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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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