Soufrière Hills Volcano had two periods of repetitive Vulcanian activity in 1997. Each explosion discharged the contents of the upper 0.5-2 km of the conduit as pyroclastic flows and fallout: frothy pumices from a deep, gas-rich zone, lava and breadcrust bombs from a degassed lava plug, and dense pumices from a transition zone. Vesicles constitute 1-66 vol.% of breadcrust bombs and 24-79% of pumices, all those larger than a few tens of µm being interconnected. Small vesicles (< few tens of µm) in all pyroclasts are interpreted as having formed syn-explosively, as shown by their presence in breadcrust bombs formed from originally non-vesicular magma. Most large vesicles (> few hundreds of µm) in pumices are interpreted as pre-dating explosion, implying pre-explosive conduit porosities up to 55%. About a sixth of large vesicles in pumices, and all those in breadcrust bombs, are angular voids formed by syn-explosive fracturing of amphibole phenocrysts. An intermediate-sized vesicle population formed by coalescence of the small syn-explosive bubbles. Bubble nucleation took place heterogeneously on titanomagnetite, number densities of which greatly exceed those of vesicles, and growth took place mainly by decompression. Development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion-decompression and surface quench in contact with air. Lava-plug fragments entered the air quickly after fragmentation (~ 10 s), so the interiors continued to vesiculate once the rinds had quenched, forming breadcrust bombs. Deeper, gas-rich magma took longer (~ 50 s) to reach the surface, and vesiculation of resulting pumice clasts was essentially complete prior to surface quench. This accounts for the absence of breadcrusting on pumice clasts, and for the textural similarity between pyroclastic flow and fallout pumices, despite different thermal histories after leaving the vent. It also allowed syn-explosive coalescence to proceed further in the pumices than in the breadcrust bombs. Uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn-explosive vesiculation even prior to magma fragmentation, probably in a zone of steep pressure gradient beneath the descending fragmentation front. Syn-explosive decompression rates estimated from vesicle number densities (> 0.3-6.5 MPa s − 1 ) are consistent with those predicted by previously published numerical models.

We demonstrate for the first time the instability of the lithium garnet Li 7 La 3 Sn 2 O 12 toward moisture. In ambient air a spontaneous Li þ /H þ exchange occurs leading to Li 7-x H x La 3 Sn 2 O 12 . The lithium released is combined with the atmospheric CO 2 under the Li 2 CO 3 form making Li 7 La 3 Sn 2 O 12 a CO 2 absorbent. The structural study of the exchanged Li 7-x H x La 3 Sn 2 O 12 phase has been performed on samples obtained by heating at reflux the mother compound in a solution of benzoic acid/ ethanol during one week. After chemical analysis of the lithium content and thermal analysis, the compound has been structurally characterized by the Rietveld method from powder and single crystal XRD experiments. It was found to crystallize in the cubic space group Ia3d with a Li þ distribution comparable to that observed in other cubic lithium garnets: three lithium sites partially occupied (24d, 48g, and 96h). The total ionic conduction of the exchanged garnet has been studied by impedance spectroscopy and compared to that of the mother form Li 7 La 3 Sn 2 O 12 . This study revealed that after Li þ /H þ exchange the conduction remains unchanged.

We have recently demonstrated the instability of Li 7 La 3 Sn 2 O 12 garnet in humid atmosphere: a spontaneous and reversible ionic exchange Li + /H + occurs rapidly, leading to the protonated garnet Li 7-x H x La 3 Sn 2 O 12 . In the present article, we show that this instability cannot be generalized to all lithium garnets. We have tested different garnets with various cell parameters, lithium quantity, and lithium distribution and have observed that the Li + /H + exchange feasibility is directly connected to the lithium stoichiometry: if the concentration of Li + ions is greater than what can be accommodated on the tetrahedral site commonly occupied, meaning more than three lithium ions per formula, the garnet is sensitive to humidity. Structures determined by powder neutron diffraction are presented for two exchanged garnets: Li 7-x H x La 3 Sn 2 O 12 and Li 5-x H x La 3 Nb 2 O 12 obtained from ionic exchange in ethanol and benzoic acid. For the Sn one, the ionic Li + /H + exchange is associated with a transition from a tetragonal (I4 1 /acd) to a cubic (Ia3̅ d) cell, while for the Nb phase, the use of a noncentrosymmetric space group (I2 1 3), confirmed by second harmonic generation (SHG) test, is essential to describe the structure. The work is completed by 6 Li, 7 Li, and 119 Sn solid state Nuclear Magnetic Resonance (NMR) of the tin compounds. The impact of the Li + /H + exchange on the dynamics of the lithium ions has been investigated by 7 Li relaxation, and the dynamics of protons and lithium ions in the exchanged phase have been compared.