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

Abstract Shimazakiite occurs as greyish white aggregates up to 3 mm in diameter. Two polytypes, shimazakiite-4 M and shimazakiite-4 O , 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 = 53º [shimazakiite-4 M ]; and α = 1.584(2), β = 1.648(2), γ = 1.670(2) and 2V calc = 54.88º [shimazakiite-4 O ]. Quantitative electronmicroprobe analyses (means of 28 and 25 determinations) gave the empirical formulae Ca 2 B 1.92 O 4.76 (OH) 0.24 and Ca 2 B 1.92 O 4.76 (OH) 0.24 for shimazakiite-4 M and shimazakiite-4 O , respectively. The crystal structure refinements: P 2 1 /c , a = 3.5485(12), b = 6.352(2), c = 19.254(6) Å , β = 92.393(13)°, V = 433.6(3) Å 3 [for shimazakiite-4 M ]; and P 2 1 2 1 2 1 , a = 3.55645(8), b = 6.35194(15), c = 19.2534(5) Å , V = 434.941(18) Å 3 [for shimazakiite-4 O ], converged into R 1 indices of 0.1273 and 0.0142, respectively. The crystal structure of shimazakiite consists of a layer containing B 2 O 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 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-4 M ]; and 3.84(33)(014); 3.02(42)(022); 2.86(100)(104); 2.79(29)(113); 1.903(44)(126,118) [for shimazakiite-4 O ].