierite.—It was not until recently that a monoclinic form 
of the soda component was even suspected. Barbier and Prost? 
found a sodium feldspar that is monoclinic and isomorphous with 
orthoclase, which Clarke? lists as ‘“‘barbierite isomeric with albite.”’ 
Schaller? says that the existence of a monoclinic soda feldspar iso- 
morphous with orthoclase must be admitted. Thus we may have a 
relationship in the soda component identical with that prevailing in 
the potash feldspar. The thermal ranges of barbierite are unknown. 
THE CALCIUM COMPONENT 
Anorthite —Anorthite (av, not, and épos, straight) is a triclinic 
lime feldspar, but unlike the other components it is usually regarded 
as a salt of orthosilicic acid (H,SiO,) instead of the trisilicic. The 
formula is commonly written: CaAl,Si,O; or CaAl,(SiO,),. 
So far as known no isomeric form of the lime component exists, 
consequently the term ‘‘lime component” and ‘‘anorthite” can be 
used indiscriminately. 
The above three components, :potash, soda, and lime, are the 
most important feldspar constituents. Two other components 
however ought to be mentioned. 
t Weinschenk-Johannsen, Fundamental Principles of Petrology, 1916, p. 41. 
2 “Sur Vexistence d’un feldspath sodique monoclinique isomorphe de 1’orthoclase,”’ 
Bull. Soc. Chem. (1908), II, 804. 
3B, W. Clarke, U.S. Geol. Surv. Bull. 588, p. 35. 
4 Bull. Soc. Min., XXXIII (1910), 320. 
