THE MINERALOGRAPHY OF THE FELDSPARS 201 
Hlawatsch,’ in a very complete review, concludes that two 
minerals are isomorphous when: (1) they possess like crystal 
forms; (2) when their chemical composition is strictly analogous; 
(3) when they are capable of forming homogeneous ‘mixed crys- 
tals.’ The etymology of the term isomorphism strictly means 
‘““same form.’”’ We know, however, that two minerals that are 
isomorphous do not possess identically the same crystal form but 
differ from one another in many details. Though they are similar 
they are not identical. In addition to the crystallographic mean- 
ing of the term a similarity in chemical composition is implied. 
On the other hand in viewing such substances mineralographically 
the term is applied to systems where complete solubility in the 
solid condition exists among the components. 
One of the difficulties in the correct use of these terms is illus- 
trated by their application to the plagioclase feldspars. Albite 
is soluble in all proportions in anorthite; both of these minerals are 
triclinic but their formulas as generally written indicate respectively 
a trisilicate and an orthosilicate. Their mutual solubility has 
suggested to mineralogists that their chemical structures ought to 
be similar, being similar salts of the same acid as suggested by 
Schwantke? and Bayley: 
But this gets us into an opposite difficulty. If the different ° 
salts are salts of the same acid, and have similar structures, how 
is it then that certain pairs are completely soluble while others are 
only partially so? To this question no direct answer is forth- 
coming, for ‘“‘very little is yet known about the physical and 
chemical conditions which determine the solubility of a substance 
in a (liquid) solvent. In fact the essential nature of the process of 
solution must be regarded as at present uncertain. It has been 
noticed that solution is more likely to occur if the [components] 
are alike chemically, but no general rule can be framed.’’* On 
the other hand when we consider solids and (crystalline) solid 
t Hlawatsch, Zeitschr. fiir Kryst., Vol. LI (1912), pp. 417-01. 
2 A. Schwantke, “Die Beimischung von ‘Ca’ in Kalifeldspath und die Myrmekit- 
bildung,” Centralbl. fiir min. Geol. und Pal., 1909, pp. 311-18. 
3 William S. Bayley, Descriptive Mineralogy, Appleton, 1917, 408. 
4W.C. D. Whetham, Theory of Solution, Cambridge University Press (1902), 
p. 78. 
