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

Published online by Cambridge University Press:  05 July 2018

I. Kusachi ,
S. Kobayashi ,
Y. Takechi ,
Y. Nakamuta ,
T. Nagase ,
K. Yokoyama ,
K. Momma ,
R. Miyawaki ,
M. Shigeoka  and
S. Matsubara
I. Kusachi
Affiliation:
Department of Earth Sciences, Faculty of Education, Okayama University, Okayama 700-8530, Japan Department of Geology and Palaeontology, National Museum of Nature and Science, Tsukuba 305-0005, Japan
S. Kobayashi
Affiliation:
Department of Applied Science, Faculty of Science, Okayama University of Science, Okayama 700-0005, Japan
Y. Takechi
Affiliation:
Kurashiki Museum of Natural History, Kurasiki 710-0046, Japan
Y. Nakamuta
Affiliation:
Kyushu University Museum, Fukuoka 812-8581, Japan
T. Nagase
Affiliation:
Tohoku University Museum, Sendai 980-8578, Japan
K. Yokoyama
Affiliation:
Department of Geology and Palaeontology, National Museum of Nature and Science, Tsukuba 305-0005, Japan
K. Momma
Affiliation:
Department of Geology and Palaeontology, National Museum of Nature and Science, Tsukuba 305-0005, Japan
R. Miyawaki*
Affiliation:
Department of Geology and Palaeontology, National Museum of Nature and Science, Tsukuba 305-0005, Japan
M. Shigeoka
Affiliation:
Department of Geology and Palaeontology, National Museum of Nature and Science, Tsukuba 305-0005, Japan
S. Matsubara
Affiliation:
Department of Geology and Palaeontology, National Museum of Nature and Science, Tsukuba 305-0005, Japan
*
* E-mail: miyawaki@kahaku.go.jp
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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 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].

Keywords

shimazakiitenew mineralcalcium borateFukaskarn
Type
Letter
Information
Mineralogical Magazine , Volume 77 , Issue 1 , February 2013 , pp. 93 - 105
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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