Meieranite, Na2Sr3MgSi6O17, a New Mineral from the Wessels Mine, Kalahari Manganese Fields, South Africa

Abstract

Abstract A new mineral species, meieranite, ideally Na2Sr3MgSi6O17, has been found in the Wessels mine, Kalahari Manganese Fields, Northern Cape Province, South Africa. It occurs in isolated aggregates embedded in a matrix mainly of sugilite, along with minor aegirine and pectolite. Crystals of meieranite are up to 0.5 × 0.5 × 0.4 mm in size. No twinning is observed. The mineral is light blue to blue in transmitted and under incident lights, transparent with white streak, and has vitreous luster. It is brittle and has a Mohs hardness of 5.5; cleavage is good on {010} and no parting was observed. The measured and calculated densities are 3.41(3) and 3.410 g/cm3, respectively. Optically, meieranite is biaxial (–), with α = 1.610(1), β = 1.623(1), γ = 1.630(1) (white light), 2V (meas.) = 70(1)°, 2V (calc.) = 72°. The calculated compatibility index based on the empirical formula is –0.007 (superior). An electron microprobe analysis yields an empirical formula (based on 17 O apfu) of Na1.96(Sr2.91Ba0.03Ca0.03Pb0.02)Σ2.99(Mg0.62Mn0.28Co0.07Fe0.01)Σ0.98Si6.03O17, which can be simplified to Na2Sr3MgSi6O17. Meieranite is orthorhombic, with space group P21nb and unit-cell parameters a 7.9380(2), b 10.4923(3), c 18.2560(6) Å, and V 1520.50(8) Å3. Its crystal structure is characterized by two kinds of layers that alternate along [010]: layers of corner-sharing SiO4 and M2+O4 tetrahedra (M2+ = Mg, Mn, Co, Fe) and layers of NaO6 and SrO8 polyhedra. The tetrahedral layers consist of eight-, five-, and four-membered rings and are composed of [Si6O17] ribbons (parallel to [101]) linked together by MO4 tetrahedra. Most remarkably, the structure of meieranite is topologically identical to that of the nordite group of minerals, which has the general chemical formula Na3SrR3+M2+Si6O17, where R = Ce and La and M = Zn, Fe, and Mn. Accordingly, chemically, meieranite may be obtained through the coupled substitution of 2Sr2+ for (Na+ + R3+) in nordite.