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BIGCREEKITE: A NEW BARIUM SILICATE MINERAL SPECIES FROM FRESNO COUNTY, CALIFORNIA

Laurel C. Basciano¹, Lee A. Groat^1,, Andrew C. Roberts², Robert A. Gault³, Gail E. Dunning⁴ and Robert E. Walstrom⁵

¹ Department of Earth and Ocean Sciences, University of British Columbia, Vancouver, British Columbia V6T 1Z4, Canada
² Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
³ Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada
⁴ 773 Durshire Way, Sunnyvale, California 94087, U.S.A.
⁵ P.O. Box 1728, Silver City, New Mexico 88062, U.S.A.

E-mail address: lgroat{at}eos.ubc.ca

The sanbornite deposits at Big Creek and Rush Creek, Fresno County, California, are host to many rare barium silicates. Bigcreekite, ideally BaSi₂O₅·4H₂O, is a newly identified mineral species that occurs along very thin transverse fractures in laminated quartz-rich sanbornite portions of the rock. It postdates the other associated barium silicates and represents either a later primary phase from infiltrated fluids or a product of alteration of pre-existing Ba-rich minerals, possibly sanbornite. It is white to colorless 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, vitreous to pearly luster, white streak, H 2–3, uneven fracture. Bigcreekite is biaxial positive, 1.537(2), ß 1.538(2), 1.541(2); X = b, Y = a, Z = c, 2V_meas 59.2(5)°, 2V_calc 60°; dispersion is moderate, r < v, and it is non-pleochroic. It is orthorhombic, space group Pnma, with unit-cell parameters refined from powder data: a 5.038(6), b 9.024(3), c 18.321(6) Å, V 833(1) ų, Z = 4. The strongest six lines of the X-ray powder-diffraction pattern [d in Å(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 on O = 9) is (Ba_1.00Na_0.01)_1.01Si_2.00H_8.00O_9.00. D_calc = 2.76 g/cm³, and D_meas = 2.66(3) g/cm³. 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 rows of silicate tetrahedra. The structure of bigcreekite has similarities to those of both sanbornite and gillespite. Bigcreekite was named after Big Creek, California, the type locality.

Keywords: bigcreekite, new mineral species, barium silicate tetrahydrate, crystal structure, inosilicate, Big Creek, California.

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