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Betafite
Betafite is a mineral group in the pyrochlore supergroup, with the chemical formula (Ca,U)2(Ti,Nb,Ta)2O6(OH). Betafite typically occurs as a primary mineral in granite pegmatites, rarely in carbonatites. Defined by the B-site atom Ti, Atencio et al.(2010) combined and considered the ideas portrayed in (Hatert and Burke)(2008) and a modernization of (Hogarth)(1977) system for nomenclature of pyrochlore and betafite in order to further rationalize the naming process of this grouping of minerals. Therefore, Atencio et al. (2010), states that only two of the mineral species that were formerly recognized under the previous nomenclature system of betafite in Hogarth (1977) are now recognized. They are ''oxyuranobetafite'' and ''oxycalciobetafite''. Now the term betafite is a synonym or varietal group name under the pyrochlore super group (Christy and Atencio 2013).
==Structure==
The pyrochlore supergroup minerals conform to the general formula, A2-m,B2,X6-w,Y1-n, where the m, w, and n variables represent the parameters that indicate incomplete occupancy of the A, Y, and X sites(Atencio et al. 2010). They crystallize isometric system with a space group of Fd3̅m or its subgroups where Betafite has a hexoctahedral class (Hogarth 1977). Site A is generally an 8-coordinated cation with a ~ 1.0 Å radius, B site is generally a 6-coordinated cation, which contains the elements Ti, Nb, and Ta usually for betafite, (Atencio et al. 2010, site X is generally O but can subjugate to OH and F, and site Y is typically an anion but can also be a vacancy, H2O, or a very large monovalent cation, like Cs, K, and Rb.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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