294 Turner — Rock-forming Biotites and Amphiboles. 



Art. XXXI. — Some Bock-forming Biotites and Amphiboles /* 

 by H. W. Turner, with analyses by W. F. Hillebrand, 

 H. N. Stokes, and William Valentine. 



The minerals referred to in this paper were separated from 

 the rocks containing them by means of the Thoulet solution. 

 The analyses were made in the laboratory of the U. 8. Geo- 

 logical Survey, the object being to furnish a basis for calculat- 

 ing the molecular composition of the rocks containing them. 

 All of the material was fresh and sensibly free from foreign 

 particles. The powder analyzed in each case was examined 

 microscopically and such impurities as were found are given. 

 Samples of minerals as well as chips of the rocks containing 

 them will be sent to any one who is investigating rock-forming 

 minerals, upon request. The specimens came from the Sierra 

 Nevada in all cases. 



Biotite. 



Biotite No. 2136 is from the biotite-granite at the base of El 

 Oapitan in Yosemite Valley. It is quite black in color, as seen 

 with a hand lens, bot under the microscope is brown. This 

 biotite-granite forms large areas in the granitic complex of the 

 Sierra Nevada, and is quite uniform in appearance and compo- 

 sition at nearly all points where I have seen it. The biotite 

 which it everywhere contains as an essential constituent, is 

 quite the same in color and degree of pleochroism in the many 

 thin sections examined, and the analysis given below may be 

 fairly assumed to represent in general the composition of the 

 biotite of this granite. Biotite-granite 2136 contains quartz> 

 plagioclase^>orthoclase^> biotite$> magnetite> titanite>apatite 

 >zircon, the last four minerals being merely accessories. The 

 aplitic granites that cut the other granites in dikes likewise 

 contain a very similar biotite in small amount. 



A microscopic examination of the material analyzed, as well as 

 the mere fact that phosphorus pentoxide is present, showed that 

 there were apatite prisms mixed with the biotite. The analysis 

 was therefore recalculated and the P 2 5 (*20) taken out together 

 with the CaO (-26) required by the P 2 5 to form apatite. The 

 H 2 below 110° C. is likewise left out in this recalculation 

 since it was no doubt adventitious. 



Biotite No. 1751 S.N. is from a quartz-monzonite collected 

 about^l klm. southwest of Blood's Station, Alpine County, in 

 the Big Trees quadrangle. The material analyzed was found, 

 on microscopic examination, to contain occasional white grains, 



* Published by permission of the Director of the U. S. Geological Survey. 



