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



quartz changes into tridymite, and pure quartz glass crystal- 

 lizes as tridjmite ; so that above this temperature tridymite is 

 unquestionably the stable phase. In the presence of fused 

 chlorides silica crystallizes as quartz at temperatures up to 760° 

 and as tridymite above 800° ; crystalline quartz inverts to tridy- 

 mite above 800° and tridymite goes back to quartz at 750°. 

 The inversion temperature is therefore about 800° and the 

 change is enantiotropic. The density of artificial tridymite 

 was found to be 2*318, and that of quartz glass 2213. The 

 pure natural quartz used had a. density of 2'654, the artificial 

 crystals, 2*650. 



Neither the salt of the trisilicic acid, Ca 2 Si 3 8 , the akerman- 

 ite analogue, 4CaO,3Si0 2 , nor the tricalcic silicate, 3CaOSi0 2 , 

 can exist in the two-component system. 



The optical evidence gained by the microscopic study of 

 the crystallized products of mixtures of silicon dioxide and 

 calcium oxide in variable proportions confirms the pyrometric 

 measurements in the following particulars : (1) That silica, 

 calcium metasilicate, calcium orthosilicate and calcium oxide 

 are the only compounds in the series ; (2) that two different 

 modifications of silica exist and correspond in their properties 

 to the minerals quartz and tridymite ; that the metasilicate 

 crystallizes in two enantiotropic varieties, one of which is 

 identical with the mineral wollastonite in its characteristics ; 

 that three enantiotropic phases of the orthosilicate exist and 

 are stable at different temperatures ; (3) that the metasilicate 

 forms solid solutions both with silicon dioxide and with ortho- 

 silicate over limited ranges. 



The experience gained in the course of the examination of 

 these and other laboratory preparations indicates that the best 

 results can be obtained by observing them in powdered form 

 and immersed in liquids of different refractive indices and* 

 not in thin sections embedded in Canada balsam. In a liquid 

 whose refractive index is equal to that of one of the com- 

 ponent substances of the product, the differences in homo- 

 geneity in the product are more readily discerned than in a 

 thin section, and at the same time one of the optical constants 

 of the substance is ascertained. 



Geophysical Laboratory, 



Carnegie Institution of Washington, June, 1906. 



