Hydrothermal synthesis of pumpellyite-okhotskite series minerals

摘要

Hydrothermal experiments to synthesize pumpellyite group minerals of the pumpellyite-okhotskite series and to investigate their stability have been carried out at 200, 300 and 400 MPa P_(fluid) and 250-500 deg C by using cold-seal pressure vessels and solid buffers of MnO_2-Mn_2O_3, Cu_2O-CuO and Cu_2O-Cu buffer assemblages. Okhotskite and pumpellyite rich in the okhotskite component crystallized from an oxide mixture starting material of Ca_4MgMn~(3+)_3Al_2Si_6O_(24.5)-oxide + excess H_2O at P_(fluid) of 200, 300 and 400 MPa and temperatures of 300 and 400 deg C. However, a single phase of okhotskite was not produced, and associated piemontite, hausmannite, wollastonite, clinopyroxene, corundum, braunite-neltnerite solid solution and alleghanyite also formed. Mn-pumpellyite of the okhotskite-pumpellyite join occurs as aggregates of needle crystals, rounded grains or flaky crystals. Chemical compositions are variable and range from pumpellyite-(Mn~(2+)) to okhotskite: 31-36 SiO_2, 13-21 Al_2O_3, 12-25 total Mn_2O_3, 0.6-4 MgO and 20-24 wt. percent CaO. Reconnaissance experiments using a starting material of synthetic Ca_2Mn~(3+)Al_2Si_3O_(12)(OH)-piemontite at 300 MPa and temperatures of 250, 300, 400 and 500 deg C indicate that Mn-rich pumpellyite can crystallize from piemontite at lower temperatures than the stability field of piemontite. The Mn-rich pumpellyite was accompanied by garnet, wollastonite and alleghanyite. The chemical compositions of the Mn-pumpellyites are 32-36 SiO_2, 18-27 Al_2O_3, 8-18 total Mn_2O_3 and 20-23 wt. percent CaO. This study shows that the stability fields of piemontite, piemontite + Mn-pumpellyite, and Mn-pumpellyite range in this order with decreasing temperature under high fO_2 conditions. The maximum stability temperature of Mn-rich pumpellyite lies between 400 and 500 deg C at 200-400 MPa in high fO_2 conditions.