Mottramite

Classic arborescent forms

Mottramite, lead-copper vanadate, PbCu(VO₄)(OH), takes on many habits, ranging from sharp, seed-like crystals to botryoidal masses, but among the most famous for collectors is the arborescent grass-green specimens from Tsumeb, Namibia.

If you want to call it dendritic or plumose rather than arborescent, no one is likely to put you in jail; all the words reflect a branching nature, spreading like leaves, feathers, or flowery clusters. This habit develops when mineral crystals have small but systematic irregularities or dislocations in their growth on particular crystal faces that predispose further growth in a slightly offset manner, producing elongate, somewhat twisted aggregates.

Mottramite can be many colors, ranging from almost black through brown to yellow, but it’s most typically some shade of green, and again I would say the bright grass-green examples from Tsumeb are among the most famous and popular.

Close up to show arborescent forms, with darker mottramite or descloizite at right, and white calcite on both sides. Cat. No. 559.

The specimen was probably mined in the 1960s or 1950s (I’ve had it since the early 1970s). The arborescent mottramite forms a substrate upon which white calcite crystals grew. There is a dark layer between the mottramite and calcite which I think is descloizite, the zinc analog of mottramite, PbZn(VO₄)(OH), but it may be dark mottramite.

The back side shows two cavities lined with mottramite, again with a layer of darker mottramite or descloizite. The small 2-mm cavity at left actually goes into the specimen at least 11 mm, so it would be a stalactitic form if it were not covered by the calcite crystals.

Mottramite’s name is from the type locality, Mottram St. Andrew, in Cheshire, England, south of Manchester, where ore was stockpiled, but I think most historical researchers think the specimens actually came from the Pim Hill Mine, Shrewsbury, Shropshire. The name Mottram Andrew for the place dates to at least 1414, and may relate to Old English mōt-trēum, "at the assembly trees."