W. T. Schaller — Notes on Powellite and Molybdite. 71 



Akt. YII. — Notes on Powellite and Molybdite • by 

 Waldemae T. Schaller.* 



1. Powellite from Llano Co., Texas. — The specimens of 

 powellite from Barringer Hill, Texas, described in this paper 

 were received through the kindness of Mr. Wm. E. Hidden 

 and represent a small lot that was found in January, 1907. 

 According to Mr. Hidden, who generously placed the material 

 at the writer's disposal, the specimens are similar to those 

 found by him in 1889 and referred to molybdite.f The rein- 

 vestigation of this mineral was undertaken as it had been sug- 

 gested that it was probably a natural occurrence of the trioxide 

 of molybdenum, the writer having shown in a previous paper;}: 

 that molybdite was a hydrous ferric molybdate. Analysis 

 showed, however, that this mineral from Barringer Hill is a 

 calcium molybdate and is referable to powellite. 



As received by the writer, the mineral is in loose pieces, 

 some over a centimeter wide, associated with, coating and often 

 entirely replacing, molybdenite. In fact, the powellite forms a 

 pseudomorph after the sulphide of molybdenum, the foliated 

 structure of the latter being frequently retained. The dirty 

 white to gray mineral, sometimes stained brown by iron oxide, 

 breaks up into small glistening scales which, when rubbed 

 between the fingers, crumble to a pearly powder which adheres 

 to the skin and resembles in appearance some varieties of fine- 

 grained talc. Under the microscope, the mineral shows double 

 refraction but no crystal outline. The individual crystal units are 

 very minute, the specimens being aggregates of very fine scales. 

 A density determination was made on about 0*8 gram of the 

 powdered sample. The pycnometer method was used and the 

 determination was carried out with care at a temperature of 25° 

 The value obtained for the sample is 4*153, which when cor- 

 rected for impurities, as described beyond, gives 4*23. Hid- 

 den! found the value 4*004, which would indicate that his 

 sample contained more impurities than the writer's. 



Analysis of this mineral gave the following results, the min- 

 eral dissolving readily in HC1. Qualitative tests failed to show 

 the presence of any tungstic oxide. The mineral is difficultly 

 fusible and gives, at the most, only a trace of water in the 

 closed tube. 



CaO 27-46 



Mo0 3 67*90 



Loss on ign. _ 2'33 



MoS .„• 1-50 



SiO„ -88 



100-07 

 * Published by permission of the Director of the U. S. Geological Survey, 

 f This Journal, xxxviii, 485, 1889. % Ibid., xxiii, 297, 1907. 



| Ibid., xxxviii, 485, 1889. 



