John E. Wolff— Chlorite from Northern Wyoming. 475 



Art. XLII. — A New Chlorite from Northern Wyoming; by 

 John E. Wolff. 



In December, 1910, the writer received from Mr. Albert F. 

 Holden of Cleveland, Ohio, a talc-like mineral which he had 

 in turn received from Mr. W. G. Swart, a mining engineer of 

 Denver, as apparently containing a large amount of alumina. 

 By correspondence with Mr. Swart it was ascertained that the 

 mineral had been brought in by prospectors who were reluc- 

 tant to disclose the actual place of the find but from whom 

 finally the following information was obtained. To quote 

 Mr. Swarfs letter: "Location — Sheridan County, Wyoming. 

 Occurrence — as nearly as I can make out from Mr. Heineman's 

 description, it occurs in a vertical vein. He describes the 

 exposed face as being ' as high as the office ceiling ' and about 

 6 to 8 feet wide. The lines of fracture run vertically in 

 the body of the mineral. This information was obtained by 

 Mr. Parmelee, who originally brought me the mineral." 



Sheridan County lies along the northern border of the state, 

 a little east of the center, and includes in its western half the 

 uplift of the Big Horn Mountains, in which the pre-Cambrian 

 crystalline rocks are exposed. In the published reports these 

 are described as mainly granite with some bands of schist, 

 hence the only deductions possible from this scanty informa- 

 tion is that the mineral occurs as a vertical schist band six to 

 eight feet wide, representing either a shear zone or an integral 

 member of a series of crystalline schists. 



Description of the Mineral. 



Macroscopic characters. — Several pieces were received, the 

 largest a slab 7 in. long, 3 wide, and \ in. thick. The material 

 is well foliated, with fine straight foliation planes like a roofing- 

 slate, has a faint silvery green color and is decidedly translucent 

 even in the thicker pieces, while thin splinters are colorless and 

 almost glassy. The specimen is homogeneous, and composed 

 of minute parallel scales of the mineral. 



Microscopic characters. — The thin sections confirm the 

 purity of the material, for with the exception of a few minute 

 prisms of zircon (?) and fluid cavities it is composed of sub- 

 parallel plates of the chlorite, sometimes interlocking with a 

 ' tendency to radial arrangement, with an average thickness 

 across the base of l/10 mm and diameter on the base of 2/10 mm . 

 No twinning is seen and no crystalline outlines, but a good 

 basal cleavage. 



Optical properties. — Dr. H. E. Merwin, of the Geophysical 

 Laboratory, Washington, has kindly made the following meas- 

 urements, entirely with the microscope : 



a = 1-580 /3 = 1-580 to 1-581 y = 1-589 



with a possible error of ±-001. 2E measured on several plates 



