Chemistry and radiation effects of thorite-group minerals from the Harding pegmatite, Taos County, New Mexico

Dark brown and yellow thorites, with compositions extending the range of natural solid solution for the zircon structure type, occur as 0.5- to 2-cm grains in the Harding pegmatite, Taos County, New Mexico. Low totals of the microprobe analyses probably indicate the presence of 7-12 wt% H₂O, in agreement with TGA. Both varieties of thorite are partially recrystallized. In the brown variety, the degree of crystallinity correlates with increasing substitution of P, V, and Ca for Si and Th. Dark brown thorites from the quartz-lath spodumene zone have an average composition near (Th_0.87U_0.08Pb_0.05)SiO₄, are fully metamict, and show only slight alteration. Thorites from a partially albitized area of the microcline-spodumene zone show slight to moderate alteration characterized by enrichment in P, V, As, and Ca. Calculated alpha-event doses (4-12 × 10⁷ α/mg) are greater than the damage saturation dose (~1 × 10¹⁶ α/mg), but substantial crystallinity is present. The energy barrier to recrystallization of alpha-decay-damaged thorite enriched in P and V is inferred to be lower than for ideal thorite or U-rich microlites from the microcline-spodumene zone. Thorite from quartz, quartz-lath spodumene, and microcline-spodumene zones crystallized as a primary magmatic phase close in composition to ideal thorite, but enriched in U. Partial alteration of primary thorite in the microcline-spodumene zone is related to partial replacement of this unit by cleavelandite. -from Authors