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



silicic acids, we find that we have located two — the metasilicate 

 and orthosilicate — and missed two — the akermanite analogue 

 4CaO, 3Si0 2 , and the tricalcic silicate. The next step was 

 therefore obviously to bring all our resources to bear upon 

 these particular compositions in order definitely to ascertain 

 whether such compounds can exist when the components are 

 pure, and if so, under what conditions and with what proper- 

 ties. 



The Akermanite Analogue. — The akermanite analogue 

 (4Ca0.3Si0 2 ) was first taken up and the neighboring concen- 

 trations investigated at intervals of 1 per cent with the great- 

 est care. A large charge of this particular composition was 

 repeatedly melted and examined under the microscope, but it 

 failed to show homogeneous structure or any characteristic 

 property of a compound. On the other hand, the pseudo- 

 wollastonite and the orthosilicate appeared in the proportion 

 appropriate to its place in the series. Furthermore, since the 

 melting temperatures of these mixtures were within the reach 

 of our platinum furnaces, and therefore of our most sensitive 

 pyrometric measurements, we were able to hold the tempera- 

 ture constant at any desired point and then by rapid cooling 

 (quenching in mercury) to fix any phase which might have 

 been present and become unstable below that temperature. 

 Here again we found that pseudo-wollastonite and the calcium 

 orthosilicate were the only phases which could be separated 

 from this or any mixture of the pure components in this 

 neighborhood. It is our belief, therefore, that the akermanite 

 mineral cannot exist between the pure components and is only 

 possible when other substances are present. This is further 

 indicated by the fact that the metasilicate of calcium in the 

 presence of magnesium forms a solid solution of which the 

 limiting concentrations are relatively wide and which would 

 easily account for the akermanite mineral produced from the 

 fusion of the three components. 



The Orthosilicate of Calcium, 20aO,SiO^ (65'00 per cent 

 CaO).* — It has long been known that the orthosilicate of cal- 

 cium, although not found in nature, can be formed by the 

 fusion of the pure components. The temperature of fusion is 

 very high and the crystalline material obtained disintegrates 

 spontaneously at the lower temperatures The cause of the 

 disintegration has not been carefully studied heretofore, and 

 optical determinations of it are difficult, owning to the extreme 

 fineness of the disintegrated product. Our investigation estab- 

 lishes the fact that the orthosilicate of calcium can exist in 



*The metasilicate of calcium has been made the subject of a special paper 

 by Allen, White and Wright (loc. cit.) and will not receive detailed con- 

 sideration here. 



