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Howard T. Evans

Publications and source records attributed to Howard T. Evans.

9 recordsLinked to original sources
USGS2018

Delindeite titanosilicates and lourenswalsite, two new from the Magnet Cove region, Arkansas

Delindeite and lourenswalsite are two new barium titanosilicate minerals found as microscopic crystals in miarolitic cavities in nepheline syenite in the Diamond Jo quarry, Hot Spring County, Arkansas. Delindeite is found as aggregates of flake-like crystallites in compact spherules, light pinkish grey in colour, with a resinous, pearly lustre. The flakes are biaxial positive with average n ∼ 1.813; the measured density is 3.3 g/cm 3 . Electron diffraction revealed a monoclinic unit cell in space group C 2 l m or subgroup, with a = 21.617(13), b = 6.816(5), c = 5.383(3) Å, β = 94.03(5)° (refined from X-ray powder data). The strongest X-ray lines are ( hkl , d obs , I rel ): (200, 10.80, 100); (311, 3.54, 24); (6̄01, 3.083, 28); (601, 2.888, 31); (2̄21, 2.806, 20); (910, 2.262,18). The crystals are submicroscopically twinned on (100) and also produce additional continuous diffraction streaks parallel to a *, which double the b and c axes. The formula derived from electron and ion probe analyses (H 2 O by difference), as constrained by density and molar volume data, is approximately (Na,K) 2.7 (Ba,Ca) 4 (Ti,Fe,Al) 6 Si 8 O 26 (OH) 14 , with Na > K, Ba ≫ Ca, Ti ≫ Fe,Al; Z = 1. Lourenswalsite occurs as very thin hexagonal plates in rosettes, silver grey to light brownish grey in colour. The crystals are biaxial negative with very low 2 V angle. Indices of refraction are n α = 1.815, n β ≈ n γ = 1.840; the measured density is 3.17 g/cm = 1.840; the measured density is 3.17 g/cm 3 . X-ray and electron diffraction show a sharp pseudohexagonal lattice with a = 5.244 Å, but extremely diffuse diffraction streaks normal to the hk 0 plane. In these streaks a period of 20.5 Å can be discerned. A hexagonal unit cell with a = 5.244(2) Å, c = 20.49(3) Å can be refined from the powder diffraction data but does not account for some lines, probably because of extreme layer disorder as shown by precession single-crystal patterns. The strong X-ray powder lines are (002, 10.22, 20); (-, 3.93, 20); (111, 2.608, 100); (300, 1.5145, 80); (220, 1.3111, 25). The formula given by microprobe analyses, constrained by density and molar volume data, is approximately (K,Ba) 2 (Ti,Mg,Ca,Fe) 4 (Si,Al,Fe) 6 O 14 (OH) 12 with K > Ba, Ti ≫ (Mg,Ca,Fe), Si > Al > Fe; Z = 1. These minerals are formed under oxidizing weathering conditions, and iron is assumed to be in the Fe 3+ state.

Arkansas↗
USGS2003

Bobjonesite, V4+ O (SO4) (H2O)3, a new mineral species from Temple Mountain, Emery County, Utah, U.S.A

Bobjonesite, V 4+ O (SO 4 ) (H 2 O) 3 , is a new mineral species from Temple Mountain, Emery County, Utah, U.S.A. It occurs as blue-green crusts and efflorescences in fractures in a fossil (Triassic) tree; individual crystals are <<1 mm and are intimately intergrown. Bobjonesite hydrates very easily, and is unstable in all but the driest atmosphere. Its structure was determined on a crystal of bobjonesite; however, the physical properties, optical properties and X-ray powder-diffraction pattern were recorded on the synthetic equivalent, and an electron-microprobe analysis was not possible. Bobjonesite has a pale blue streak, a vitreous luster and no observable fluorescence under ultraviolet light. It has no cleavage or parting. The Mohs hardness is ~1, and the calculated density is 2.28 g/cm 3 . Bobjonesite is biaxial positive, with α 1.555(2), β 1.561(1), γ 1.574(2), 2 V (obs.) = 72(1)°, 2 V (calc.) = 69°; it is non-pleochroic, X = b , Y ≈ a , Z ∧ c ≈ 19° (in β obtuse). Bobjonesite is monoclinic, space group P 2 1 / n , cell dimensions from single-crystal data: a 7.3940(5), b 7.4111(3), c 12.0597(9) Å, β 106.55(1)°, V 633.5(1) Å 3 , Z = 4. The strongest seven lines in the X-ray powder-diffraction pattern [ d in Å( I )( hkl )] are as follows: 5.795(100)(002), 3.498(90)(112), 3.881(48)(1̅03), 5.408(37) (101), 4.571(20)(012), 6.962(11)(1̅01) and 6.254(11)(011). The chemical formula was derived from crystal-structure analysis; the end-member formula is V O (SO 4 ) (H 2 O) 3 . The crystal structure of bobjonesite was refined to an R index of 3.6% for 1105 observed (| F o | > 5> F ) reflections measured with an automated four-circle X-ray diffractometer using Mo K α X-radiation. There is one V site occupied by V 4+ and surrounded by three O atoms and three (H 2 O) groups in an octahedral arrangement, with one short vanadyl bond (1.577 Å), four similar equatorial bonds (<2.022 Å>), and one longer V–O bond (2.278 Å) trans to the vanadyl bond. The structure consists of isolated [V 4+ 2 O 2 (H 2 O) 6 (SO 4 ) 2 ] clusters linked by hydrogen bonds.

Utah↗
USGS2002

Molecular structure of the decamolybdodicobaltate(III) ion

When solutions of cobalt(II) ion and heptamolybdate ion (Mo₇O₂₄⁶⁻) are mixed in the presence of an oxidizing agent (such as bromine), Kurnakov, Hall, and others found that an emerald green complex is formed as the main product, namely, the heteropoly complex ion hexamolybdocobaltate(III), which forms salts such as (NH₄)₃[H₆CoMo₆O₂₄]·12H₂O. Friedheim and Keller showed that at the same time a small amount of another complex is also formed, having a Co:Mo ratio of 1:5. Tsigdinos, et al., found that if a hot solution of the hexamolybdocobaltate complex ion is treated with activated charcoal or Raney nickel the 1:6 complex is converted almost entirely to the 1:5 complex. These workers proved the latter to be a dimer, and formulated the ammonium salt (earlier prepared by Friedheim and Keller) as (NH₄)₆[Co₂Mo₁₀O₃₆]·10H₂O. We have now completed a crystal structure analysis of this interesting compound, which shows the proper formula to be (NH₄)₆[H₄Co₃Mo₁₀O₃₈]·7H₂O.

Journal of the American Chemical Society↗
USGS1979

The thermal expansion of anhydrite to 1000° C

The thermal expansion of anhydrite, CaSO 4 , has been measured from 22&deg; to 1,000&deg; C by X-ray diffraction, using the Guinier-Lenn&eacute; heating powder camera. The heating patterns were calibrated with Guinier-H&auml;gg patterns at 25&deg; C, using quartz as internal standard. Heating experiments were run on natural anhydrite (Bancroft, Ontario), which at room temperature has lattice constants in close agreement with those of synthetic material. The orthorhombic unit cell at 22&deg; C (space group Amma ) has a =7.003 (1) &Aring;, b =6.996 (2) &Aring; and c =6.242 (1) &Aring;, V =305.9 (2) &Aring; 3 . At room temperature, the thermal expansion coefficients &alpha; and &beta; (&alpha; in &deg;C &minus;1 &times;10 4 , &beta; in &deg;C &minus;2 &times;10 8 ) are for a , 0.10, &minus;0.69; for b , 0.08, 0.19; for c , 0.18, 1.60; for V , 0.37, 1.14. Second-order coefficients provide an excellent fit over the whole range to 1,000&deg; C.

Physics and Chemistry of Minerals↗
USGS1976

Definitive X-ray powder data for covellite, anilite, djurleite, and chalcocite

By using the Guinier-Hägg focusing X-ray powder camera with Cr Kα 1 and Cu Kα 1 radiations, new powder data for covellite (CuS), anilite (Cu 7 S 4 , djurleite (Cu 1.934 S), and low chalcocite (Cu 2 S) have been obtained from synthetic and natural materials. The powder data were treated by least-squares analysis to obtain the best unit-cell parameters consistent with the data. These powder data are intended to replace inferior and incomplete data previously published and to serve as standards for the study of copper sulflde minerals.

Journal of Research of the U.S. Geological Survey↗
USGS1975

Crystal structure of the heptamolybdate(VI)(paramolybdate) ion, [Mo7O24]6–, in the ammonium and potassium tetrahydrate salts

The crystal structures of the isomorphous salts M I 6 [Mo 7 O 24 ],4H 2 O (M = NH 4 or K) have been refined by three-dimensional X -ray diffraction methods. Unit cell dimensions of these monoclinic compounds, space group P 2 1 / C with Z = 4, are, ammonium salt: a = 8·3934 ± 0·0008, b = 36·1703 ± 0·0045, c = 10·4715 ± 0·0011 Å, β= 115·958°± 0·008°; and potassium salt: a = 8·15 ± 0·02, b = 35·68 ± 0·1, c = 10·30 ± 0·02 Å, β= 115·2°± 02°. By use of multiple Weissenberg patterns, 8197 intensity data (Mo- K α radiation) for the ammonium compound and 2178 (Cu- K α radiation) for the potassium compound were estimated visually and used to test and refine Lindqvist's proposed structure in the space group P 2 1 / c . Lindqvist's structure was confirmed and the full matrix least-squares isotropic refinement led to R 0·076 (ammonium) 0·120 (potassium), with direct unambiguous location of the cations and water molecules in the potassium compound.

Journal of the Chemical Society, Dalton Transactio↗
USGS1966

Shattuckite and plancheite: A crystal chemical study

The orthorhombic crystal structures of shattuckite, Cu 5 ( SiO 3 ) 4 (OH) 2 and planch&eacute;tite, Cu 8 (Si 4 0 11 ) 2 (OH) 4 H 2 O, have been solved. Shattuckite contains silicate chains similar to pyroxene in a complex association with copper atoms, while the closely related planch&eacute;ite contains silicate chains similar to amphibole.

Science↗