G. D. Gatta

A comparative study of fibrous zeolites under pressure

Fibrous zeolites (FZ) are the only group of natural zeolites that have been well investigated under pressure. The high-pressure (H P ) behaviour of several FZ has been studied by means of in situ H P -single-crystal/powder diffraction experiments. Here I report a comparative study on lattice compressibility, structural deformation mechanisms and the role played by the framework (Si/Al-distribution, cross-linking of the building unit chains) and extra-framework content on the H P -behaviour of FZ. The structural analogy among the FZ group, due to the 4 = 1 secondary building unit (SBU), induces similar elastic behaviour and a “FZ-average bulk modulus” can be calculated: K T0 = 50±10 GPa. The bulk modulus value changes as function of the extra-framework content, following the sequence: K T0 (Ba-FZ)> K T0 (Ca-FZ)> K T0 ((Ca+Na)-FZ)> K T0 (Na-FZ). Another interesting result is related to the axial compressibility. The experiments on natrolite, scolecite, edingtonite and thomsonite show that the elastic anisotropy, represented by the axial bulk moduli, is strongly influenced by the tetragonal topological symmetry. The H P -structural refinements performed show one main deformation mechanism for all these zeolites: the cooperative rotation (anti-rotation) of the SBU. This mechanism strongly reduces the free volume of the 8-membered ring channels, parallel to the SBU-chain direction.

Zeolites at high pressure: A review

Abstract This is a review of the elastic behaviour and pressure (P)-induced structural evolution of zeolites and presents a comparative analysis of the deformation mechanisms of the Si/Al-framework and the rearrangement of the extra-framework species in response to applied pressure. The interaction between P-transmitting fluids and zeolites, which can lead to phenomena such as ‘P-induced over-hydration’, is described. The comparative elastic analysis and the high-P structural data of zeolites reported so far allow us to make some generalizations: (1) The range of compressibility among this class of openframework silicates is large, with bulk moduli ranging between 15 and 70 GPa; (2) Microporosity does not necessarily imply high compressibility, as several zeolites are less compressible than other non-zeolitic rock-forming minerals; (3) Compressibilities of zeolites do not seem to be directly related to microporosity, at least if we model microporosity with the ‘framework density’; (4) The flexibility observed in zeolites under hydrostatic compression is mainly governed by tilting of rigid tetrahedra around O atoms that behave as hinges within the framework. Pressure-induced tilting commonly leads to continuous rearrangement of the framework without any phase transition. More rarely, tilting induces displacive phase transitions and isothermal P-induced reconstructive phase transitions (i.e. with change in framework topology), have not been reported in this class of materials; (5) Deformation mechanisms in response to applied pressure are generally dictated by the topological configuration of the framework rather than the Si/Al-distribution or the extra-framework content. The channel content governs the compressibility of the cavities, leading to different unit-cell-volume compressibilities in isotypic structures.

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.

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.

First accurate location of two proton sites in tourmaline: A single-crystal neutron diffraction study of oxy-dravite

Abstract A single-crystal neutron diffraction study of oxy-dravite from Osarara (Narok district, Kenya) was performed. Intensity data were collected in Laue geometry at 10 K and anisotropic-structure refinement was undertaken. For the first time, two independent H sites were refined unambiguously for a mineral belonging to the tourmaline supergroup and located at 0.26, 0.13, 0.38 (labelled as H3, site occupancy ~98%) and at 0, 0, 0.9 (labelled as H1, site occupancy ~25%). The H-bonding scheme can thus be defined as follows: (1) the O at the O3 site acts as a ‘donor’ and the O at the O5 site as ‘acceptor’, the refined O3-H3 bond distance is 0.972(2) Å (and 0.9946 Å corrected for ‘‘riding motion’’), H3···O5 = 2.263(2) Å, O3···O5 = 3.179(1) Å and O3-H3···O5 = 156.6(1)°; (2) the oxygen at the O1 site acts as a ‘donor’ and the O atoms at O4 and O5 as ‘acceptors’, the refined O1-H1 bond distance is 0.958(8) Å (and 0.9833 Å corrected for ‘‘riding motion’’), H1···O4 = 2.858(6) Å, O1···O4 = 3.378(1) Å and O1···H1-O4 = 115.12(1)°, whereas H1···O5 = 2.886(6) Å, O1···O5 = 3.444(1) Å and O1-H1···O5 = 118.23(1)°. A further test refinement was performed with the H1 site out of the three-fold axis (at 0.02, 0.01, 0.90); this leads to O1-H1 = 0.995(8) Å (and 1.0112 Å corrected for ‘‘riding motion’’), H1···O4 = 2.747(6) Å and O1-H1···O4 = 121.7(4)°, whereas H1···O5 = 2.654(9) Å and O1-H1···O5 = 136.5(6)°. Bond-valence analysis shows that the H-bonding strength involving O3 is stronger than that involving O1: ~0.11 and <0.05 valence units, respectively. The refined angle between the O3-H3 vector and [0001] is 3.40(9)°. Such a small angle is in line with a pleochroic scheme for the OH-stretching absorption bands measured by infrared spectroscopy.

High-pressure study of a natural cancrinite

Abstract The high-pressure elastic behavior and the P-induced structure evolution of a natural cancrinite from Cameroun {Na6.59Ca0.93[Si6Al6O24](CO3)1.04F0.41·2H2O, a = 12.5976(6) Å, c = 5 .1168(2) Å, space group: P63} were investigated by in situ single-crystal X-ray diffraction under hydrostatic conditions up to 6.63(2) GPa with a diamond-anvil cell. The P-V data were fitted with an isothermal Birch-Murnaghan type equation of state (BM EoS) truncated to the third order. Weighted fit (by the uncertainty in P and V) gave the following elastic parameters: V0 = 702.0(7) Å3, KV0 = 51(2) GPa, and K´V = 2.9(4). A linearized BM EoS was used to fit the a-P and c-P data, giving the following refined parameters: a0 = 12.593(5) Å, Ka0 = 64(4) GPa, Ka´ = 4.5(9), for the a-axis, and c0 = 5.112(3) Å, Kc0 = 36(1) GPa, Kc´ = 1.9(3) for the c-axis (elastic anisotropy: Ka0:Kc0 = 1.78:1). A subtle change of the elastic behavior appears to occur at P > 4.62 GPa, and so the elastic behavior was also described on the basis of BM EOS valid between 0.0001-4.62 and 5.00-6.63 GPa, respectively. The high-pressure structure refinements allowed the description of the main deformation mechanisms responsible for the anisotropic compression of cancrinite on (0001) and along [0001]. A comparative analysis of the structure evolution in response of the applied pressure and temperature of isotypic materials with cancrinite-like topology is carried out.

Gemmological investigation of a synthetic blue beryl: a multi-methodological study

Abstract A multi-methodological investigation of a synthetic Cu/Fe-bearing blue beryl [IV(Be2.86Cu0.14)∑=3.00VI(Al1.83Fe0.143+Mn0.032+Mg0.03)∑=2.03IV(Si5.97Al0.03)∑=6.00O18·(Li0.12Na0.04·0.40H2O)] has been performed by means of gemmological standard testing, electron microprobe chemical analyses, laser ablation inductively coupled plasma mass spectroscopy, thermo-gravimetric analyses, infrared spectroscopy and single-crystal X-ray diffraction in order to determine the gemmological properties, crystal structure and crystal-chemistry of this material. The increasing production of marketable hydrothermal synthetic beryls with ‘exotic’ colours and the small number of studies on the accurate location of chromophores in the crystal structure inspired this multi-methodological investigation. The X-ray structural refinements confirm that the space group of the Cu/Fe-bearing blue beryl is P6/mcc, with unit-cell parameters: 9.2483 ≤ a ≤ 9.2502 Å and 9.2184 ≤ c ≤ 9.2211 Å. The analysis of the difference Fourier maps of the electron density suggests that Cu is located at the tetrahedral site (Wyckoff 6f position) along with Be, whereas Fe shares the octahedral site with Al (4c position). No evidence of extra-framework Cu/Fe-sites (i.e. channel sites) has been found. The Li is probably located at the extra-framework 2b site. Infrared spectra show that the H2O molecules are present with two configurations: one with the H⋯H vector oriented ‖[0001] and the other with H⋯H vector oriented ⊥[0001].

On the thermo-elastic behaviour of kyanite: a neutron powder diffraction study up to 1200°C

Abstract The high-temperature (HT) behaviour of kyanite (Al2SiO5) was investigated by in situ neutron powder diffraction up to 1200°C. Within the investigated T range, no phase transition was observed. The axial and volume thermal expansion coefficient (αj = lj−1(∂lj/∂T), αV = V−1 (∂V/∂T)), calculated by weighted linear regression through the data points, are: αa = 5.5(2)×10−5, αb = 5.9(2)×10−5, αc = 5.18(8)×10−5, αV = 7.4(1)×10−3 °C−1, with αa:αb:αc = 1.06:1.14:1. All three angles of the kyanite lattice show a slight decrease with T, with ∂α/∂T = −2(2)×10−5, ∂β/∂T = −4(1)×10−5, ∂γ/∂T = −10(2)×10−5 °C. The magnitudes of the principal Lagrangian unit-strain coefficients (Ɛ1, Ɛ2, Ɛ3) and the orientations of the thermal strain-ellipsoids, between the ambient temperature and each measured T, were calculated. The magnitude and the orientation of all the three unit-strain coefficients are almost maintained constant with T. At T-T0 = 1177°C , Ɛ1^a = 76(2)°, Ɛ1^b = 70(2)°, Ɛ1^c = 38(3)°, Ɛ2^a = 49(3)°, Ɛ2^b = 66(3)°, Ɛ2^c = 127(4)°, Ɛ3^a = 135(3)°, Ɛ3^b = 31(3)°, Ɛ3^c = 91(2)° with Ɛ1:Ɛ2:Ɛ3 = 1.57:1.29:1. The structural refinements, performed at 23, 600, 650, 700, 750, 800, 900, 950, 1050 and 1200°C allowed the description of the structural evolution and the main T-induced deformation mechanisms, which are mainly represented by the polyhedral distortions of the AlO6 octahedra.

The effect of pressure on open-framework silicates: elastic behaviour and crystal–fluid interaction

The elastic behaviour and the structural evolution of microporous materials compressed hydrostatically in a pressure-transmitting fluid are drastically affected by the potential crystal–fluid interaction, with a penetration of new molecules through the zeolitic cavities in response to applied pressure. In this manuscript, the principal mechanisms that govern the P-behaviour of zeolites with and without crystal–fluid interaction are described, on the basis of previous experimental findings and computational modelling studies. When no crystal–fluid interaction occurs, the effects of pressure are mainly accommodated by tilting of (quasi-rigid) tetrahedra around O atoms that behave as hinges. Tilting of tetrahedra is the dominant mechanism at low-mid P-regime, whereas distortion and compression of tetrahedra represent the mechanisms which usually dominate the mid-high P regime. One of the most common deformation mechanisms in zeolitic framework is the increase of channels ellipticity. The deformation mechanisms are dictated by the topological configuration of the tetrahedral framework; however, the compressibility of the cavities is controlled by the nature and bonding configuration of the ionic and molecular content, resulting in different unit-cell volume compressibility in isotypic structures. The experimental results pertaining to compression in “penetrating” fluids, and thus with crystal–fluid interaction, showed that not all the zeolites experience a P-induced intrusion of new monoatomic species or molecules from the P-transmitting fluids. For example, zeolites with well-stuffed channels at room conditions (e.g. natural zeolites) tend to hinder the penetration of new species through the zeolitic cavities. Several variables govern the sorption phenomena at high pressure, among those: the “free diameters” of the framework cavities, the chemical nature and the configuration of the extra-framework population, the partial pressure of the penetrating molecule in the fluid (if mixed with other non-penetrating molecules), the rate of P-increase, the surface/volume ratio of the crystallites under investigations and the temperature at which the experiment is conducted. An overview of the intrusion phenomena of monoatomic species (e.g. He, Ar, Kr), small (e.g. H2O, CO2) and complex molecules, along with the P-induced polymerization phenomena (e.g. C2H2, C2H4, C2H6O, C2H6O2, BNH6, electrolytic MgCl2·21H2O solution) is provided, with a discussion of potential technological and geological implications of these experimental findings.