Esquireite, BaSi6O13·7H2O, is a new mineral from the Esquire #1 claim along Rush Creek, eastern Fresno County, California, USA, and a Ba-silicate lens on the NW slope of Trumbull Peak, Mariposa County, California, USA. Esquireite is a low-temperature hydrothermal alteration product of sanbornite and is found on surfaces of sanbornite. The mineral occurs as colorless rectangular blades, elongated and striated parallel to [010] and flattened on {001}. The streak is white, the luster is vitreous to pearly, and the crystals are transparent. The Mohs hardness is about 2. The tenacity is somewhat flexible, but not elastic, with brittle failure. The fracture is irregular and there are two cleavages: perfect on {001} and fair on {100}. The measured and calculated densities are 2.18(2) g/cm3 and 2.237 g/cm3, respectively. Esquireite is biaxial (+), with α 1.477(1), β 1.481(1), γ 1.492 (calc.) (white light); 2Vmeas = 63.8(6)°; no dispersion or pleochroism; optical orientation: Y = b; Z ^ c ≈ 22° in obtuse β. Six electron-microprobe analyses gave the following empirical formula, based on 20 O apfu: Ba0.95Si6.00O20H14.10. Esquireite is monoclinic, space group C2, with cell parameters a 13.601(4), b 4.9222(10), c 15.092(5) Å, β 111.578(19)°, V 939.6(4) Å3, and Z = 2. The eight strongest lines in the X-ray powder diffraction patterns are [dobs Å(I)(hkl)]: 7.02(38)(002), 5.11(33)(201), 4.649(66)(003,2¯03), 4.191(100)(111), 3.339(65)(4¯02,3¯11,3¯12), 2.967(32)(2¯05,1¯14,311), 2.343(33)(multiple), and 2.261(35)(multiple). The crystal structure (R1 = 10.8% for 323 reflections with Fo > 4σ F) contains four-tetrahedra-thick [Si6O13]2– silicate layers with Ba(H2O)82+ polyhedra between the layers.

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