THE MINERALOGRAPHY OF THE FELDSPARS 255 
the CaO in CaAl,Si,0s. The small amounts of extraneous bases. 
and water, such as Fe,0,, FeO, MgO, H.O, etc., that are very fre- 
quently present in natural specimens, have been ignored. The 
following factors were used in calculating the percentage of each 
of the three components: 
Component Formula Oxide Factor 
Rotashwees ane KAISi,Og K,0O 16.85 
Sodium....... NaAlSi,Og Na,O 11.83 
Mime sec — CaAl,Si,03 CaO 20.21 
The sum of these three components should be too per cent theo- 
retically but as a matter of fact it rarely was. The majority of 
these totals was about 96 per cent, indicating if the analyses were 
accurately made, that the specimens were only 96 per cent pure 
feldspar. The inferior quality of many of the analyses or the 
probable presence of additional components in the system were 
emphasized when the sum of the feldspar components was con- 
siderably above or below 100. All those showing a total below 85 
and above 110 have been rejected. The three feldspar components 
were then proportionately raised or lowered to 100 per cent. All 
the calculations were performed on a 20-inch slide rule; the errors 
resulting from its use being within the limits of the chemical 
analyses and within plotting range upon the triangular co-ordinate 
base used (Fig. 19). 
It is known that not all of the Na,O is necessarily in NaAl1Si,Os 
in every case. ‘This base may be present in the form of nephelite 
or carnegieite in addition to the albite. Foot and Bradley’ have 
pointed out such a possibility and say: “‘Albite sometimes occurs, 
associated with an excess of the constituents . . . . either in free 
condition, as corundum or silica, or in combination as nephelite.”’ 
When the sum of the three feldspar components, calculated from 
a reliable chemical analysis, exceeds 100 per cent then the number 
of components is probably in excess of the three assumed. This con- 
dition is far more common than is usually supposed. Washington 
t Foot and Bradley, ‘‘On Solid Solutions in Minerals, III,” ‘‘The Constant Com- 
position of Albite,” Amer. Jour. Sci. (4), XXXVI, 47. 
