Composition of Staurolite. 



83 



never been previously analyzed. The material from Sugar 

 Hill in Lisbon, N. H., was collected in the summer of 1893 by 

 Prof. G. J. Brush. As observed by him extensive ledges of 

 gray, staurolitic mica schist occur, extending several miles 

 north from Pearl Lake, better known as Mink Pond, and in- 

 cluding the ledges on Garnet Hill and Cowen Hill. In the 

 ledges on Cowen Hill unusually large and fresh crystals are 

 found, measuring up to 115 mm long by 40 mm broad. Thin sec- 

 tions of these crystals revealed the fact that they are remark- 

 ably free from inclusions of quartz and garnet, which are so 

 common in staurolite, but they contain carbonaceous material 

 arranged in certain definite planes, as described later. The 

 staurolite from near Burnsville was collected by the writers in 

 the summer of 1892, while engaged in work for the North 

 Carolina Geological Survey. It was found at and near a 

 prospect pit on the property of Mr. D. M. Hampton, which 

 had been dug in exploiting for iron ore. The associated mine- 

 rals are magnetite, menaccanite and corundum. The staurolite 

 occurs in crystalline aggregates, often intimately associated 

 with the iron ores. 



In the preparation of material for analysis the carefully 

 selected crystals were pulverized and sifted to a uniform grain. 

 In the case of the North Carolina mineral the magnetite and 

 menaccanite were removed by means of an electro-magnet. 

 In order to separate a powder of uniform specific gravity the 

 use of fused silver nitrate, which can be diluted with potassium 

 nitrate was resorted to, as recommended by J. "W. Retgers.* It 

 was found convenient to use a double , 



walled, cylindrical, copper air bath, 

 shown in section in the accompanying 

 figure. The outer cylinder a stands on 

 legs which are not represented. The 

 inner bath is supported by brackets, b, 

 and is provided with several perforated 

 discs near the bottom, which serve to 

 disseminate the heat of the lamp. The 

 well A holds a test tube containing the 

 silver nitrate, which can readily be kept 

 in a state of fusion and at a constant 

 temperature for any desired length of 

 time ; this latter condition being very 

 essential in order to avoid circulating 

 currents. The fusing point of silver 

 nitrate is 198° C. but the temperature 

 which was found most convenient for 

 work was about 250° C. The specific gravity of fused AgNO s 



* Jahrb. f. Min., 1889, ii, p. 190. 



k 



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