Volborthite and tangeite from Milpillas mine (Mexico)

The Milpillas mine (Municipio de Santa  Cruz, Sonora, Mexico) benefits a porphyry copper deposit, characterized by a potent oxidation zone and enrichment zone with secondary copper sulfides. The primary ore is constituted by chalcopyrite and bornite. The oxidation lead to the formation of a complex secondary mineral assemblage, with a lot of well crystallized and rare species.

Recently, we studied some copper vanadates from the Milpillas mine. Both labelled as Vésigniéite, a barium and copper vanadate, the samples are formed by olive green to yellowish green aggregates of scales and pseudohexagonal crystals forming lamellae. Actually, the analysis revealed that the label assignation was incorrect:

volborthita milpillas

Volborthite from Milpillas mine

 

The result of analysis of this sample, with a centimeter size aggregate of scaly crystals, indicates that is one of the best Volborthite specimens we observed. The Volborthite (a copper polyvanadate) is not distinguishable de visu from Vésigniéite, thus requiring analytic confirmation in any case.

volborthita milpillas2

crystalline aggregates of Volborthite from Milpillas mine

 

2015-02-19_02-19-45 M=B R=6 S=4

Volborthite from Milpillas mine. FOV of all pictures: 1.2 cm.

 

Other mineral found was the Tangeite, also known as calciovolborthite in the literature. The Tangeite forms scaly aggregates, rosettes and occasionally aggregates of small crystals, with color between yellowish green to dark green.  The Tangeite is a copper and calcium vanadate.

tangeita milpillas

Tangeite on malachite from Milpillas mine

The results obtained suggests that the non analyzed specimens of supossed Vésignéite from Milpillas mine should be examined to clarify its identification.

 

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