Abstract

Omongwaite, Na2Ca5(SO4)6.3H2O, is a new mineral, found as inclusions in gypsum crystals in recent salt lake deposits at Omongwa pan, Namibia. It is monoclinic, with space group C2, a = 12.08(3) Å, b = 6.96(1) Å, c = 6.39(2) Å, ß = 90.2(3)°, V = 537(2) Å3, and Z = 1. The six strongest lines in the X-ray powder diffraction pattern [dobs.3 (Å), (I/Io meas.), (hkl)] are: 6.028, (40), (110); 3.484, (29), (310); 3.019, (51), (400); 3.014, (100), (220); 2.824, (34), (112); and 2.820, (65), (

\({\bar{1}}\)
12). Electron microprobe analysis, recalculated on the basis of 3H2O per formula unit (p.f.u.), gave 56.16 wt.% SO3, 30.82 wt.% CaO, 5.25 wt.% Na2O, 3.21 wt.% K2O, 6.25 wt.% H2O, totalling 101.69 wt.%. The empirical formula, based on 24 anhydrous oxygens p.f.u., is (Na1.47K0.59)Σ=2.06Ca4.76S6.07O24.3H2O, yielding Na2Ca5(-SO4)6.3H2O as the end-member formula. Na/K ratios are variable, with an average of ∼2.5. The crystals are elongated, with pseudohexagonal transversal cross-sections and with sphenoidal terminations that are commonly developed at one end. The crystal structure of omongwaite is similar to that of bassanite, CaSO4.0.5H2O. Published studies of the synthetic phase show that it can be described as a bassanite structure in which one out of six Ca2+ ions are replaced by Na+ and a second Na+ ion occupies a position near those sites. The crystals are parallel to the [001] axis of the gypsum crystals in which they occur as inclusions. The mineral formed by topotactic replacement during interaction of gypsum with concentrated solutions. It is preserved where the affected surface became covered by gypsum by rapid growth shortly after the formation of omongwaite. The mineral is named after the locality where it was found.

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