End-member ferrian kanonaite: An andalusite phase with one Al fully replaced by (Mn, Fe)3+ in a quartz vein from the Ardennes

Kanonaite, with compositions plotting on the join Mn3+AlSiO5-Fe3+AlSiO5, was discovered in a late quartz vein of the Le Coreux metamorphic manganese deposit. A typical structural formula is (Mn3+3.69Fe3+0.36)Al3.95Si4.00O20, representing maximum solid solution within the system Al2SiO5-Mn2SiO5-Fe2SiO5. Refractive indices are =1.777; =1.855. The end-member compositions form the outermost, latest products in zoned crystals ranging to less manganiferous kanonaite. A crystal structure determination of a Mn-rich kanonaite confirms that about 96% of all the Mn3+ present is located in the strongly Jahn-Teller-distorted octahedron of the andalusite-type structure. Combining all relevant mineral-chemical and petrological data available on the deposit, a speculative model is presented in which kanonaite crystals with successively higher Mn3+ contents form during decreasing temperatures in the course of the anticlockwise PTt path of extensional metamorphism. Kaolinite occurring in zones within composite kanonaite porphyroblasts of adjacent phyllites is regarded here as by-product of a continuous retrograde breakdown reaction of less manganiferous kanonaite. In places, kanonaite was peripherally replaced by muscovite and Mn- and Fe-oxides. [PUBLICATION ABSTRACT]