Bastnasite

Rare Earth Elements (REEs), which comprise of only 17 elements from the entire periodic table, however, are the vitamins of modern industry. Although REEs occur in the upper continental crust at concentrations comparable to copper, they rarely form economically viable deposits. Today, the most significant sources are magmatic carbonatites and their associated silicate magmas, as well as A-type granites (alkali-rich, extension-related). Among the minerals, bastnäsite dominates global REE production.

Gem-quality bastnäsite, however, is an extraordinary rarity. Typically, bastnäsite forms in fine-grained aggregates intimately mixed with other REE minerals. Crystals that are both well-formed and transparent are seldom seen. One of the few localities where such specimens occur is Zagi Mountain, part of the North West Frontier Province of Pakistan. There, gemmy bastnäsite crystals are hosted within A-type granite gneisses. Formation of A-type granite and it's REE enrichment is one of the most fundamental questions in the earth science. Detailed study of Zagi Mountain is scarce, based on my knowledge and understanding of granites, two possibilities are: 1) extremely fractionation of granitic magma to concentrate REE, F, and CO2 in the melt; 2) REE-bearing granitic magma originate from partial melting of metasomatized (maybe by subducted sediment or derived fluid) mantle, which also enriched incompatible elements. Growth large bastnäsite crystals require later hydrothermal fluid. Fluids mobilize REEs from the granite and reprecipitate them in fractures or miarolitic cavities. In the case of Zagi Mountain, these fluids may have been generated during Himalayan orogenesis, which also metamorphosed the original granites into gneisses.

This complete crystal features a great size and quality compare to the most finds, which are not as gemmy. Black neddle-like crystals are tourmaline and the white fibers phase could be amphibole.

Bastnäsite, 2.5cm, Zagi Mountain, Hameed Abad Kafoor Dheri, Peshawar District, Khyber Pakhtunkhwa Province, Pakistan

Jingnan (Tom) Zhang Coll & Photo