WASHINGTON: CALCIUM ORTHOSILICATE IN ROCKS 347 



of olivine (2(Mg,Fe)O.Si02), and c = molecules of calcium 

 orthosilicate (2CaO.Si02)/ we have the equations: 

 (l)2d+f + c = available silica. 



(2) d + 2f = amount of MgO and ''residual" FeO.^ 



(3) d + 2c = amount of ''residual" CaO.« 



These equations can be solved, of course, in several ways, but. 

 that which I have adopted is as follows: Subtracting (1) from 

 the sum of (2) + (3), we get the value of f + c. Substituting 

 this in (1) d is found and then from (2) and (3) f and c are found 

 successively. 



In the second case, where there is sufficient tentative wollas- 

 tonite to meet the deficiency in Si02, the total amount of silica 

 in the rock is subtracted from the sum of the silica which has been 

 allotted to leucite, nephelite, anorthite, diopside, and to tentative 

 wollastonite. The deficit thus shown is the number of molecules 

 of necessary 2CaO.Si02, and also the amount of silica to be 

 assigned for it. This requires twice as much CaO. The rest of 

 the CaO remains in wollastonite and takes an equal amount of 

 Si02, while the diopside remains unchanged. 



Two examples will be given of the calculation of norms con- 

 taining calcium orthosilicate, the figures being placed in the 

 positions which they uniformly occupy on the backs of the cards 

 on which the analyses of my collection are written (pages 348, 349). 



Regarding the mechanical procedure of calculation, it may be 

 observed that a columnar arrangement and the writing down of 

 all the constituent oxides in the minerals of fixed composition, 

 the feldspars and lenads, acmite, magnetite and ilmenite, recom- 

 mended in the original publication on the Quantitative System, 

 are time-consuming, and needless after a little practice. I have 

 also, to save time, slightly altered the order of procedure, though 

 this does not depart, in any respect, from the principles on which 

 the calculation of the norm is based. The various molecular 



'' This is represented by the sjonbol cs, and has a molecular weight of 172. 



* This is the FeO + MnO remaining after the formation of magnetite and 

 ilmenite. 



* This is the CaO remaining after the formation of anorthite and apatite (and 

 rarely perofskite). 



