Iezzi, Gianluca

The density, colour and texture, plus mineral and chemical features of 18 ceramic-like CDW samples from the Abruzzo region (Central Italy) were characterised. The concretes, natural stones, tiles, roof-tiles, bricks and perforated bricks are either aphanitic to porphyric. Concretes and natural stones are grey to white and tend to be > 2.0 g/cm3; the masonries are brown to reddish and close to < 2.0 g/cm3. Concrete and natural stone are rich or even exclusively made up of calcite, with high amounts of CaO (>40 wt%) and LOI (volatiles, CO2 + H2O). The masonries are instead calcite-, CaO- (<25 wt%) and LOI-poor (<8 wt%) but enriched in SiO2 (45 to 70 wt%) stabilised as quartz and/or cristobalite, with significant amount of Al2O3 (12 to 20 wt%). S and Cl contents are similar among concrete, bricks and perforated bricks. The petrography of CDW concretes is similar among geographical areas with abundance of limestones used as aggregates. However, in limestone-poor areas CDW are SiO2- and Al2O3-rich, reflecting the prevalent use of masonry and/or silicate-rich construction materials, implying that each geographical area is characterised by peculiar CDW composition. Therefore, the knowledge of mesoscopic, physical and petrographic aspects has to be known for planning adequate sorting methods, promoting upcycling reusing applications. Some of the studied CDW samples are susceptible to release relative high Cr and As content.

Analyses of three oriented rock samples collected in the footwall of a major normal fault in Central Italy provide insights into textural properties of a lithified carbonate fault core. Data from lithified fault rocks are very rare and we document here the grain size distribution in a fault core at an unprecedented scale range, which complements already existing observations obtained from sieve, sedimentation and/or laser diffraction methods in loose deposits. X-ray powder diffraction analysis shows that the samples, which are located at a mutual distance of few tens of meters, are exclusively made of calcite. Mesoscopic samples and polished thin sections oriented normal to the fault plane have been analysed with high resolution scanner (HRS), transmission optical microscopy (TOM) and scanning electron microscopy in back-scattered mode (BS-SEM). Textural features of tectonised calcite crystals have been quantified using image analysis on digitalised photographs at magnifications from 1:1 to 9000:1, documenting size dimensions of grains from cm to sub-μm. The obtained D values have average values of 1.65 in 2D, overlapping with those independently obtained by box-counting methods in the core of the same fault by previous authors. Textures of calcite clasts in tectonised and lithified rocks can be appropriately quantified by the analytical protocols proposed here. This work represents the first multi-scale image analysis, from sub- μm to cm sizes, of the core portion of a normal fault that cut into carbonate rocks. Our study provides a template for similar investigations to be carried on other faults that will help to better understand the relations between fault textures and deformation processes.

In this study, we have investigated the control of cooling rate on the composition of titanomagnetite formed from a trachybasaltic melt. Results show that disequilibrium growth conditions exert a primary control on the abundance, texture, and composition of the crystals. As the degree of cooling is increased, titanomagnetites show immature textures and are progressively enriched in Al ? Mg and depleted in Ti. Thus, early-formed titanomagnetite nuclei do not re-equilibrate with the melt over faster cooling rates; instead, their compositions are far from equilibrium. On the basis of the different intra-crystal redistribution rates for Ti, Al, and Mg, we have calibrated a geospeedometer that represents the first quantitative description of the effect of cooling rate on titanomagnetite composition. This model was tested using the compositions of titanomagnetites in lava and dike samples from Mt. Etna volcano whose crystallization conditions resemble those of our experiments. Cooling rates calculated for lava samples are comparable with those measured in several volcanic complexes. At Mt. Etna, compositional variations of titanomagnetite grains from the innermost to the outermost part of a dike testify to progressively higher degrees of cooling, in agreement with numerical simulations of thermal gradients in and around magmatic intrusions.

The inherent instability of volcanic edifices, and their resultant propensity for catastrophic collapse, is a constant source of volcanic risk. Structural instability of volcanic edifices may be amplified by the presence of carbonate rocks in the sub-volcanic strata, due to the debilitating response of carbonates to thermally-induced alteration. Nonetheless, decarbonation reactions (the primary weakening mechanism), may stall when the system becomes buffered by rising levels of a reaction product, carbon dioxide. Such thermodynamic stalling might be inferred to serve to circumvent the weakness of volcanic structures. However, the present study shows that, even when decarbonation is halted, rock physical properties continue to degrade due to thermal microcracking. Furthermore, as a result, the pathways for the escape of carbon dioxide are numerous within a volcanic edifice. Therefore, in the case of an edifice with a subvolcanic sedimentary basement, the generation of carbon dioxide via decarbonation is unlikely to hinder its impact on instability, and thus potentially devastating flank collapse.

The in situ differential scanning calorimetry (DSC) technique has been applied to investigate the solidification paths of a basaltic liquid. The starting glass was heated up to 1300°C, kept at this superliquidus temperature for 2 h and cooled at rates (ΔT/Δt) of 7, 60, 180, 1000, and 1800°C/h, down to 800 and 600°C. Glass transition temperature (Tg), crystallization temperature (Tx_HR) and melting temperature (Tm) were measured by in situ DSC spectra on heating. Tx measured along the cooling paths (Tx_CR) shows exothermic peaks that change from a single symmetric shape (7 and 60°C/h) to multi-component patterns (180, 1000, and 1800°C/h). The recovered products characterized by field emission gun source of the scanning electron microscopy and electron probe micro-analyzer-wavelength dispersive spectrometers show a phase assemblage of spinel (sp), clinopyroxene (cpx), melilite (mel), plagioclase (plg), and glass. Moreover, crystal size distributions (CSDs) and growth rates (Gmax and GCSD) were also determined. The crystal content slightly increases from 7 to 1800°C/h. Faceted sp are present in all the run products with an amount always <2 area%. Cpx increases from 7 to 1800°C/h, changing its texture from almost faceted to dendritic between 60 and 180°C/h. The area% of mel follows an asymmetric Gaussian trend, while plg nucleates only at 7°C/h with a content <2 area%. The coupling of DSC and SEM outcomes indicate that sp nucleate first, followed by cpx and mel (and/or plg). The increment of ΔT/Δt causes an increase of the CSD slope (m) and crystal population density per size (n0), as well as a decrease of the crystal size, for both cpx and sp. The log-linear CSD segments with different slopes at 7 and 60°C/h suggest multiple nucleation events and crystal growth by coarsening. Gmax and GCSD for cpx and sp directly measured on the actual crystallization time by DSC spectra, both increase with the increasing of ΔT/Δt. The onset temperature of crystallization (Txi) decreases as ΔT/Δt increases, following an exponential trend that defines the uppermost portion of a time-transformation-temperature-like curve. This analytical model allows us to quantitatively model the kinetic crystallization paths of dry basalts.

Glass stability (GS) indicates the glass reluctance or ability to crystallise upon heating and it can be characterised by several methods and parameters. GS is frequently used to retrieve glass-forming ability (GFA) of corresponding liquids: the likelihood of obtaining a crystal-free glass through melt-quenching. In the present study, GS has been determined for the first time on six sub-alkaline glasses having complex (natural) compositions, the most widespread and abundant on Earth. The two end-members are a basalt and a rhyolite, B100 and R100, plus intermediate compounds B80R20, B60R40, B40R60, B20R80. Each glass was heated in a differential scanning calorimetry (DSC) at a rate of 10 °C/min (600 °C/h) to measure in-situ Tg (glass transition), Tx (onset of crystallization) and Tm (melting) temperatures, from ambient to their liquidus temperatures. The ex-situ run-products quenched at Tm have been characterised by scanning electron microscope (SEM) and electron probe micro-analyzer (EPMA) techniques, in order to quantify textures and compositions of phases, respectively. R100 and B20R80 do not shown any DSC peaks, whereas B40R60, B60R40, B80R20 and B100 thermograms display progressively more resolvable peaks. As SiO2 (wt%) in the melt increases from B100 to B40R60, Tx linearly increases, Tm first decreases and then levels off, whereas Tg weakly changes. R100 and B20R80 run-products are completely glassy, while from B100 to B40R60 the amount of glass (gl) increases from 48.5 to 97 area%, counterbalanced by a decrease of clinopyroxene (cpx) content from 47.7 to 16 area%. The spinel (sp) content is constrained within a narrow range of 0.9–3.8 area%. Conversely, plagioclase (plg) crystallizes heterogeneously on the Al2O3 holders only in B100 and B80R20 and at distance < 100 μm from it. R100, B20R80, B40R60 and B60R40 ex-situ glasses exhibit chemistries very close to their starting compositions, according to the absence (or scarcity) of crystals formed during heating. Instead, B100 and B80R20 glasses are enriched in Si, Al, and Na but depleted in Fe, Mg, and Ca due to internal crystallization of sp and mostly cpx. Specifically, the composition of cpx from B100 is enriched in M2Ca, M1Mg, M2,M1Fe, and M1,TAl. The values of KT, KH, KW, KLL, and w2 (as GS parameters) increase linearly and monotonically as a function of SiO2, showing high correlation coefficients (R2 = 0.93–0.95). Moreover, Tx values and GS parameters highly correlate with GFA via Rc (critical cooling rate), as previously determined by ex-situ cooling-induced experiments. This leads to the conclusion that GS scales with GFA for natural silicate compositions. In addition, the in-situ Rc value of B100 measured with DSC is > 45 °C/min (> 2700 °C/h), corroborating the value of Rc of ~ 150 °C/min (9000 °C/h) determined by ex-situ experiments. In turn, relevant solidification parameters on heating or cooling obtained by DSC investigations, also for chemically complex (natural) systems, extend the results from previous observations conducted on simple silicate systems. These outcomes are relevant for lavas or magmas that re-heat glass-bearing volcanic rocks, as well as for fabricating glass-ceramic materials with desirable texture and composition of phases starting from abundant and very cheap raw volcanic rocks.

Direct structural characterization of the changes in the local environment of Mg occurring in the garnet structure as a function of the Ca content are determined by Mg K-edge X-ray absorption fine structure on synthetic samples along the pyrope-grossular join. With increasing Ca content, the short Mg-O2 distance of the dodecahedron slightly decreases, while the long Mg-O4 distance tends to increase, so that the dodecahedron is more distorted in grossular-rich garnets than in end-member pyrope. This quantitative direct description of the changes in the local environment of Mg in the pyrope-grossular solid solution confirms and better defines previous experimental and recent computational results.

The thermodynamic and physical properties of magmas have been extensively investigated as a function of T, P, fO2 and composition allowing the development of accurate phase stability, viscosity, and diffusion models. However, how the silicate melt properties are influenced by kinetic effects is still an open question. The most important transformation of a magma is its solidification due to cooling, i.e. the transition from a silicate melt to a rock. Solidified magmas may be crystalline, vitreous, or a mixture of glass and crystals. If the cooling rate is larger enough to prevent crystallization, a magma can encompass the supercooling region without crystallisation. The smallest cooling rate that suppresses or strongly limited the nucleation of crystals is the critical cooling rate Rc. Melts with low Rc persist in a metastable liquid state and have a good glass forming ability (GFA). GFA and Rc of melts can be quantitatively estimated using (1) the reduced glass transition parameter Trg = Tg/Tm (Tg, temperature of glass transition; Tm, temperature of melting), and (2) the viscosity fragility concept. As stated by the theory, strong liquids with high Trg values have good GFA and low Rc, whereas fragile liquids with low Trg have a poor GFA and high Rc. Using available experimental data and theoretical models, we analyze the kinetic effects in dry magmas of different composition. The obtained results are relevant for the formation of lava flows and domes. In sub-alkaline magmas, Trg linearly increases and Rc decreases as the Si and Al content increases. Rc of basalts range between 101 and 103 K/s. In dacitic and rhyolitic melts, Rc is between 10-3 and 10-5 K/s. Alkaline melts have Trg values lower than those of sub-alkaline compositions. Results are consistent with the available experimental data. The sluggish kinetics of nucleation determined by using the relation Rc vs Trg is also in agreement with the experimental and theoretical data for synthetic silicate melts. The outlined solidification behaviour of magmatic melts has a profound influence on the viscosity paths of magmas. Depending on the Trg and Rc values, less evolved magmas may have a viscosity larger than that of more evolved magmas due to the rapid crystallization induced by the cooling during their flowing on the Earth. The glassy portion of poorly evolved magmas is indicative of rapid cooling, whereas the glassy fraction of evolved magmas is not unequivocally indicative of rapid cooling being their typical Rc values low. Basaltic lavas may flow on the Earth surface for long times only if they have a temperature close to Tm, whereas more evolved lavas can flow for longer periods with temperatures well below Tm. Fully glassy lavas like obsidians have invariably rhyolitic or trachytic compositions.

Emissivity and reflectance spectra have been investigated on two series of silicate glasses, having compositions belonging to alkaline and subalkaline series, covering the most common terrestrial igneous rocks. Glasses were synthesized starting from natural end-members outcropping at Vulcano Island (Aeolian Islands, Italy) and on Snake River Plain (USA). Results show that the shift of the spectra, by taking Christiansen feature (CF) as a reference point, is correlated with SiO2 content, the SCFM factor and/or the degree of polymerization state via the NBO/T and temperature. The more evolved is the composition, the more polymerized the structure, the shorter the wavelength at which CF is observable. CF shift is also dependent on temperature. The shape of the spectra discriminates alkaline character, and it is related to the evolution of Qn structural units. Vulcano alkaline series show larger amount of Q4 and Q3 species even for mafic samples compared to the subalkaline Snake River Plain series. Our results provide new and robust insights for the geochemical characterization of volcanic rocks by remote sensing, with the outlook to infer origin of magmas both on Earth as well as on terrestrial planets or rocky bodies, from emissivity and reflectance spectra.

The OH–F substitution in “synthetic Rb-richterite” has been investigated along the join Rb(NaCa)Mg5Si8O22(OH)2–Rb(NaCa) Mg5Si8O22(F)2. Syntheses were done by conventional hydrothermal techniques (Tuttle-type vessels) at 800 °C, 1 kbar P( H2O). SEM microscopy showed very high yields of acicular to prismatic amphibole crystals up to XF = F/(F + OH) = 0.6. Beyond this value, a micaceous phase and a very fine-grained granular phase were present. Powder X-ray diffraction patterns show a single amphibole phase below XF = 0.6; above XF = 0.6, a distinct peak at d = 12.18 Å indicates the presence of a mica and there is a broad hump starting at ~ 20o 2θ and ~ 15o wide, both features increasing in intensity with increasing XF. Cell dimensions at XF = 0 are compatible with an ideal amphibole composition Rb(NaCa)Mg5Si8O22(OH)2 and evolve with increasing XF up to XF = 0.6, where there is a sharp discontinuity in a, β and V. The infrared OH-stretching spectrum of the OH end-member shows a main band at 3732 cm−1 which is assigned to the local MgMgMg–OH→ Rb arrangement, and a minor band at 3670 cm−1 assigned to the local MgMgMg–OH→ □ arrangement. This latter band indicates a slight departure toward tremolite. Intermediate OH–F compositions show the appearance of a second band at 3718 cm−1, whose intensity is proportional to the F content in the system, in accord with OH−OH and OH–F arrangements across the filled A-site. For XF > 0.6, the OH-stretching spectra are complicated by the appearance of two more peaks at 3705 and 3685 cm−1. Additional bands at lower wavenumbers, centered around 3595, 3540 and 3475 cm−1, respectively, are better resolved by collecting the spectra on disks heated at 250 °C to remove the adsorbed moisture in the pellet. Combining the behavior of unit-cell dimensions and the infrared spectra with mass-balance arguments indicates that at high XF values, Na replaces part of the Rb at the A-site in the amphibole and the tremolite component of the amphibole increases, while “Rb tetrasilicic magnesium mica” crystallizes, along with semi-amorphous nanophases. The variation in band intensities as a function of XF indicates that OH and F randomly occupy local pairs of O(3) sites across a (filled) A-site, and that there is no short-range order of OH and F.