Shimazakiite-4M and shimazakiite-4O, Ca (sub 2) B (sub 2) O (sub 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): alpha =1.586(2), beta =1.650(2), gamma =1.667(2) and 2V (sub calc) =53 degrees [shimazakiite-4M]; and alpha =1.584(2), beta =1.648(2), gamma =1.670(2) and 2V (sub calc) =54.88 degrees [shimazakiite-4O]. Quantitative electron-microprobe analyses (means of 28 and 25 determinations) gave the empirical formulae Ca (sub 2) B (sub 1.92) O (sub 4.76) (OH) (sub 0.24) and Ca (sub 2) B (sub 1.92) O (sub 4.76) (OH) (sub 0.24) for shimazakiite-4M and shimazakiite-4O, respectively. The crystal structure refinements: P2 (sub 1) /c, a=3.5485(12), b=6.352(2), c=19.254(6) Aa, beta =92.393(13) degrees , V=433.6(3) Aa (super 3) [for shimazakiite-4M]; and P2 (sub 1) 2 (sub 1) 2 (sub 1) , a=3.55645(8), b=6.35194(15), c=19.2534(5) Aa, V=434.941(18) Aa (super 3) [for shimazakiite-4O], converged into R (sub 1) indices of 0.1273 and 0.0142, respectively. The crystal structure of shimazakiite consists of a layer containing B (sub 2) O (sub 5) units (two near-coplanar triangular corner-sharing BO (sub 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 Aa(I)(hkl)] are: 3.02(84)(022); 2.92(100)(1041) 2.81(56)(104); 2.76(32)(113); 1.880(32)(1181,1261,126,118) [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].