Bunnoite, a new hydrous manganese aluminosilicate from Kamo Mountain, Kochi prefecture, Japan

Abstract

A new mineral, bunnoite, originating from Kamo Mountain in Ino, Kochi Prefecture, Japan, has been identified. Bunnoite occurs as veins and lenses in hematite-rich ferromanganese ore, is dull green in color, and forms foliated subhedral crystals up to 0.5 mm in length. Its hardness is 5½ on the Mohs scale and its calculated density is 3.63 g cm−3. The mineral is optically biaxial (+), with α = 1.709(1), β = 1.713(1), γ = 1.727(1) (white light), 2V meas = 54° and 2V calc = 57°. The empirical formula of bunnoite is (Mn2+ 5.36Mg0.27Fe2+ 0.25Fe3+ 0.11)Σ6.00(Al0.60Fe3+ 0.40)Σ1.00(Si5.89Al0.11)Σ6.00O18(OH)3, and its simplified ideal formula is written as Mn2+ 6AlSi6O18(OH)3. The mineral is triclinic P $$ \overline{1} $$ , and the unit cell parameters refined from powder X-ray diffraction data are a = 7.521(5) A, b = 10.008(8) A, c = 12.048(2) A, α = 70.46(5)°, β = 84.05(6)°, γ = 68.31(6)° and V = 793.9(9) A3. The crystal structure of bunnoite has been solved by the charge flipping method in conjunction with single-crystal X-ray diffraction data and refined to R1 = 3.3 %. Bunnoite was found to have a layered structure with alternating tetrahedral and octahedral sheets parallel to the ( $$ \overline{1} $$ 11). The silicate tetrahedra form sorosilicate [Si6O18(OH)] clusters in the tetrahedral sheets, while the octahedra share edges to form continuous strips linked by [Mn2O8] dimers in the octahedral sheets. This mineral is classified as 9.BH according to the Nickel-Strunz system and has been named in honor of the Japanese mineralogist Michiaki Bunno (b. 1942).