W. H. Melville — Powellite : Calcium Molyhdate. 139 



Crystallized dark brown garnet is found in considerable 

 abundance throughout this locality. The crystals exhibit the 

 usual combination of rhombic dodecahedron and tetragonal 

 trisoctahedron. 



That which proved to be the most important constituent of 

 the specimen, about 1*5 grams, somewhat resembled scheelite at 

 first sight, but a careful study of its characters excluded that 

 mineral species from consideration. The strong reactions for 

 molybdenum suggested a new species. The mineral was well 

 crystallized and easily detached in almost absolutely pure con- 

 dition from its friable matrix. Angular measurements were 

 obtained on a number of crystals, from which the crystallo- 

 graphic elements were calculated. The 

 fundamental angle (111) a (111) was 

 chosen because of its great accuracy, 

 the signals on the goniometer being per- 

 fectly reflected from these planes. Other 

 angles were read oftentimes between 

 reflected signal and reflected light, and 

 again between merely reflected light 

 from the crystal faces. The best crystals 

 were about 0*04 (l mm ) long, and others 

 attained the maximum length of 010 

 inch. It was found that the crystals belonged to the pyra- 

 midal (tetragonal) system of crystallization, and were closely 

 allied in habit and development to scheelite. In the following 

 table of measurements this analogy is shown. 





Powellite. 





■ Scheelite.* 



a: c 



1 : 1-5445 





1: 1-5369 



Between normals. 



Observed. 



Calculated. 





111a 111 



49° 12' 



fundamental 



49° 27' 



m^ooi 



65° 24' 



65° 24' 



65° 16' 



IIIa'UI 



79° 561' 



80° V 



79° 56' 



101 ^ 101 



65° 55' 



65° 51' 



66° 6' 



IOIaIII 



40° IV 



40° 1' 



39° 58' 



From this comparison of angles and axial ratio it is evident 

 that sharp and accurate observations must be obtained in order 

 to distinguish by crystallographic means alone between these 

 two species. Many crystals were examined and many trials 

 were necessary before any difference in these angles from those 

 of scheelite could be made out. 



The following forms were observed : { 001 ( a a : a a : c 



{111} a : a : c 

 { 101 } a : oca: c 

 {110} a: a: cue 



Small rudimentary planes appear on some crystals at the 

 lower portion of the combination edges (111) (101), thus sug- 



* Dana's System of Mineralogy, 1883, p. 605. 



