THE MINERALOGRAPHY OF THE FELDSPARS 241 
Their formula, upon the basis that the end members are orthoclase 
and celsian, is misleading. 
Groth? discusses the difficulty of distinguishing between isomor- 
phous “mixtures”? and compounds when there appears to be a 
definite and fixed ratio between the end members of a given series. 
In referring to the “triclinic feldspars”’ [plagioclase ?] he says: 
The predilection toward certain definite mixture ratios in the series named 
is probably connected with the fact that apparently a regular distribution 
of the two kinds of atomic groups provides a particularly stable equilibrium 
of the crystal structure, since it occurs also with isomorphous substances of 
completely analogous chemical constitution.? 
The fact that many ‘isomorphous mixtures [occur] in simple stoichio- 
metric proportions appear in certain cases to possess greater sta- 
bility than do those in other proportions.”” The modern “view of 
Pace structure .- - shows . +. that the” formation of a 
crystal from two different kinds of chemical molecules,? even though 
these differ very slightly from each other, will give a particularly 
stable structure when the molecules? take part in this formation in 
regularly alternating manner; since such a substance has as much 
right to the name of ‘molecular compound’ as to that of ‘isomor- 
phous mixture,’ it is evident that that view does not permit of any 
sharp boundary between the two ideas.’4 
Groth points out the inherent failure of the usual mineralogical 
methods of attack to distinguish compounds from isomorphous 
mixtures, solid solutions. Upon such problems mineralography 
sheds considerable light. Unless a compound is unstable at its 
melting temperature, the liquidus curve assumes a maximum in 
the form of an arch. In all cases there is an abrupt change in the 
direction and slope of the curves showing the physical properties 
of the series. This is clearly emphasized by the CaSiO,-MgSiO, 
(Pseudowollastonite-Clinoenstatite) diagram.s Both of the end 
members of the series form eutectiferous mixtures with the com- 
mon double salt, diopside. If the specific-gravity curve is plotted 
t™P. Groth, Chemical Crystallography, translation by H. Marshall, 1906, p. 98. 
2P. Groth, Neues Jahrb. f. Mineral., II (1903), 93 ff. 
3 Components. 4P. Groth, Chemical Crystallography (1906), p. 105. 
5 Allen and White, Amer. Jour. Sci. (4), Vol. XXVII (1909); Ferguson and Mer- 
win, Amer. Jour. Sci. (4), Vol. XX XVIII, August, 1919. 
