THE MINERALOGRAPHY OF THE FELDSPARS 227 
Regers’ found that the ferrous salt, when pure, is monoclinic in 
crystal habit while the magnesium salt is orthorhombic. 
From aqueous solutions, at ordinary temperature it is possible to obtain 
. [solid solutions] containing up to 54 per cent of magnesium sulphate, 
and possessing the monoclinic form of ferrous sulphate. There must, there- 
fore, be a second (a monoclinic) form of magnesium sulphate, which can form 
[limited] isomorphous mixtures with ferrous sulphate. There is a gap between 
the mixture containing 54 per cent of the magnesium salt and the next higher 
one, which contains 81 per cent, no intermediate mixtures being known. 
Mixtures from this point up to pure magnesium sulphate exist, and they 
exhibit the orthorhombic form of the latter salt. If the densities of these 
various mixtures are plotted against the corresponding percentage composi- 
tion, as in Figure 6, it is seen that the values lie upon separate and distinct 
straight lines, not parallel to each other. 
The gap referred to above undoubtedly implies a limited 
solubility between the components; that is, limited isodimorphism. 
It is strongly suspected that the potash-soda feldspars are of this 
type. If so Hintze’s data can be treated in the manner shown in 
Figure 6. The upper of these two non-parallel lines represents 
gravities of the monoclinic modifications, or orthoclase-barbierite; 
the lower line the triclinic forms, microcline-albite. If Hintze’s 
data can be relied upon, the lines in the figure strongly suggest the 
dimorphism of each component, for it is known that a dimorphous 
substance possesses different gravities depending upon its modifi- 
cation, and that the density of the higher temperature form is 
almost always higher than that of the lower. Consequently we 
would expect to find that orthoclase has a higher specific gravity 
than microcline. This is apparently the case. A factor, how- 
ever, that may nullify the latter interpretation, is that natural 
specimens of orthoclase almost invariably contain more sodium 
feldspar dissolved in them than is contained in microcline. Since 
the gravity of the sodium component is higher than either ortho- 
clase or microcline this would increase the gravity of natural 
specimens of orthoclase above that possessed by microcline irre- 
spective of any dimorphism that may exist. Until laboratory 
experiments are undertaken final conclusions are impossible. 
t Regers, Various articles in Zeitschr. fiir Chem., 1889, 1890, and Jour. C.S., 1891, 
2 Paul Groth, English translation of Hinleitung in die chemische Krystallographie, 
Chemical Crystallography (1906), p. 92. 
