326 Penfield and Sperry — Mineralogical Notes. 



7. Barium Feldspar from Blue Hill, Delaware Co., Pa. ; the 

 Cassixite of Dr. I. Lea* 



Professor F. A. Genth f was the first to show that this was 

 in any way an interesting and distinct variety by his analysis, 

 which showed that the feldspar contained nearly four per cent 

 of BaO ; his results, however, yielded no definite formula, 

 and could not be correctly interpreted without an optical 

 examination. 



Onr investigations were made on very good cleavage speci- 

 mens in the Yale University cabinet. A section parallel to the 

 base (001) shows that the mineral is a mixture, composed 

 chiefly of a monoclinic feldspar with interpositions of a plagio- 

 clase (albite) with fine striations and small extinction angle. 

 The albite appears as streaks running through the section par- 

 allel to the ortho-axis Tj / these streaks have nearly a uniform 

 width of about 0'05 mm , a varying length of usually less than 

 I™™, and taper off slightly toward the ends, which are rounded. 

 They are distributed qnite evenly through the section, with 

 usually a clear space of 0'5 mm between the streaks. In the sec- 

 tion parallel to the clinopinacoid (010), the streaks are mnch 

 more conspicuous. The plagioclase has an extinction of +18°, 

 which wonld indicate that it is nearly pure albite ; the mono- 

 clinic feldspar has an extinction of +6° like orthoclase, it also 

 shows in convergent light an obtuse bisectrix. The difference 

 in the extinction angles makes the albite streaks more con- 

 spicuous than in the basal section, the streaks being parallel to 

 the vertical axis c y they have the same width as in the basal 

 section, 0'05 mni , but usually a greater length, the longest being 

 2 mm ; they are also more tapering. The albite is therefore inter- 

 posed in thin plates parallel to the orthopinacoid, with the cleav- 

 age continuous in the two minerals. From the frequency and 

 thickness of the albite streaks it seems as though it would make 

 np not more than one-tenth of the total bulk of the feldspar. 

 In the clinopinacoid section it is also seen that the material in 

 which the albite is imbedded is not homogeneous but is streaked 

 by a series of very narrow bands parallel to the vertical axis, ef 

 these are very evenly distributed through the orthoclase and 

 show the same extinction as the albite. It is probable that 

 these microscopic interpositions, parallel to the larger ones, are 

 also albite and they give in polarized light, when one of the 

 minerals is dark, a curious vein-like, striated appearance to the 

 section. Each of the larger streaks of albite is surrounded for 

 a distance of about 0*01 mm with a zone of pure feldspar, which 

 contains none of these minute streaks of albite ; this makes the 

 contrast between the wider streaks of albite and the mono- 



* Proc. Acad. Nat. Sci. Philad., 1866, 110. f Report Min. Perm.. 1876, 224. 



