Original paper

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

Dörsam, Guido; Liebscher, Axel; Wunder, Bernd; Franz, Gerhard; Gottschalk, Matthias

Abstract

Ca, Sr)-walstromite with the mean composition Ca0.43 Sr0.57 [SiO3] was synthesized at 4 GPa/600°C andX 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 Ca0.43 Sr0.57 -walstromite are = 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 Ksite1-site2D(Sr Ca) defined as (Sr/Ca)site1/(Sr/Ca)site2 of KA3-A1D-Sr Ca) = 21, K A3- A2D(Sr-Ca) = 2400, and K A1-A2D(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 Kphase1 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.

Keywords

walstromitecasio3 &ndashsrsio3 solid solutionhigh-pressure synthesiskd -mineral-fluidcrystal structure