Abstract
Bradleyite, a sodium phosphate-magnesium carbonate Na3Mg(PO4)(CO3), occurs in sedimentary salt rocks and igneous, carbonatitic and kimberlitic rocks. In this paper, we present the characteristics of a bradleyitefound ina new geological environment as an inclusion in a diamond from the Córigo Sorriso placer deposit in Mato Grosso State, Brazil, where other unusual mineral inclusions in diamond were earlier identified. Bradleyite is a part of a polymineral inclusion, comprising a porous aggregate of grains with size less than 150 nm, hosted within a dolomite crystal. The studied bradleyite is characterized by the highest MgO + FeO concentrations and the lowest Na content in comparison with bradleyite from other localities. It demonstrates significant variability of the composition, particularly in Na (28.75-37.84 mass %Na2O). Nitrogen by the EDS analysis was also detected. We also report for the first time the ab initio crystal structure of natural bradleyite. The studied mineral has monoclinic symmetry, with cell parameters a = 8.684 Å, b = 6.804 Å, c = 5.074 Å, β = 90.34°. The structure was solved ab initio and refined using dynamical scattering theory in space group P21/m, confirming the model obtained by powder refinement of synthetic analogues. The final structure model converged with to a formula Na3(Mg0.86Fe0.14)(PO4)(CO3), Z = 2. Bradleyite is a polygenetic mineral. In continental salt deposits, it forms under atmospheric pressure as the result of sedimentation. In deep-formed igneous rocks, such as kimberlites and carbonatites, bradleyite occurs as a product of late-stage crystallization of carbonatitic melt and as a primary-crystallized phase in deep-seated minerals, such as olivine, ilmenite, chrome spinel, magnetite and others. Our find demonstrates its stability in diamond and in diamond formation environment conditions, and may be considered as a product of crystallization from primary melt inclusion.
