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Synthesis and characterization of K₂Ca₅(SO₄)₆· H₂O, the equivalent of görgeyite, a rare evaporite mineral

¹ Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, 2 George Street, G.P.O.Box 2434, Brisbane, Qld 4001, Australia
² Analytical Electron Microscopy Facility, Faculty of Science, Queensland University of Technology, 2 George Street, GPO Box 2434, Brisbane, Qld 4001, Australia

Correspondence: ^* E-mail:t.kloprogge{at}qut.edu.au

Görgeyite, K₂Ca₅(SO₄)₆··H₂O, is a very rare monoclinic double salt found in evaporites related to the slightly more common mineral syngenite. At 1 atmosphere with increasing external temperature from 25 to 150 °C, the following succession of minerals was formed: first gypsum and K₂O, followed at 100 °C by görgeyite. Changes in concentration at 150 °C due to evaporation resulted in the formation of syngenite and finally arcanite. Under hydrothermal conditions, the succession is syngenite at 50 °C, followed by görgyeite at 100 and 150 °C. Increasing the synthesis time at 100 °C and 1 atmosphere showed that initially gypsum was formed, later being replaced by görgeyite. Finally görgeyite was replaced by syngenite, indicating that görgeyite is a metastable phase under these conditions. Under hydrothermal conditions, syngenite plus a small amount of gypsum was formed, after two days being replaced by görgeyite. No further changes were observed with increasing time. Pure görgeyite showed elongated crystals approximately 500 to 1000 µ m in length. The infrared and Raman spectra are mainly showing the vibrational modes of the sulfate groups and the crystal water (structural water). Water is characterized by OH-stretching modes at 3526 and 3577 cm^–1 , OH-bending modes at 1615 and 1647 cm^–1 , and an OH-libration mode at 876 cm^–1 . The sulfate ₁ mode is weak in the infrared but showed strong bands at 1005 and 1013 cm^–1 in the Raman spectrum. The ₂ mode also showed strong bands in the Raman spectrum at 433, 440, 457, and 480 cm^–1 . The ₃ mode is characterized by a complex set of bands in both infrared and Raman spectra around 1150 cm^–1 , whereas ₄ is found at 650 cm^–1.