Penfield and Sperry — Mineralogical Notes. 327 



clinic feldspar all the more marked in polarized light, for if 

 the albite is dark it will be surrounded by a border of homo- 

 geneous light material, the whole being imbedded in a gray 

 streaked ground-mass which has the optical effect of an ortho- 

 clase, because the microscopic albite interpositions are subordi- 

 nate to the monoclinic feldspar. 



We hoped that at least a partial separation of the albite and 

 the monoclinic feldspar could be made by means of the heavy 

 solutions, but in this we were disappointed ; the cleavage being 

 continuous in the two minerals prevents a good mechanical 

 separation by powdering, and the difference in specific gravity 

 was too slight to yield good results. All of the powder had 

 nearly a uniform specific gravity. That which was taken for 

 analysis sank in the Thoulet solution of sp. gr. 2*642, leaving 

 about half of the powder floating. The following analysis, 

 which was made by Mr. Sperry, showed traces of Mn, Mg and 

 Sr, but they were too minute to be determined. The mean of 

 three analyses by Professor Genth is also given for comparison : 



BaK 



Genth. Sperry. Ratio. Anorthite. Albite. feldspar. 



Si0 2 62-60 62-95 1-049 -010 (2) -384 (6) -655 



A1 9 3 -- - 19-97 19-82 '192 '005 (1) '066 (1) '121 



Fe 2 8 -12 -17 



CaO -19 -25 -005^1 -005 (1) 



Na 2 0__ 4-31 4-01 -064 ! . Qft -064(1) 



BaO __ 3-71 3-95 -026 f 18b ) * 



K 2 895 8-57 -091 J \ LLi 



Ign -19 -11 



100-04 99-83 



In the above the ratio of A1 2 3 : KO=-192 : 486, almost 1 : 1. 

 Regarding the small amount of CaO as belonging to anorthite, 

 and uniting it with the albite to form a plagioclase, we can 

 divide our analysis into two parts which will probably repre- 

 sent the mixture very closely : 



Na,Ca feldspar, K.Ba feldspar, K, Ba feldspar, Ba feld- 



Albite. Monoclinic. Calculated to 100. Ratio. K 2 Al 2 Si 6 16 . spar. 



SiOa-- 23-64 Si0 2 ._ 39-31 61*12 1-019 -852(6) -167(4) 



A1 2 8 . 7-33 A1 2 3 . 12-49 19-42 .'188 -147(1) -041(1) 



CaO__ -25 BaO_. 3'95 6-14 -040 -040(1) 



Na 2 0_ 4-01 K a O._ 8-57 13-32 -142 -142(1) 



35-23 64-32 100-00 



In the 64*32 per cent of the monoclinic K, Ba feldspar the 

 ratio of SiO, : A1 2 3 : RO=5'60 : 1-03 : 1, which gives no simple 

 formula if we regard it as a homogeneous mineral. If, how- 

 ever, we regard it as composed of a potash feldspar K 2 Al 2 Si 6 16 

 and a barium feldspar, deducting the ratio of K 2 Al 2 Si 6 16 we 

 have left Si0 2 : A1 2 3 : BaO =167 : '041 : -040, almost exactly 

 4:1:1. 



