Crystal chemistry of synthetic lawsonite solid-solution series CaAl2[(OH)2/Si2O7]{middle dot}H2O-SrAl2[(OH)2/Si2O7]{middle dot}H2O and the Cmcm-P21/m phase transition

Abstract

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 P 2 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 (A), b = 0.197· x Sr + 8.787 (A), c = 0.263· x Sr + 13.130 (A), and v = 4.62· x Sr + 101.46 (cm 3 /mol) for orthorhombic lawsonite and a = 0.119· x Sr + 5.306 (A), b = 0.118· x Sr + 13.160 (A), c = 0.025· x Sr + 5.833 (A), 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 Δ v 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–P 2 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–P 2 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.