Kanonaite has been discovered in a metachert pebble derived from the metamorphic sole of the Oman ophiolite in United Arab Emirates. The kanonaite rims (50 m in maximum width) manganian andalusite with a distinct chemical discontinuity, and is also found intergrown with muscovite. It contains up to 65 mol.% Mn3+AlSiO5, and the Fe analogue, Fe3+AlSiO5, typically accounts for 3–12 mol.% of the total. Petrographic features suggest that kanonaite formed by reaction between manganian andalusite and quartz, with the addition of K introduced by an aqueous fluid, according to: 6(Al0.62Mn3+0.30Fe3+0.08)AlSiO5 + 3SiO2 + 2K+ + 3H2O → 3(Mn3+0.60Al0.36Fe3+0.04)AlSiO5 + K2(Al3.64Fe3+0.36)(Si6Al2)O20(OH)4 + 2H+. The formation of kanonaite can be attributed to a retrograde hydration event under high-f(O2) buffering according to the solid–solid reactions: 14Mn2O3 + 4SiO2 = 4Mn7SiO12 + O2 and Mn2O3 + SiO2 + Al2SiO5 = 2MnAlSiO5. The formation of kanonaite suggests that the retrograde metamorphism in the sole of the Oman ophiolite involved localized conditions of high f(O2).
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مادة فرعية
The Canadian Mineralogist
