Original paper

Kobokoboite, Al6(PO4)4(OH)611H2O, a new mineral from the Kobokobo pegmatite, Democratic Republic of the Congo

Mills, Stuart J.; Birch, William D.; Kampf, Anthony R.; van Wambeke, Leopold

Abstract

Kobokoboite, ideally Al6(PO4)4(OH)611H2O, is a new mineral (IMA2009-057), from the Kobokobo pegmatite, South Kivu Province, Democratic Republic of the Congo. Kobokoboite forms 0.1 to 0.15-mm thick crusts and coatings of pearly white, glistening, platy crystals, usually on a matrix of frondelite and keckite. Other associated minerals reported include a fine-grained mixture of planerite, variscite and wavellite (“coeruleolactite”), as well as muscovite-2M2, evansite and apatite-(CaOH). Individual crystals of kobokoboite are rectangular plates or flattened blades on (001). The dominant form observed is {001}. The crystals have a pearly lustre, are translucent but transparent on edge, have a white streak and are non-fluorescent. Mohs hardness is about 2 (estimated). The fracture is irregular and the tenacity is brittle. Measured density is 2.21(3); calculated density is 2.287 gcm-3. Kobokoboite crystals are biaxial (-), with the indices of refraction β = 1.558(2) and γ = 1.562(2) measured in white light and with 2Vmeas. = 60-80°. Orientation: X ≈ c; Y and Z could not be determined. Crystals are non-pleochroic. The empirical formula (based on (O + OH + F) = 33) is: (Al5.85Fe3+0.14Ca0.03Na0.02)Σ6.04(PO4)3.97[(OH)5.80F0.34]Σ6.1410.98H2O. Kobokoboite is triclinic, space group P1 or P1, a = 7.460(1), b = 7.737(1), c = 12.385(5) Å, α = 102.79(2), β = 90.20(3), γ = 116.33(2)°, V = 620.6(3) Å3 and Z = 1. The five strongest lines in the powder X-ray diffraction pattern are [dobs in Å, (I)]: 11.990, (100); 6.868, (45); 3.552, (39); 6.006, (33); 3.081, (29). The chemistry and cell parameters of kobokoboite suggest a structural relationship with planerite and the new mineral IMA2005-025a, where the same sheet of octahedra and tetrahedra parallel to {001} are responsible for a, b and γ. The name is in reference to the type locality.

Keywords

kobokoboitenew mineralaluminium phosphateoptical propertieskobokobo pegmatitedemocratic republic of the congo