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Davidite (A1-xM21O38) samples from five different geological localities contain approximately 0.2 to 9.5 wt% UO2 (0.02 to 0.65 atoms per formula unit) and <0.1 to 1.3 wt% ThO2 (<0.01 to 0.09 atoms per formula unit). Maximum amounts of other notable cations include 3.7 wt% V2O3, 4.1 wt% Cr2O3, 2.5 wt% Y2O3, 5.6 wt% La2O3, 6.0 wt% Ce2O3, 4.0 wt% MnO, 2.4 wt% ZnO, 2.7 wt% SrO, and 4.9 wt% PbO. As a result of the variation in age and Th-U content, the calculated α decay dose ranges from ~0.2 to 44 x 1016 α/mg (~0.06 to 14.5 dpa). For samples with ages of 275--295 Ma, the critical dose for amorphization based on electron diffraction is ~0.8 x 1016 α/mg. Natural davidite is commonly altered to rutile, ilmenite, titanite, and other minor phases. [ABSTRACT FROM AUTHOR]