THE NORM IN IGNEOUS ROCKS 85 



In P, therefore, we first allot for ilmenite, leucite, nephelite, 

 anorthite, and magnetite. Then, for ackermanite, 96 CaO units are 

 taken from diopside previously made, and used with 72 SiOa for 

 ackermanite in accordance with the formula 4CaO. 38102. In 

 looking up the percentage weight of ackermanite (am), in the table 

 on p. 256 the unit of calculation is 24, not 96; i. e., it is one-fourth the 

 molecular proportion of CaO in the ackermanite. The 55 units of 

 CaO left over from the original diopside assignment are allotted for 

 new diopside, and the MgO and FeO thus set free go to olivine. 



It should be noted that in this analysis after making diopside there 

 is no excess of CaO to be set aside for the making of wollastonite (cf. 

 Analysis F). In the following example (Q), 95 molecular-proportion 

 units of CaO are left over after the making of diopside for wollastonite. 



The calculation of Analysis Q differs from that of the preceding 

 example, as was noted just above, in that CaO in the tentative allot- 

 ment is found to cover the MgO and FeO for diopside and to remain 

 over after this in amount equal to 95 units. These are given to 

 wollastonite (wo), CaO. Si O^. The silica deficit is 78. By turning 

 92 of the 95 CaO of the wollastonite into ackermanite (making in such 

 a case 23 molecules of ackermanite calling for 92 CaO and 69 SiOa), 

 we can do away with a deficit of silica equal to 23. If then our silica 

 deficit were not 78 as it is in this analysis but only 23 or less, enough 

 CaO set aside for wollastonite could be converted into ackermanite in 

 this manner to do away with the silica deficit. The formula used is, 

 where 3'= the silica deficit, y=as well the number of ackermanite 

 molecules to be made. This alternative is not open to us in this 

 analysis but it is clear how such calculations are to be treated when 

 they arise. In this case, with insufficient molecules of wollastonite 

 to satisfy the deficit of SiO^ by their conversion into ackermanite, 

 the molecules of both diopside and wollastonite are to be recalculated 

 to make new diopside, new olivine, and ackermanite by the following 



equations. 



Let x^\ht molecules of new diopside 

 ■y=the molecules of ackermanite 

 s=the molecules of new olivine. 



Then 2X-\-2,y-\ — =the available SiOa 



:x;+43'=the available CaO 

 x+z=i\\t available (MgFe)O. 



