Vapor-phase Crystallization Research Articles

Red mud treated with KOH: synthesis of sustainable materials from waste for water treatment.

Solid waste resulting from bauxite ore (red mud) was converted into useful products consisting in hydrogarnet together with zeolite. Red mud (RM) transformation from disposal material into new source was carried out using potassium hydroxide as an activator and hydrothermal process (HY) or vapor phase crystallization (VPC) approach. HY process was performed at 60, 90, and 130°C whereas during the VPC method, red mud was contacted only with vapor from the distilled water heated at 60 and 90°C. The results indicate the formation of katoite and zeolite L (LTL topology) with both approaches. All the synthetic products display magnetic properties. In addition, a preliminary investigation on arsenic removal from drinking water (from 59 to 86%), makes the synthetic materials appealing for environmental applications. Finally, the synthesis of a large amount of very useful newly-formed phases using vapor molecules confirms the efficiency of the innovative and green VPC process in waste material transformation.

Determining the Role of Water Molecules in Sodalite Formation Using the Vapor Phase Crystallization Method

The efficiency of the vapor phase crystallization (VPC) process in zeolite formation using mixtures of a natural source (obsidian) and common waste materials (red mud and fly ash) was analyzed. The aim was to demonstrate that water molecules available during this treatment control mainly the synthesis of sodalite, regardless of the raw material used, as long as it is rich in amorphous silica and alumina pre-fused with NaOH. The data indicate that increasing the temperature to generate steam from distilled water during the VPC process results in the continuous transformation of amorphous material into sodalite and, subordinately, cancrinite. The formation of the newly formed phases was monitored by powder XRD and SEM.

A green process for zeolite synthesis: low-temperature vapor phase treatment of natural bauxites

In this work, three samples of bauxite were used as inexpensive sources for the zeolite formation by a green process based on vapor-phase crystallization (VPC) method. The synthesis was carried out using a water bath with deionized water heated at 35, 45, 60 or 90 °C. During the process, NaOH pre-fused bauxites were contacted only with vapor from the liquid. The results indicate that sodalite formed in all the samples after VPC at higher temperatures (60 °C and 90 °C). Large amount of zeolite A was instead synthesized in the sample characterized by SiO2/Al2O3 ratio next to 1 and after vapor treatment at 35 °C and 45 °C. These data highlight the determining role of both raw material chemical composition and amount of water molecules in type of zeolite formed by vapor crystallization method. Moreover, the results indicate that VPC process made it possible to synthesize zeolite generating no water waste.

Synthesis of ZIF-8-coated Pt/SiO2 by vapor deposition for alkyne semi-hydrogenation

In this work, ZIF-8/Pt/SiO2 catalysts were prepared by combining atomic layer deposition (ALD) and vapor phase conversion methods. First, Pt metal nanoparticles were deposited on SiO2 nanowires by ALD. Then, ZnO was further deposited, also by ALD. Subsequently, the ZnO film was converted into ZIF-8 film by vapor phase crystallization to form a sandwich structure (ZIF-8/Pt/SiO2). The microstructures of the catalysts were characterized by XRD, TEM, BET, IC-MS, XPS, and CO-DRIFT. It was shown that the Pt particles were highly dispersed on the SiO2 nanowires before and after coating with ZIF-8, and the ZIF-8 film was coated continuously on the entire catalyst with high conformity. The performance of the catalyst was studied by using the semi-hydrogenation of 1-heptyne as a probe reaction. The ZIF-8 film induces an electron density increase in the Pt component, leading to an increase of the olefin selectivity from 14% to 70% in the 1-heptyne hydrogenation reaction. A reduced thickness of the ZIF-8 film increases the catalytic activity but does not affect the selectivity of 1-heptylene.

Significance of tridymite distribution during cooling and vapor-phase alteration of ignimbrites

Abstract Thick sequences of silicic ignimbrites contain complex emplacement and cooling histories, often masking contacts between ignimbrite flow packages. Mineralogical and textural variations in these sequences are primarily a function of emplacement temperature and cooling time. Here, we focus on the use of the silica polymorph tridymite to understand the association of vapor-phase crystallization and devitrification within ignimbrite flow packages. As opposed to the common occurrence of cristobalite, the restricted domains in which we observe tridymite may provide more relevant constraints for interpreting post-emplacement devitrification and vapor-phase alteration. This study examines sections through the Whakamaru (New Zealand), Bishop (U.S.A.), and Grey’s Landing (U.S.A.) ignimbrites by combining textural observations with measurements of density, groundmass crystallinity, and the distribution and proportion of tridymite to cristobalite. The rheomorphic Grey’s Landing ignimbrite represents a high-temperature end-member scenario, with widely distributed tridymite (up to 20%) resulting from a high-magmatic temperature and rapid devitrification in a low-porosity deposit. In the welded Whakamaru and Bishop ignimbrites, metastable tridymite (up to 13%) is concentrated along boundaries between flow packages. Here tridymite is interpreted to crystallize in transient permeable zones, forming during vapor-phase alteration prior to compaction, where upper denser-welded flow materials serve as vapor seals. Our results suggest that tridymite may link the initial cooling and welding history of ignimbrites to vapor-phase alteration and devitrification, and may serve as a potential mineralogical fingerprint of depositional contacts, important for consideration of lateral transport of fluids in geothermal reservoirs.

MEL Zeolites: Synthesis, Properties, and Catalytic Applications

The review provides an analysis of the synthesis design features and the acidic and catalytic properties of MEL-type zeolites. Practicable methods for MEL zeolite synthesis are discussed, including one-step and two-step implementations of hydrothermal crystallization, crystal seed assisted synthesis, dual template synthesis, and vapor-phase crystallization to obtain nanocrystalline zeolites as isolated nanocrystals or binderless pellets. The study also provides an overview of methods for targeted control of the phase purity, chemical composition, crystal morphology/size, and acidic properties of MEL zeolite. Relationships are demonstrated between the MEL synthesis conditions and the catalytic properties of these zeolites in alkylation and disproportionation of aromatic compounds, oligomerization of lower olefins, oxidation of glycerol, amination of isobutylene, and conversion of lower alcohols.

Preparation of Binderless Silicoaluminophosphates by Vapor-Phase Crystallization of Kaolin–Phosphoric Acid Grains

Transformations of granules based on kaolin and phosphoric acid under conditions of vapor-phase crystallization in a mixture of water vapor and a structure-directing agent have been studied. It has been shown that silicoaluminophosphate granules obtained in the presence of dipropylamine, triethylamine, or tetramethylammonium hydroxide as a structure-directing agent consist of a shell formed by dense nonporous cristobalite, tridymite, and berlinite phases and the core made mainly of microporous crystalline silicoaluminophosphates. The formation of the shell, which ensures the strength of the silicoaluminophosphate granules, is due to the interaction of steam with the material of the granules at the initial stages of vapor-phase crystallization. It has been established that the selectivity of the structure-directing action of the amines under vapor-phase crystallization conditions basically corresponds to the template hydrothermal synthesis. It has been assumed that the specific features of the structure-directing action of dimethylamine during vapor-phase transport synthesis are due to its low boiling point, which ensures the primary contact of the granules with template, rather than water molecules. As a result, the products of the transformation of granules in the presence of a mixture of dimethylamine and water do not contain dense nonporous phases and are cocrystallized silicoaluminophosphate and calcium aluminosilicate with the gismondine structure.

Specific Features of Formation of Crystalline Silicoaluminophosphates in Grains Based on Kaolin and Phosphoric Acid

Transformations of granules based on kaolin and phosphoric acid under the conditions of vapor-phase crystallization at 170°C were studied. Specific features of the individual and cooperative action of a water vapor and a template dipropylamine on the granules were determined. It is shown that the regulation of competing processes of formation of dense non-porous phases (tridymite, cristobalite, berlinite) and microporous crystalline silicoaluminophosphates of structural types ATL and ATN can be carried out by creating transport pores in the volume of the initial granules. The formation of transport pores was provided by using of porogens (hydroxyethyl cellulose and carbon nanospheres) with the subsequent calcination of the granules. The presence of 40- and 60- nm macropores in the porous structure favored intensification of mass-exchange processes occurring within the granules under the vapor-phase crystallization conditions. This led to an increase in the selectivity of crystallization of the materials with crystal structure types AEL and ATN. The morphology of the crystals with the AEL and ATN structure and their chemical composition depend on the porogen used to prepare the starting granules.

Nanocrystalline zeolites beta: Features of synthesis and properties

The effect of synthesis conditions for BEA zeolite nanocrystals (BEA-ZNC) on the combination of their morphological, textural, and acidic properties has been studied. BEA-ZNC have been synthesized by the hydrothermal or vapor-phase crystallization of reaction mixtures with a high template concentration using the following variable synthesis parameters: the SiO2/Al2O3 ratio the presence of mineralizing additives, the SiO2 source and template type, and the synthesis pH. It has been found that an increase in the aluminum content in BEA-ZNC from 2.7 to 7.8 Al atoms per unit cell is accompanied by a decrease in the size of the ZNC from 250–350 to 100 nm, an increase in the acid site concentration, and an increase in the fraction of weak acid sites in the acidity range of the samples. It has been shown that the specific feature of the synthesis of BEA-ZNC is a low degree of inclusion of silica and the template into the zeolite composition, which is 28–62 and 3.4–5.5%, respectively, of the amounts initially introduced into the reaction mixture. It has been proposed that silica gel should be used as the most promising SiO2 source for the preparation of BEA-ZNC; the use of silica gel provides formation of a material in the form of isolated nanocrystals with a size of 150–300 nm, a developed pore structure, and a high concentration of acid sites. It has been shown that the BEA-ZNC synthesized by vapor-phase crystallization are inferior to the samples prepared by conventional hydrothermal crystallization with respect to the pore structure characteristics and acidic properties.

Major element and oxygen isotope geochemistry of vapour-phase garnet from the Topopah Spring Tuff at Yucca Mountain, Nevada, USA

AbstractTwenty vapour-phase garnets were studied in two samples of the Topopah Spring Tuff of the Paintbrush Group from Yucca Mountain, in southern Nevada. The Miocene-age Topopah Spring Tuff is a 350 m thick, devitrified, moderately to densely welded ash-flow tuff that is zoned compositionally from high-silica rhyolite to latite. During cooling of the tuff, escaping vapour produced lithophysae (former gas cavities) lined with an assemblage of tridymite (commonly inverted to cristobalite or quartz), sanidine and locally, hematite and/or garnet. Vapour-phase topaz and economic deposits associated commonly with topaz-bearing rhyolites (characteristically enriched in F) were not found in the Topopah Spring Tuff at Yucca Mountain. Based on their occurrence only in lithophysae, the garnets are not primary igneous phenocrysts, but rather crystals that grew from a F-poor magmaderived vapour trapped during and after emplacement of the tuff. The garnets are euhedral, vitreous, reddish brown, trapezohedral, as large as 2 mm in diameter and fractured. The garnets also contain inclusions of tridymite. Electron microprobe analyses of the garnets reveal that they are almandinespessartine (48.0 and 47.9 mol.%, respectively), have an average composition of (Fe1.46Mn1.45Mg0.03Ca0.10)(Al1.93Ti0.02)Si3.01O12and are comparatively homogeneous in Fe and Mn concentrations from core to rim. Composited garnets from each sample site have δ18O values of 7.2 and 7.4%. The associated quartz (after tridymite) has δ18O values of 17.4 and 17.6%, values indicative of reaction with later, low-temperature water. Unaltered tridymite from higher in the stratigraphic section has a δ18O of 11.1% which, when coupled with the garnet δ18O values in a quartz-garnet fractionation equation, indicates isotopic equilibration (vapour-phase crystallization) at temperatures of ∼600°C. This high-temperature mineralization, formed during cooling of the tuffs, is distinct from the later and commonly recognized low-temperature stage (generally 50−70°C) of calcite, quartz and opal secondary mineralization, formed from downward-percolating meteoric water, that locally coats fracture footwalls and lithophysal floors.

Vapor-phase crystallization route to oxidized Cu foils in air as anode materials for lithium-ion batteries

Download options Please wait... Supplementary files Supplementary information PDF (299K) Article information DOI https://doi.org/10.1039/C2CE26544C Article type Paper Submitted 25 Jun 2012 Accepted 08 Oct 2012 First published 11 Oct 2012 Download Citation CrystEngComm, 2013,15, 144-151 BibTex EndNote MEDLINE ProCite ReferenceManager RefWorks RIS Permissions Request permissions Vapor-phase crystallization route to oxidized Cu foils in air as anode materials for lithium-ion batteries K. Chen, S. Song and D. Xue, CrystEngComm, 2013, 15, 144 DOI: 10.1039/C2CE26544C To request permission to reproduce material from this article, please go to the Copyright Clearance Center request page. If you are an author contributing to an RSC publication, you do not need to request permission provided correct acknowledgement is given. If you are the author of this article, you do not need to request permission to reproduce figures and diagrams provided correct acknowledgement is given. If you want to reproduce the whole article in a third-party publication (excluding your thesis/dissertation for which permission is not required) please go to the Copyright Clearance Center request page. Read more about how to correctly acknowledge RSC content. Social activity Tweet Share Search articles by author Kunfeng Chen Shuyan Song Dongfeng Xue

Columnar jointing in vapor-phase-altered, non-welded Cerro Galán Ignimbrite, Paycuqui, Argentina

Columnar jointing is thought to occur primarily in lavas and welded pyroclastic flow deposits. However, the non-welded Cerro Galan Ignimbrite at Paycuqui, Argentina, contains well-developed columnar joints that are instead due to high-temperature vapor-phase alteration of the deposit, where devitrification and vapor-phase crystallization have increased the density and cohesion of the upper half of the section. Thermal remanent magnetization analyses of entrained lithic clasts indicate high emplacement temperatures, above 630°C, but the lack of welding textures indicates temperatures below the glass transition temperature. In order to remain below the glass transition at 630°C, the minimum cooling rate prior to deposition was 3.0 × 10−3–8.5 × 10−2°C/min (depending on the experimental data used for comparison). Alternatively, if the deposit was emplaced above the glass transition temperature, conductive cooling alone was insufficient to prevent welding. Crack patterns (average, 4.5 sides to each polygon) and column diameters (average, 75 cm) are consistent with relatively rapid cooling, where advective heat loss due to vapor fluxing increases cooling over simple conductive heat transfer. The presence of regularly spaced, complex radiating joint patterns is consistent with fumarolic gas rise, where volatiles originated in the valley-confined drainage system below. Joint spacing is a proxy for cooling rates and is controlled by depositional thickness/valley width. We suggest that the formation of joints in high-temperature, non-welded deposits is aided by the presence of underlying external water, where vapor transfer causes crystallization in pore spaces, densifies the deposit, and helps prevent welding.

Growth of tetragonal CdP2 single crystals and the properties of barriers on their basis

Large (50 mm long, 20 mm in diameter) CdP2 single crystals with tetragonal symmetry are grown by vapor-phase crystallization for the first time. The unit cell parameters of the single crystals are determined, and it is shown that they have the stoichiometric composition of cadmium diphosphide. Photosensitive In(Cu)/CdP2 structures are fabricated, and their photosensitivity spectra are investigated. It is found that these structures exhibit broadband photosensitivity. The mechanism of photoelectric conversion and the type of interband transitions are discussed. From indirect and direct interband transitions, the bandgap values that are responsible for edge absorption are determined. It is concluded that CdP2 single crystals are feasible for broadband optical converters.

Photoelectric properties of In/CdP2 surface barrier structures

We used vapor-phase crystallization to grow homogeneous single crystals of the compound CdP 2 of diameter 20 mm and length up to 50 mm. We determined the unit cell parameters and the composition of the single crystals grown. We created photosensitive In/CdP 2 surface barrier structures and studied their photosensitivity spectra. Based on these studies, we established the broadband character of the photosensitivity of the structures, the nature of the photoconversion processes, and the type of interband transitions. We conclude that it is possible to use CdP2 single crystals as broadband photoconverters for natural light.

Thermodynamic aspects of formation of quantum-dimensional GaN-based heterostructures

From the viewpoint of the Gibbs classical thermodynamics, the mechanism of GaN vapor-phase crystallization is analyzed. When investigating the formation of neutral complexes of acceptor impurities with hydrogen, the key statements of the Rice and Debye-Huckel theory are used. On the basis of the molecular-interaction analysis, the instability regions in the GaN-InN and GaN-AlN systems are revealed. Under the MOC-hydride-technology conditions, the dependences of the change in the composition of crystallizing solid solutions on the vapor-phase composition are determined.

Transient processes in Stromboli’s shallow basaltic system inferred from dolerite and magmatic breccia blocks erupted during the 5 April 2003 paroxysm

We describe the mineralogy, geochemistry, and mesomicrostructure of fresh subvolcanic blocks erupted during the 5 April 2003 paroxysm of Stromboli (Aeolian Islands, Italy). These blocks represent ∼50 vol.% of the total erupted ejecta and consist of fine- to medium-grained basaltic lithotypes ranging from relatively homogeneous dolerites to strongly or poorly welded magmatic breccias. The breccia components are represented by angular fragments of dolerites entrapped in a matrix of vesiculated (lava-like to scoriae) crystal-rich (CR) basalt. All of the studied blocks are cognates with the CR basalt of the normal Strombolian activity or lavas and they are often coated by a few-centimeter thick layer of crystal-poor (CP) basaltic pumice erupted during the paroxysm. We suggest that they result from the rapid increase of pressure and related subvolcanic rock failure that occurred shortly before the 5 April 2003 explosion, when the uppermost portion of the edifice inflated and suffered brecciation as the result of the sudden rise of the gas-rich CP basalt that triggered the eruption. Dolerites and magmatic matrix of the breccias show major and trace element compositions that match those of the CR basalts erupted during normal Strombolian activity and effusive events at Stromboli volcano. Dolerites consist of (a) phenocrysts normally found in the CR basalts and (b) late-stage magmatic minerals such as sanidine, An60-28 plagioclase, Fe–Mn-rich olivines (Fo68-48), phlogopite, apatite, and opaque mineral pairs (magnetite and ilmenite), most of which are never found both in lava flows and scoriae erupted during the persistent explosive activity that characterizes typical Strombolian behavior. Subvolcanic crystallization of the Stromboli CR magma, leading to slowly cooled equivalents of basalts, could result from transient drainage of the magma from the summit craters to lower levels. Fingering and engulfing of the material that collapsed from the summit crater floor into the shallow basaltic system during the late evening of 28 December 2002 coupled with the short break in the summit persistent explosions between December 2002 and March 2003 permitted the CR magma pockets to solidify as dolerites, which were confined to the uppermost portion of the system and thus not involved in the ongoing flank effusive activity. Crystal size distribution of the basaltic blocks and crystallization of the finer-grained (<0.1 mm) mafic minerals of the dolerites over a time interval of ∼100 days closely agrees with the above interpretation. Vesicle filling (miarolitic cavities) locally found in some dolerites, with minerals deposited as vapor-phase crystallization is a result of continuous gas percolation through the rocks of the uppermost portion of the volcanic system. Poorly welded magmatic breccias formed during syn-eruptive processes of 5 April 2003, when the paroxysm strongly shattered the shallow subvolcanic system and many dolerite fragments were entrapped in the CR magma. In contrast, the high degree of welding between the dolerite clasts and the CR basaltic matrix in the strongly welded magmatic breccias provides a snapshot of subvolcanic intrusions of the CR basalt into the dolerite when, after a 2-month break in activity, CR magmas started to rise again to the summit craters. Blocks similar to these subvolcanic ejecta of 5 April 2003 were also erupted during previous paroxysms (e.g., 1930) suggesting that changes in the usual Strombolian activity (e.g., short breaks in the persistent mild explosions and/or flank effusive activity) lead to transient crystallization of dolerites in the shallow plumbing system.

Geochemical characteristics of the alteration of volcanic and volcaniclastic rocks in the Feres Basin, Thrace, NE Greece

AbstractThe Tertiary basin of Feres consists of sedimentary rocks, andesitic-rhyolitic volcanic rocks of K-rich calc-alkaline affinities, rocks with calc-alkaline and shoshonitic affinities and volcaniclastic fall and flow deposits. Volcanic and volcaniclastic rocks have variable concentrations of LIL elements (Ba, Sr, Rb, Th) and HFS elements (Zr, V) due to their mode of origin. The pyroclastic flows frequently show more or less intense devitrification, vapour-phase crystallization and, in some cases, evidence of fumarolic activity, as is indicated by the presence of scapolite. The volcanic and volcaniclastic rocks display various types of alteration including formation of zeolites (clinoptilolite, heulandite, mordenite, and laumontite) and smectite, as well as hydrothermal alteration (development of silicic, argillic, sericitic and propylitic zones) associated with polymetallic mineralization. The behaviour of chemical elements during alteration varies. Some are immobile and their distribution is controlled by the conditions prevailing during parent-rock formation and emplacement, but others, such as Ba and Sr, are mobile and selectively fractionate in zeolite extra-framework sites. The formation of zeolite from alteration of volcanic glass is accompanied by an increase in Mg and Al content, and a decrease in Si and Na content, whereas Ca is not affected by alteration. In certain pyroclastic flows, there is a significant difference in K-content between incipient glass and altered rock, due to K-feldspar formation during devitrification and vapour-phase crystallization.

Zeolitic alteration in the Tertiary Feres volcano-sedimentary basin, Thrace, NE Greece

AbstractThe Tertiary volcano-sedimentary sequence of the Feres basin (Thrace, NE Greece), includes lavas of andesitic–rhyolitic composition as well as volcaniclastic rocks, pyroclastic flows and pyroclastic fall deposits principally of dacitic–rhyodacitic composition. The pyroclastic flow deposits frequently show intense devitrification, vapour-phase crystallization and evidence of fumarolic activity, which involves deposition of scapolite in pore spaces. The Feres basin can be subdivided on the basis of mineral alteration assemblages: (1) the Pefka region; characterized by intense hydrothermal alteration of the volcanic rocks and mineral zoning(silicic, argillic, sericitic and propylitic zones) with polymetallic mineralization, and (2) the remainder of the basin; where the volcaniclastic rocks are characterized by the alteration of volcanic glass to zeolites (clinoptilolite, heulandite, mordenite, analcime), clay minerals (smectite, illite, celadonite, chlorite), SiO2polymorphs (cristobalite, opal-CT, quartz), K-feldspar and calcite. Laumontite is also present as an alteration product of plagioclase, with stilbite sporadically occurringin veinlets. Locally, rhyolites are also altered to zeolites (clinoptilolite and/or mordenite). The zeolitization process has occurred rapidly with the depositional environment, temperature, rate of cooling(of the volcanic rocks), nature and temperature of the mineral-forming fluids and composition of the parent material controllingthe formation of zeolites.

Fabrication of hollow spheres composed of nanosized ZSM-5 crystals via laser ablation

Zeolite spheres with a core-shell structure were fabricated by a combination of pulsed laser deposition (PLD) and vapor-phase crystallization. Spherical mesoporous DAM-1 or SBA-15 was used as the silicon source and substrate. After vapor phase treatment hollow spherical shells consisting of nanosized ZSM-5 crystals were formed. In addition, the size of the ZSM-5 crystals in the shell ranges from 5 to 500 nm which can be adjusted by changing the crystallization time (3–5 days) or the PLD coating thickness. The deposition rate was 13 nm/min at 120 mJ and 6.5 nm/min at 70 mJ. The ZSM-5 core–shell structure was characterized by XRD, IR, SEM and N 2 absorption. The dissolution of the mesoporous silica substrate and transport of the resulting silica species to the growing ZSM-5 film during vapor-phase crystallization is discussed in terms of a solution mediated transport mechanism.

Experimental and textural investigation of welding: effects of compaction, sintering, and vapor-phase crystallization in the rhyolitic Rattlesnake Tuff

The abrupt changes in character of variably welded pyroclastic deposits have invited decades of investigation and classification. We conducted two series of experiments using ash from the nonwelded base of the rhyolitic Rattlesnake Tuff of Oregon, USA, to examine conditions of welding. One series of experiments was conducted at atmospheric pressure (1 At) in a muffle furnace with variable run times and temperature and another series was conducted at 5 MPa and 600 °C in a cold seal apparatus with variable run times and water contents. We compared the results to a suite of incipiently to densely welded, natural samples of the Rattlesnake Tuff. Experiments at 1 At required a temperature above 900 °C to produce welding, which is in excess of the estimated pre-eruptive magmatic temperature of the tuff. The experiments also yielded globular clast textures unlike the natural tuff. During the cold-seal experiments, the gold sample capsules collapsed in response to sample densification. Textures and densities that closely mimic the natural suite were produced at 5 MPa, 600 °C and 0.4 wt.% H 2O, over run durations of hours to 2 days. Clast deformation and development of foliation in 2-week runs were greater than in natural samples. Both more and less water reduced the degree of welding at otherwise constant run conditions. For 5 MPa experiments, changes in the degree of foliation of shards and of axial ratios of bubble shards and non-bubble (mainly platy) shards, are consistent with early densification related to compaction and partial rotation of shards into a foliation. Subsequent densification was associated with viscous deformation as indicated by more sintered contacts and deformation of shards. Sintering (local fusion of shard–shard contacts) was increasingly important with longer run times, higher temperatures, and greater pressures. During runs with high water concentrations, sintering was rare and adhesion between clasts was dominated by precipitation of sublimates in pore spaces. A few tenths wt.% H 2O in the rhyolite glass promote the development of welding by sharp reduction of glass viscosity. Large amounts of water inhibit welding by creating surface sublimates that interfere with sintering and may exert fluid pressure counter to lithostatic load if sintering and vapor-phase sublimates seal permeability in the tuff.