Calcium Borate Minerals in the CaO-B2O3-H2O System at Fuka, Okayama Prefecture, Japan.

AbstractData on the dehydration of pentahydroborite, CaB2O(OH)6·2H2O and nifontovite, Ca3B6O6(OH)12·2H2O from the Fuka mine, Japan are presented. Critical temperatures of the dehydration of the borates were determined by thermogravimetric analysis/differential thermal analysis measurements. The untreated mineral samples and their heating products were investigated by X-ray diffraction and Raman spectroscopy. Upon dehydration, both minerals decompose and undergo amorphization, and at greater temperatures crystallize as an orthorhombic calcium borate, CaB2O4 (Pnca). The dehydration paths of the two minerals are different, with nifontovite showing a greater resistance to decomposition and amorphization than pentahydroborite. Differences in the dehydration processes are related to the residuals of the water content and structural accommodation of the borate polyanion.

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Nifontovite and olshanskyite from Fuka, Okayama Prefecture, Japan

Mineralogical Magazine ◽

10.1180/minmag.1994.058.391.10 ◽

1994 ◽

Vol 58 (391) ◽

pp. 279-284 ◽

Cited By ~ 20

Author(s):

Isao Kusachi ◽

Chiyoko Henmi

Keyword(s):

Hydrothermal Alteration ◽

Chemical Analyses ◽

Calcium Borate ◽

Unit Cell Parameters ◽

X Ray ◽

Cell Parameters ◽

Crystalline Limestone ◽

Wet Chemical ◽

Borate Minerals ◽

Okayama Prefecture

AbstractNifontovite 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 P1̄ 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.

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Shimazakiite-4M and shimazakiite-4O, Ca2B2O5, two polytypes of a new mineral from Fuka, Okayama Prefecture, Japan

Mineralogical Magazine ◽

10.1180/minmag.2013.077.1.09 ◽

2013 ◽

Vol 77 (1) ◽

pp. 93-105 ◽

Cited By ~ 10

Author(s):

I. Kusachi ◽

S. Kobayashi ◽

Y. Takechi ◽

Y. Nakamuta ◽

T. Nagase ◽

...

Keyword(s):

Crystal Structure ◽

Diffraction Pattern ◽

Powder Diffraction ◽

Refractive Indices ◽

New Mineral ◽

Calcium Borate ◽

Crystalline Limestone ◽

Okayama Prefecture

AbstractShimazakiite occurs as greyish white aggregates up to 3 mm in diameter. Two polytypes, shimazakiite-4M and shimazakiite-4O, have been identified, the former in nanometre-sized twin lamellae and the latter in micrometre-sized lamellae. Shimazakiite was discovered in an irregular vein in crystalline limestone near gehlenite-spurrite skarns at Fuka mine, Okayama Prefecture, Japan. Associated minerals include takedaite, sibirskite, olshanskyite, parasibirskite, nifontovite, calcite and an uncharacterized hydrous calcium borate. The mineral is biaxial (–), with the following refractive indices (at 589 nm): α = 1.586(2), β = 1.650(2), γ = 1.667(2) and 2Vcalc = 53º [shimazakiite-4M]; and α = 1.584(2), β = 1.648(2), γ = 1.670(2) and 2Vcalc = 54.88º [shimazakiite-4O]. Quantitative electronmicroprobe analyses (means of 28 and 25 determinations) gave the empirical formulae Ca2B1.92O4.76(OH)0.24 and Ca2B1.92O4.76(OH)0.24 for shimazakiite-4M and shimazakiite-4O, respectively. The crystal structure refinements: P21/c, a = 3.5485(12), b = 6.352(2), c = 19.254(6) Å , β = 92.393(13)°, V = 433.6(3) Å3 [for shimazakiite-4M]; and P212121, a = 3.55645(8), b = 6.35194(15), c = 19.2534(5) Å , V = 434.941(18) Å3[for shimazakiite-4O], converged into R1 indices of 0.1273 and 0.0142, respectively. The crystal structure of shimazakiite consists of a layer containing B2O5 units (two near-coplanar triangular corner-sharing BO3 groups) and 6- and 7-coordinate Ca atoms. Different sequences in the c direction of four layers are observed in the polytypes. The five strongest lines in the powder-diffraction pattern [listed as d in Å (I)(hkl)] are: 3.02(84)(022); 2.92(100)(10) 2.81(56)(104); 2.76(32)(113); 1.880(32)(11,12,126,118) [for shimazakiite-4M]; and 3.84(33)(014); 3.02(42)(022); 2.86(100)(104); 2.79(29)(113); 1.903(44)(126,118) [for shimazakiite-4O].

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