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Official Journal of the International Union of Speleology |
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| | << Volume 35 (2) - Year 2006 |
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| Vashegyite from Gaura cu Muscă Cave (Locvei Mountains, Romania): a new and rare phosphate occurrence |
| Bogdan Petroniu Onac - Luminiţa Zaharia - Joe Kearns - Daniel Veres. |
| Pages: from 67 to 73 |
| This study investigated the occurrence of vashegyite from a guano-rich deposit located in the Gaura cu Muscă Cave, Romania. Analytical methods used include optical microscopy, X-ray powder diffraction (XRD), scanning electron-microscopy (SEM), inductively coupled plasma-atomic emission spectrometry (ICP-AES), thermal investigations and Fourier-transform infrared (FT-IR) analyses. Vashegyite occurs as friable, chalky white, irregular nodules of up to 2.5 cm in diameter, within a 15 cm thick sequence of organic and minerogenic sediments. The chemical structural formula is: (Al10.91Fe3+0.06Na0.1Ca0.02Mg0.08)Σ=11.17[(PO4)8.78(SiO4)0.056]Σ=8.83(OH)6.17ˇ43.79H2O. Electron microscope images show vashegyite crystals to be flattened on (001). The orthorhombic lattice constants of vashegyite determined by XRD are a = 10.766(2) Ĺ, b = 15.00(4) Ĺ, c = 22.661(1) Ĺ, and V = 3660.62 Ĺ3 (Z = 4). The major weight loss, reflected in 3 endothermic peaks, was observed between 40° and 200°C, corresponding to the removal of water molecules. Vashegyite FT-IR absorption bands are comparable in position and relative intensity to other Al-phosphates.
Water percolating through guano becomes strongly acidic and reacts with the clay-rich sediment laid down by the underground stream to form vashegyite. In the lower part of the investigated profile, crandallite and ardealite were also found.
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