Strontium in the collinsite structure: Rietveld refinement

The crystal structure of a strontium variety of a rare phosphate—mineral collinsite (Ca_{2 ? x}Sr_x)₂MgPO₄] · 2H₂O was solved from powder X-ray diffraction data (λ CuK_α radiation, Ni filter, 12.36° ≤ 2θ ≤ 100.00°, scan step 0.02°, exposure time per step 15 s) by the Rietveld method (R_wp = 4.15%, R_F = 1.03%, R_B = 2.46%); a = 5.8219(1) Å, b = 6.8319(2) Å, c = 5.4713(1) Å, α = 96.965(2)°, β = 108.846(2)°, γ = 107.211(2)°, sp. gr., \(\bar P1\), Z = 1, ρ_calcd = 3.12 g/cm³ (at x = 0.72). The new mineral was discovered in carbonatites from the Kovdor alkaline-ultrabasic massif. The crystallochemical data for collinsite were analyzed and compared with those for isotypic minerals of the fairfieldite group. Characteristic features of the low-temperature geochemistry of strontium were established.