Fukalite
Fukalite is a rare calcium silicate-carbonate mineral with the chemical formula (CO3)(OH,F)2. It was first discovered in 1977 in the Fuka mine located in Okayama Prefecture, Japan, after which the mineral is named.
Discovery and occurrence
Fukalite was first described by Chiyoko Henmi, Isao Kusachi, Akira Kawahara, and Kitinosuke Henmi in 1977. It occurs as a secondary alteration product of spurrite in high-temperature contact-metamorphic skarn deposits, specifically within spurrite gehlenite skarns.
At its type locality in Fuka, it is found closely associated with calcite and xonotlite. It has also been identified in skarn deposits in Hiroshima Prefecture, Japan, as well as the Gumeshevsk deposit in the Ural Mountains, Russia.
Physical and optical properties
Fukalite typically forms fine-grained crystalline aggregates, with individual crystals measuring up to 0.2 mm in length. Its color ranges from translucent white to light brown, featuring a vitreous (glassy) luster and a white streak.
It has a Mohs hardness scale rating of 4 (comparable to fluorite) and a measured density of 2.77 g/cm³. Optically, fukalite is biaxial with refractive indices of α = 1.595, β = 1.605, and γ = 1.626, and a 2V angle of approximately 90°.
Crystallography
Fukalite crystallizes in the orthorhombic crystal system with unit cell dimensions of a = 5.48 Å, b = 3.78 Å, and c = 23.42 Å (Z = 2). Its crystal structure consists of alternating calcium polyhedral layers and silicate-carbonate modules exhibiting structural Order-Disorder (OD) polytypism.