166 A. L. DAY MINERAL RELATIONS FROM LABORATORY VIEWPOINT 



than for a single mineral, and much greater when three components are 

 brought together; but they have not been found to differ in character 

 from those offered by way of illustration in the preceding discussion of 

 the behavior of a single mineral when heated. Their increased number 

 invites caution and greatly enhances the value of laboratory experience 

 in prescribing the conditions which shall surround the measurements, 

 but they are in no sense prohibitive, nor do they threaten to become so. 

 Beyond this their elaboration is more properly left to discussions of par- 

 ticular cases in which they occur, of which the literature of the Geo- 

 physical Laboratory already contains several.^^ 



The Isomorphous Eelation 



The crystallization of a two-component mixture in which the compo- 

 nent minerals are in isomorphous relation to each other is of somewhat 

 different character. Here, again, the general case may be found in any 

 desired detail, and with appropriate illustrations in general treatises on 

 physical chemistry and in the more recent handbooks of petrology (Id- 

 dings, Harker — loc. cit.) ; also in the publications of Vogt (loc. cit.), 

 Doelter,22 and the Geophysical Laboratory.^^ Some particular phases of 

 the subject are brought out by laboratory study which do not immediately 

 appear in the investigation of natural minerals with the microscope, and 

 there is little in the low temperature researches of the physical chemist to 

 suggest looking for them. The cooling of an isomorphous mixture of defi- 

 nite proportions (for example, a lime-soda feldspar), even in the simplest 

 case of unlimited mutual solubility and no intermediate compounds, offers 

 at least two cases which the petrologist will find it necessary to differen- 

 tiate — the case in which under-cooling occurs, and one in which it does 

 not. If under-cooling does not enter (the simplest theoretical case), a 

 solution of A and B (figure 6) of composition a, cooling from a molten 

 condition to the temperature represented by h, begins to separate at once, 

 but not in crystals of one of the original components, as in the case of a 



21 Day and Allen : Isomorphism and thermal properties of the feldspars, loc. cit. ; 

 Arthur L. Day and E. S. Shepherd : The lime-silica series of minerals, American Journal 

 of Science (4) 22, 1906, pp. 265-302 ; E. T. Allen, Fred. Eugene Wright, and J. K. 

 Clement : Minerals of the composition MgSiOg ; A case of tetramorphism, American Jour- 

 nal of Science (4) 22, 1906, pp. 385-438; E. T. Allen and J. K. Clement: The role of 

 water in tremolite and certain other minerals, American Journal of Science (4) 26, 

 1908, pp. 101-118 ; E. T. Allen and W. P. White : Diopside and its relations to calcium 

 and magnesium metasilicates, American Journal of Science (4) 27, 1909, pp. 1-47 ; E. S. 

 Shepherd and G. A. Rankin : The binary systems of alumina with silica, lime, and mag- 

 nesia, American Journal of Science (4) 28, 1909, pp. 293-333. 



22 C. Doelter : Physikalisch-chemische Mineralogie, Leipzig, 1905. 



23 Day and Allen, loc. cit. 



