Abstract

Ferri-annite is a new, naturally occurring mica with a representative composition of K (sub 1.82) (Mg (sub 1.83) Fe (super 2+) (sub 4.23) )(Fe (super 3+) (sub 1.69) Al (sub 0.28) Si (sub 6.02) )O 20 (OH) 4 . Occurrence as flaky to tabular grains or as massive aggregates of fine acicular grains near or within a riebeckite-rich zone. Coexists with hematite, magnetite, quartz, ankerite, stilpnomelane, and riebeckite and can be chemically subdivided into two groups, A and B. A variety contains 4 to 7 wt.%Al 2 O 3 and has light reddish brown (= X) to pale yellow green (= Y,Z) pleochroism; the B variety, with the lowest Al 2 O 3 content (1 to 2wt.%), has brownish red (= X) to pale greenish brown (QY,Z) pleochroic colors. Twinning is frequent. Cell parameters (a,b,c,beta ,V) of (group B) were calculated as 5.402(6), 9.237(4), 10.306(7)Aa, 99 degrees 16'(10), and 507.54(67)Aa 3 using X-ray powder diffraction analysis.--Modified journal abstract.

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