Vesuvianite rich in U, Th, and REE occurs in a syenite and nepheline syenite in southeastern Seward Peninsula, Alaska. The vesuvianite occurs as large (up to 1.5 cm) tabular crystals; in thin section it is brownish-yellow and commonly zoned with an isotropic (metamict) core and an anisotropic (non-metamict) rim. The chemical composition can be approximated by the formula (Ca,Na,La,Ce,Pr,Nd,U,Th)18.1(Al,Ti,Fe,Mn,Mg)12.9Si18(O,OH,F)76. UO2 content ranges from 0.39 (non-metamict) to 0.84 weight percent (metamict), and ThO2 ranges from 0.72 (non-metamict) to 2.70 weight percent. Coexisting allanite has UO2 and ThO2 contents that range from 0.29 to 0.40 and 0.77 to 1.30 weight percents, respectively. The vesuvianite probably formed by metasomatic activity prior to intrusion of subsilicic alkaline dikes.

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