LARSEN AND STEIGERI MINERALOGIC NOTES 7 



and is so faintly birefracting that it is easily mistaken for an 

 isotropic mineral. It is probably very finely crystalline. Its 

 index of refraction as measured by the immersion method is 

 1.625 ± 0.003. No further optical data could be determined. 

 No cleavage was observed. Aphrosiderite from Weilburg, 

 Prussia, 3 is optically positive, and has the following indices of 

 refraction and pleochroism: 



a and /3 = 1 . 612 ± 0. 003; pale olive-green. 

 y = 1.616 ± 0.003; colorless. 



Chemical analysis. The aphrosiderite gelatinizes with hydro- 

 chloric acid. Material for an analysis was carefully selected, 

 but the best sample contained a good deal of the mica slate 

 and a little pyrite and calcite. The calcite was removed by 

 dilute acetic acid and the analysis was then made on that 

 portion of the remaining sample which was soluble in HC1. 

 The results of the analysis by George Steiger are given in column 

 3 of table 1. Column 3a gives the results for the acid soluble 

 portion computed to 100 per cent. Column 3b gives the molec- 

 ular ratios and columns 4 and 5 give the analyses of related 

 chlorites for comparison. The analysis shows that the chlorite 

 is near aphrosiderite and that it has the empirical formula 

 64(Fe,Mg)O.24Al 2 O 3 .42SiO 2 .60H 2 O, which is near 5(Fe,Mg)0. 

 2Al2O3.4SiO2.5H2O and between the formulas assigned to aphro- 

 siderite and delessite. 



II. THURINGITE FROM COLORADO 



A chlorite, differing from that described above as aphrosiderite 

 chiefly in the higher content of both ferrous and ferric iron and 

 in the presence of considerable manganous oxide, occurs in large 

 amount as a primary mineral in several of the veins near Creede, 

 Colorado. The veins are of considerable size and occupy great 

 fault fissures. The filling of the vein on which are the Ridge 

 and Solomon mines in East Willow Creek consists largely of 

 this chlorite with a little quartz, fluorite, and the ore minerals, 

 galena and sphalerite. The great Amethyst vein is made up 



3 Unpublished manuscript by Esper S. Larsen. 



