The chemical composition of uranospathite, arsenuranospathite, and associated minerals revisited: the peculiarity of fluorine incorporation in autunite group minerals
Theye, Thomas; Walenta, Kurt; Markl, Gregor
published: Jan 1, 2016
ArtNo. ESP154019301003, Price: 29.00 €
In contrast to most or all other members of the autunite group ("uranium micas"), the Al-bearing members uranospathite and arsenuranospathite can contain F. To investigate the content of fluorine and its relation with other ions, the compositions of these minerals from their respective type localities and of related minerals were analyzed with the electron microprobe. The analyses of uranospathite from Redruth, Cornwall, show that all analyzed crystals contain F, however, in variable amounts from 0.3 to 1.0 F per formula unit. The F content of the analyzed arsenuranospathite crystals is also variable, amounting from 0.2 to 1.0 F per formula unit. The average value for samples from Wittichen (Black Forest, Germany), one of the type localities, is 0.4 F per formula unit. In the second type locality, Menzenschwand (Black Forest, Germany), a higher mean value of 0.9 F per formula unit was analyzed. The incorporation of (OH) is necessary to achieve charge balance. In general, the analyses of uranospathite and arsenuranospathite show a relatively consistent Al content of 0.8 to 1.0 Al per formula unit. In contrast, the F content is stronger variable, between 0.2 and 1.0 F per formula unit. Therefore, the presented analy- ses are not compatible with an ideal coupled exchange involving Al and F as suggested for uranospathite by Locock et al. (2005), and the IMA-approved formula Al1 -x x [(UO2)(PO4)]2 (H2 O)20 + 3x F1 – 3x, 0 < x < 0.33 has therefore to be reconsidered. Also the ideal formula of arsenuranospathite proposed by Chukanov et al. (2009), Al[(UO2)(AsO4)]2 F * 20H2 O, does not conform to this mineral in general. We propose new simplified formulae which account for an independent variability of Al und of F. Chemical analyses of a related Al-rich member of the autunite group, sabugalite, from two type localities in Portugal, revealed that F is not present in significant amounts. The same is true for other analyzed autunite group minerals, even if associated with either uranospathite or arsenuranospathite. It seems that the presence of F in substantial amounts is restricted to aluminum-bearing and water-rich members of the autunite group.