350 



WASHINGTON: CALCIUM ORTHOSILICATE IN ROCKS 



the equations on page 194 of the book on the system. If there 

 i? still a deficit, all the remaining FeO and MgO are assigned to 

 olivine, and the salic Na202 and Si02 divided between albite and 

 nephehte by the equations in (a) on the same page. If there is 

 still not enough Si02 to form albite, orthoclase and leucite are 

 calculated, the salic Na20 being all assigned to nephehte; and if 

 there is not enough Si02 for orthoclase, the equations described 

 above are resorted to. 



This procedure may seem to be lengthy and comphcated, but 

 in reality, after a httle experience, knowing the mineral compo- 

 sition of the rock and the amount of Si02, it will often happen 

 that a simple inspection will lead one to adopt at once the correct 

 stage of process. Excess Si02 is present in the norm of most 

 rocks, except those with modal lenads, occurring not infrequently 

 in those with modal olivine, so that its presence can usually be 

 assumed in such rocks as diorites, andesite, and even not a few 

 basalts. But if, for example, the rock is a nephehte-syenite, 

 tinguaite or tephrite, the presence of normative nephelite may be 

 assumed as probable, and all the excess MgO and FeO assigned 

 at once to olivine. Such facts permit short cuts in the calcula- 

 tion of the norm, and any errors in the assumptions are detected 

 in the calculation. 



A table is here given for the calculation of calcium orthosihcate 

 which may be "cut out and pasted over that for akermanite in 

 the book, which it replaces. 



