Possible shock-induced crystallization of skeletal quartz from supercritical SiO2-H2O fluid: A case study of impact melt from Kamil impact crater, Egypt

Since its discovery, the Kamil crater (Egypt) has been considered a natural laboratory for studying small-scale impact cratering. We report on a previously unknown shock-related phenomenon observed in impact melt masses from Kamil; that is, the shock-triggered formation of skeletal quartz aggregates from silica-rich fluids. These aggregates are unshocked and characterized by crystallographically oriented lamellar voids and rounded vesicles. The distribution of the aggregates can be correlated with former H2O- and impurity-rich heterogeneities in precursor quartz; i.e., fluid inclusions. The heterogeneities acted as hot spots for local melting. Due to the presence of H2O and the high impact pressure and temperature, the formation of a localized supercritical fluid is plausible. Below the upper critical end point of the SiO2–H2O system (temperature <1100 °C and pressure <1 GPa), SiO2 melt and H2O fluid become immiscible, leading to the rapid and complete crystallization of skeletal quartz.

Permeability variation in crystalline rocks due to low-grade solution phenomena

Permeability of crystalline rocks depends on parameters such as density and interconnectivity of fractures and pores. While in pristine crystalline rocks porosity is usually considered to be low, low-grade solution phenomena such as the formation of episyenites occur occasionally and may cause a local dramatic increase in porosity and permeability. These solution phenomena can be effective in otherwise unaltered rocks and may result in the preferential removal of certain mineral phases, especially of quartz so that porosities correspond to the spatial distribution of the previously existing mineral phase if no subsequent mineralization occurs (e.g., Pennacchioni et al., 2016). Using light-optical and scanning electron microscopy, X-ray tomography, micro-XRD, as well as digital image analysis, the differences in connectivity and hence permeability between, for example, quartz-depleted granite, gneiss, and schist can be characterized and quantified. We demonstrate that such porosities do not necessarily result in high permeabilities in an undeformed granodiorite from the Central Gneiss unit of the Tauern Window (Lago di Neves area, Italy), since former quartz aggregates are not interconnected due to their relatively late crystallization age and the preservation of the magmatic fabric; however, in the case of moderate mylonitic deformation, quartz as rheologically weak phase forms interconnected aggregates and layers. Its dissolution results in an extremely increased permeability. Therefore, not only the content and grain size but also the distribution, shape and alignment of minerals are crucial for rock permeability and need to be carefully investigated when searching for a final repository of highly radioactive waste in crystalline rocks. Especially since local shear zones may form in otherwise undeformed intrusive bodies, a detailed structural analysis beyond the exclusion of the presence of fractures is required to mitigate the risk of a long-lasting nuclear waste disposal.

Effect of Aggregate Types on the Mechanical Properties of Traditional Concrete and Geopolymer Concrete

For the same concrete quality, different types of coarse aggregates may result in different mechanical properties. This paper presents a study on the effect of aggregate types on the mechanical properties of two concretes, namely, geopolymer concrete (GP) and traditional Portland cement (TC) concrete. The mechanical properties were investigated through several large-scale tests. Moreover, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and laser scanning microscope (LSM) images were obtained to study the microstructure of tested mixes. The results revealed that the aggregate type has different effects on the mechanical properties of TC and GP, as they were behaving opposite to quartz and limestone aggregates. Microstructure analysis further confirmed the growth of well-bonded regions between the paste and aggregate in the GP with limestone aggregates, and the formation of several weak interfacial zones in concrete mixtures made with quartz aggregates. It was concluded that the mechanical properties of GP are very sensitive to the stiffness of aggregate, concentrations of stress, and the physical and chemical reactions occurring in the interfacial transition zone which may lead to improved or weakened bond strength between paste and aggregates.

Surface microstructures developed on polished quartz crystals embedded in wet quartz sand compacted under hydrothermal conditions

Intergranular pressure solution plays a key role as a deformation mechanism during diagenesis and in fault sealing and healing. Here, we present microstructural observations following experiments conducted on quartz aggregates under conditions known to favor pressure solution. We conducted two long term experiments in which a quartz crystal with polished faces of known crystallographic orientation was embedded in a matrix of randomly oriented quartz sand grains. For about two months an effective axial stress of 15 MPa was applied in one experiment, and an effective confining pressure of 28 MPa in the second. Loading occurred at 350 °C in the presence of a silica-saturated aqueous solution. In the first experiment, quartz sand grains in contact with polished quartz prism ($$\overline10{1 }0$$ 1 ¯ 010 ) faces became ubiquitously truncated against these faces, without indenting or pitting them. By contrast, numerous sand-grain-shaped pits formed in polished pyramidal ($$17\overline{6 }3$$ 17 6 ¯ 3 ) and ($$\overline{4 }134$$ 4 ¯ 134 ) crystal faces in the second experiment. In addition, four-leaved and (in some cases) three-leafed clover-shaped zones of precipitation formed on these prism faces, in a consistent orientation and pattern around individual pits. The microstructures observed in both experiments were interpreted as evidence for the operation of intergranular pressure solution. The dependence of the observed indentation/truncation microstructures on crystal face orientation can be explained by crystallographic control of stress-induced quartz dissolution kinetics, in line with previously published experimental and petrographic data, or possibly by an effect of contact orientation on the stress-induced driving force for pressure solution. This should be investigated in future experiments, providing data and microstructures which enable further mechanism-based analysis of deformation by pressure solution and the effect of crystallographic control on its kinetics in quartz-rich sands and sandstones.

Novel Magnetic Inorganic Composites: Synthesis and Characterization

The addition of magnetic particles to inorganic matrices can produce new composites exhibiting intriguing properties for practical applications. It has been previously reported that the addition of magnetite to concrete improves its mechanical properties and durability in terms of water and chloride ions absorption. Here we describe the preparation of novel magnetic geopolymers based on two different matrices (G1 without inert aggregates and G2 with inert quartz aggregates) containing commercial SrFe12O19 particles with two weight concentrations, 6% and 11%. The composites’ characterization, including chemical, structural, morphological, and mechanical determinations together with magnetic and electrical measurements, was carried out. The magnetic study revealed that, on average, the SrFe12O19 magnetic particles can be relatively well dispersed in the inorganic matrix. A substantial increase in the composite samples’ remanent magnetization was obtained by embedding in the geopolymer SrFe12O19 anisotropic particles at a high concentration under the action of an external magnetic field during the solidification process. The new composites exhibit good mechanical properties (as compressive strength), higher than those reported for high weight concretes bearing a similar content of magnetite. The impedance measurements indicate that the electrical resistance is mainly controlled by the matrix’s chemical composition and can be used to evaluate the geopolymerization degree.

Microstructure of Vein Quartz Aggregates as an Indicator of Their Deformation History: An Example of Vein Systems from Western Transbaikalia (Russia)

We investigated the microstructural and crystallographic features of quartz from complex vein systems associated with the development of thrust and shear deformations in Western Transbaikalia using electron back scatter diffraction (EBSD) and optical microscopy. Vein quartz systems were studied to obtain insights on the mechanisms and localization of strains in quartz, in plastic and semibrittle conditions close to the brittle–ductile transition, and their relationship to the processes of regional deformations. Five types of microstructures of vein quartz were distinguished. We established that the preferred mechanisms of deformation of the studied quartz were dislocation glide and creep at average deformation rates and temperatures of 300–400 °C with subsequent heating and dynamic and static recrystallization. The formation of special boundaries of the Dauphiné twinning type and multiple boundaries with angles of misorientation of 30° and 90° were noted. The distribution of the selected types in the differently oriented veins was analyzed. The presence of three generations of vein quartz was established. Microstructural and crystallographic features of vein quartz aggregates allow us to mark the territory’s multi-stage development (with the formation of syntectonic and post-deformation quartz).

Technological analysis of material from Areal Morro Branco, Porto Grande, Amapá, by fractionation

Natural sand is extracted using mining methods, even for the manufacture of artificial sands. Sand is widely used in several areas. As construction aggregates; material processing industries; water and sewage treatment. The Areal of the company Morro Branco, where the sand samples were collected, is located around the headquarters of the municipality of Porto Grande. The municipality of Porto Grande in Amapá is located in the center of the state 108 kilometers from the capital Macapá. The objective of this research was to make the technological analysis of material from Areal Morro Branco, Porto Grande, Amapá, by fractionation. The sand was removed from a sandy area in the municipality of Porto Grande – AP with the use of hand tools for collection. 03 (three) samples from different points with different composition were collected, due to the existence of organic matter in its composition. Sodium silicate with a concentration of 10% and bromoform was used for the breakdown of the organic matter of the sample. These procedures were determined in the dry sample in an oven for 24 hours at approximately 110ºC, and the percentage of humidity (%U), this, was also determined in the original sample. It was verified through the tests and procedures performed that the moisture of the sand of The Areal Morro Branco is influenced by the organic matter resulting from the forest areas and water resources of the surroundings of The Areal. The granulometry of the points from which the sand was collected in the sand has characteristics of angled and sub-angled grain. With the aid of bromoform density, it was feasible to realize that the sand from which the sample was collected has quartz aggregates, feldspar with a density lower than 2.89 g/cm3. It has been suggested that the sand may also have mineral derivatives of sand with density greater than 2.89g/cm3, which are Olivine and Pyroxene.

Experimental grain growth of quartz aggregates under wet conditions and its application to deformation in nature

Grain growth of quartz was investigated using two quartz samples (powder and novaculite) with water under pressure and temperature conditions of 1.0–2.5 GPa and 800–1100 ∘C. The compacted powder preserved a substantial porosity, which caused a slower grain growth than in the novaculite. We assumed a grain growth law of dn-d0n=k0fH2Orexp⁡(-Q/RT)t with grain size d (µm) at time t (seconds), initial grain size d0 (µm), growth exponent n, a constant k0 (µmn MPa−r s−1), water fugacity fH2O (MPa) with the exponent r, activation energy Q (kJ mol−1), gas constant R, and temperature T in Kelvin. The parameters we obtained were n=2.5±0.4, k0=10-8.8±1.4, r=2.3±0.3, and Q=48±34 for the powder and n=2.9±0.4, k0=10-5.8±2.0, r=1.9±0.3, and Q=60±49 for the novaculite. The grain growth parameters obtained for the powder may be of limited use because of the high porosity of the powder with respect to crystalline rocks (novaculite), even if the differences between powder and novaculite vanish when grain sizes reach ∼70 µm. Extrapolation of the grain growth laws to natural conditions indicates that the contribution of grain growth to plastic deformation in the middle crust may be small. However, grain growth might become important for deformation in the lower crust when the strain rate is < 10−12 s−1.