46 Allen, etc. — Diopside and its Relations 



posed of about 95*5 per cent of a mix-crystal containing about 

 62*5 per cent of diopside, 37*5 per cent magnesium silicate, 

 and 4'5 per cent a-MgSi0 3 . Its melting temperature is 1375°. 

 Microscopically eutectic textures were observed rarely if at all. 



4. Six solid solutions appear in this system. Only two of 

 them contain more than three or four per cent of the lesser 

 component, and only these will be mentioned here. 



a. ^-calcium silicate (wollastonite) forms a saturated solution 



of wollastonite containing about ll per cent diopside (8 per cent 



MgSi0 3 ) : 83 per cent CaSi0 3 , when crystallization takes place 



in the neighborhood of 1050°, i. e., wollastonite is capable of 



17 

 dissolving about — =20 per cent of its own weight. This series 



83 



of solutions is interesting from the fact that the inversion point 

 of pure calcium silicate (1190°) appears to be raised by the addU 

 tion of MgSi0 3 , up to 100° in the most concentrated solutions. 

 This is probably largely, if not wholly, an apparent rise in the 

 inversion point due to viscosity, for, as is well known, an inver- 

 sion point should be raised only when the concentration of the 

 solution below the point is greater than that above, while here 

 there is a rise in the weaker solutions which suffer no change 

 in concentration when they invert. Again, the concentration of 

 solutions just below the inversion point cannot be determined 

 with accuracy on account of the difficulty of establishing an 

 equilibrium in solid silicate solutions. 



b. Diopside dissolves about 60 per cent of its own weight, 

 forming a solution which contains 66"5 per cent MgSiO„ : 33'5 per 

 cent CaSi0 3 . This saturated solution is very similar to diopside 

 in all its properties. Its melting point is only 3° lower (i. e., the 

 maximum heat absorption falls there, the melting interval is 

 unknown). The specific gravity changes very little ; the optic 

 data show slight but noticeable changes : the refractive indices, 

 the birefringence, the optic axial angle and the extinction angles 

 all falling continuously with the addition of MgSi0 3 from diopside 

 up to the limit of solid solution at 66-5 per cent MgSi0 3 . 



5. In the series of limited solid solution between diopside 

 and clino-enstatite, the effect of the addition of MgSi0 3 to 

 diopside is, furthermore, clearly shown by etch figures on the 

 prismatic cleavage faces. On passing from diopside to the 

 limit of solid solution at about 66*5 per cent MgSi0 3 , the 

 shape of the etch pits changes gradually, their character, on 

 preparations of intermediate composition, being intermediate 

 between those of the two compounds, diopside and pure 

 /3-MgSi0 3 , thus proving that actual solid solution does exist in 

 the series, and that the effects of the end members are felt 

 crystallographically in the solid solutions of the same. — For 

 the observation, under the microscope, of changes which take 

 place in substances at high temperatures, a special micro- 



