Nifontovite and Olshanskyite from Fuka, Okayama Prefecture, Japan

Isao Kusachi and Chiyoko Henmi
Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700, Japan
Department of Earth Sciences, Faculty of Science, Okayama University, Okayama 700, Japan

Abstract: 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 Ca3.052B5.991O6.038(OH)12·1.96H2O and Ca2.888B3.997(OH)18 on the basis of O = 20 for nifontovite and OH=18 for olshanskyite, respectively. The formulae are consistent with those from type localities.

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 P1¯ or P1 and a = 9.991(5), b = 14.740(11), c = 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−3 (meas.) obtained for olshanskyite agrees with the estimated ideal value 2.31 g cm−3 (calc.). Nifontovite was formed by hydrothermal alteration of an anhydrous borate, and olshanskyite was formed by hydrothermal alteration of nifontovite and the anhydrous borate.

Keywords: nifontovite • olshanskyite • borate • skarn • crystalline-limestone • Fuka • Japan

Mineralogical Magazine; June 1994 v. 58; no. 391; p. 279-284; DOI: 10.1180/minmag.1994.058.391.10
© 1994, The Mineralogical Society
Mineralogical Society (