Ferri-annite, a new, naturally occurring mica with a representative composition of K1.82(Mg1.83Fe4.232+)(Fe1.693+Al0.28Si6.02)O20(OH)4, probably expressed by the quadrilateral components annite, phlogopite, ferri-annite (K2Fe62+Fe23+Si6O20(OH)4), and ferri-phlogopite (K2Mg6Fe23+Si6O20(OH)4), is described from the Wittenoom area, Western Australia. The mineral occurs as flaky to tabular grains or as massive aggregates of fine acicular grains near or within a riebeckite-rich zone of banded iron-formation, the Dales Gorge Member of the Hamersley Group. It coexists with hematite, magnetite, quartz, ankerite, stilpnome-lane, and riebeckite. The ferri-annite can be chemically subdivided into two groups, A and B. The group A variety contains 4 to 7 wt.% Al2O3 and has light reddish brown (=X) to pale yellow green (=Y,Z) pleochroism, and the B variety, with the lowest Al2O3 content (1 to 2 wt.%), has brownish red (=X) to pale greenish brown (=Y,Z) pleochroic colors. The latter variety generally contains about 10 wt.% more Fe than the former. (001) cleavage is perfect. Twinning is frequent. Cell parameters (a, b, c, β, V) of the mica (group B) were calculated as 5.402(6), 9.237(4), 10.306(7)Å, 99°16′(10), and 507.54(67)Å3 using X-ray powder diffraction analysis. Formation of much of the ferri-annite appears to be the result of potassium enrichment in stilpnomelane concomittant with the formation of associated riebeckite.

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