Ferrokinoshitalite, a New Species of Brittle Mica from the Broken Hill Mine, South Africa: Structural and Mineralogical Characterization

Fenokinoshitalite, ideally BaFe2+3(Si2Al2)Ot0(OH)2, is the iron-rich analogue of kinoshitalite, and thus a new species of the brittle mica group. Samples commonly have considerable chemical substitutions, with large cations (e g., K, Na) for Ba, mediumsize cations (e.g., Mg, Mn, Ti, Al, Fe3*) in octahedral sites, and F for OH. The color is dark green, the luster is vitreous, thin grarns are translucent, the streak is green, the Mohs hardness is 3, the density is 3.69(8.) g/cm3 lmeasured.l and 3.59 g/cm3 lcalculated), and it has perfect (001) cleavage. From single-crystal determinations, ferrokinoshitalite is monoclinic, space group C2/m., with unit-cell parameters a 5.389(1), b 9.337(2), c 10.O54(2) A, B 100.53(2)'. The polytype is |M, and Zis equal to 2. The strongest five powder X-ray diffraction lines [d in A(I)] are: 2.651(100), 2.176(40),1.551(30), 1.659(25), and 1.529(25). The mean index of refraction, as determined from the Gladstone-Dale relationship, is 1.6939. The mineral is biaxial negative, and strongly pleochroic. Thermal gravimeffic analysis shows a net gain in weight of -2 4 wtTo ftom 400 to 1000"C, after a small loss in weight of -0.2Vo from 24 to 400"C. The weight gain is attributed to iron oxidation, which produces hematite. The crystal structure of ferrokinoshitalite was refined to an agreement factor R = 0.032 and wR = 0 036 using 750 unique observed reflections on a sample with a structural fomula of (Ba6 aTKj 33Na6 a) (Fe2+1 72Mg6 74l\4ns 6gFe3+6 15Ti6 17) (Si2 aaAll 56)O16(OH1 35F6 65). The two independent octahedral sites are marginally different in size (Ml:2.l2}versus M2: 2.106 A). The effect ofFe content on the structures of Ba-rich micas is discussed. The tetrahedral rotation angle, ct, is small at 3 95', owing to the relatively large size ofFe2* and Ba. Fenokinoshitalite occurs in massive Pb-Zn-Cu-Ag sulfide orebodies at the Broken Hill mine, northern Cape Province, South Africa, in a mineral assemblage containing quartz + magnetite + spessartine-rich garnet + apatite + sillimanite t ferroan gahnite + sulfides, with Mn-rich grunerite (in places, manganogrunerite), manganoan fayalite and Mn-rich pyroxferroite also present in amphibole-rich layers. On the basis of previous studies, ferrokinoshitalite is believed to have formed at or near the peak of metamorphism (at 6'70 + 20"C and 4.5 + 1.0 kbar).