296 Day and Shepherd — Lime-Silica Series of Minerals. 



gressive paramorpliic change of this form into the 7-form can 

 be followed under the microscope and is interesting to watch. 

 Shortly after cooling, irregular interference colors appear, as 

 though induced by internal strains, and soon the mass resem- 

 bles a crystalline aggregate of minute fibers. Toward the end 

 of the process, the larger masses usually disintegrate as fine 

 powder owing to the enormous increase in volume (about 10 

 per cent) during the inversion. 



(c) The yform is stable at low temperatures and into it the 

 preceding forms usually pass on cooling to ordinary tempera- 

 tures. It is noteworthy that the properties of this form are. 

 unlike those of the above, particularly in specific gravity and 

 refractive index. The density of the a and /3 forms is about 

 ■3*27, while that of the a-form is only 2*97. As a result, on 

 paramorpliic change in the solid state a great increase in vol- 

 ume takes place which at once shatters the larger fragments 

 of the original material and causes the preparation to u dust." 

 The 7-form can be had, therefore, only in the form of powder. 

 It is prismatic in habit, cleaves well parallel to the long direc- 

 tion, is colorless and transparent, and occasionally shows 

 indications of twinning with small extinction angles ; c: c = 3° 

 was measured in one instance. The form is probably, there- 

 fore, monoclinic in system. The twinning is recognized only 

 rarely, and the extinction usually appears parallel. The 

 refractive indices were determined by the method of refrac- 

 tive liquids : a = l-640db-003, /3 = l'645±-003, 7 = l-654±-003. 

 The birefringence is weak, about -014; biaxial with an optic 

 axial angle in air 2E = 52°, measured by the graphical method 

 of Becke on a section nearly normal to an optic axis ; optical 

 character negative ; plane of optic axes perpendicular to the 

 prism axis in contradistinction to the a and ft forms. Com- 

 pared with the a and @ forms this form is readily distinguished 

 by its lower refractive index, its optical character, optic 

 axial angle, and position of the plane of optic axes to the 

 prism axis. 



The metasilicate occurs in two enantiotropic modifications, 

 one of which corresponds to the mineral wollastonite. The 

 second form has been called pseudo-wollastonite, and is, stable 

 above 1200°. Both these forms have been discussed so thor- 

 oughly in a preceding paper* that repetition here is unneces- 

 sary. The properties of the artificial wollastonite counterpart 

 those of the mineral, while the pseudo-wollastonite is pseudo- 

 hexagonal, probably monoclinic in crystal system. It is opti- 

 cally positive and nearly uniaxial ; its refractive indices, a = 



*Loc. cit. 



