140 



WARREN. 



was taken for chemical analysis. The analyses may, therefore, be 

 expected to represent fairly the same material upon which the micro- 

 scopic measurements were made. The results of the analyses are col- 

 lected in Table II, together with two by Makinen.^° The figures for all 

 of the radicals with the exception of the alkalies are the average of 

 closely agreeing duplicates. The figures for the alkalies are in all 

 cases but one ^^ the average of three separate determinations. In 

 estimating the alkalies the perchlorate method of separating the soda 

 and potash was employed substantially as described by Gooch,^^ and 

 was found to be very satisfactory, in fact, the writer prefers it for 

 several reasons to the usual platinic chloride method. Blank determi- 

 nations for alkalies were carried out under precisely the same condi- 

 tions as the real determinations and furnished a slight correction which 

 was applied. 



The total amounts of the three feldspar molecules (Mic, Ab., An.) 

 for each sample as computed from the analytical data are given in 

 Table I. 



In order to ascertain the amount of the potassic feldspar molecule 

 actually present in solid solution in the albite member, a number of 

 attempts were made by heavy solution separations to obtain the albite 

 in a pure condition for analysis. As a matter of fact it was found 

 extremel}^ difficult to get anything like a clean separation of the two 

 feldspars in most cases. After repeated trials it was found that by 

 using acetylene tetrabromide diluted with benzene or xylene, the 

 perthite from Grafton, N. H., could be made to yield an albite fraction 

 that contained only a small amount of attached microcline. Careful 

 microscopic measurements of the amount of microcline present, indi- 

 cated that its amount could not be far from 3 to 4 per cent. As this 

 material appeared to be the best that could be obtained, it was used 

 for duplicate alkali determinations whose results are given in column 9, 

 of Table I. In using these results to determine the amount of potassic 

 feldspar in the albite an allowance was made for the presence of three 

 per cent of the microcline as such. Such a calculation can, of course, 

 be only approximate, nor is it possible to estimate the exact amount 

 of the error. It seems likely, however, that the error is no greater 

 than that introduced into the calculations from other sources, and it 

 may be noted that this error is of about the same order as that intro- 



lOLoc. cit., 66. . 



11 In this one, No. 5, one of the determinations was rejected as bemg obvi- 

 ously considerably too high for soda. 



12 Methods of Analysis, F. A. Gooch, John Wiley & Sons, (1913). 



