F. E. Wright — Optical Properties of Eoscoelite. 305 



Art. XXY. — The Optical Properties of Roscoelite ; by Fred. 

 Eugene Wright. 



The mineral roscoelite has long interested chemists and 

 mineralogists, because of its high vanadium content. At first 

 it was considered by J. Blake, its discoverer, to be a chromium 

 mica, but an analysis by G-enth* of the material collected by 

 Blake from the Stockslager Mine, Eldorado Co., Cal., proved 

 it to be a vanadium mica of so unusual composition that Blakef 

 gave it a special name. A second analysis of the mineral was 

 made by RoscoeJ in the same year with results similar to, but 

 not identical with, those obtained by Genth. Two decades 

 later Hillebrand§ established, by a series of careful determina- 

 tions on pure material, its chemical character; a discussion by 

 Clarke] of Hillebrand's results brought out the relation of 

 roscoelite to the other members of the mica group. 



Turner,^ Ransome,*"* Lindgrenff and others have investi- 

 gated its geological mode of occurrence not only in California, 

 but also in Colorado, Oregon and Australia. In these papers its 

 optical properties are mentioned briefly, but nowhere, so far as 

 I have been able to find, has an effort been made to ascertain 

 these properties more accurately. The object of the present 

 note is to present the results of measurements on two samples 

 of material from the Stockslager Mine, the original locality. 

 For these I am indebted to Dr. Hillebrand, and desire to 

 express, herewith, my appreciation of his kindness. This mate- 

 rial is not part of the sample which served for the chemical 

 analysis, but it is from the same mine and is similar in every 

 respect to the original. The first sample was given to Dr. 

 Hillebrand by Dr. Geo. F. Becker, to whom it had been sent 

 by Mr. Ryland J. Becker; the second sample was presented to 

 Dr. Hillebrand by the late Prof. A. H. Chester. Of these 

 samples the first is coarser-grained and more homogeneous, 

 and better adapted for optical work. 



Sample No. 1 consists of loose flakes of practically pure 

 roscoelite. Occasional inclusions of a dark red to opaque sub- 

 stance, possibly iron oxide, are scattered through the flakes, 

 but they are so intergrown that separation by heavy solutions 



* This Journal, (3), xii, 32, 1876. 

 + Ibid., p. 31. 



jProc. Eoy. Soc, xxv, 109, 1876. 

 § This Journal, (4), vii, 451, 1899. 

 || Ibid., p. 454. 

 TIbid., p. 455. 



** U. S. Geol. Surv. Bull., 262, 9, 1905. 



ft Mining and Sci. Press, lxxxii, 252, 1901. U. S. Geol. Surv., 22d Ann. 

 Rept., pt. 2, 643, 1901. 



