Meieranite, Na₂Sr₃MgSi₆O₁₇, a new mineral from the Wessels mine, Kalahari manganese fields, South Africa

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 3 0.5 3 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 a ¼ 1.610(1), b ¼ 1.623(1), c ¼ 1.630(1) (white light), 2V (meas.) ¼ 70(1)8, 2V (calc.) ¼ 728. 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 Na_1.96(Sr_2.91Ba_0.03Ca_0.03Pb_0.02)_R2.99(Mg_0.62Mn_0.28Co_0.07Fe_0.01)_R0.98Si_6.03O₁₇, 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) ų. Its crystal structure is characterized by two kinds of layers that alternate along [010]: layers of corner-sharing SiO₄and M^2þO₄tetrahedra (M^2þ¼ Mg, Mn, Co, Fe) and layers of NaO₆and SrO₈polyhedra. The tetrahedral layers consist of eight-, five-, and four-membered rings and are composed of [Si₆O₁₇] ribbons (parallel to [101]) linked together by MO₄tetrahedra. Most remarkably, the structure of meieranite is topologically identical to that of the nordite group of minerals, which has the general chemical formula Na₃SrR^3þM^2þSi₆O₁₇, where R ¼ Ce and La and M ¼ Zn, Fe, and Mn. Accordingly, chemically, meieranite may be obtained through the coupled substitution of 2Sr^2þfor (Na^þþ R^3þ) in nordite.