(Ca,Sr)-walstromite with the mean composition Ca 0.43 Sr 0.57 [SiO 3 ] was synthesized at 4 GPa/600 °C and X Sr bulk = 0.875 in presence of a 1 molar aqueous (Ca,Sr)Cl 2 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) A, b = 9.464(3) A, c = 6.7507(16) A, α = 83.22(2)°, β = 76.83(2)°, γ = 70.33(2)°, and V = 395.46(17) A 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 A, [VI] A2–O>= 2.369 A, and [VII] A3–O>= 2.660 A. 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 D (Sr – Ca) site1 – site2 defined as (Sr/Ca) site1 /(Sr/Ca) site2 of K D (Sr–Ca) A3–A1 = 21, K D (Sr–Ca) A3–A2 = 2400, and K D (Sr–Ca) A1–A2 = 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 D(Sr–Ca) phase1–phase2 = [(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.