scheelite (w. chalcocite, quartz and siderite)

A shard of sulphide matrix encrusted with slender prismatic quartz crystals (to 6mm), supplanted by well-formed crystals of silver-grey chalcocite (to 9 mm), aggregates of discoidal orange-yellow siderite crystals, and sparse bi-pyramidal crystals of translucent, honey-brown scheelite (to circa 1 mm). The scheelite fluoresces green in short wavelength ultraviolet. The entire specimen is shown in Exhibit 1, while the scheelite crystals is better seen in exhibits 2 and 3. In Exhibit 3 a little snow-white, microcrystalline fraipontite (?) is visible at upper left of centre and an undetermined sulphide phase is present between the two prominent chalcocite crystals. Exhibit 4 reveals the habit and colour variation - zoning - of the siderite crystals.

Scheelite was confirmed by EDS at the University of Nevada. An almost identical specimen at Harvard, MGMH 2022.4.10385, has been studed in greater detail and the scheelite found to be Mo-rich, with a molar ratio of roughly 3:2 of tungstate to molybdate (Mark Feinglos, pers.comm. to M. Southwood, 2015). It therefore lies towards the middle of the scheelite - powellite series. Fraipontite was also confirmed on the MGMH specimen by XRD.

This specimen was purchased from Chris Amo in 2016. Amo acquired it from Isaias Casanova and it was formerly in the collections of Carl Acosta and Ben Schumer. The paragenesis has been described in detail by Gebhard (1999) as a unique find from 46 Level in the third oxidation zone.

chalcocite; fraipontite (?); quartz; siderite; undetermined