294: Day and Shepherd — Lime-Silica Series of Minerals. 



on powdered particles by Schroeder van der Kolk's method of 

 refractive liquids and numerical constants thus obtained on 

 grains measuring even less than '001 irim in linear dimensions. 



Optically, four different compounds were recognized in the 

 lime silica series ; three of these appeared in different poly- 

 morphic modifications according to the conditions of forma- 

 tion. Pure silica crystallized either as quartz or tridymite ; 

 calcium metasilicate (CaSiO,) either as wollastonite* or pseudo- 

 wollastonite ; calcium orthosilicate (Ca 2 Si0 4 ) in three forms, a, 

 /3, and 7 ; and calcium oxide only in one form, so far as 

 known. The experiments indicate that for each compound 

 the different modifications bear enantiotropic relations to one 

 another. 



Calcium oxide. — Well-developed crystals of this substance 

 are rare and were observed only in preparations from large 

 melts in the furnace. In intermediate mixtures between the 

 oxide and the orthosilicate it occurs usually as small rounded 

 colorless grains which are easily recognized under the micro- 

 scope by their high refractive index and isotropism. The 

 crystals are colorless to pale yellow, transparent, and crystal- 

 lize in unmodified cubes of the isometric system. Their 

 hardness is between 3 and 4. Attempts were made to etch 

 these crystals by immersing them in water, but the etch fig- 

 ures obtained w T ere not sufficiently distinct to be of value. 

 They pass rapidly into Aetzhugel which cover the entire 

 etched face. The crystals cleave perfectly after the cube 

 (100). Their refractive index, 1'82, was determined by im- 

 mersing fragments in a high refractive liquid solution of 

 arsenic sulphide in arsenic bromide. Owing to rapid superficial 

 decomposition, the crystals soon became coated with a thin 

 crust which tends to decrease the accuracy of this determina- 

 tion of the refractive index. Optical anomalies were observed 

 occasionally, and were due probably to internal strains. On 

 exposure to air and moisture, the crystals slowly become 

 hydrated and disintegrate. 



The orthosilicate. — The microscopic examination of the 

 various preparations of this compound revealed the existence 

 of three distinct forms, a, /3, and 7, which are stable over 

 different temperature ranges. The optical properties of the 

 a and j3 forms are similar and their microscopic discrimina- 

 tion is a difficult problem, while the 7-form, which is stable 

 for emperatures below 675°, differs considerably from the a 

 and forms and can be recognized with ease. 



(a) The a-form is stable only at temperatures above 1410° 

 and on cooling has a strong tendency to invert to the /3 and 7 

 forms. It was found by experiment that if chilled very 

 * Compare this Journal, xxi, 89-108, 1906. 



