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Crystals of the solid-solution series of (Ca,Sr)-lawsonite were synthesized hydrothermally at 4 GPa and 600 and 800 °C in piston-cylinder experiments. Synthesis products were analyzed with SEM, EMP, and powder-XRD. Lawsonite was observed in both the orthorhombic space group Cmcm and in the monoclinic space group P2 1 /m. It is exclusively orthorhombic at low x Sr  bulk  but monoclinic at high x Sr  bulk ; in the range x Sr  bulk  = 0.18 to 0.4 both polymorphs coexist and the data suggest a two-phase field between x Sr  ortho  ~0.1-0.2 and x Sr  mono  ~0.3-0.4 at 4 GPa/600 °C. Linear regression to the refined lattice parameters yields a = 0.017·x Sr  + 5.841 (Å), b = 0.197·x Sr  + 8.787 (Å), c = 0.263·x Sr  + 13.130 (Å), and v = 4.62·x Sr  + 101.46 (cm 3 /mol) for orthorhombic lawsonite and a = 0.119·xSr + 5.306 (Å), b = 0.118·x Sr  + 13.160 (Å), c = 0.025·x Sr  + 5.833 (Å), β = 0.38·x Sr  + 124.07 (°), and v = 3.20·x Sr  + 101.59 (cm 3 /mol) for monoclinic lawsonite. The data suggest an increasingly negative Δv ortho-mono  with increasing x Sr . In monoclinic lawsonite, structural expansion due to the incorporation of Sr is primarily accomplished by tilting and rotation within the Si 2 O 7 -group, whereas in orthorhombic lawsonite this tilting and rotation is prohibited by symmetry restrictions and expansion is mostly accomplished by an increase in lattice parameters. Combining the extrapolated Ca end-member volume for monoclinic lawsonite with published high-P data yields K 0  mono  = 137(3) GPa (K′ = 4.4). Contrary to the Ca end-member system, the Cmcm-P2 1 /m phase transition is quenchable within the Sr-bearing system. A tentative phase diagram for (Ca,Sr)-lawsonite at 600 °C indicates a narrow orthorhombic-monoclinic two-phase field that shifts significantly to lower pressure with increasing x Sr . The Cmcm-P2 1 /m phase transition in the Sr end-member system is located at ≤1 GPa at ~400 to 600 °C, 6 to 9 GPa below the transition in the Ca-system, and has a negative P-T slope.

american mineralogist

Crystal chemistry of synthetic lawsonite solid-solution series CaAl 2 [(OH) 2 /Si 2 O 7 ]·H 2 O– SrAl 2 [(OH) 2 /Si 2 O 7 ]·H 2 O and the Cmcm–P2 1 /m phase transition