Kannanite, a new mineral from Kannan Mountain, Japan

Abstract

Kannanite, a new Ca–dominant member of the ardennite series, was obtained from Kannan Mountain, Ozu, Ehime Prefecture, Japan. Kannanite occurs in fine quartz veins crossing the hematite–rich regions of an iron–manganese ore included in the metachert. These veins are pale orange in color, while the kannanite itself is brownish–orange to orange, with a thin section exhibiting weak pleochroism. The mineral was found in grain sizes ranging from several to 15 µm and has a Mohs hardness of 6 and a calculated density of 3.43 g cm⁻³. The mean refractive index obtained from the Gladstone–Dale relationship for this mineral is 1.788. The empirical formula for kannanite is
(Ca_3.60Mn²⁺_0.40)_Σ4(Al_3.00Mn³⁺_1.31Fe³⁺_0.69Mg_0.71Mn²⁺_0.19Ni_0.06Cu_0.05)_Σ6
[(V⁵⁺_0.70Si_0.16As_0.14)_Σ1O_3.84(OH)_0.16](SiO₄)₂(Si₃O₁₀)(OH)₆,
while the simplified formula is Ca₄[(Al,Mn³⁺,Fe³⁺)₅Mg] (VO₄)(SiO₄)₂(Si₃O₁₀)(OH)₆. Kannanite has an orthorhombic structure with a space group Pnmm and unit cell parameters a = 8.8802(14) Å, b = 5.9919(13) Å, c = 18.882(3) Å and V = 1004.7(3) ų. Kannanite is considered to be formed by the activities of metamorphic fluid accompanied with the Sanbagawa metamorphism.