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Mineralogy and crystal structures of barium silicate minerals from Fresno County, California Basciano, Laurel Christine


The sanbornite deposits at Big Creek and Rush Creek, Fresno County, California are host to many rare barium silicates, including bigcreekite, UK6, walstromite and verplanckite. As part of this study I described the physical properties and solved the crystal structures of bigcreekite and UK6. In addition, I refined the crystal structures of walstromite and verplanckite. Bigcreekite, ideally BaSi₂0₅-4H₂0, is a newly identified mineral species that occurs along very thin transverse fractures in fairly well laminated quartz-rich sanbornite portions of the rock. It post dates the other associated barium silicates and represents either a later primary phase from intruded fluids or a product of alteration of pre-existing Ba-rich minerals, possibly sanbornite. It is colourless and forms poorly developed crystalline masses parallel to the fracture direction. There are two perfect cleavages {010} and {001}. Bigcreekite is biaxial negative, with indices of refraction α 1.537(2), β 1.538(2), γ 1.541(2); X = b, Y = a, Z = c and 2V[sub meas] = 59.2(5)°, 2V[sub calc] = 60.1 °. The crystal structure of bigcreekite was solved in space group Prima to R = 3.65%, with cell parameters a 5.038(6), b 9.024(3), c 18.321 (6) Å, and Z = 4. The empirical formula of bigcreekite (based on 9 O) is (Ba₁.₀Na₀.₀₁)₁.₁Si₁.₉₉H₈.₀₂O₉ . D[sub calc] = 2.739 g/cm³ and D[sub meas] = 2.66(3) g/cm³. Bigcreekite is a hydrous chain silicate containing four-membered rings which form chains of silica tetrahedra, parallel to [100] and staggered in the [001] direction. Water molecules fill the large spaces between the rows of silicon tetrahedra. Bigcreekite has similarities to sanbornite and gillespite. Bigcreekite was named for Big Creek, California, its type locality. UK6, ideally (Si, Al)₄Ba₃Cl0₉(Cl, H₂0)₄ , is a newly identified mineral species that is light blue-grey, with one perfect cleavage on {0001}, and forms irregular masses up to 10 mm enclosed in parallel-bedded sanbornite-quartz rock. UK6 is uniaxial negative, with indices of refraction ε 1.594(2), ω 1.642(2). The crystal structure of UK6 was solved in space group P6₃mc to R = 4.85%, with cell parameters a 5.2432(7), c 29.859(6) and Z = 2. The empirical formula of UK6 (based on 7 cations and 2 H₂0 per formula unit) is (Ba₂.₉₇ , Na₀.₀₃ , Ca₀.₀₁)₃.₀₁(Si₂.₆₃ , Al₁.₃₄ , Ti₀.₀₃)₄ClO₉(Cl₁.₆₁, 2H₂O, F₀.₀₅)₃.₆₆, and D[sub calc] = 3.709 g/cm³. The UK6 structure is based on double tetrahedral layers, [T₄O₈][sub ∞∞], consisting of six-membered rings, with three layers of barium polyhedra connecting the tetrahedral layers. UK6 is part of the Monteregianite-(Y)- Wickenburgite series (Struntz classification) and is structurally and chemically similar to cymrite. The crystal structure of walstromite was originally solved by Glasser & Glasser (1968) in space group P1 using photographic methods to an R index of 16 %. It was redetermined in space group P1 with unit cell parameters a 6.733(1), b 9.608(2), c 6.685(1) Å, α 69.64(3), β 102.29(3), γ 96.89(3)°, Z = 2 to an R index of 3.8 %. The empirical formula of walstromite (based on 9 anions per formula unit) is (Ca₁.₉₇, Na₀.₀₁₁)₁.₉₅₈Ba₁.₁₁ (Si₂.₉₉₅, Al₀.₀₂₃ )₃. ₀₁₈O₉ which is similar to the formula determined by Glasser & Glasser (1968). The walstromite structure consists of trigonal rings of silica tetrahedra arranged in layers on (101) with barium and calcium atoms lying approximately halfway between the silicon tetrahedral rings, perpendicular to the a axis. Walstromite is isostructural with margarosanite, as predicted by Glasser & Glasser (1968). The crystal structure of verplanckite was originally solved by Kampf et al. (1973) in space group P6/mmm to an R index of 10.2 %. It was redetermined in P6/mmm with unit cell parameters a 16.300(2) c 7.169(1) Å, Z = 3 to an R index of 4.0 %. The empirical formula of verplanckite (based on 12 Si and Al per formula unit) is (Ba₁₂.₇₆, C a ₀.₀₁ Na₀.₂₁)₁₂.₉₈(Mn₄.₅₄, Ti₁.₁₉ , Mg₀.₀₃Fe₀.₂)₆.₁₇ (Si₁₁.₆₆, Al₀.₃₄)₁₂O₃₁.₇₈(Cl₂₃.₆₄, F₀.₄₉) ₂₄.₁₃, which is significantly different than analyses obtained by Alfors et al. (1965) and Kampf et al. (1973) due to the large variation in chemistry. Verplanckite is a framework structure consisting of four-membered tetrahedral rings and square pyramid polyhedra which form open hexagonal rings, with barium polyhedra located along the inner edges of the rings. There is a large degree of disorder in the verplanckite structure. Verplanckite is similar to the zeolite group of minerals with a very open framework.

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