IngentaConnect Crystal structure refinement of synthetic Ca0.43 Sr0.57 [SiO3 ]-w...

Abstract:

Ca, Sr)-walstromite with the mean composition Ca_0.43 Sr_0.57 [SiO₃] was synthesized at 4 GPa/600°C and^X ^bulk _Sr = 0.875 in presence of a 1 molar aqueous (Ca, Sr)Cl₂ fluid together with monoclinic (Ca, Sr)-lawsonite, grossular and strontianite. Intracrystalline as well as walstromite-mineral and walstromite-fluid Ca-Sr distribution was determined by analysing the product fluid with ICP-OES for Ca and Sr concentrations, solids by electron-microprobe analysis, powder XRD-analysis with Rietveld refinement and single-crystal X-ray diffraction. Based on the single-crystal X-ray diffraction data the crystal structure of (Ca, Sr)-walstromite was solved in space group P -1 with refined lattice parameters a = 6.7580(9) Å, b = 9.464(3) Å, c = 6.7507(16) Å, α = 83.22(2)°, β = 76.83(2)°, γ = 70.33(2) , and V = 395.46(17) Å ³. The data indicate that (Ca, Sr)-walstromite has slightly smaller volume and A-sites than the polymorph (Ca, Sr)-wollastonite-II over the entire compositional range. Coordination numbers and mean bond lengths for the three A-sites in Ca_0.43 Sr_0.57 -walstromite are < [VIII] A1-O> = 2.588 Å, <^[VI] A2-O = 2.369 Å, and < ^[VII]A3-O = 2.660 Å. The smaller volume suggests that (Ca, Sr)-walstromite is the high- P/low- T polymorph. The data show extreme intracrystalline fractionation of Sr within the (Ca, Sr)-walstromite solid solution series with exchange coefficients K^site1-site2_{D(Sr Ca)} defined as (Sr/Ca)^site1/(Sr/Ca)^site2 of K^A3-A1_{D-Sr Ca}) = 21, K^{A3- A2}_D(Sr-Ca) = 2400, and K A1-A2_{D(Sr Ca)} = 114 . The walstromite-mineral and walstromite-fluid Ca-Sr distribution indicates notable fractionation of Sr into the coexisting phases lawsonite, strontianite and fluid compared to (Ca, Sr)-walstromite. Grossular has only very minor amounts of Sr. Exchange coefficients K^{phase1 phase2}_(DSr-Ca) =[(Sr/Ca)^phase1]ⁿ/[(Sr/Ca)^phase2]^m with n and m being the stoichiometric coefficients of the respective exchange reaction are 0.00068 for walstromite-fluid, 0.000011 for walstromite-lawsonite and walstromite-strontianite, and 31.8 for walstromite-grossular.

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Crystal structure refinement of synthetic Ca0.43 Sr0.57 [SiO3 ]-walstromite and walstromite-fluid Ca-Sr distribution at upper-mantle conditions

Crystal structure refinement of synthetic Ca_0.43 Sr_0.57 [SiO₃ ]-walstromite and walstromite-fluid Ca-Sr distribution at upper-mantle conditions