Shimazakiite-4M and shimazakiite-4O, Ca2B2O 5, two polytypes of a new mineral from Fuka, Okayama Prefecture, Japan

I. Kusachi, S. Kobayashi, Y. Takechi, Y. Nakamuta, T. Nagase, K. Yokoyama, K. Momma, R. Miyawaki, M. Shigeoka, S. Matsubara

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Abstract

Shimazakiite 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 2V calc = 53° [shimazakiite-4M]; and α = 1.584(2), β = 1.648(2), γ = 1.670(2) and 2Vcalc = 54.88° [shimazakiite-4O]. Quantitative electron-microprobe analyses (means of 28 and 25 determinations) gave the empirical formulae Ca2B 1.92O4.76(OH)0.24 and Ca2B 1.92O4.76(OH)0.24 for shimazakiite-4M and shimazakiite-4O, respectively. The crystal structure refinements: P2 1/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 B 2O5 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)(104̄) 2.81(56)(104); 2.76(32)(113); 1.880(32)(118̄,126̄,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].

Original languageEnglish
Pages (from-to)93-105
Number of pages13
JournalMineralogical Magazine
Volume77
Issue number1
DOIs
Publication statusPublished - 2012 Feb 1

Keywords

  • Calcium borate
  • Fuka
  • New mineral
  • Shimazakiite
  • Skarn

ASJC Scopus subject areas

  • Geochemistry and Petrology

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    Kusachi, I., Kobayashi, S., Takechi, Y., Nakamuta, Y., Nagase, T., Yokoyama, K., Momma, K., Miyawaki, R., Shigeoka, M., & Matsubara, S. (2012). Shimazakiite-4M and shimazakiite-4O, Ca2B2O 5, two polytypes of a new mineral from Fuka, Okayama Prefecture, Japan. Mineralogical Magazine, 77(1), 93-105. https://doi.org/10.1180/minmag.2013.077.1.09