Meieranite, Na (sub 2) Sr (sub 3) MgSi (sub 6) O (sub 17) , a new mineral from the Wessels Mine, Kalahari manganese fields, South Africa

A new mineral species, meieranite, ideally Na (sub 2) Sr (sub 3) MgSi (sub 6) O (sub 17) , 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 0.5X0.5X0.4 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 (super 3) , respectively. Optically, meieranite is biaxial (-), with alpha =1.610(1), beta =1.623(1), gamma =1.630(1) (white light), 2V(meas.)=70(1) degrees , 2V(calc.)=72 degrees . 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 (sub 1.96) (Sr (sub 2.91) Ba (sub 0.03) Ca (sub 0.03) Pb (sub 0.02) ) (sub Sigma 2.99) (Mg (sub 0.62) Mn (sub 0.28) Co (sub 0.07) Fe (sub 0.01) ) (sub Sigma 0.98) Si (sub 6.03) O (sub 17) , which can be simplified to Na (sub 2) Sr (sub 3) MgSi (sub 6) O (sub 17) . Meieranite is orthorhombic, with space group P2 (sub 1) nb and unit-cell parameters a 7.9380(2), b 10.4923(3), c 18.2560(6) Aa, and V 1520.50(8) Aa (super 3) . Its crystal structure is characterized by two kinds of layers that alternate along [010]: layers of corner-sharing SiO (sub 4) and M (super 2+) O (sub 4) tetrahedra (M (super 2+) =Mg, Mn, Co, Fe) and layers of NaO (sub 6) and SrO (sub 8) polyhedra. The tetrahedral layers consist of eight-, five-, and four-membered rings and are composed of [Si (sub 6) O (sub 17) ] ribbons (parallel to [101]) linked together by MO (sub 4) 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) SrR (super 3+) M (super 2+) Si (sub 6) O (sub 17) , where R=Ce and La and M=Zn, Fe, and Mn. Accordingly, chemically, meieranite may be obtained through the coupled substitution of 2Sr (super 2+) for (Na (super +) +R (super 3+) ) in nordite.