WASHINGTON: CALCIUM ORTHOSILICATE IN ROCKS 345 



GEOLOGY. — The calculation of calcium orthosilicate in the 

 norm of igneous rocks. Henry S. Washington, Geophys- 

 ical Laboratory. 



When the calculation of the norm from the analysis of an 

 igneous rock was first worked out some fifteen years ago in con- 

 nection with the Quantitative System, provision had to be made 

 for magmas so low in silica that there remained a deficit for the 

 femic minerals, after salic potash, soda and hme had been as- 

 signed their minimal amounts to form leucite, nephelite and 

 anorthite respectively. Nearly all melilite-bearing rocks and 

 some nephelinites fall under this category. At that time the 

 physical chemistry of the calcium silicates had been scarcely 

 studied and, though the use of calcium orthosilicate was consid- 

 ered, the normative calcium silicate molecule selected was 

 4Ca0.3Si02, as it corresponded to the slag mineral akermanite 

 of Vogt, which is apparently isomorphous with and related to 

 melilite. Equations for the calculation of this molecule in the 

 norm were accordingly given in the publication of the quantita- 

 tive system. 



Subsequently, investigations in the Geophysical Laboratory^ 

 showed the non-existence of 4Ca0.3Si02 but the existence of 

 a compound of the formula 3Ca0.2Si02. It seemed probable 

 that this last formula, rather than Vogt's, represented the com- 

 position of akermanite. Consequently the new formula was 

 adopted, and a corresponding modification of the norm and its 

 calculation was proposed. ^ 



Very recently the study of the ternary system CaO-Al203-Si02 

 has been completed by Rankin and Wright,-^ who show that the 

 compound 3Ca0.2Si02 is unstable and occupies only a small 

 field, while the orthosilicate, 2CaO.Si02, exists in three stable 

 forms, depending on the temperature, and occupies a large field; 

 that is, the orthosilicate may be formed within a much wider 



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1 A. L. Day, E. S. Shepherd and F. E. Wright, Am. J. Sci. (4) 22: 280. 1906; 

 E. S. Shepherd, G. A. Rankin and F. E. W right, J. Ind. Eng. Chem., 3: 214. 1911. 



2 Cross, Iddings, Pirsson,\\ ashington, J. Geol., 20: 558. 1912. 



3 G. A. Rankin and F. E. Wright, Am. J. Sci., 39: 1. 1915. 



