HIGH-PRESSURE BEHAVIOR OF MICROPOROUS MATERIALS: CRYSTAL-FLUID INTERACTIONS AND DEFORMATION MECHANISMS AT THE ATOMIC SCALE

Zeolite are crystalline, hydrated aluminosilicates characterized by a tetrahedral framework of TO4 units connected in such a way that sub-nanometric channels and cages occur. These structural cavities host the so-called extra-framework population, which mainly consists of alkali and alkaline-earth cations and small molecules, such as H2O. In the last decades, the scientific community showed a rising interest on the behavior of microporous and mesoporous compounds (e.g., zeolites) at high-pressure conditions, and in particular on the crystal-fluid interaction phenomena occurring at extreme conditions. As zeolites could act as an ideal carrier of H2O and others small molecules or monoatomic species (e.g., CO2, CH4, H2S, He, Ar, Kr, Xe,\u2026), experiments on zeolites compressed (and ambient to low/high T) in aqueous mixtures have important implications in the Earth Sciences. Furthermore, high-pressure experiments on synthetic zeolites may pave the way for new routes of tailoring new functional materials (made by hybrid host-guest architecture), bearing a potentially relevant technological impact. In this experimental thesis, after an overall introduction and a section on the high-pressure experimental techniques (Chapter 1 and 2 , respectively), the high-pressure behavior and the crystal-fluid interaction at the atomic scale of a selected series of natural and synthetic zeolites (i.e., AlPO4-5, leonhardite, laumontite, phillipsite) and a zeolites-like mineral (i.e., armstrongite) have been investigated by means of in-situ single-crystal X-ray diffraction, using \u201cpenetrating\u201d and \u201cnon-penetrating\u201d pressure-transmitting fluids. Into details: 1. AlPO4-5 (Chapter 3): the high-pressure behavior of AlPO4-5 has been studied by single crystal XRD using synchrotron radiation and a diamond anvil cell (DAC), with crystals compressed in silicone oil and methanol:ethanol:water =16:3:1 (m.e.w.) mixture. The high-pressure evolution of the crystalline structure and the deformation mechanism at atomic scale have been described on the basis of high-quality structure refinements, revealing adsorption phenomena of H2O (and likely methanol) already at 2 Kbar. Moreover, evidence of an incommensurately modulated structure of AlPO4-5 have been found. 2. Leonhardite and laumontite (Chapter 4): the H2O adsorption kinetics, at ambient pressure and temperature, of leonhardite to give laumontite has been investigated using single crystal XRD techniques. In-situ high-pressure XRD experiments, using synchrotron radiation and a DAC, have been performed in order to obtain the bulk moduli of the two minerals (previously unknown). A detailed description of the atomic deformation mechanisms has been addressed. 3. Phillipsite (Chapter 5): the pressure-induced deformation mechanisms, at the atomic scale, have been studied via single crystals XRD-experiments, using synchrotron radiation and a DAC. Despite no pressure-induced adsorption was observed, the experimental findings suggest a change in the deformation mechanisms induced by a re-arrangement of the extra-framework population. 4. Armstrongite (Chapter 6): the high-pressure evolution of this zeolite-like mineral has been studied in the m.e.w. as nominally penetrating fluid. A first-order phase transition has been detected between 4 and 5 GPa. In the Chapter 7 a detailed discussion of the aforementioned experimental findings has been addressed, along with their technological and geological implications. The results of the present studies have been published in peer-reviewed journals

Phase transitions and crystal structure evolution of hydrated borates at non-ambient conditions

Hydrated borates are a class of minerals composed of clusters or chains of Bφx groups (where φ represents an oxygen atom, a H2O molecule, or an OH- group) organized either in tetrahedra or planar triangular groups. Hydrated borates are considered a more cost-effective alternative to B4C in radiation-shielding concretes [1], primarily due to the significant cross-section (~3840 barns) for thermal neutrons of the 10B isotope, which represents approximately 20% of natural boron. It is advisable to comprehensively characterize the crystal chemistry, elastic properties, P-T phase stability fields, and structural behaviour of natural borates under varying temperature and pressure conditions to model and understand their role as aggregates in radiation-shielding concretes [2], where the components experience pressure (via static compression) and temperature (via irradiation). Since 2018, my research group has conducted an extensive study of economically valuable hydrated borates, as well as common complementary phases occurring in borates deposits. High-pressure investigations of all studied hydrated borates have revealed one or more phase transitions occurring at pressures below 11 GPa, and the occurrence of these transitions appears to be highly correlated with the H2O content of the minerals (e.g., [3-4]). In response to the phase transitions, the most significant structural change observed in our experiments is the increase in the coordination number of alkali/alkaline-earth cations as well as of part of the boron population, from IIIB to IVB, due to the interaction between IIIB and H2O molecules. This, on the other hand, emphasizes the importance of the hydrogen bond network, usually with complex and pervasive configuration, in preserving the stability of the crystalline edifice of this class of materials. References 1. Okuno K. (2005). Radiat. Prot. Dosimetry. 115, 258–261. 2. Torrenti J. and Nahas G. (2010) Int. Conf. Concr. under Sev. Cond., Merida, Yucatan. 3–18 3. Comboni D., Pagliaro F., Gatta G. D., et al. (2020) J. Am. Ceram. Soc. 103:5291–5301 4. Comboni D., Poreba T., Pagliaro F., et al. (2021) Acta Crystallogr. B 77:940–945

High-pressure behavior and pressure-driven phase transitions in hydrated borates

Hydrated borates (e.g., colemanite, ulexite, kernite and borax) are the most common ore minerals of boron, an important geochemical marker, in particular in pegmatitic and granitic systems, for petrogenetic processes and a strategic element in a series of technological applications. In hydrated borates, the main structural units are Bφx units (tetrahedra and planar trigonal group where φ is an anion, O2- or OH-), connected in such a way to form clusters of polyions connected to alkaline/Earth alkaline (mainly Na+, K+, Ca2+, Mg2+) polyhedra. In these structures, H2O molecules and OH- form a complex and pervasive hydrogen-bond network, often enhancing the connection between the polyions clusters and the cations-polyhedra, therefore playing a paramount role in the stability of the crystalline edifice. The aim of this contribution is to analyze and provide insides on the high-pressure behavior and structure evolution of a number of hydrate borate minerals, unveiling the phase transition driving deformation mechanisms that lead to the formation of their high-pressure polymorphs. A common pattern, that could be used to predict the high-pressure phase stability of this class of minerals, has been detected

Unveiling the high-pressure transitions in hydrated borates at ID15b, ESRF

Natural borates (e.g., ulexite, colemanite, kernite and borax) are the most common ore minerals of boron, strategic element in a series of technological applications. In hydrated borates, the main structural units are Bφx units (tetrahedra and planar trigonal group where φ is an anion, O2- or OH-), connected in such a way to form clusters of polyions connected to alkaline/Earth alkaline (mainly Na+, K+, Ca2+, Mg2+) polyhedra. In these structures, H2O molecules and OH- form a complex and pervasive hydrogen-bond network, often enhancing the connection between the polyions clusters and the cations-polyhedra, therefore playing a paramount role in the stability of the crystalline edifice. Hydrated borates can act as neutron-shielding materials, due to the isotope 10B (which accounts for about 20% of the natural boron) high cross-section for thermal neutrons (~3840 barns) [1]. Enhanced neutron radiation shielding capacity is achievable by using boron-containing minerals as aggregates in concretes. Notably, a comprehensive characterization of the crystal-chemistry, elastic parameters, phase-stability and structural behaviour (at the atomic scale) at different T and P conditions, is still missing for most hydrated borates. The aim of this contribution is to analyze and provide insides on the high-pressure behavior and structure evolution of a number of hydrate borate minerals, unveiling the phase transition driving deformation mechanisms that lead to the formation of their high-pressure polymorphs

Pressure-driven phase transitions in hydrated borates

Hydrated borates (e.g., colemanite, ulexite, kernite and borax) are the most common ore minerals of boron, an important geochemical marker, in pegmatitic and granitic systems, for petrogenetic processes and a strategic element in a series of technological applications. Hydrated borates, which have been listed as critical raw materials by the EU [1], could be used as aggregate in neutron-shielding Sorel or Portland concretes, enhancing the adsorption towards thermal neutrons. In hydrated borates, the main structural units are Bφx units (tetrahedra and planar trigonal group where φ is an anion, O2- or OH-), connected in such a way to form clusters of polyions connected to alkaline/Earth alkaline (mainly Na+, K+, Ca2+, Mg2+) polyhedra. In these structures, H2O molecules and OH- form a complex and pervasive hydrogen-bond network, often enhancing the connection between the polyions clusters and the cations-polyhedrons, therefore playing a paramount role in the stability of the crystalline edifice [2, 3]. The aim of this contribution is to analyze and provide insides on the high-pressure behavior and structure evolution of several hydrate borate minerals, unveiling the phase transition driving deformation mechanisms that lead to the formation of their high-pressure polymorphs. A common pattern, that could be used to predict the high-pressure phase stability of this class of minerals, has been detected

Chemical reactivity of pozzolans from Sardinia for the industrial production of hydraulic limes

The research is aimed at studying the chemical reactivity between lime and volcanic rocks belonging to different Sardinian outcrops, for a use as raw material in the production of hydraulic / pozzolanic limes. On the basis of preliminary geochemical and mineralogical-petrographic investigations, several volcanic rocks from basic-intermediate to acid in composition (substantially from andesitic, to dacitic, to rhyolitic) have been selected and used for to perform laboratory reactivity tests. These rocks differ in the variable content of glass (from 15% to about 95% in volume), due to the presence of secondary minerals, and to physical properties (density, porosity, water absorption, etc.). The physical characteristics are essentially linked to the different compositional incidence of the crystalline, crystal-clastic, lithic (present in some pyroclastic facies), type and quantity of glass phases, to their different methods of installation (conditioned by temperature, chemical composition, grade welding, etc.), and to the different degree of alteration. The results of the investigations on the pozzolan materials (by polarized light microscopy, XRD, SEM, EPMA-WDS, Chapelle test) show that following parameters affect the chemical reactivity of the volcanic products with lime: i) quantity and type of amorphous phases (glass), linked to the different emplacement of volcanic rocks (affected by temperature, chemical composition, welding grade, etc.), ii) compositional incidence of the crystalline phases, crystal-clasts, lithics (these latter present in some pyroclastic facies), iii) alteration grade of the rocks and presence of secondary minerals (e.g., zeolites, phyllosilicates, etc.)

A comparative study of crystal-fluid interaction phenomena in ABC-6 zeolites group: the case of ERI, OFF and EAB topologies

The pressure-mediated intrusion of molecules, along with solvated ions, into the nano-cavities of microporous or layered materials is one of the most spectacular routes to promote a mass transfer from fluids to crystalline solids. This phenomenon, observed for example in synthetic and natural zeolites, is now exploited in order to expand their industrial utilization, developing new functional materials and enhancing catalytic performance [1,2]. From a geological perspective, a full understanding of the Pinduced crystal-fluid interaction could lead to a re-evaluation of the role played by zeolites as fluid carriers during the early stages of subduction, considering that this class of open-framework silicates can have even up to 20 wt.% H2O. In this study, we have investigated the crystal-fluid interaction, promoted by pressure, of three different natural zeolites belonging to the ABC-6 group: erionite (ERI framework, 6-membered ring sequence of AABAAC), offretite (OFF, sequence of AAB), and bellbergite (EAB, sequence of AABCCB). The goals of the experiments were twofold: 1) to understand the potential role of erionite as a fluid carrier during subduction, given its status as one of the alteration minerals in oceanic floor basalts, and 2) to compare the mechanisms used by structurally similar frameworks in accommodating bulk compression and adsorbing new molecules. Synchrotron XRD experiments were conducted to investigate erionite, offretite, and bellbergite single crystals, using a diamond anvil cell and both potentially penetrating and non-penetrating Ptransmitting fluids. The latter were used as a benchmark for evaluating crystal-fluid interaction, as the adsorption of new molecules decreases the bulk compressibility due to the "pillar" effect played by guest species in structural voids [1]. The results showed that erionite experiences the highest adsorption magnitude among the three zeolites. Furthermore, the occurrence and magnitude of the crystal-fluid interaction phenomena were found to be strongly governed by the H2O content of the hydrous P-transmitting fluids used for the experiments. Ne atoms were observed to penetrate into the offretite framework, making weak Van der Waals interactions with the extra-framework population. Natural bellbergite was found to be almost impenetrable for guest molecules from the transmitting fluids, highlighting the key role played by "secondary factors", such as the extra-framework content of the mineral, in crystal-fluid interaction phenomena

High-pressure phase trasition and crystal structure evolution of inderite, MgB3O3(OH)5 5H2O

Inderite, ideally [MgB3O3(OH)5∙5H2O], is a light (1.80 g/cm3) Na-free hydrated borate, discovered in the Inder deposit (Kazakhstan), which could be efficiently employed in radiation-shielding concretes due to its relatively high B2O3 content (⁓37 wt%). The crystal structure of inderite is made by [B3O3(OH)5]2- polyions, organized in 3-membered rings of 2 Bφ4 tetrahedra and one Bφ3 unit (where φ is an anion; O2-or OH-). Prior to any utilization, is advisable to correctly characterized the thermodynamic parameters of any aggregate, if used in neutron-shielding concretes, where temperature can increase due to the interactions with the highly energetic neutron beam. Overall, phase transitions occurring at different pressures (and temperatures) were discovered in all the hydrous borates investigated so far (e.g., [1, 2]), suggesting that the high-pressure stability of hydrated borates having polyions organized in isolated units (e.g., inderite) is directly correlated with the total H2O content of the mineral itself. Inderite is the ideal case-scenario to validate this model and here we report the results of this study that leads to: 1) track the isothermal compressional path, based on the experimental P-V data, 2) derive the elastic parameters, currently unavailable in the literature; 3) investigate the phase-stability field of inderite at high-pessure; 4) describe the high-pressure structural re-arrangement of inderite at the atomic scal

High-pressure behavior of intermediate scapolite : compressibility, structure deformation and phase transition

Scapolites are common volatile-bearing minerals in metamorphic rocks. In this study, the high-pressure behavior of an intermediate member of the scapolite solid solution series (Me47), chemical formula (Na1.86Ca1.86K0.23Fe0.01)(Al4.36Si7.64)O24[Cl0.48(CO3)0.48(SO4)0.01], has been investigated up to 17.79 GPa, by means of in situ single-crystal synchrotron X-ray diffraction. The isothermal elastic behavior of the studied scapolite has been described by a III-order Birch\u2013Murnaghan equation of state, which provided the following refined parameters: V0 = 1110.6(7) \uc53, KV0 = 70(2) GPa (\u3b2V0 = 0.0143(4) GPa 121) and KV\u2032 = 4.8(7). The refined bulk modulus is intermediate between those previously reported for Me17 and Me68 scapolite samples, confirming that the bulk compressibility among the solid solution increases with the Na content. A discussion on the P-induced structure deformation mechanisms of tetragonal scapolite at the atomic scale is provided, along with the implications of the reported results for the modeling of scapolite stability. In addition, a single-crystal to single-crystal phase transition, which is displacive in character, has been observed toward a triclinic polymorph at 9.87 GPa. The high-pressure triclinic polymorph was found to be stable up to the highest pressure investigated