662 CROSS, IDDINGS, PIRSSON, WASHINGTON 



ber this may be based on optical investigation. The com- 

 position of minerals with constant or comparatively simple 

 molecules may be taken as that of the ideal molecule, as, for 

 example, in such minerals as quartz, orthoclase, albite, anorthite,. 

 leucite, nephelite, apatite, zircon, titanite, etc. 



The proportions of the chemical components reckoned as 

 oxides belonging to each of these minerals must be multiplied 

 by the percentage weight of each mineral to furnish the chem- 

 ical components of the whole rock. Thus 35 per cent, of quartz 

 = 35 per cent, of Si0 2 ; 10 per cent, of orthoclase = 6.47 per 

 cent, of SiO s , 1.83 per cent. A1 2 3 , 1.69 per cent. K 2 0. 



When the mineral has no fixed chemical molecule, as, for 

 example, olivine, in which Mg and Fe are variable, it is neces- 

 sary to consider the composition most likely to obtain for the 

 mineral in a rock of about the character of the one under inves- 

 tigation, or to observe more specifically the optical properties 

 of the mineral where these are characteristic of the chemical 

 composition. 



For the plagioclase feldspars the optical properties have 

 been elaborately investigated and are well known. It is neces- 

 sary to determine as accurately as possibly by Michel-Levy 1 

 methods, aided by Becke's 2 method, the composition of the stri- 

 ated feldspars, noting the variation in zones, and estimating 

 approximately the average composition of the crystals. The 

 ratios of Ab to An must be transposed into ratios between 

 Na 3 and CaO by halving the value of Ab. For the reason 

 that Ab stands for the formula NaAlSi 3 O g and An for the for- 

 mula CaAl 2 Si 3 8 . In the first there is only one Na, hence Ab : 

 An : : Na : Ca. When we express the composition of albite by 

 Na s O . A1 2 3 . 6Si0 2 , we are using 2Ab. And in estimating 

 Na s O from the albite combined with anorthite in a plagioclase 

 feldspar we obtain one-half as many molecules of Na s O as we 

 have Ab derived from the familiar symbol of a plagioclase^ 

 Ab n An m . 



1 Elude sur la determination des feldspaths (Paris), i re fasicule, 1894; 2 me fasi- 

 cule, 1896. 



2 Sitzungsb. Akad. Whs. Wien., Vol. CII, Part I, pp. 358-76, 1893. 



