346 WASHINGTON: CALCIUM ORTHOSILICATE IN ROCKS 



range of conditions than the 3:2 siHcate. It has therefore been 

 thought best by the authors of the quantitative system to 

 substitute the calcium orthosihcate molecule for that of 

 3Ca0.2Si02. This seems the more advisable, since calcium 

 orthosihcate exists in the mineral monticellite, and is analogous 

 chemically to the ferrous and magnesium orthosilicates, fayalite 

 and forsterite, while 3Ca0.2Si02 is unknown in nature . 



This change necessitates new equations for the calculation of 

 the norm, and, though these may be figured out by anyone 

 acquainted with the principles involved, it would seem to be 

 advisable to present them. It will, however, be rarely necessary 

 to use them, for among nearly six thousand rock analyses already 

 assembled for a new edition of the Collection of Rock Analyses, 

 of which the norms have been calculated, only about 80 are of 

 such low silicity and other chemical characters as to demand 

 the calculation of calcium orthosihcate. It may be remarked 

 that, judging from the cases in which I have had to calculate 

 calcium orthosihcate, the substitution here proposed changes 

 the classificatory position of very few rocks. This is to be ex- 

 pected, as it amounts only to the shifting of a very small amount 

 of silica, and affects only the relation of pyric to olic normative 

 minerals. The present opportunity also permits the illustration 

 of a simplified procedure in calculating the norm which I have 

 finally adopted as most economical of time and labor. 



It will be assumed that a preliminary study of the mol numbers 

 and successive trials have shown a deficiency in silica, that is, an 

 amount insufficient to form leucite, nephelite, anorthite, acmite, 

 diopside, and olivine, so that there must be a readjustment of 

 the silica assigned to diopside (and wollastonite), and a certain 

 amount of calcium orthosihcate must be calculated. 



The most frequent case is that in which there is no wollasto- 

 nite, or its amount is insufficient to satisfy the deficit in silica. 

 Here, after allotting Si02 to form leucite, nephelite, anorthite, and 

 possibly acmite, the amount thus used is deducted from the total 

 (molecular) amount, the residue being the "available silica." Let- 

 ting d = molecules of diopside (CaO.(Mg,re)0.2Si02),/ = molecules 



