Calcium-strontium-solid solutions of margarite, anorthite, slawsonite, calcite and strontianite were synthesized from oxide-hydroxide-fluid mixtures by way of hydrothermal experiments at 400 to 500 °C and 390 to 500 MPa. The fractionation of Ca and Sr between the coexisting phases was investigated via electron microprobe and X-ray diffraction analyses of the solids and inductively coupled plasma-optical emission spectrometry of the fluids. A complete solid solution series of margarite is indicated and the lattice parameters a, b, c, and V increase linearly with increasing Sr content. The lattice parameters of (Ca,Sr)-anorthite show a likewise linear increase with increasing Sr content. The formation of Sr-Ca slawsonite and Sr-Ca strontianite is restricted to high bulk Sr contents. The fractionation of Ca and Sr between minerals and fluid expressed as KD(Sr-Ca)mineral-fluid increases in the following sequence: calcite-fluid < anorthite-fluid ≤ margarite-fluid ≈ 1≤ strontianite-fluid < slawsonite-fluid. At low xSr (i.e., for common metamorphic bulk rock compositions), KD(Sr-Ca)calcite-fluid = 0.09, KD(Sr-Ca)anorthite-fluid = 0.51, and KD(Sr-Ca)margarite-fluid = 0.56; the Sr/Ca ratio of the fluid is therefore generally higher than that of the coexisting minerals. This suggests that during prograde metamorphism and probably continuous dehydration, the Sr/Ca ratio of the rock decreases continuously with time. But owing to the roughly equal Ca-Sr mineral-fluid fractionation for margarite and anorthite, which was also determined for zoisite/clinozoisite and lawsonite, this decrease will result in only moderate changes of whole-rock and fluid Sr-composition as long as these phases are the main Sr-bearing minerals.

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Liebscher, A.; Thiele, M.; Franz, G.; Doersam, G.; Gottschalk, M. (2009): Synthetic Sr-Ca margarite, anorthite and slawsonite solid solutions and solid-fluid Sr-Ca fractionation. European Journal of Mineralogy, 21, 2, 275-292.