HYDROTHERMAL SYNTHESIS OF PUMPELLYITE-OKHOTSKITE SERIES MINERALS

Abstract

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 Pfluid and 250-500 °C by using cold-seal pressure vessels and solid buffers of MnO2-Mn2O3, Cu2O-CuO and Cu2O-Cu buffer assemblages. Okhotskite and pumpellyite rich in the okhotskite component crystallized from an oxide mixture starting material of Ca4MgMn3+3Al2Si6O24.5-oxide+excess H2O at Pfluid of 200, 300 and 400 MPa and temperatures of 300 and 400 °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-(Mn2+) to okhotskite: 31-36 SiO2, 13-21 Al2O3, 12-25 total Mn2O3, 0.6-4 MgO and 20-24 wt.% CaO. Reconnaissance experiments using a starting material of synthetic Ca2Mn3+Al2Si3O12(OH)-piemontite at 300 MPa and temperatures of 250, 300, 400 and 500 °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 SiO2, 18-27 Al2O3, 8-18 total Mn2O3 and 20-23 wt.% 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 fO2 conditions. The maximum stability temperature of Mn-rich pumpellyite lies between 400 and 500 °C at 200-400 MPa in high fO2 conditions.