Naslov
Crystal-chemical properties of meta-autunite from Baixa, Brazil

Autori
Bermanec, Vladimir ; Kniewald, Goran ; Tomašić, Nenad

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
Eleventh Slovenian-Croatian Crystallographic Meeting - Book of Abstracts / - Ljubljana : Slovenian Crystallographic Society, 2002

Skup
Eleventh Slovenian-Croatian Crystallographic Meeting

Mjesto i datum
Bohinj, Slovenija, 27.06.2002. - 30.06.2002

Vrsta sudjelovanja
Predavanje

Vrsta recenzije
Međunarodna recenzija

Ključne riječi
meta-autunite; crystal-chemical properties; thermal behaviour

Sažetak
Meta-autunite belongs to the autunite series. The degree of hydration determines the crystal structure properties of the mineral series members. The chemical composition of meta-autunite is usually expressed as Ca [(UO2)2(PO4)2]×2½-6½H2O thus being less hydrated than autunite with ten to twelve molecules of water. By the means of X-ray powder diffraction the investigated mineral sample originating from Baixa, Brazil, was identified as meta-autunite (PDF 39-1351) crystallizing in space group P4/nmm with unit cell parameters a = 6.980(1), c = 8.441(2) Å, V = 411.3(2) Å3. The chemical analysis yields formula (Ca, Ba, Sr, Na, Mg)[(UO2)1.86(P, As, Bi)O4)2.08]×6.14H2O. The thermogravimetric analysis shows total mass loss of 18.74% in the temperature range from room temperature to 1500°C. The mass loss due to the dehydration is likely to take place at lower temperatures occurring stepwise at 62.35, 107.23 and 164.84°C. The powder diffraction data obtained at these temperatures indicate phase transformations. No transformation to orthorhombic meta-autunite II (PDF 14-73) was observed at 80°C as suggested in the literature (Leo, 1960). The phases appearing at 82 and 114°C are similar to not well defined NaUO2AsO4×4H2O (PDF 13-49) and could be indexed with orthorhombic cell a = 7.253(4), b = 6.537(3), c = 7.008(4) Å, V = 332.3(3) Å3 and a = 7.092(8),b = 6.532(5), c = 6.972(8) Å, V = 323.0(5) Å3 respectively. This process is represented with decreasing of parameter a. Another two diffraction patterns obtained at 173 and 204°C have been failed to be indexed as a single phase. The diffraction pattern obtained at 1050°C suggests the presence of UO3 (PDF 45-857) as a part of the material melts. The calculated unit cell parameters for UO3 are a = 6.66(4), b = 3.98(2), c = 4.27(3) Å, b = 99.6(5)° and V = 111.6(9) Å3. The mass loss occurring above 1000°C could be probably attributed to the loss of phosphorous. The endothermic peaks of the DTA curve clearly correspond to stepwise dehydration. A wide exothermic peak could be related to the oxidation of uranium being probably partly present as U4+ or U5+. The thermal behavior of meta-autunite witnesses the high structural sensitivity to environmental conditions. References: Leo, G. W. (1960): Autunite from Mt. Spokane, Washington, American Mineralogist, 45, 99-128

Izvorni jezik
Engleski

Znanstvena područja
Geologija, Kemija

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