Lipuite, a new manganese phyllosilicate mineral from the N'Chwaning III mine, Kalahari Manganese Fields, South Africa

Abstract

AbstractA new phyllosilicate mineral, lipuite (IMA2014-085), has been discovered from the N'Chwaning III mine, Kalahari Manganese Fields, Northern Cape Province, Republic of South Africa. It occurs as platy, tabular, or granular crystals and veined agglomerate in association with Mn-bearing sugilite, taniajacoite, pectolite, richterite, norrishite and namansilite. Lipuite is dark red–brown with vitreous lustre, red streak, an estimated Mohs hardness of 5 and the measured density is 2.83(3) g/cm3. It is biaxial (+) and characterised by bright red to dark red colour in thin section with measured refractive indices in white light: α = 1.635(1), β = 1.653(1), γ = 1.670(1) and 2V = 86(2)°. The Raman spectra of lipuite are composed of over 21 bands at 109, 146, 162, 183, 206, 244, 288, 342, 362, 455, 496, 520, 552, 613, 669, 886, 930, 971, 1097, 3487 and 3540 cm–1. The empirical formula from microprobe analyses is (based on total number of cations = 27.5 and structural refinement): K1.12Na8.16(Mn4.77Fe0.07)Σ4.84Mg0.44[Si11.97O30(OH)4](PO4)0.94O2(OH)2·4H2O. The idealised formula is: KNa8Mn3+5Mg0.5[Si12O30(OH)4](PO4)O2(OH)2·4H2O.Lipuite is orthorhombic, space groupPnnm,a= 9.080(3),b= 12.222(3),c= 17.093(5) Å,V= 1897.0(9) Å3andZ= 2. The strongest powder X-ray diffraction peaks [d, Å (I) (hkl)] are: 9.965(40)(011), 2.938(33)(310), 2.895(100)(311), 2.777(38)(224), 2.713(53)(320), 2.483(32)(126), 2.086(35)(046) and 1.534(40)(446). The crystal structure of lipuite is characterised by sheets of SiO4tetrahedra that are linked together along [010] by K+, Na+, Mn3+, Mg2+and P5+cations, as well as hydrogen bonds. These tetrahedral sheets consist of 14-membered rings of SiO4tetrahedra that zigzag along [100]. The two independent Mn3+cations are both octahedrally coordinated. They form five-membered, edge-shared octahedral clusters between the SiO4tetrahedral sheets. Lipuite represents a rather unique structure type and its silicate tetrahedral sheets can be considered a derivative of the silicate sheets in mica.