Nifontovite and olshanskyite from Fuka, Okayama Prefecture, Japan

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<jats:title>Abstract</jats:title><jats:p>Nifontovite and olshanskyite, two rare hydrous calcium borate minerals, have been found in crystalline limestone near gehlenite-spurrite skarns at Fuka, Okayama Prefecture. Nifontovite occurs as aggregates of tabular crystals up to 5 cm long and 1.5 cm wide, and rarely as euhedral crystals up to 1 mm long. Olshanskyite occurs as anhedral masses, or as micro-twinned platy crystals up to 1 cm long. Wet chemical analyses give the empirical formulae Ca<jats:sub>3.052</jats:sub>B<jats:sub>5.991</jats:sub>O<jats:sub>6.038</jats:sub>(OH)<jats:sub>12</jats:sub>·1.96H<jats:sub>2</jats:sub>O and Ca<jats:sub>2.888</jats:sub>B<jats:sub>3.997</jats:sub>(OH)<jats:sub>18</jats:sub> on the basis of O = 20 for nifontovite and OH=18 for olshanskyite, respectively. The formulae are consistent with those from type localities.</jats:p><jats:p>The X-ray powder data for these minerals were determined with accuracy. The unit cell parameters of nifontovite agree closely with those published previously. X-ray studies show that olshanskyite is triclinic with the possible space group <jats:italic>P</jats:italic>1̄ or <jats:italic>P</jats:italic>1 and <jats:italic>a</jats:italic> = 9.991(5), <jats:italic>b</jats:italic> = 14.740(11), <jats:italic>c</jats:italic> = 7.975(3) Å, α = 94.53(4), β = 69.08(3), γ = 112.44(5)° and Z = 3. The density 2.19 g cm<jats:sup>−3</jats:sup> (meas.) obtained for olshanskyite agrees with the estimated ideal value 2.31 g cm<jats:sup>−3</jats:sup> (calc.). Nifontovite was formed by hydrothermal alteration of an anhydrous borate, and olshanskyite was formed by hydrothermal alteration of nifontovite and the anhydrous borate.</jats:p>

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