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

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 2V_calc = 54.88° [shimazakiite-4O]. Quantitative electron-microprobe analyses (means of 28 and 25 determinations) gave the empirical formulae Ca₂B _1.92O_4.76(OH)_0.24 and Ca₂B _1.92O_4.76(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) ų [for shimazakiite-4M]; and P2₁2₁2₁, a = 3.55645(8), b = 6.35194(15), c = 19.2534(5) Å, V = 434.941(18) ų [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].