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



lastonite. The optic axial angle was found to increase and 

 reached a maximum of about 25° in air, whereas pure pseudo- 

 wollastonite is nearly uniaxial. In many of the preparations 

 hexagonal outlines of this phase are noticeable, and two sets 

 of imperfect cleavage lines intersecting at an angle of 120° 

 can then be seen on sections normal to the acute positive bisec- 

 trix. Mixtures containing over 45 per cent of calcium oxide 

 appear homogeneous, and the crystals from the preparations 

 show a continuous variation in their optic properties from that 

 point on as the composition of the metasilicate is approached. 



The microscopic study of the preparations ranging in com- 

 position between calcium metasilicate and calcium orthosilicate 

 showed that the metasilicate is capable of -absorbing a consid- 

 erable amount of the orthosilicate and still appear homogene- 

 ous, the resultant crystals expressing the fact of solid solution 

 by the change in their optic properties. The limit of homo- 

 geneity of the mass appeared to be reached at abont 50 per 

 cent lime and in crystals from fusions of that composition the 

 refractive index a was found to have increased from 1*615 of 

 pure pseudo-wollastonite to about 1*630 ; the optic axial angle 

 was also larger (2 E about 20°-30°) ; the plane of optic axes 

 was normal to a direction of cleavage in contrast to its position 

 in crystals of solid solutions of the metasilicate and silica in 

 which the plane of optic axes was observed parallel to the 

 direction of cleavage. In the 50 per cent preparations, twin- 

 ning lamellae with small extinction angles can often be seen 

 on sections normal to the acute positive bisectrix, thus demon- 

 strating their monoclinic or triclinic nature, notwithstanding 

 the hexagonal outline of the basal section and systems of 

 cleavage lines intersecting at angles of about 120°. 



In products containing a little more than 50 per cent of cal- 

 cium oxide the fine-grained eutectic begins to appear in small 

 patches between the crystallites of the metasilicate. Fusions 

 from mixtures of about 55 per cent lime and 45 per cent silica 

 are usually fine-grained, and differences in size between crystals 

 of the meta- and orthosilicate are less apparent. 



Preparations with more than 55 per cent lime ordinarily 

 disintegrate to fine powder on cooling, due to the great 

 increase in volume of the orthosilicate on inverting to the 7- 

 form at low temperatures. Studies in texture could not there- 

 fore be made in the loose powder, and evidences of solid solu- 

 tion near the orthosilicate were sought for by means of optical 

 constants alone. The refractive indices of the minute crystal- 

 lites from the products approaching the orthosilicate in com- 

 position were not observed to differ appreciably from those of 

 the orthosilicate. 



