Davidite from the Pandora Prospect, Pima County, Arizona, crystallizes in the space group R3 with lattice parameters a = 9.190(3)A, α = 68.73(1)°. Its formula, determined from microprobe analysis, is REE0.91U0.33Ca0.20Y0.25(Sr,Th,Pb)0.09 Ti12.53Fe5.97Mg0.24Cr0.21Al0.14(Mn,V,Zr,Nb,Sc)0.23O38. In its natural state the mineral is metamict. The crystal structure was reconstituted by heating in air at 920°C for two hours. Davidite is isostructural with the crichtonite-group minerals, having a structure based on a close-packed anion lattice with a mixed stacking sequence (hhc …), and with rare-earth elements (REE), M(0), occupying a site in the anion framework. The uranium atom does not substitute for REE in M(0), but occupies an octahedral site M(l) along with yttrium and small REE. The remaining small metal atoms, mainly titanium and iron, occupy a tetrahedral site, M(2), and three other octahedral sites, M(3), M(4), and M(5). A comparison is made of the site occupancies and average metal-oxygen bond lengths for five crichtonite-group minerals—senaite, crichtonite, davidite, loveringite, and landauite. A linear correlation is observed between the unit-cell edge, arh, and the M(l)–O bond length.

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