powellite

The first record of powellite in the Tsumeb-related literature is by Pinch and Wilson (1977), who described straw-yellow tetragonal crystals (to 3 mm) on scorodite with tennantite. The composition and vintage of these specimens implies that they were almost certainly recovered from the second oxidation zone. Gebhard (1991) noted that gerdtremmelite was also present in the powellite paragenesis described by Pinch and Wilson (1977), although at the time of their description gerdtremmelite had not been fully described.

Powellite was named as a component of the type assemblage for gerdtremmelite (Schmetzer and Medenbach 1985), along with betpakdalite-CaCa, hematite, kaolinite, quartz, and scorodite.

Powellite forms a series with scheelite (CaWO₄), and crystals of intermediate composition (c. 60% scheelite : 40% powellite) have been determined (by the University of Arizona) on a specimen in the Feinglos Collection (Mark Feinglos, pers. comm. to M. Southwood, September, 2015). These golden-brown, pseudo-octahedral crystals, to a few mm in size, are associated with quartz and fraipontite, on a sulphide matrix. The specimen is now in the collections at Harvard University (MGMH 2022.4.7770L).

Powellite is readily confused with stolzite, scheelite or stottite, but it is rarer than the first two of these minerals while stottite belongs to a very different paragenesis.

arseniosiderite; betpakdalite-CaCa; chalcocite; fraipontite; gerdtremmelite; kaolinite; quartz; scorodite; siderite; tennantite-(Zn)