24 Allen, etc. — Diopside and its delations 



when heated, which in the beginning of the work was quite 

 confusing. The normal behavior of a solution of wollaston- 

 ite containing more than 2-4 per cent of diopside, (the 

 limit of solubility of the latter in pseudo-wollastonite) would 

 be as follows : First, an inversion should occur somewhat above 

 1190°, giving crystals of pseudo-wollastonite saturated with 

 diopside, and an excess of free diopside. At 1348° some 

 eutectic melting would be noted, and finally, when the melting 

 point curve is reached, another thermal point would be found. 

 Under ordinary experimental conditions, however, where the 

 rate of heating is about 3° per minute, solutions containing 5 

 per cent MgSi0 3 (10'8 per cent diopside) showed no melting at 

 the eutectic temperature. Evidently no inversion had taken 

 place. When the rate of heating was considerably slower a 

 slight eutectic melting was noted, while if the crystals were 

 previously held for some time at 1360°, there was a strong 

 absorption of heat at the eutectic temperature. In the 8 per 

 cent MgSi0 3 solution (1T'3 per cent diopside) the eutectic failed 

 in one instance, i. e., there was no melting at the eutectic 

 temperature. Mixtures of saturated mix-crystals of wollaston- 

 ite-diopside give, when the heating is not too slow, three points, 

 the eutectic at about 1348°, a further melting accompanying 

 the inversion at a higher temperature,* and finally, the point 

 on the melting point curve where all becomes liquid. Thus 

 the 10 per cent mixture (containing about 4 per cent of free 

 diopside) showed heat absorptions at 1340°, 1377° and 1451°. 

 The 8 per cent solution showed a similar behavior, indicating 

 that some inversion may have occurred below the eutectic points. 

 Another explanation is perhaps more probable, viz. : that the 

 lowest point may be due to an unstable eutectic between wollas- 

 tonite and diopside, since the latter was found to separate from 

 the more concentrated solutions at the higher temperatures. 

 If so the point lies very near the pseudo-wollastonite diopside 

 eutectic. A fact that seems to favor the explanation is that 

 the 10 per cent solution always gives this lowest point although 

 it has been proved that the more concentrated solutions invert 

 with greater difficulty. Moreover, when the 28 per cent solu- 

 tion was crystallized below 1190° and therefore contained no 

 pseudo-wollastonite (a conclusion also verified by the micro- 

 scope), all melted at 1358° as usual. 



Specific-volume curve. — In 1890 Retgersf stated clearly two 

 arguments to prove that diopside was a chemical compound in 

 distinction from a mix-crystal. The first was that the minerals 

 in nature which contain the metasilicates of calcium and 



* Since the solution of the diopside in the wollastonite crystals prevents 

 partly or wholly the eutectic melting at the proper temperature, this melting 

 will at once occur when the crystals are inverted. 



f Ann. Ecole Polytech. de Delft, iv, p. 186, 1890. 



