NOTES ON MIMETITE, THAUMASITE, AND WAVELLITE. 



By Edgar T. Wherry, 



Of the Bureau of Chemistry, United States Department of Agriculture? 



The following brief papers are the results of studies made in the 

 mineral collections of the United States National Museum. 



MIMETITE FROM UTAH. 



A specimen labeled " Penfieldite, Tintic District, Utah,"' in a 

 United States Geological Survey collection, transmitted to the 

 museum in 1902 (No. 85013), was examined by Mr. E. S. Larsen 

 in the course of his optical study of all available minerals and 

 found to be quite distinct from penfieldite in its optical properties.^ 

 It has therefore been further investigated, and proves to be mimetite 

 in a rather unusual form — transparent, colorless, acicular crystals. 

 Crystals from what is evidently the same occurrence have been 

 described and figured by Farrington and Tillotson,^ but very few 

 forms were observed upon them. The crystals on the United States 

 National Museum specimen being rich in forms, this account of them 

 has seemed desirable. 



The specimen is a 5 by 5 by 8 cm. mass of siliceous rock, containing 

 numerous small cavities lined with drusy quartz, and on one face 

 several imbedded galena crystals in an advanced state of alteration. 

 The mimetite crystals occur in the cavities, being especially abundant 

 on the galena-bearing side, and are subsequent to both galena and 

 quartz. 



The thinner crystals are colorless and transparent, with an adaman- 

 tine luster; thicker ones have a faint yellowish hue and are more 

 resinous. The mean index of refraction of one of the needles, 

 measured on the goniometer by allowing sodium light to be refracted 

 through faces lying 30° apart, proved to be 2.14±:0.02. Mr. Larsen 

 found by the immersion method in selenium-sulfur mixtures (0= 

 2.14. £=2.13, both ±0.02, agreeing essentially with the results given 

 in the literature for mimetite. 



1 This paper was prepared while the writer held the position of Assistant Curator of 

 the Division of Mineralogy and Petrology in the United States National Museum. 

 =* American Mineralogist, vol. 2, 1917, p. 20. 

 ■• Field Columb. Mus. Publ. 129, Geol. Ser., vol. 3, No. 7, 1908, p. 150, pi. 50. 



Proceedings U. S. National Museum, Vol. 54— No. 2240. 



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