ALBERT

Description: 〈span〉〈div〉Abstract〈/div〉A new mineral species, meieranite, ideally Na〈sub〉2〈/sub〉Sr〈sub〉3〈/sub〉MgSi〈sub〉6〈/sub〉O〈sub〉17〈/sub〉, 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/cm〈sup〉3〈/sup〉, 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 〈span〉apfu〈/span〉) of Na〈sub〉1.96〈/sub〉(Sr〈sub〉2.91〈/sub〉Ba〈sub〉0.03〈/sub〉Ca〈sub〉0.03〈/sub〉Pb〈sub〉0.02〈/sub〉)〈sub〉Σ2.99〈/sub〉(Mg〈sub〉0.62〈/sub〉Mn〈sub〉0.28〈/sub〉Co〈sub〉0.07〈/sub〉Fe〈sub〉0.01〈/sub〉)〈sub〉Σ0.98〈/sub〉Si〈sub〉6.03〈/sub〉O〈sub〉17〈/sub〉, which can be simplified to Na〈sub〉2〈/sub〉Sr〈sub〉3〈/sub〉MgSi〈sub〉6〈/sub〉O〈sub〉17〈/sub〉.Meieranite is orthorhombic, with space group 〈span〉P〈/span〉2〈sub〉1〈/sub〉〈span〉nb〈/span〉 and unit-cell parameters 〈span〉a〈/span〉 7.9380(2), 〈span〉b〈/span〉 10.4923(3), 〈span〉c〈/span〉 18.2560(6) Å, and 〈span〉V〈/span〉 1520.50(8) Å〈sup〉3〈/sup〉. Its crystal structure is characterized by two kinds of layers that alternate along [010]: layers of corner-sharing SiO〈sub〉4〈/sub〉 and 〈span〉M〈/span〉〈sup〉2+〈/sup〉O〈sub〉4〈/sub〉 tetrahedra (〈span〉M〈/span〉〈sup〉2+〈/sup〉 = Mg, Mn, Co, Fe) and layers of NaO〈sub〉6〈/sub〉 and SrO〈sub〉8〈/sub〉 polyhedra. The tetrahedral layers consist of eight-, five-, and four-membered rings and are composed of [Si〈sub〉6〈/sub〉O〈sub〉17〈/sub〉] ribbons (parallel to [101]) linked together by 〈span〉M〈/span〉O〈sub〉4〈/sub〉 tetrahedra. Most remarkably, the structure of meieranite is topologically identical to that of the nordite group of minerals, which has the general chemical formula Na〈sub〉3〈/sub〉Sr〈span〉R〈/span〉〈sup〉3+〈/sup〉〈span〉M〈/span〉〈sup〉2+〈/sup〉Si〈sub〉6〈/sub〉O〈sub〉17〈/sub〉, where 〈span〉R〈/span〉 = Ce and La and 〈span〉M〈/span〉 = Zn, Fe, and Mn. Accordingly, chemically, meieranite may be obtained through the coupled substitution of 2Sr〈sup〉2+〈/sup〉 for (Na〈sup〉+〈/sup〉 + 〈span〉R〈/span〉〈sup〉3+〈/sup〉) in nordite.〈/span〉