Pier Francesco Zanazzi

High-pressure structural study of muscovite

The compressibility and structural variations of two 2M1 muscovites having compositions (Na0.07K0.90 Ba0.01□0.02)(Al1.84Ti0.04Fe0.07Mg0.04)(Si3.02Al0.98) O10 (OH)2 (7 mole % paragonite) and (Na0.37K0.60□0.03)(Al1.84Ti0.02 Fe0.10Mg0.06)(Si3.03Al0.97) O10(OH)2 (37 mole % paragonite) were determined at pressures between 1 bar and 35 kbar, by single-crystal X-ray diffraction using a Merrill-Bassett diamond anvil cell.Isothermal bulk moduli, setting K′ = 4, were 490 and 540 (± 30) kbar for the Na-poor and Na-rich samples respectively. Both samples show highly anisotropic compressibility patterns, with βa∶βb∶βc= 1∶1.15∶3.95 for the Na-poor sample and βa∶βb∶βc= 1∶1.19∶3.46 for the Na-rich one.HP structural refinements showed that the different compressibility was largely due to the partial substitution of Na for K in the interlayer region. Moreover, the different compressibility of the tetrahedral and octahedral layers, observed in both micas, increased the a rotation of the tetrahedral layer by about 2° in 28 kbar, as also indicated by the evolution of interlayer cation bond lengths. This increases the repulsion of oxygens of the basal layers and between the high-charged cations of the tetrahedral layer. As a consequence, phengitic substitution, reducing α rotation, would increase the baric stability of mica.Comparison between the HP structures of muscovite and phlogopite indicated the lower compressibility of the latter, mainly due to the greater compressibility of the dioctahedral layer with respect to that of the trioctahedral layer.The HT and HP behaviour of di- and trioctahedral micas showed an anisotropy in the compressional pattern which was markedly greater than that observed in the dilatation pattern. This unexpected result was explained by the different evolution with P and T of alkaliO bond lengths. By combining HP and HT data, a tentative equation of state of muscovite is proposed.

High-pressure behavior of kyanite; decomposition of kyanite into stishovite and corundum

Abstract The pressure stability of kyanite was experimentally reversed with the use of a multianvil apparatus. Kyanite was found to decompose into its oxides stishovite and corundum between 14 ± 0.5 GPa (at 1000 °C) and 17.5 ±1.0 GPa (at 2000 °C). Reliable thermodynamic calculations can be performed to temperatures of approximately 1500 °C. Up to this temperature, the location of the equilibrium kyanite = corundum + stishovite. determined in this study, constrains the equilibrium coesite = stishovite. A set of thermodynamic data was calculated by linear programming from the kyanite breakdown reaction and the coesite = stishovite equilibrium. Feasible values for the fitted thermodynamic properties are -28.5 to -26.3 MPa/K for the temperature derivative of the bulk modulus [(dK/dT)p] of kyanite, -815254 to -813635 J/mol for G0(1298) of stishovite. and 24.6 to 26.3 J/mol · K for S0(1298) of stishovite. The experimental results indicate (1) that in peraluminous eclogites of basaltic or sedimentary origin, stishovite may coexist stably with corundum at a depth greater than 420- 450 km and (2) that in an inhomogeneous Al-emiched mantle, corundum could be a minor constituent in the lower mantle.

High-pressure behavior of kyanite; compressibility and structural deformations

Abstract The lattice parameters of kyanite were measured at various pressures up to about 60 kbar by single-crystal X-ray diffraction in a diamond anvil cell. Unit-cell dimensions decreased linearly with an almost uniform rate: βa = 2.00(8) × 10-4, βb = 1.90(4) × 10-4, βc = 2.00(4) × 10 4 kbar-1. The principal compressibility coefficients were β1 = 2.23 × 10-4, β2 = 2.04 ×10-4, β3 = 1.65 × 10-4 kbar-1, with β1 forming an angle of 35° with the c axis. K0, calculated by fitting pressure- volume data to a third-order Birch-Mumaghan equation of state, was 1560(100) kbar. with K′ 5.6(5.5): when K′ was set at 4, K0 became 1600(30) kbar. Structural refinements were carried out on data collected at 0.001 kbar with the crystal m air and at 0.1. 25.4. 37. and 47 kbar with the crystal in the diamond anvil cell. Whereas the Si tetrahedra and A14 octahedron were incompressible in this P range, the polyhedral bulk modulus for Al1 and Al2 was 1280(150) kbar and 2380(200) kbar for Al3. These octahedra became more regular with increasing pressure. The almost isotropic compression pattern was due to the many shared edges between the polyhedra. uniformly distributed in the cell. The evolution of Al-Al separation showed that the largest reduction regarded the Al2-Al3 and Al2-Al4 distances, whereas the average Al1-Al2 distance was almost unchanged, resulting from linkage with Si tetrahedra having rigid edges. The result was that the largest reduction did not occur along the c axis but along the Al4-Al1-Al2-Al3 directions. The geometrical structural invariance, expressed by the βv/αv ratio and obtained from the average compressibility and average thermal expansion of the cell volume (Winter and Ghose 1979), was 23 °C/kbar. The following equation of state, which applies in crustal P-T conditions, may be defined as: V/V0 = 1 + 3.00(7) × 10-5T- 5.8(1) × 10-4 P, where T is in °C and P in kbar. The present volume-pressure data support multi-anvil experiments by Schmidt et al. (1997) defining the P-T conditions necessary for decomposition of kyanite into stishovite + corundum.

Effects of temperature and pressure on the structure of lawsonite

Abstract Independent isobaric data on thermal expansion and isothermal compressibility data for lawsonite, CaAl2Si3O7(OH)2 ·H2O, were determined between 23 and 598 °C and 0.001 and 37.7 kbar using single-crystal X-ray diffraction in a microfumace and a diamond- anvil cell, respectively. The crystal structures of lawsonite were also refined from intensity data collected at 23, 444, and 538 °C and 0.5 and 28.7 kbar. Both expansion and compression patterns are slightly anisotropic, with minor changes along c with respect to a and b. Unit-cell dimensions vary linearly with T and P: αa = 1.26(4) × 10-5, αb= 1.12(4) × 10 -5, αc = 7.6(3) × 10-6°C-1, and βa = 3.4(1) × 10-4, βb = 3.0(1) × 10-4, βc = 2.8(2) × 10-4 kbar-1. Bulk modulus, calculated as the reciprocal of cell-volume compressibility, is 1100(40) kbar. The cavities of the framework accommodating Ca and H2O molecules change by only about ± 5% in the investigated T and P ranges. Thus, at room pressure H2O molecules can be hosted in the lawsonite structure at least up to above 500 °C, where the reduction of reflection intensities shows the beginning of dehydration and breakdown of the phase. Like other dense phases, structural changes with T and P essentially affect bond lengths, whereas interpolyhedral variations mainly concern the Si-O-Si′ angle between tetrahedral pairs, which increases with temperature and decreases with pressure. The present data define the “geometric” equation of state for lawsonite on the basis of cell-volume variations: V/V0 = 1 + 3.13(9) × 10-5T - 9.1(3) × 10-4 P, where T is in degrees Celsius, P is in kilobars, and the α/β ratio is 34 bar/°C. This indicates that the cell volume of lawsonite remains unchanged with geothermal gradients of about 10 °C/km, a condition actually observed in down-going subduction slabs. Therefore, the results of high- T and high-P structure refinements are in agreement with results from multi-anvil experiments and confirm that lawsonite is a good candidate for carrying water down to mantle depths.

The 10 Å phase: Crystal structure from single-crystal X-ray data

Abstract Here we report the results of the first three-dimensional refinement of the 10 Å phase performed with single-crystal X-ray data. The 10 Å phase, Mg3Si4O10(OH)2H2O, is monoclinic, space group C2/m, a = 5.323(1)Å, b = 9.203(1)Å, c = 10.216(1)Å, β = 99.98(1)°, V = 492.9(2) Å3; the calculated density, assuming Z = 2, is 2.676 g.cm-3. The structure has been solved by direct methods and refined by least-squares method with anisotropic displacement parameters. The final agreement index (R1) was 0.088 for 54 refined parameters and 499 unique observed reflections collected with a diffractometer with a CCD detector. The structure of the 10 Å phase is very similar to that of a homo-octahedral, 1 M trioctahedral mica: it is a silicate consisting of 2:1 tetrahedral-octahedral layers parallel to (001). The mean Si-O, Mg1-O, and Mg2-O bond lengths are 1.626, 2.082, and 2.081 Å, respectively. The ditrigonal rotation angle α is 0.53°. The interlayer of the 10 Å phase is occupied by water molecules. According to the oxygen occupancy, 1 H2O p.f.u. is assumed in the investigated sample. Although the average water oxygen position is in the mid-plane, structural refinement suggests disorder along c*. Twelve hydrogen bonds are located between the water molecule and the 6 + 6 oxygen atoms of the basal rings of adjacent tetrahedral sheets (water-oxygen distances averaging 3.19 Å). Therefore there are six possible orientations for the water molecule, with six hydrogen bonds pointing toward the upper basal ring and six pointing toward the lower ring of tetrahedral sheets. The orientational disorder of water, in agreement with previous Raman spectroscopy data, is a feature relevant to the evaluation of thermodynamic functions and thermal stability of the 10 Å phase, which is a possible water carrier (9.1 wt%) in subducting slabs at high pressure.

Preparation and physico-chemical properties of the ternary complexes formed between adenosine 5′-triphosphoric acid, bis(2-pyridyl)amine, and divalent metal ions. Crystal and molecular structures of the compounds containing MgII and CaII

Ternary compounds formed between M (MgII, CaII, SrII, MnII, CoII, CuII, or ZnII), adenosine 5′-triphosphate [adenosine 5′-triphosphate(4–)= atp], and bis(2-pyridyl)amine(bipyam) have been prepared. The solid compounds are crystalline and have a stoicheiometry described by the formula M(Hatp)(Hbipyam)·nH2O (n= 2–9). X-Ray powder diffraction patterns are similar. Potentiometric titrations in aqueous solution show the presence of two ionizable protons. Visible spectra suggest an octahedral co-ordination geometry. I.r. spectra indicated essentially the same type of metal–ligand interactions in all the complexes and show that Hatp3– co-ordinates to the metal through the oxygen atoms of the α, β, and γ phosphate groups. The ternary compounds where M = MgII(1) or CaII(2) have been studied by single-crystal X-ray diffraction techniques and their molecular structures determined. The two species are isostructural and can be formulated as [Mg(H2O)6][Hbipyam]2[Mg(Hatp)2]·12H2O (1) and [Ca(H2O)6][Hbipyam]2[Ca(Hatp)2]·9H2O (2). Both (1) and (2) crystallise in space group C2221(Z= 4), with a= 22.734(3), b= 10.233(3), c= 30.997(4)A for (1) and a= 22.965(3), b= 10.154(3), c= 32.390(4)A for (2). X-Ray diffraction data were collected on a Philips automatic diffractometer and the structures solved by direct methods using the SIR (Semi-invariant Representation) package and refined by full-matrix least squares to final R values of 0.111 and 0.124 (1 088 and 1 008 independent observed reflections) for (1) and (2) respectively. In the [M(Hatp)2]4– units the metal ions lie on a two-fold axis with an octahedral co-ordination geometry completed by the oxygen atoms of the α, β, and γ phosphate groups of two symmetry-related Hatp3– molecules. The co-ordination polyhedron of (1) is nearly regular but in (2) it is significantly distorted. The phosphate chains have a folded configuration in both (1) and (2). In both complexes there are no bonding interactions between the metal ions and the adenine base. The metal atoms of the [M(H2O)6]2+ cations are also located on two-fold axes while the six co-ordinated water molecules form hydrogen bonds with the phosphate chains. The Hbipyam+ molecules do not co-ordinate to the metal ions and are disordered around two-fold axes. Strong stacking interactions exist between Hbipyam+ and purine rings.

New insights on high-pressure behaviour of microporous materials from X-ray single-crystal data

Abstract The main deformation mechanisms induced by pressure on different structural types of zeolites were analysed by comparing experimental data and theoretical models. Data of single-crystal X-ray diffraction obtained with the sample in a Merrill–Bassett diamond anvil cell on a four-circle diffractometer were collected at different pressures for samples of heulandite, scolecite and bikitaite, using non-penetrating pressure transmitting media (glycerol or silicon oil), up to 5 GPa. The results indicated that, at first approximation, the theoretical approach reproduces the structural evolution of zeolites under pressure. However, the flexibility possessed by framework microporous silicates resulted more complex than that which can be modelled by undeformable “rigid-unit modes”, being completely flexible in the oxygen hinges. Moreover, the compressibility of the zeolites under study does not appear to be directly related to the microporosity represented by the framework density (FD): the bulk moduli (simply defined as the inverse of volume compressibility coefficients) of heulandite (27.5(2) GPa) and scolecite (54.6(3) GPa) were different even though their FD’s were quite similar. Single crystal data have shown that the structural evolution of the open-framework silicates, is strongly controlled by the framework, whereas the role of the extra-framework content was less important. In all three zeolites the position of the extra-framework water molecules and cations was maintained approximately and their coordination numbers remained unchanged within the pressure range investigated.

High-pressure structural behaviour of scolecite

The HP structural evolution of a natural scolecite from Iceland (space group Cc ) was studied up to 5 GPa using in situ single-crystal X-ray diffraction data from a diamond-anvil cell (DAC) with silicon oil as non penetrating pressure transmitting medium. Linear regressions yielded mean axial compressibilities for a , b and c axes of β a = 4.4(2)·10–3, β b = 6.1(2)·10–3, β c = 6.0(1)·10–3 GP a-1 . K 0 , refined with a second-order Birch-Murnaghan equation, fixing K 0 ’ at 4, is 54.6(7) GPa. The bulk scolecite structure compression was the result of the “soft” behaviour of the channels (K ≅ 17 GPa for [100]-channels; K ≅ 50 GPa for [001]-channels) and the more rigid behaviour of the tetrahedral framework (K ≅ 96 GPa), which underwent kinking of the Secondary Building Unit (SBU) along [100]-chains. The angle between the SBUs (φ), increased from 20.80(2)° at 0.0001 GPa, to 22.00(6)° at 3.38 GPa. Within the investigated pressure range, the position of the extra-framework cations and water molecules remained almost unchanged. Up to 4.2 GPa no phase transition was observed.

High-pressure behavior of gypsum: A single-crystal X-ray study

Abstract High-pressure X-ray diffraction was carried out on a single crystal of gypsum compressed in a diamond anvil cell. The sample maintained its crystal structure up to 4.0 ± 0.1 GPa. The fit of pressure dependence of the unit-cell volume to the third-order Birch-Murnaghan equation yielded KT0 = 44(3) GPa and (∂KT/∂P)0 = 3.3(3), where KT0 and (∂KT/∂P)0 are the isothermal bulk modulus and its pressure derivative in ambient conditions. The axial compressibility values, fitting data collected up to 3.94 GPa, were β0aEoS = 6.1(1) and β0cEoS = 5.6(1) 10-3 GPa-1. The value of β0bEoS was 6.2(8) 10-3 GPa-1 fitting the data collected up to 2 GPa, due to non-linearity above this pressure; axial compressibility of gypsum is almost isotropic (β0a:β0b:β0c = 1:1:0.9). This behavior is partly unexpected for a layered mineral based on alternate layers of Ca- and S-polyhedral chains separated by interlayers occupied by water molecules. Above 4.0 GPa the compression curve of gypsum shows a discontinuity with a 2.5% contraction in volume. Structural refinements indicate that SO4 volume and average S-O bond distances remain almost unchanged from room pressure to 3.9 GPa [range 1.637(4)-1.66(9) Å3; 1.4733-1.48 Å]. The SO4 tetrahedron undergoes distortion: the smaller distance decreases from 1.4731(9) to 1.45(2) Å and the larger increases from 1.4735(9) to 1.51(2) Å. In contrast, the calcium polyhedra show expected high-pressure behavior, becoming more regular and decreasing in volume from 25.84(8) Å3 at ambient P to 24.7(1) Å3 at 3.9 GPa. The largest variations were observed in the interlayer region where the water molecules are located. Along the b axis, the two structural layers have very different compressibilities: the polyhedral layer is almost incompressible in the pressure range studied, whereas water layer compressibility is 9.7(3) 10-3 GPa-1, about twice that of the other two lattice parameters. At ambient conditions, water molecules form weak hydrogen bonds with the O atoms of Ca and S polyhedra. With increasing pressure, the weakest hydrogen bond becomes the strongest one: from 0.001 to 4 GPa, the distance changes from 2.806(1) to 2.73(2) Å for OW-H1···O2, and from 2.883(2) to 2.69(3) Å for OW-H2···O2. Structure refinements show that water remains in the structure when P increases. The observed distortion of sulfate tetrahedra explains the splitting of the ν1 sulfate stretching mode, and the various measured compressibilities of the two hydrogen bonds and the coalescence of the Raman stretching mode observed at pressures over 5 GPa.

Effects of pressure on the structure of bikitaite

The structural behaviour of bikitaite, Li 2 (Al 2 Si 4 O 12 ). 2H 2 O, was investigated under hydrostatic pressure using X-ray single-crystal diffraction data. A Merrill-Bassett diamond anvil cell was mounted with glycerol, as non penetrating pressure-transmitting medium, ruby chips and a small crystal of quartz as the calibrant. A strong anisotropic compression was observed by linear regressions of lattice parameters against P, bikitaite being softer along the c axis (βc = 9.3(1) 10 -3 GPa -1 ), than along b (β b = 6.6(1) 10 -3 GPa -1 ) and a (β a = 2.4(1) 10 -3 GPa -1 ) (β a : β b : β c = 1 : 2.75 : 3.9). Fitting the cell-volume — pressure data to a second order Birch-Murnaghan equation of state, as indicated by the finite strain-stress plot, yielded K 0 = 44.2(4) GPa, with K’ = 4 and V 0 = 295.58(2) A 3 . The evolution of the bikitaite structure with P was studied by comparing the results of refinements with data collected at room conditions, at 3.2 GPa and after decompression. The structure can be described as sheets of six-membered rings parallel to (001), connected by pyroxene-like chains. 8-ring and 5-ring channels run along [0 10] and inside the 8-ring channel there is a onedimensional chain of water molecules, which is linked to the framework through the extra-framework Li atoms. Under pressure, the kinking of the pyroxene-like chain decreased the free diameters of the 5-ring channels, strongly reducing the distance between the ab planes. On the contrary, the tridymite-like planes with 6-membered rings were more rigid. The positions of the extra-framework cations and water were maintained at HP even though the configuration of the water chains changed slightly: the distances between the water molecules decreased, whereas the kinking angle of the chain increased.