Hydrothermal synthesis of pumpellyite–okhotskite series minerals |
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Authors: | M Akasaka Y Suzuki H Watanabe |
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Institution: | (1) Department of Geoscience, Faculty of Science and Engineering, Shimane University, Matsue, Japan, JP;(2) Department of Earth Science, Faculty of Science, Kanazawa University, Kanazawa, Japan, JP |
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Abstract: | Summary ?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 °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 P
fluid 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.
Received March 3, 2000; revised version accepted December 28, 2001 |
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