Calcium Meiasilicate. 99 



These numbers show very satisfactory agreement with the 

 specific gravity 2 - 915 of the wollastonite which crystallized 

 from an under-cooled melt, and may be regarded as the true 

 specific gravity of pure wollastonite. 



Some further light has been thrown on the formation of 

 wollastonite by the unpublished work of Messrs. Day and 

 Shepherd of this laboratory. They have examined the entire 

 series of lime-silica mixtures and have obtained wollastonite 

 easily in a variety of mixtures. There appears to be little dif- 

 ficulty in obtaining true wollastonite as soon as an excess of 

 either component is present in the charge. The metasilicate 

 first crystallizes in the hexagonal form, but the inversion to 

 wollastonite occurs during cooling with little or none of the 

 difficulty which we encountered in pure CaSi0 3 preparations. 

 True wollastonite can, in fact, be obtained more readily out of 

 concentrations with a slight excess of CaO than by the use of 

 vanadic acid, but the crystals so formed are not large enough 

 for convenient microscopic study, and cannot be readily sepa- 

 rated. 



In accord with nearly all of our laboratory experiments, both 

 crystallization and inversion go on more slowly in the presence 

 of an excess of silica, due probably to mechanical inertness or 

 viscosity. 



Pseudo- Wollastonite. — This form may be obtained by heat- 

 ing wollastonite above 1180°, or by crystallizing a melt above 

 this temperature. It is only rarely that anything but pseudo- 

 wollastonite is obtained on cooling a melt, but to insure its 

 formation the melt needs only to be slightly agitated to over- 

 come the instability. Pseudo-wollastonite has been described 

 optically by Bourgeois.* It shows a basal cleavage, is optically 

 positive, and very nearly uniaxial, though Bourgeois regards it 

 as really monoclinic. Doelterf combats this view, but Mr. 

 Wright in the microscopic part of this paper finds additional 

 arguments in support of it. It crystallizes, under such con- 

 ditions as have obtained in our experiments, in fibrous, fan- 

 shaped aggregates. The density of the inverted but still 

 unmelted crystals is variable, owing, no doubt, to the presence 

 of bubbles, and not to be distinguished with certainty from 

 the wollastonite, showing again that the volume change which 

 accompanies the inversion is very small. 



Specific Gravity op Wollastonite at 25° compared with Water at 25°. 

 1. 



erted but not 



2. Inverted but not 





melted. 



melted. 



3. Melted, 



2-886 



2-896 



2913 



2-886 



2-896 



2-912 



* Bull. Soc. Min., v, 14-15. 



f N. Jahrb. f. Min., 1886, i, 120 and 122. 



