Mbobomkulite, hydrombobomkulite and nickelalumite, new minerals from Mbobo Mkulu Cave, Eastern Transvaal

Three new cave minerals from the Mbobo Mkulu Cave, Eastern Transvaal, are described. The structure of these minerals, which is probably layered, is discussed and models are proposed. Mbobomkulite, NiAl4(NO3)2(OH)12.3H2O is monoclinic with parameters a0=10,171, b0=8,865, c0=17,145 A, β=95,370, Z=4, density=2,30g/cm3 and optical properties α=1,515, γ=1,585, maximum extinction α' and perfect cleavage [001]=100. Hydrombobomkulite with composition NiAl4(NO3)2(OH)12.13-14 H2O is monoclinic ( ) with parameters a0=10,145, b0=17,155, c0=20,870 A, β=90,550, Z=8( ). Nickelalumite, NiAl4SO4(OH)12.3H2O, is monoclinic with a0=10,175, b0=8,860, C0=17,174 A, β=95,950, Z=4,density 2,24g/cm3 and optical properties α=1,532, γ=1,543,α' and [001]=300. The three minerals, of supergene origin, are microcrystalline, forming powdery pale-blue nodules in the soil of the cave and coatings on gypsum crystals. Mbobomkulite and nickelalumite are isomorphous to chalcoalumite and solid solutions exist between the three minerals