0. Andersen — Aventurine Feldspar. 381 



index of the feldspar glass, produced by melting the powdered 

 feldspar, was determined.* 



In the descriptions of the hematite lamellae only the features 

 most characteristic for each specimen are given. Qualities 

 like thickness, absorption colors, etc., have already been 

 described, and as they were practically the same for all varie- 

 ties they will be mentioned only exceptionally in the follow- 

 ing descriptions. 



Albite from Fisher Hill Mine, Mineville, Essex County, 

 New Ybrk.f 



The feldspar. — The cleavage pieces were rather fresh, trans- 

 parent, of a strong red color, with patches of a green substance 

 not identified. 



Extinction angle on (001) = + 3° 

 " " " (010) = +18° 



/3 = 1-535 

 Composition : Ab 92 An 8 , a comparatively pure albite. 



Polysynthetic twinning after the albite law with the one set 

 of lamellae comparatively broad and the other very narrow. 

 The twinning striation on (001) was accordingly very fine. 



The hematite lamellae. — The aventurization was rather sub- 

 dued, silky, produced by a great number of very small lamellae. 



Most of the lamellae form very narrow, linear strips of maxi- 

 mal dimensions - 3 X - 01 mm ; some form larger flakes with 

 more equal diameters (maximum - 3 mm ). The narrow lamellae 

 are generally irregularly rounded at the ends, seldom showing 

 edges that indicate six-sided outlines. The larger flakes are 

 sometimes approximately hexagonal, but more often irregularly 

 rounded or tongued. Figs. 1 and 2, PI. I show the most char- 

 acteristic shape of the lamellae. 



Orientation of the lamellce. — Thegoniometric measurements 

 are given in Table 2. _ The planes^)!' orientation were: (001), 

 (010), (112), (112),_(021), (150) ; (150). Most prominent were 

 (112), (150), and (150), all of which contained numerous lamellae. 

 After (010) there were also many Jamellae, often larger than 

 the others ; after (001), (112) and (021) there were few. 



The lamellae after (112) and (112) were generally oriented 

 with the projections of the elongated edges on (001) parallel 

 to the a-axis. Most of these lamellae were of the type of nar- 

 row strips described above. Other lamellae after (112) were 

 oriented with the projections of their elongated edges on (001) 



* This method for determining plagioclase feldspars is very convenient, 

 when a high temperature furnace is available, and probably as accurate as 

 the best of the other optical methods. See E. S. Larsen, this Journal (4), 

 xxviii, 265, 1909. 



\ Specimens from U. S. National Museum, No. 47773. 



