Parasibirskite, a New Mineral from Fuka, Okayama Prefecture, Japan

I. Kusachi, Y. Takechi, C. Henmi and S. Kobayashi
Department of Earth Sciences, Faculty of Education, Okayama University, Okayama, 700-8530, Japan
Department of Earth Sciences, Faculty of Science, Okayama University, Okayama, 700-8530, Japan
Division of Earth Sciences, Kurashiki University of Science and the Arts, Kurashiki, 712-8505, Japan

Abstract: Parasibirskite, with the ideal formula Ca2B2O5·H2O, is a new mineral species found at Fuka, Okayama Prefecture, Japan. It is a polymorph of sibirskite, CaHBO3, and occurs as subparallel aggregates of tabular crystals up to 40 × 20 × 3 µm in size. Associated minerals are takedaite, olshanskyite, sibirskite, frolovite and calcite. The mineral is white, and has a weak pearly luster. Optically, the mineral is biaxial positive, α 1.556(2), β 1.593(2), γ 1.663(2) (λ, 589 nm). The Vickers microhardness of aggregates is 121 kg mm−2. The mineral is monoclinic with space group of P21/m, a 6.722(4), b 5.437(2), c 3.555(2) Å, β 93.00(5)°, V 129.8(2), Å3. The strongest lines in the X-ray powder pattern [d in Å (I)(hkl)] are 2.237(100)(300), 6.73(70)(100), 2.975(60)(011), 3.354(30)(200), 2.855(20)(210) and 1.776(20) (002). Wet chemical analysis, electron-microprobe analysis and ICP emission spectrometry give the values CaO 56.06 %, B2O3 34.10 %, H2O 9.97 % and total 100.13%. The empirical formula calculated on the basis of O = 6 is Ca1.985B1.945O4.901·1.099H2O, for Z = 1, Dcalc 2.54 and Dmeas 2.50(1) g cm−3. Parasibirskite is formed by hydrothermal alteration of takedaite.

Keywords: parasibirskite • sibirskite • gehlenite-spurrite skarns • borate • Fuka • Japan

Mineralogical Magazine; August 1998 v. 62; no. 4; p. 521-525; DOI: 10.1180/002646198547891
© 1998, The Mineralogical Society
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