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Scheelite


Chemistry: CaWO4
Size: 42 mm
Owner: Malcolm Southwood
Photographer: Malcolm Southwood

An open vug in massive sulphide is lined with slender prismatic quartz crystals (to 6 mm). These are in turn supplanted by well-formed crystals of silver-grey chalcocite (to 9 mm), aggregates of discoidal yellow microcrystals of siderite, and sparse bi-pyramidal crystals of dark-brown scheelite (to c. 2 mm).  Photograph 2 (FOV 15 mm) shows scheelite crystals (to 2 mm) which appear brownish black in this image. 

The scheelite has been verified by EDS, which detected the presence of molybdenum as well as tungsten, indicating an intermediate composition in the solid solution series between scheelite (the tungstate end-member) and powellite (the molybdate).  The scheelite crystals fluoresce bright greenish-yellow in short-wave UV.       

The paragenesis is identical to that described by Gebhard (1999) from what is believed to be a unique occurrence on 46 Level, in the third oxidation zone.  According to Gebhard the siderite in this assemblage is nickel-rich, although the siderite on this specimen has not been analysed.