16 Allen, etc. — Diopside and its Relations 



1 Chilled from an initial temperature of 1346°; all orthorhom- 



bic crystals (a-form). 



2 Chilled from an initial temperature of 1337°; all the crystals 



were orthorhombic. 



3 Chilled from an initial temperature of 1324°; all monoclinic 



crystals (/?-form). 



4 Chilled again from 1324° with the same result. 



After the existence of the a-form had been established, the* 

 inversion point was repeatedly sought for by the Frankenheim 

 method and a very small minimum between 1405° and 1435° 

 was found in many cases bnt by no means in all, and the point 

 was so small that one might well have hesitated to interpret it 

 as an inversion, had other evidence been lacking. Taken in 

 connection with the sharp heat change which occurs in the 

 mixtures (see below) at about 1365°, it appears that the inver- 

 sion point is overstepped about. 40° in both directions, which 

 is in perfect accord with the sluggish behavior of solid silicates. 

 The inversion- point in pure magnesium silicate is represented 

 in the diagram (fig. 1) as falling somewhat as it dissolves cal- 

 cium silicate, because this is the relation which commonly 

 holds, but it is manifestly impossible to settle the question at 

 present. 



a-Magnesium Silicate. — This form, which has already been 

 described, crystallizes in the orthorhombic system in equant 

 crystals on short doubly terminated prisms which resemble 

 forsterite in habit, index of refraction and birefringence. 

 Its specific gravity, judging from its index of refraction, varies 

 little from that of /3-magnesium silicate (3*192). It was found, 

 by applying the floating method to a single small crystal, to 

 be about 3*16. Fortunately, in several instances a few well- 

 formed, separately developed crystals were found on the sur- 

 face of charges of magnesium silicate which had been melted 

 and crystallized in the furnace, though usually all was com- 

 pletely inverted into the magnesium pyroxene. The measure- 

 ments of these crystals wdiich are recorded in Part II of this 

 paper prove conclusively that this is a form entirely distinct 

 from forsterite or enstatite. 



Inversion in the Mixtures. — The inversion line is traced in 

 all the mixtures from 68 per cent to 98 per cent magnesium 

 silicate and varies little from 1365.° Some typical curves 

 showing both the inversion and eutectic points are plotted in 

 figs. 4 and 5, curves II, V, VI, VII, and VIII. The signifi- 

 cant feature of these curves is the gradual diminution of the 

 heat absorption at the latter (eutectic) temperature and the 

 gradual increase of that at the former (inversion), as we pass 

 toward 100 per cent magnesium silicate. Thus in the 70 per 

 cent mixture the eutectic melting is large, while in the 95 per 



