Ishmael Hassan

Cation disorder in dolomite, CaMg(CO3)2, and its influence on the aragonite + magnesite ↔ dolomite reaction boundary

Abstract The structure of dolomite, CaMg(CO3)2, was determined from 298 to 1466 K at a constant pressure of about 3 GPa using in situ synchrotron X-ray diffraction data to investigate the state of disorder. An order parameter s, defined as 2 xCa - 1, varies from s = 1 (where xCa = 1) for a completely ordered dolomite to s = 0 (where xCa = 0.5) for a completely disordered dolomite. On heating, there is no measured change in s until the temperature is high enough to cause exchange of Ca2+ and Mg2+ cations. Significant disorder began to occur at about 1234 K [s = 0.83(1)] and increases along a smooth pathway to T = 1466 K [s = 0.12(5)]. The R3 - ↔ R3 - c transition in dolomite is described by a modified Bragg-Williams thermodynamic model with the following molar free energy of disorder, G̅d (T; s) = RTc[1 - s2 + 1/2 a(s4 - 1) - (T/Tc) {2 ln2 - (1+s) ln(1 + s) - (1 - s) ln(1 - s)}]. Using Tc = 1466 K and a = -0.29, this model provides an excellent agreement with experimental data. Moreover, the maximum enthalpy of disorder, H̅d(s = 0) = RTc(1 - 1/2 a) ~ 14 kJ/mol, agrees with published calorimetric data. A thermodynamic description of the aragonite + magnesite ↔ dolomite reaction boundary is also presented and it reproduces the main qualitative features correctly.

Cancrinite: Crystal structure, phase transitions, and dehydration behavior with temperature

Abstract The structural behavior of a cancrinite, Na5.96Ca1.52[Al6Si6O24](CO3)1.57·1.75H2O, was determined by using in situ synchrotron X-ray powder diffraction data [λ = 0.91806(5) Å] at room pressure and from 25 to 982 °C. The sample was heated at a rate of about 9.5 °C/min, and X-ray traces were collected at about 15 °C intervals. The satellite reflections in cancrinite were lost at about 504 °C, where a phase transition occurs. All the unit-cell parameters for cancrinite also show a discontinuity at 504 °C. Initially, the [Ca·CO3] clusters and their vacancies are ordered in the channels, and this ordering is destroyed on heating to give rise to the phase transition. Cancrinite loses water continuously until about 625 °C; thereafter an anhydrous cancrinite phase exists. From 25 to 952 °C, a minimal amount of CO2 is lost from the structure. Over this temperature range, the average bridging angle, which is an indication of the degree of rotation of the tetrahedra, increases from 143.7(4) to 147.7(5)°. Rotations of the tetrahedra are caused by expansion of the Na1-O2 bond lengths.

Sodalite: High-temperature structures obtained from synchrotron radiation and Rietveld refinements

Abstract The structural behavior of sodalite, ideally Na8[Al6Si6O24]Cl2, at room pressure and from 28 to 982 °C on heating, was determined by using in situ synchrotron X-ray powder diffraction data (λ = 0.92007(4) Å) and Rietveld refinement. The sample was heated at a rate of about 9.5 °C/min and X-ray spectra were collected at intervals of about 15 °C. The cubic unit-cell parameter for sodalite increases smoothly and non-linearly to 982 °C. The percent volume change between 28 and 982 °C is 4.8(2)%. Between 28 and 982 °C, the Al-O and Si-O distances are constant, while the Al-O-Si angle increases from 138.29(1) to 146.35(2)° by 5.06(2)°. Simultaneously, the angle of rotation of the AlO4 tetrahedron, φAl, decreases from 22.1 to 16.9°, a difference of 5.2°, while the angle of rotation of the SiO4 tetrahedron, φSi, decreases from 23.6 to 18.0°, a difference of 5.6°. Moreover, the [Na4Cl]3+ clusters expand with increases in the Na-Cl bond length by 0.182(4) Å, and corresponding increases in the short Na-O bond length by 0.093(2) Å, and decreases in the longer Na-O* distance by 0.108(1) Å. Large displacement parameters occur for the Na and Cl atoms, and as the weaker Na-Cl bond expands with temperature, the Na atoms move toward the plane of the framework six-membered rings, which causes the framework tetrahedra to rotate and results in a relatively high rate of expansion of the structure. The framework TO4 tetrahedra distort slightly with temperature. If the Na atom reaches approximately the plane of the six-membered ring, the expansion will be retarded, but sodalite melts before this occurs. Sodalite melts at about 1079 °C and begins to lose NaCl. The NaCl component is lost in two stages: about 4.5 wt% of NaCl is lost slowly at about 1150 °C, and about 7.0 wt% of NaCl is lost at a faster rate at about 1284 °C.

Transmission electron microscopy and differential thermal studies of lazurite polymorphs

Abstract Transmission electron microscopy (TEM) and thermal (DTA-TG) analyses of lazurite, (Na,Ca)8[Al6Si6O24](SO4,S)2, indicate three forms: cubic, modulated, and orthorhombic. Cubic lazurite contains mainly [Na3Ca·SO4]3+ clusters that are disordered together with the O1 and O2 oxygen atoms, and no satellite reflections occur. In the modulated lazurite structure, [Na3Ca·SO4]3+ and [Na3Ca·S]3+ clusters are ordered and possibly cause ordering of the framework oxygen atoms on the O1 and O2 positions and produce satellite reflections. Different ordering of these clusters gives an orthorhombic supercell with parameters of 2d110 × 6d11̄0 × c with respect to the cubic subcell. The DTA-TG study indicates that the orthorhombic to the modulated phase transition occurs at a peak temperature of 489 °C. A single-crystal fragment of lazurite may contain all three phases, as was observed by TEM in this study. Such intergrowths indicate a continuous framework with different regions of the crystal containing different ordering and chemistries. Two new lazurite superstructures were observed with dimensions of 6d110 × 3d001, and 3d112 × 3d1̄11 with respect to the cubic subcell.

Manganese carbonyl compounds of N,N-bidentate di-2-pyridylketone (dpk) and N,O,N-tridentate hydroxybis(2-pyridyl)methanolato (dpkO,OH). The structure of fac-[Mn(CO)3(dpkO,OH)]

Abstract Electrochemical measurements on fac -[Mn(CO) 3 (dpk)Br] and fac -[Mn(CO) 3 (dpkO,OH)] revealed solvent dependence and rich redox properties and X-ray studies on fac -[Mn(CO) 3 (dpkO,OH)] show distorted octahedral geometry around manganese with the major distortion is due to the binding of hydroxybis(2-pyridyl)methanolato (dpkO,OH) anion and anti-parallel tapes of fac -[Mn(CO) 3 (dpkO,OH)] interlocked via a network of hydrogen bonds. When di-2-pyridyl ketone (dpk) was allowed to react with [Mn(CO) 5 Br] in dry diethyl ether under ultrasonic conditions fac -[Mn(CO) 3 (dpk)Br] was isolated in good yield and when the same reaction was carried out under reflux conditions in toluene fac -[Mn(CO) 3 (dpkO,OH)] was isolated. Infrared spectra of the isolated compounds confirmed their fac -geometry and the presence and absence of the ketonic group of dpk. Electrochemical measurements on fac -[Mn(CO) 3 (dpk)Br] reveal sensitivity to solvents and the presence of reversible and irreversible electronic transfers. In contrast to fac -[Re(CO) 3 (dpk)Cl] where noteworthy electrochemical reactions with CO 2 were observed, the electrochemical reactions of CO 2 with fac -[Mn(CO) 3 (dpk)Br] disclosed no significant reaction. However, when fac -[Mn(CO) 3 (dpk)Br] was allowed to electrochemically interact with group I and II metal ions considerable electrochemical changes were noted on the second reduction wave that may point to the possible use of fac -[Mn(CO) 3 (dpk)Br] as an electrochemical sensor for group I and II metal ions. The electrochemical properties of fac -[Mn(CO) 3 (dpkO,OH)] show the presence of closely spaced irreversible oxidations and probable electrochemical oxidation of coordinated dpkO,OH anion to dpk in dmf. Crystals of fac -[Mn(CO) 3 (dpkO,OH)] obtained from dimethyl sulfoxide (dmso) solution of fac -[Mn(CO) 3 (dpkO,OH)] are in the monoclinic C2/c space group. Structural analysis on fac -[Mn(CO) 3 (dpkO,OH)] disclosed distorted octahedral coordination about manganese with the major distortion due to the tridentate coordination of dpkO,OH anion and the packing of molecules show stacks of anti-parallel tapes of fac -[Mn(CO) 3 (dpkO,OH)] interlocked via a network of hydrogen bonds.

Haüyne: phase transition and high-temperature structures obtained from synchrotron radiation and Rietveld refinements

Abstract The structural behaviour of a haüyne with a chemical composition of Na4.35Ca2.28K0.95[Al6Si6O24]-(SO4)2.03, at room pressure and from 33 to 1035ºC on heating, was determined by using in situ synchrotron X-ray powder diffraction data (λ = 0.92249(5) Å). The satellite reflections in haüyne are lost at ~400ºC and a true substructure results because of this phase transition. There is a discontinuity in the a unit-cell parameter at ~585ºC. The a parameter increases rapidly and non-linearly to 585ºC, but above 585ºC, the expansion rate decreases. The percent volume change between 33 and 576ºC is 2.0(3)%, and 0.6(3)% between 593 and 1035ºC. Between 33 and 1035ºC, the Al-O, Si-O and S-O distances are constant. Between 33 and 576ºC, the angle of rotation of the AlO4 tetrahedron, φAl, changes from 11.5 to 5.8º, while the angle of rotation of the SiO4 tetrahedron, φSi, changes from 12.4 to 6.3º. The Al-O-Si bridging angle changes from 150.05(2) to 153.08(1)º from 33 to 576ºC. Beyond 585ºC, φAl and φSi angles remain nearly constant even though the maximum rotation of the tetrahedra is not achieved. Moreover, the Al-O-Si angle continues to increase at a slower rate from 585 to 1035ºC by 1.05(2)º. From 33 to ~585ºC, the K atom position migrates at a slower rate than the Na and Ca atoms, and the structure expands at a high rate. Beyond 585ºC, all the atomic positions of the interstitial cations (Na+, K+, Ca2+) remain nearly constant and the expansion of the structure is retarded.

Synthesis and structure of the first molybdenum compound of N,O,N-hydroxybis(2-pyridyl)methanolato (η 3-dpkO, OH). The structure of [Mo(O)2(μ-O)(η 3-dpkO,OH)]2 · 2dmso

When di-2-pyridyl ketone (dpk) was allowed to react with [Mo(CO)6] in toluene under reflux in air [Mo(O)2(μ-O)(η 3-dpkO,OH)]2 where η 3-dpkO,OH is N,O,N-hydroxybis(2-pyridyl)methanolato was isolated. Infrared and 1H-NMR spectral data measured on solutions of the isolated compound indicate the absence of the carbonyl groups and the presence of coordinated η 3-dpkO,OH, terminal and bridging oxo-groups and elemental analysis confirmed the formulation of the product as [Mo(O)2(μ-O)(η 3-dpkO,OH)]2. Crystals of [Mo(O)2(μ-O)(η 3-dpkO,OH)]2·2dmso obtained from a dimethyl sulfoxide (dmso) solution of [Mo(O)2(μ-O)(η 3-dpkO,OH)]2 are in the monoclinic P21/c space group. X-ray structural analysis confirmed the identity of [Mo(O)2(μ-O)(η 3-dpkO,OH)]2 and shows a dioxomolybdenum oxo-bridged dimer with two terminal oxygen atoms and one tridentate N,O,N-dpkO,OH occupying the coordination sites of the high-valent molybdenum(VI) in a pseudo-octahedral coordination geometry. The molecular packing shows parallel stacks of [Mo(O)2(μ-O) (η 3-dpkO,OH)]2·2dmso and disclose an extensive network of non-covalent interactions within each stack.

Mikanolide from Jamaican Mikania micrantha.

Mikanolide [systematic names: 1,10:2,3-diepoxy-6,8-dihydroxy-11-vinylgermacr-4-ene 12,14-di-gamma-lactone and 7,10a-dimethyl-1a,1b,2a,6a,7,9a,10,10a-octahydro-4H-6,3-methenofuro[3,2-c]bisoxireno[f,h]oxacycloundecin-4,8(6H)-dione], C(15)H(14)O(6), derived from a variety of Mikania micrantha growing in Portland, Jamaica, contains a methylcyclodecane ring fused to an unsaturated planar alpha,gamma-lactone, an envelope-type near-planar vinyl-beta,gamma-lactone and two epoxide moieties. The crystal packing shows stacks of mikanolide molecules interlocked via a network of non-classical C-H...O hydrogen bonds between the lactone units.

Unusual Al-Si ordering in calcic scapolite, Me79.6, with increasing temperature

Abstract A scapolite sample, Me79.6, from Slyudyanka, Siberia, Russia, has been studied using in situ synchrotron powder X-ray diffraction (XRD) and Rietveld structure refinements on heating from 26 to 900 °C and on cooling to about 300 °C. The structure was modeled and refined in space group I4/m. An accurate room-temperature structure was also obtained by using synchrotron high-resolution powder X-ray diffraction (HRPXRD) data and Rietveld structure refinement. From HRPXRD, the cell parameters are a = 12.16711(2), c = 7.575466(5) Å, and V = 1121.461(3) Å3; and are 1.643(1) and 1.672(1), respectively, so the T1 (Al0.25Si0.75) and T2 (Al0.46Si0.54) sites are partially ordered at room temperature. On heating, the distances indicate that the T1 and T2 sites become more Si- and Al-rich, respectively, and therefore, ordering increases unusually with increasing temperature. This increase in Al-Si ordering occurs from 892 to 900 °C. At 900 °C, the T1 site becomes fully ordered with only Si atoms, while the T2 site contains Al0.51Si0.49 and therefore, is fully disordered. On cooling, the sample does not fully revert back to the original partially ordered state. At 300 °C, all the cell parameters are smaller because of the increased Al-Si ordering that is quenched in.

Tugtupite: High-temperature structures obtained from in situ synchrotron diffraction and Rietveld refinements

Abstract The structural behavior of tugtupite, (ideally Na8[Al2Be2Si8O24]Cl2), a member of the sodalitegroup minerals, at room pressure and from 33 to 982 °C on heating, was determined by using in situ synchrotron X-ray powder diffraction data [λ = 0.91997(4) Å] and Rietveld refinement. The sample was heated at a rate of 9.5 °C/min and X-ray traces were collected at intervals of 16 °C. The unit-cell parameters for tugtupite increase smoothly and non-linearly to 982 °C. The percent volume change between 33 and 982 °C is 2.97(3)%. In tugtupite, large displacement parameters occur for the Na and Cl atoms, and the Na-Cl bond expands with temperature. The [Na4⋅Cl]3+ clusters expand with increases of the Na-Cl bond length by 0.073(3) Å between 33 and 982 °C. This forces the Na atoms toward the plane of the framework six-membered rings, and causes the framework tetrahedra to rotate. The framework TO4 (T = Al3+, Be2+, or Si4+) tetrahedra distort slightly with temperature, but the T-O distances remain nearly constant. This mechanism causes a fairly high-rate of expansion in tugtupite. If the Na atom reaches approximately the plane of the six-membered ring, because of the increase in bonding to the Na atom, the expansion will be retarded, but tugtupite melts before this occurs. Tugtupite melts at 1029 °C. The NaCl component in tugtupite is lost in two main stages; 1.8 wt% NaCl is first lost at about 1007 °C, and 8.2 wt% NaCl is lost in several steps between 1019 and 1442 °C.