to Calcium and Magnesium Metasilicates. 39 



aggregates of cliopsicle to dull, lusterless, white granular masses 

 of much finer grain and higher MgSi0 3 content. 



Beyond the 68 per cent the preparations appear inhomoge- 

 neous, the /3-MgSi0 3 appearing in increasing amounts as its 

 composition is approached. In the preparations ranging from 

 68 to 90 per cent MgSi0 3 in composition, /3-MgSi0 3 appears 

 almost without exception intergrown with the diopside and 

 usually occupies the center of the large diopside sections. The 

 /3-MgSiO s is invariably twinned polysynthetically after 100 and 

 on sections approximately normal to the prism axis, the pris- 

 matic cleavage lines can be seen cutting across both the diopside 

 and the /3-MgSi0 3 , thus indicating, the close crystallographic 

 symmetry of the two compounds. In such sections the /3-MgSi0 3 

 is characterized by its weak birefringence and by the position 

 of its optic axial plate parallel with the twinning lamellae, while 

 for the enclosing diopside substance the optic axial plane is at 

 right angles to the twinning lamellae. — The limit of crystal 

 miscibility of diopside in /3-MgSi0 3 is above 98 per cent MgSi0 3 , 

 since preparations of that composition are clearly inhomoge- 

 neons. The optic properties of the /3-MgSi0 3 change slightly 

 between 98 and 100 per cent Mg$i0 3 , but only enough to indi- 

 cate very slight solid solution, probably not over 2 per cent 

 of diopside. 



Etch figures. — Proof of the fact of solid solution for composi- 

 tions ranging from pure diopside to about 67 per cent MgSi0 3 

 was also gained by etching the crystals with hydrofluoric acid. 



At the present time isomorphism is a much discussed subject 

 and its final definition has not yet been agreed upon. Emphasis 

 has been placed on similarity of the crystal form of the two end 

 members, on analogous chemical composition, on complete misci- 

 bility, and on the fact that for some of the physical properties, 

 as specific volumes, the properties of the intermediate mixtures 

 are additive functions of those of the end members. This 

 last assumption of "Retgers has been questioned recently, * while 

 the qualification of analogous chemical composition has long 

 been considered unnecessary by certain investigators. 



Briefly stated, the tendency seems to exist for crystallizing 

 substances to absorb, during the process of crystallization, large 

 or small amounts of other material. The more closely similar 

 the absorbed material is to the absorbing crystal in crystal 

 structure, dimensions and tendencies, the greater the amount 

 in general which can be thus taken up. In certain instances, 

 the properties of two substances are so similar and their molec- 

 ular volumes so nearly equal, that the solid solution or crystal 

 miscibility extends from the one compound without break to 

 the second, and the physical properties vary continuously 

 throughout the series. In such a case of complete miscibility 

 no doubt can exist as to the isomorphic relations of the two end 

 *B. Gossner, Zeitschr. Kryst., xliv, 417-519, 1908. 



