Parasibirskite, with the ideal formula Ca 2 B 2 O 5 .H 2 O, is a new mineral species found at Fuka, Okayama Prefecture, Japan. It is a polymorph of sibirskite, CaHBO 3 , and occurs as subparallel aggregates of tabular crystals up to 40X20X3 mu m 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, alpha 1.556(2), beta 1.593(2), gamma 1.663(2) (lambda 589 nm). The Vickers microhardness of aggregates is 121 kg mm (super -2) . The mineral is monoclinic with space group of P2 1 /M, a 6.722(4), b 5.437(2), c 3.555(2) Aa, beta 93.00(5) degrees , V 129.8(2) Aa 3 . The strongest lines in the X-ray powder pattern [d in Aa (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 %, B 2 O 3 34.10 %, H 2 O 9.97 % and total 100.13%. The empirical formula calculated on the basis of O = 6 is Ca (sub 1.985) B (sub 1.945) O (sub 4.901) .1.099H 2 O, for Z = 1, Dcalc 2.54 and Dmeas 2.50(1) g cm (super -3) . Parasibirskite is formed by hydrothermal alteration of takedaite.

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