Howard T. Evans

Uranyl ion coordination

A review of the known crystal structures containing the uranyl ion shows that plane-pentagon coordination is equally as prevalent as planesquare or plane-hexagon. It is suggested that puckered-hexagon configurations of OH- or H2O about the uranyl group will tend to revert to plane-pentagon coordination. The concept oft of pentagonal coordination is invoked for possible explanations of the complex crystallography of the natural tiranyl hydroxides and the unusual behavior of polynuclear ions in hydrolyzed uranyl solutions.

Djurleite (Cu1.94S) and Low Chalcocite (Cu2S): New Crystal Structure Studies

Additional x-ray structure studies on low chalcocite generally confirm the previously reported structure but show that either disorder is present or the true space group is not P21/c but Pc, four of the 96 copper atoms in the monoclinic unit cell taking on twofold (linear) coordination. The crystal structure of djurleite has been solved in space group P21/n, the monoclinic cell having parameters a = 26.897, b = 15.745, and c = 13.565 angstroms; β = 90.13�; and a content of 248 copper and 128 sulfur atoms. Of the 62 different copper atoms in the structure, 52 are in threefold, triangular coordination with sulfur, nine in tetrahedral, and one in linear coordination.

Lunar Troilite: Crystallography

Fine, euhedral crystals of troilite from lunar sample 10050 show a hexagonal habit consistent with the high-temperature NiAs-type structure. Complete three-dimensional counter intensity data have been measured and used to confirm and refine Bertauts proposed low-temperature crystal structure.

An Appalachian isochron: a kaolinized Carboniferous air-fall volcanic-ash deposit (tonstein)

The Fire Clay tonstein is a kaolinized, airfall volcanic ash bed that was deposited in a widespread late Carboniferous peat-forming mire. Eleven samples from Kentucky and West Virginia, spanning a distance of 200 km, and two samples from Tennessee and Virginia indicate a characteristic mineralogical signature, as compared with other Appalachian tonsteins, consisting of well-crystallized kaolinite, beta-quartz crystal paramorphs, sanidine, ilmenite, zircon, and brookite. Detrital illite and quartz are rarely present or are in very small amounts, which indicates rapid deposition in a mire. Several normal graded cycles in this tonstein suggest repeated episodes of pyroclastic activity that produced a composite ash layer. A high-silica alkalic rhyolitic source is suggested by the geochemistry of immobile elements and by electron-probe analyses of glass inclusions in volcanic quartz from the Fire Clay tonstein. The rare-earth-element plots (chondrite normalized) of the tonstein show a pronounced negative Eu anomaly and relatively high concentrations of Zr and Th, which are both indicative of a rhyolitic source. Probe analyses of the Fire Clay glass inclusions from four states indicate a chemically identical high-silica rhyolite with peraluminous affinities. 40 Ar/ 39 Ar sanidine plateau dating indicates an age of 312 ± 1 Ma for the Fire Clay tonstein, which is consistent with previous 40 Ar/ 39 Ar dates for this tonstein. This age is in agreement with a late Westphalian B age in the European Carboniferous chronostratigraphy on the basis of an age of 311 Ma for the Westphalian B/C boundary. A new isopachous map of the Fire Clay ash-fall deposit indicates an area of 37,000 km 2 and a probable source to the present-day southwest. The deposit has a minimum preserved compacted volume of 2.8 km 3 , which corresponds to an original uncompacted volume of about 20 km 3 . This preserved volume indicates an ultraplinian volcanic explosion. Pindell and Dewey (1982) proposed an Andean-type arc in this block during the late Carboniferous, prior to South American-North American plate collision. We hypothesize an associated back-arc caldera system in the Yucatan block to explain the high-silica, potassic rhyolitic ash that gave rise to the Fire Clay tonstein.

The crystal structure and crystal chemistry of valleriite

The crystal structure of valleriite has been determined to consist of alternate interlayering of layers of two kinds: a brucite-like layer of composition [Mg.68A1.32(OH)2]; and a novel sulfide layer of composition [Fe!.07 CU.93S2].

The crystal structure of tetranatrolite from Mont Saint-Hilaire, Québec, and its chemical and structural relationship to paranatrolite and gonnardite

Abstract The structure of tetranatrolite from Mont Saint-Hilaire, Québec (U.S. National Museum sample R1830) with a = 13.197(7) Å, c = 6.630(9) Å, and space group I4 - 2d, was refined using single-crystal X-ray data. A representative formula of tetranatrolite determined from electron microprobe analysis is Na12.50K0.01Ca2.93Sr0.11Al19.09Si20.91O79.74·nH2O. The structure has the basic natrolite Si-Al-O framework configuration with Na, Ca, Sr, and K residing within inter-framework cages. Aluminum is disordered over the T1 and T2 tetrahedral sites, with T2 > T1. Water molecules O4 and O5 coordinate the intercage atoms and have high displacement parameters, indicating disorder within the cages. The Mont Saint-Hilaire tetranatrolite structure is compared to four previously determined structures, two tetranatrolite samples from Khibiny and Lovozero, Russia and two “gonnardite” samples from Tvedalen, Norway and Gignat, France. Observations are given to indicate that the Norwegian sample deduced to be tetranatrolite rather than gonnardite. Although the crystal structures of tetranatrolite and gonnardite are very similar, it is shown that the tetranatrolite compositions differ significantly from those of gonnardite. The tetranatrolite composition series varies along the join Na16Al16Si24O80-Na12Ca4Al20Si20O80, and is represented by the formula Na16-xCaxAl16+xSi24-x O80·nH2O, where x extends from approximately 2.4 to 3.9. In contrast, gonnardites from Arkansas and Austria have compositions that vary along the join Na16Al16Si24O80-Na4Ca8Al20Si20O80, which are represented by the formula ⃞ xNa16-3xCa2xAl16+xSi24-xO80·nH2O and where ⃞ indicates vacant intercage cation sites and x varies from approximately 0.3 to 3.2. Tetranatrolite is a dehydration product of paranatrolite and probably does not have a true stability field

Thermophysical properties of ilvaite CaFe 2 2+ Fe3+Si2O7O (OH); heat capacity from 7 to 920 K and thermal expansion between 298 and 856 K

The heat capacity of ilvaite from Seriphos, Greece was measured by adiabatic shield calorimetry (6.4 to 380.7 K) and by differential scanning calorimetry (340 to 950 K). The thermal expansion of ilvaite was also investigated, by X-ray methods, between 308 and 853 K.At 298.15 K the standard molar heat capacity and entropy for ilvaite are 298.9±0.6 and 292.3±0.6 J/(mol. K) respectively. Between 333 and 343 K ilvaite changes from monoclinic to orthorhombic. The antiferromagnetic transition is shown by a hump in Cp0with a Néel temperature of 121.9±0.5 K. A rounded hump in Cp0between 330 and 400 K may possibily arise from the thermally activated electron delocalization (hopping) known to take place in this temperature region.

Phosphovanadylite; a new vanadium phosphate mineral with a zeolite-type structure

Abstract Phosphovanadylite, whose simplified formula is (Ba,Ca,K,Na)x[(V,Al)4P2(O,OH)16] · 12H2O, is a new vanadium phosphate zeolite mineral found in the Phosphoria Formation at Monsanto’s Enoch Valley Mine, Soda Springs, Idaho. Its formula in more detail is (Ba0.38Ca0.20K0.06Na0.02)∑0.66[P2(V3.44Al0.46)∑0.39O10.34(OH)5.66]·12H2O. The drusy mineral occurs as pale greenish-blue euhedral cubes (20-50 μm edge) coating phosphatic, organic-rich mudstone. The chemical composition determined by electron microprobe is (in weight percent) V-28.02, P-9.91, Al-1.97, Ca-1.31, Ba-8.28, Cd-0.09, Zn-0.34, Na-0.15, K-0.73, O-46.57, and F-0.03. The index of refraction is nD = 1.566 (4) and specific gravity is 2.16 (3). The X-ray powder pattern shows strong reflections at 3.16 Å (422), 2.58 (600), 2.44 (620), and 7.73 (200), which are indexed on the basis of a cubic body-centered unit cell with a = 15.470 (4) Å. From the single-crystal structure analysis, its space group was determined to be I4̅3m, Z = 6, and its structure consists of V4O16 octahedral clusters linked to each other by P atoms to form a cubic lattice, creating cavities 7.0 and 5.5 Å in diameter where mainly H2O resides. Final residual indexes are R = 0.066, Rw = 0.061, goodness- of-fit = 0.75, and 93 observations and 24 parameters.

A crystal chemical study of the system CsCl-NaCl-H2O. Structures of the CsCl derivative compounds Cs1−x(Na.H2O)xCl, CsNa2Cl3.2H2O, and Cs2CaCl4.2H2O

A restudy of the system CsCI-NaCI-H20 {previously described by Plyushchev, Tulinova, Kutznetsova, Korovin & Petrova [Zh. Neorg. Khim. (1957). 2, 22122220; J. lnorg. Chem. USSR (1957). 2, 357-368]} yielded a new compound, which was found by crystal

Shattuckite and plancheite: a crystal chemical study.

The orthorhombic crystal structures of shattuckite, Cu5( SiO3)4(OH)2 and planch�tite, Cu8(Si4011)2(OH)4 H2O, have been solved. Shattuckite contains silicate chains similar to pyroxene in a complex association with copper atoms, while the closely related planch�ite contains silicate chains similar to amphibole.