6 LARSEN AND STEIGER: MINERALOGIC NOTES 



color match will in general be impossible, and difficulty may 

 then arise from the Purkinje effect. 



MINERALOGY. — Miner alogic notes. 1 Esper S. Larsen and 

 George Steiger, Geological Survey. 



I. APHROSIDERITE FROM BRITISH COLUMBIA 



Introduction. A highly fossiliferous Cambrian shale or slate 

 from the Burgess shale near Field, B. C., examined by the 

 authors for Dr. Charles D. Walcott, carries numerous veinlets 

 of pale green chlorite with some associated pyrite and calcite. 

 A study of this chlorite shows that it is related to aphrosiderite. 

 The authors are indebted to Dr. Walcott for the material here 

 described. 



The slate 2 is very fine-textured, so fine that much of the 

 materia] shows aggregate polarization. It is made up largely 

 of muscovite with some kaolinite, a very little quartz, apatite, 

 and pyrite. Numerous dark brown to black streaks arranged 

 parallel to the cleavage represent carbonaceous matter. An 

 analysis of the slate, made by George Steiger, is given in column 

 1, table 1. It closely resembles the analysis of sericite from 

 Diirrberg shown in column 2. The chlorite occurs in a system 

 of rudely parallel veinlets less than a millimeter across, which 

 are normal to the slaty cleavage; the rock tends to break through 

 the centers of these veinlets, leaving surfaces lined with small 

 grains of calcite and blotches of cupriferous pyrite. 



Physical properties. The chlorite is light-green in color, it 

 has a hardness of about 1 or 2, and the powder analyzed had a 

 specific gravity, as determined by the picnometer method, of 2.959, 

 which was probably not changed by the admixed muscovite by 

 more than ±0.01. It fuses with difficulty. 



In thin section the chlorite makes up most of the veinlets, but 

 there is more or less calcite in irregular crystals in the center of 

 the veins and some pyrite. It is nearly colorless in thin section 



1 Published with the permission of the Director of the U. S. Geological Survey. 



2 Walcott, C. D., Cambrian Geology and Paleontology — II, Smithsonian Misc. 

 Coll. 57: 149-51. 1914. 



