Sytle M. Antao

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.

Evidence for monazite-, barite-, and AgMnO4 (distorted barite)-type structures of CaSO4 at high pressure and temperature

Abstract Using laser-heated diamond-anvil cells, we have observed CaSO4 undergoing phase transitions from its ambient anhydrite structure to the monazite type, and at highest pressure and temperature to crystallize in the barite-type structure. On cooling, the barite structure distorts from an orthorhombic to a monoclinic lattice to produce the AgMnO4-type structure. The barite-structured form of CaSO4 that we encounter at high pressure and temperature has been, in particular, long expected as a highpressure phase of CaSO4-anhydrite from systematic trends of similar AIIBVIO4-type sulfates, selenates, and tellurates, but has not been observed before. Similarly, the monoclinic distortion of the barite structure has itself been proposed as an intermediate phase between anhydrite and barite types through comparison with the phase diagrams of NaBF4 and NaClO4. This result has important consequences for identifying structural trends between different ABO4-type phases of Group II sulfates, selenates, tellurates, chromates, molybdates and tungstates that crystallize in anhydrite, zircon, monazite, barite and scheelite-type structures at ambient and high pressures.

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.

Quantitative high-pressure pair distribution function analysis

The collection of scattering data at high pressure and temperature is now relatively straightforward thanks to developments at high-brightness synchrotron radiation facilities. Reliable data from powders, that are suitable for structure determination and Rietveld refinement, are routinely collected up to about 30 GPa in either a large-volume high-pressure apparatus or diamond anvil cell. In those cases where the total elastic scattering is of interest, as it is in the case of nano-crystalline and glassy materials, technical developments, including the use of focused high-energy X-rays (>80 keV), are advantageous. Recently completed experiments on nano-crystalline materials at the 1-ID beamline at the Advanced Photon Source suggest that quantitative data, suitable for pair distribution function analysis, can be obtained.

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.