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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).

KANONAITE FROM THE METAMORPHIC SOLE OF THE OMAN OPHIOLITE: EVIDENCE FOR HIGH-f (O2) CONDITIONS DURING RETROGRADE METAMORPHISM