Mbobomkulite

mbobomkulite

hydrombobomkulite

nickelulumite

oswaldpeetersite

Images

Formula: (Ni,Cu)Al4(NO3,SO4)2(OH)12.3H2O
Nitrate containing hydroxyl, chalcoalumite group
Crystal System: Monoclinic
Specific gravity: 2.30 measured, 2.344 calculated
Streak: Light blue
Colour: Pale sky-blue
Environments

Sedimentary
Hydrothermal environments
Cave deposits

Localities

At the type locality, the Mbobo Mkulu Cave, Mbombela, Mbombela Local Municipality, Ehlanzeni District Municipality, Mpumalanga, South Africa, the cave is developed immediately below karst residual breccia. The dolostone in which the cave has formed is chert-free and contains unusual stalactites and stalagmites of hematite and goethite attaining lengths of several metres. Allophane forms spectacular blue draperies up to 10 m in length on the walls of the final chamber; the blue colour is due to small amounts of copper and nickel. Gelatinous allophane, associated with halloysite, also forms transparent layers, 50 mm thick, on the floor. Other minerals present in the cave include opal (as a glaze coating blue allophane), alunite (as powdery pale-green nodules in the soil), aluminite (as white chalky efflorescences), jarosite (as yellow micro-crystalline coating on hematite-goethite stactites and stalacmites), atacamite (as a green staining in the soil), a potassium-rich sampleite (forming finely crystalline blue nodules in the soil), gypsum (as crystalline crusts on the walls), kalinite (forming granular "cave flowers" growing on the floor), sasaite (as white powdery nodules in the soil), and leucophosphite (as purple coatings on chert slabs).
In two places in the third chamber of the cave the new minerals mbobomkulite, hydrombobomkulite and nickelalumite occur as sky-blue, friable nodules in a matrix of powdery allophane, forming mounds on the floor.
Macroscopically mbobomkulite is a powdery, very finely grained, pale sky-blue mineral. Under the microscope it is colourless. It forms pseudohexagonal, very thin plates about 10 microns across, as well as forming rosettes (Annals of the Geological Survey of South Africa 14.2.1-10).
It is a rare product of oxidation and leaching of nickel-copper from sulphides in the cave roof, reacting with aluminum from phyllosilicates (sheet silicates) and nitrate from bat guano (HOM).

At the Jomac Mine, White Canyon Mining District, San Juan County, Utah, USA, mbobomkulite occurs in a sedimentary uranium-vanadium deposit associated with oswaldpeetersite, cuprite, antlerite, goethite, lepidocrocite, hydrombobomkulite, sklodowskite and gypsum (HOM).

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