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Omongwaite, Na2Ca5(SO4)6.3H2O, a new mineral from recent salt lake deposits, Namibia

Published online by Cambridge University Press:  05 July 2018

F. Mees ,
F. Hatert  and
R. Rowe
F. Mees*
Affiliation:
Department of Geology and Mineralogy, Royal Museum for Central Africa, Leuvensesteenweg 13, B-3080 Tervuren, Belgium
F. Hatert
Affiliation:
Laboratory of Mineralogy, Department of Geology, University of Liége, Building B-18, B-4000 Liége, Belgium
R. Rowe
Affiliation:
Earth Sciences Section, Canadian Museum of Nature, PO Box 3443, Station D, Ottawa Ontario K1P 6P4, Canada
*
*E-mail: [email protected]
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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) A, b = 6.96(1) Å, c = 6.39(2) Å, B = 90.2(3)°, V= 537(2) Å3, and Z = 1. The six strongest lines in the X-ray powder diffraction pattern [dobs. (Å), (I/I° meas), (hkl)] are: 6.028, (40), (110); 3.484, (29), (310); 3.019, (51), (400); 3.014, (100), (220); 2.824, (34), (1̄12); and 2.820, (65), (112). 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.

Keywords

omongwaitebassanitenew mineralOmongwa panNamibia
Type
Research Article
Information
Mineralogical Magazine , Volume 72 , Issue 6 , December 2008 , pp. 1307 - 1318
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2008

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References

Autenrieth, H. (1958) Untersuchungen im Sechs-Komponenten-System K+, Na+, Mg2+, Ca2+, SO2,”, (Cl ), H2O mit Schlussfolgerungen fur die Verarbeitung der Kalisalze. Kali und Steinsalz, 2, 181–200.Google Scholar
Autenrieth, H. and Braune, G. (1959) Das Sechs-Komponenten-System K+, Na+, Mg2+, Ca2+, SO2,”, (Cl ), H2O bei 90° und seine Anwendung auf Schlammprobleme der Kalisalzverarbeitung. Kali und Steinsalz, 2, 395–405.Google Scholar
Bellanca, A. (1942) L'aftitalite nel sistema ternario K2SO4-Na2SO4-CaSO4 . Periodico di Mineralogia, 13,21–86.Google Scholar
Bensted, J. and Prakash, S. (1968) Investigation of the calcium sulphate-water system by infrared spectro-scopy. Nature, 219, 60–61.CrossRefGoogle Scholar
Chang, H., Huang, PJ. and Hou, S.C. (1999) Application of thermo-Raman spectroscopy to study dehydration of CaSO4.2H2O and CaSO4.0.5H2O. Materials Science and Physics, 58, 12–19.Google Scholar
Chio, C.H., Sharma, S.K. and Muenow, D.W. (2004) Micro-Raman studies of gypsum in the temperature range between 9 K and 373 K. American Mineralogist, 89, 390–395.CrossRefGoogle Scholar
Dickinson, R.G. and Dillon, R.T. (1929) The Raman spectrum of gypsum. Proceedings of the National Academy of Sciences, 15, 695–699.CrossRefGoogle ScholarPubMed
Du, H. (2000) Thermodynamic assessment of the K2SO4-Na2SO4-MgSO4-CaSO4 system. Journal of Phase Equilibria, 21, 6–18.CrossRefGoogle Scholar
Eipeltauer, E. (1956) Die Bedeutung kalorimetrischer Messungen fur die Gipserzeugung und Gipspriifung. Zement-Kalk-Gips, 9, 501–505.Google Scholar
Freyer, D., Reck, G., Bremer, M. and Voigt, W. (1999) Thermal behaviour and crystal structure of sodium-containing hemihydrates of calcium sulfate. Monatshefte fur Chemie, 130, 1179–1193.Google Scholar
Freyer, D., Ziske, S. and Voigt, W. 2002.Thermoanalytische und FT-ramanspektroskopische Untersuchungen zur Bildung und Umwandlung von CaSO4-Halbhydraten. Freiberger Forschungshefte, E3, 127–136.Google Scholar
Greenberg, J.P. and Moller, N. (1989) The prediction of mineral solubilities in natural waters: a chemical equilibrium model for the Na-K-Mg-Ca-Cl-SO4-H2O system to high concentration from 0 to 250°C. Geochimca et Cosmochimica Ada, 53, 2503–2518.CrossRefGoogle Scholar
Gudowius, E. and von Hodenberg, R. (1979) ‘Natriumpolyhalit', eine dem Bassanit und dem y-CaSO4 verwandte Phase. Kali und Steinsalz, 7, 501–504.Google Scholar
Harvie, C.E. and Weare, J.H. (1980) The prediction of mineral solubilities in natural waters: the Na-K-Mg-Ca-Cl-SO4-H2O system from zero to high concentration at 25°C. Geochimica et Cosmochimica Ada, 44, 981–997.CrossRefGoogle Scholar
Hass, M. and Sutherland, G.B.B.M. (1956) The infra-red spectrum and crystal structure of gypsum. Proceedings of the Royal Society of London, 236, 427–445.Google Scholar
Heijnen, W.M.M. and Hartman, P. (1991) Structural morphology of gypsum (CaSO4.2H2O), brushite (CaHPO4.2H2O) and pharmacolite (CaHAsO4. 2H2O). Journal of Crystal Growth, 108, 290–300.CrossRefGoogle Scholar
Hill, A.E. and Wills, J.H. (1938) Ternary systems. XXIV. Calcium sulfate, sodium sulfate and water. Journal of the American Chemical Society, 60, 1647–1655.CrossRefGoogle Scholar
Kloprogge, J.T. and Frost, R.L. (2000) Raman spectro- scopy at 77 K of natural gypsum CaSO4.2H2O. Journal of Materials Science Letters, 19, 229–231.CrossRefGoogle Scholar
Kushnir, J. (1982) The partitioning of seawater cations during the transformation of gypsum to anhydrite. Geochimica et Cosmochimica Ada, 46, 433–446.CrossRefGoogle Scholar
Lager, G.A., Armbruster, T., Rotella, F.J., Jorgensen, J.D. and Hinks, D.G. (1984) A crystallographic study of the low-temperature dehydration products of gypsum, CaSO4.2H2O: hemihydrate CaSO4. 0.50H2O, and y-CaSO4 . American Mineralogist, 69, 910–919.Google Scholar
Lepeshkov, I.N. and Fradkina, Kh.B. (1959) Salt solubilities in the CaSO4-Na2SO4-NaCl-H2O system at 35° and 55°. Russian Journal of Inorganic Chemistry, 4, 1297–1301.Google Scholar
Mees, F. (1999) Distribution patterns of gypsum and kalistrontite in a dry lake basin of the southwestern Kalahari (Omongwa pan, Namibia). Earth Surface Processes and Landforms, 24, 731–744.3.0.CO;2-0>CrossRefGoogle Scholar
Pouchou, J.L and Pichoir, F. (1988) A simplified version of the ‘PAP’ model for matrix corrections in EPMA. Pp. 315–318 in: Microbeam Analysis (Newbury, D.E., editor). San Francisco Press, San Francisco, USA.Google Scholar
Powell, D.A. (1962) Calcium sulphate hemihydrate prepared in sodium chloride solution. Australian Journal of Chemistry, 15, 868–874.Google Scholar
Prasad, P.S.R. (1999) Raman intensities near gypsum- bassanite transition in natural gypsum. Journal of Raman Spectroscopy, 30, 693–696.3.0.CO;2-8>CrossRefGoogle Scholar
Prasad, P.S.R., Ravikumar, N., Krishnamurthy, A.S.R. and Sarma, L.P. (1998) Role of impurities in gypsum-bassanite phase transition: a comparative Raman study. Current Science, 75, 1410–1414.Google Scholar
Prasad, P.S.R., Chaitanya, V.K., Prasad, K.S. and Rao, D.N. (2005) Direct formation of the y-CaSO4 phase in dehydration process of gypsum: in situ FTIR study. American Mineralogist, 90, 672–678.CrossRefGoogle Scholar
Putnis, A., Winkler, B. and Fernandez-Diaz, L. (1990) In situ IR spectroscopic and thermogravimetric study of the dehydration of gypsum. Mineralogical Magazine, 54, 123–128.CrossRefGoogle Scholar
Rassonskaya, I.S. and Semendyaeva, N.K. (1961) Phase transformations of calcium and sodium sulphates and their double salts. Russian Journal of Inorganic Chemistry, 6, 891–895.Google Scholar
Reisdorf, K. and Abriel, W. (1987) Reactionen im System CaSO4-H2O und iiber Na-Polyhalite Na2Ca5(SO4)6(H2O)3 . Neues Jahrbuch fur Mineralogie Abhandlungen, 157, 35–46.Google Scholar
Rogozovskaya, M.Z., Kononchuk, T.I. and Lukjanova, N.K. (1980) Phase transformations of gypsum and sodium pentasalt 5CaSO4.Na2SO4.3H2O in concentrated sodium chloride solutions. Russian Journal of Inorganic Chemistry, 25, 608–612.Google Scholar
Rogozovskaya, M.Z. and Kononchuk, T.I. (1981) Crystallization of gypsum and of sodium pentasalt during desulfation of sodium chloride solutions. Journal of Applied Chemistry of the U.S.S.R., 54, 1460–1463.\Google Scholar
Rowe, J.J., Morey, G.W. and Zen, C.S. (1972) The quinary reciprocal salt system Na,K,Mg,Ca/Cl,SO4— A review of the literature with new data. U.S. Geological Survey Professional Paper, 741, 37 pp.Google Scholar
Sarma, L.P., Prasad, P.S.R. and Ravikumar, N. (1998) Raman spectroscopic study of phase transitions in natural gypsum. Journal of Raman Spectroscopy, 29, 851–856.3.0.CO;2-S>CrossRefGoogle Scholar
Viljoen, JJ. and Kamupingene, T.K (1983) Otjiherero. Gamsberg Publishers, Windhoek, Namibia.Google Scholar
Weijnen, M.P.C., van Rosmalen, G.M., Bennema, P. and Rijpkema, JJ.M. (1987) The adsorption of additives at the gypsum crystal surface: A theoretical approach. I. Determination of the interfacial bond energies. Journal of Crystal Growth, 82, 509–527.CrossRefGoogle Scholar