Bigcreekite: A new barium silicate mineral species from Fresno County, California

The sanbornite deposits at Big Creek and Rush Creek, Fresno County, Califor nia, are host to many rare barium silicates. Bigcreekite, ideally BaSi2O5. 4H(2)O, is a newly identified mineral species that occurs along very thin t ransverse fractures in laminated quartz-rich sanbornite portions of the roc k, It postdates the other associated barium silicates and represents either a later primary phase from infiltrated fluids or a product of alteration o f pre-existing Ba-rich minerals, possibly sanbornite. It is white to colorl ess and forms poorly developed crystalline masses parallel to the fracture direction. There are two perfect cleavages, {010} and {001}. Other physical properties are: tabular habit, elongate [100], brittle, non-fluorescent, v itreous to pearly luster, white streak, H 2-3, uneven fracture. Bigcreekite is biaxial positive, alpha 1.537(2), beta 1.538(2), gamma 1.541(2); X = b, Y = a, Z = c, 2V(meas) 59.2(5)degrees, 2V(calc) 60 degrees; dispersion is moderate, r < <nu>, and it is non-pleochroic. It is orthorhombic, space gro up Pnma, with unit-cell parameters refined from powder data: a 5.038(6), b 9.024(3), c 18.321(6) Angstrom, V833(1) Angstrom (3), Z = 4. The strongest six lines of the X-ray powder-diffraction pattern [d in Angstrom (I)(hkl)] are: 5.068(100)(013), 4.054(85)(022), 2.974(45)(031), 2.706(60)(124), 2.327 (40)(035) and 2.257(75)(126). The empirical formula of bigcreekite (based o n O = 9) is (Ba1.00Na0.01)(Sigma1.01)Si2.00H8.00O9.00-D-calc = 2.76 g/cm(3) , and D-meas = 2.66(3) g/cm(3). The structure has been refined to R = 3.5%. Bigcreekite is a hydrous inosilicate; the SiO4 tetrahedra are arranged in four-membered rings that form chains parallel to [100] and are staggered in the [001] direction. Molecules of H2O fill the large spaces between the ro ws of silicate tetrahedra. The structure of bigcreekite has similarities to those of both sanbornite and gillespite, Bigcreekite was named after Big C reek, California, the type locality.