Shimazakiite-4M and shimazakiite-4O, Ca_2B_2O_5, two polytypes of a new mineral from Fuka, Okayama Prefecture, Japan

摘要

Shimazakiite occurs as greyish white aggregates up to 3 mm in diameter. Two polytypes, shimazakiite-AM 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-40]. Quantitative electron-microprobe analyses (means of 28 and 25 determinations) gave the empirical formulae Ca_2Bi_(1.92)O_(4.76)(OH)_(0.24) and Ca_2B_(1.92)O_(4.76)(OH)_(0.24) for shimazakiite-4M and shimazakiite-40, respec tively. The crystal structure refinements: P21/c, a = 3.5485(12), b = 6.352(2), c = 19.254(6) A, β = 92.393(13)°, V= 433.6(3) A~3 [for shimazakiite-4M]; and P2_12_12_1 a = 3.55645(8), b = 6.35194(15), c = 19.2534(5) A, V = 434.941(18) A~3[for shimazakiite-40], converged into R_1 indices of 0.1273 and 0.0142, respectively. The crystal structure of shimazakiite consists of a layer containing B_2O_5 units (two near-coplanar triangular corner-sharing BO_3 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 A(I)(hkl)] are: 3.02(84)(022); 2.92(100)(104) 2.81(56)(104); 2.76(32)(113); 1.880(32)(l 18,126,126,118) [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-40].