Granite Samples Research Articles

On the use of laser-induced breakdown spectroscopy data for mineralogical investigations – constraints and application of a clustering method

This study examines how laser-induced breakdown spectroscopy (LIBS) data collected using a downhole deployable LIBS prototype for geochemical analysis in a fashion that imitates downhole deployment may be used for mineralogical investigations. Two chemically and mineralogically practically identical felsic rocks, namely granite and microgranite, are used to assess the effects of rock texture on mineral classification and high-resolution SEM-TIMA (scanning electron microscope coupled with TESCAN's integrated mineral analyser) mineral maps are used to reveal mineralogical composition of each LIBS ablation crater. Additionally, in order to extend the LIBS application for fast mineralogical studies to a greenfield scenario (i.e. no previous knowledge) a clustering methodology is presented for mineralogical classification from LIBS data. Results indicate that most LIBS spot analyses sample mineral mixtures, 91.2 and 100% for granite and microgranite, respectively, which challenges mineralogical classification, particularly for fine-grained rocks. Positive identification and classification of minerals of slightly different compositions relative to the bulk rock (i.e. fluorite and biotite in granitic rocks) demonstrates how minerals or minerals groups of distinct and interesting chemical compositions (e.g. sulfides or oxides in silicate-dominated rocks) can be rapidly recognized in a mineral exploration scenario. Strategies for overcoming mineral mixture issues are presented and recommendations are given for effective workflows for mineralogical analysis using LIBS data in different mineral exploration stages. Supplementary material : Complete LIBS analyses for granite and microgranite samples (Appendix A); modal mineralogy results for each LIBS ablation crater based on TIMA analysis (Appendix B); and TIMA mineral maps for granite and microgranite samples are available at https://doi.org/10.6084/m9.figshare.c.6444482

Exposure of Dose Rate due to Granites of Bangalore Metropolis Area, India

Introduction: The majority of total radiation dose received by the world's population comes from natural radiation. The distinguishing features of this radiation include relatively persistent exposure to a population at a specific place. Inhalation of 222Rn (radon), 220Rn (thoron), and their offspring contributes the majority of the dose from natural radiation in background regions that are typically present. In light of this, a measurement of dose rate exposure due to some granite floorings has been attempted in Bangalore Metropolis, Karnataka, India. For a period of three years, fifteen distinct granite samples that were used as flooring for construction in buildings were examined.
 Objectives: Measurement of exposure of dose rate due to building construction materials by the inhabitants of Bangalore city
 Methods: Solid state nuclear track detector based dosimeters, CAN techniques and radiation survey meters were used and all the experimental observations were made as per the standard protocols provided by the Bhabha atomic research centre, Mumbai, India were followed.
 Results: The arithmetic mean values of dose rate varied between 2.09 ± 0.10 and 1.39 ± 0.06 mSvy-1. The observed dose rates were found to be higher in granite and lower in coarse-grained dolerite.
 Conclusions: All 14 types of granite samples have undergone a minimum of 30 measurements. The greater activity concentration of radionuclides found in granite samples leads to higher dose rates.

Relationship between radiogenic heat production in granitic rocks and emplacement age

Granites play a crucial role in the Earth's thermal regime and its evolution. Radiogenic heat production (RHP) by the decay of radioactive elements (U, Th, K) in granites is a significant parameter in estimating the thermal structure of the lithosphere. RHP variability of granites with their emplacement ages could provide insights for thermal modeling in different geological epochs. An aggregated RHP from 2877 globally-distributed granitic samples of continental crust are analyzed for this study; these sample cover the entire geological history. The average bulk RHP in all types of granitic rocks of all ages is 2.92 ± 1.86 μW/m3. The RHP tends to increase gradually with progressively younger geologic emplacement age, based on a statistical analysis of the data. However, the youngest granites do not necessarily have the highest RHP. The mean RHP in 181 representative Cenozoic Himalayan leucogranites— which are the youngest granites found on Earth, is as low as 1.84 μW/m3. This is probably related to the initial conditions of magma formation, magmatic source material, and differentiation processes in the Himalayan–Tibetan plateau. By correcting the decay factor, variations of the RHP in the emplaced granitic rocks are obtained, indicating the changing levels of heat production and different thermal regimes on Earth in various geological epochs. The highest RHP in granitic rocks emplaced in the Archean and Early Proterozoic corresponds to two global-scale collisional events during supercontinent cycles, at 2.7 and 1.9 Ga respectively. RHPs of granites can be an important indicator in the study of Earth's thermal regime and its evolution.

Coupled microplane and micromechanics model for describing the damage and plasticity evolution of quasi-brittle material

Quasi-brittle materials exhibit significant heterogeneity due to their complex mesoscopic components and microdefects, and their macroscopic mechanical behavior depends strongly on crack initiation, propagation and friction-slip. In this study, by coupling microplane and micromechanics, a new constitutive model that can consider the mesoscopic damage and plasticity mechanisms of quasi-brittle materials is proposed. The microplane is used throughout the whole construction process for the model: first, a representative volume element (RVE) is established to characterize the mesoscopic structure of quasi-brittle materials, and the advantages of microplanes are fully exploited to characterize the equivalent cracks‒matrix system. Then, the relationship between macroscopic and microplane strains (stresses) is elucidated. Second, considering the unilateral effect for cracks, the expressions for the free energy potential functions for a microplane and for the RVE are established. Third, for each microplane, the damage criterion characterized by the damage driving force is constructed for the open and closed cracks, and in particular, a new plasticity criterion characterizing the static friction property coupled damage and dilatancy behavior during strain softening deformation is constructed. By considering the characteristics of the microplane to relate macroscopic and mesoscopic stress and strain, an efficient return mapping algorithm based on the Newton‒Raphson method is derived. Finally, the stress‒strain curves and the evolution of damage and plasticity are simulated under conventional triaxial compression, uniaxial tensile, direct shear and true triaxial compression stress paths, and the effectiveness of the model is verified by comparison to the experimental data. To better reflect the inherent correlation between mesoscopic damage and macroscopic mechanical behavior, a microplane configuration that can visually characterize the mesoscopic evolution characteristics of damage is established, and it is shown that the simulated results correspond well to the macroscopic deformation and failure pattern of granite samples under different loading paths.

Evaluation of Environmental Radioactivity and Hazard Impacts Saudi Arabia Granitic Rocks Used as Building Materials

People use granite in residential buildings on a large scale all around the world. Knowing granite’s radiological characteristics allows for the safe use of its properties to be maintained over time. Our findings are significant for two reasons: first, they may increase public awareness of the naturally radioactive properties of the materials under investigation, and second, they are necessary to establish standards, regulations, and management for the building materials used in Saudi Arabia. In this study, twenty-four granitic rock samples were collected from the Hai’l area in Saudi Arabia, and these samples are used as building material. Gamma spectrometry was used to measure the amounts of 238U, 232Th, 226Ra, and 40K in the collected granitic rock samples. The obtained data was also used to calculate some environmental hazard parameters, such as the absorbed gamma dose rate (AGDR), annual effective dose rate (AEDR), radium equivalent (Raeq), external and internal hazard indexes (Hex and Hin), gamma index (Iγ), alpha index (Iα), and excess lifetime cancer index (ELCR). We concluded that the collected granitic samples are harmful and need more attenuation if used as building materials.

Size effect of failure mode of thermally damaged torus granite

The rocks around wellbores in energy-storage areas are affected by high temperatures and thermal stresses during the extraction of geothermal resources. In this study, the mechanical properties of torus granite samples with different aperture ratios and sizes are experimentally investigated at high temperatures. Further, Acoustic Emission (AE) and Digital Image Correlation (DIC) technology are used to monitor the deformation and failure process of torus granites. The research results show that the radial peak load decreases exponentially with increases in the aperture ratio, temperature, and thermal damage. 400 °C is the critical threshold for the strength instability of the torus granite. According to the failure characteristics. When the aperture ratio λ < 0.3, linear tensile cracks appear in the granite, and the “double fan” failure mode is finally formed. When the heating temperature reaches 400 °C, torus granite successively formed linear cracks and horizontal tensile cracks, and finally a ‘four fan’ failure mode. When the heating temperature reaches 600 °C, the horizontal tensile cracks produced when the torus granite fails evolve from asymmetric to symmetrical. Based on the deformation and failure results of the torus granite under compression, the dynamic failure criterion of the inner and outer pore walls of the torus granite under radial compression is derived. Finally, the theoretical analysis and experimental results are compared, the scale and temperature effects of the failure mode of the torus granite are deeply analysed. This study results can provide theoretical reference for surrounding rock support and shaft diameter ratio design.

Granite alteration as the origin of high lithium content of groundwater in southeast Hungary

The study aimed to find the origin of the high lithium content of groundwater in southeast Hungary. To be concerned with assessing the economic potential of lithium in these waters, it is necessary to investigate the lithium-releasing geochemical processes. For this purpose, granite and pegmatite samples were analysed for lithium content and alteration from various Battonya Complex locations. The key alteration processes are the chloritisation of biotite and the sericitisation of feldspar, which are hypothesised to mobilise Li from minerals into the geothermal waters. Whole-rock analyses show that the 36 ppm Li concentration in one sample (S1) exceeded the average crustal Li concentration (∼20 ppm), while three samples have near-average crustal concentrations. The Li content of rock samples decreases along with the increasing intensity of alteration. The Li concentration of biotite, muscovite, chlorite, feldspar and quartz from granites and pegmatite was measured using laser-induced-breakdown-spectrometry (LIBS). A total of 180 data points were analysed. Biotite and muscovite of the relatively fresh granites contain Li up to ∼3800 and ∼2500 ppm, respectively. Chloritisation is a hydrothermal alteration process in granites that can be described by the following reaction: biotite + plagioclase → chlorite + epidote + titanite + muscovite. As a function of chloritisation, Li concentration decreases two- and three orders of magnitude from biotite to chlorite in the more altered cases. A similar trend is observed for muscovite in parallel. Feldspars have Li of 17–141 ppm, while there is no correlation between the Li content of feldspars and the degree of alteration. Quartz contains constant low concentrations around the 1 ppm detection limit, with one outlier value of 48 ppm. Chloritisation of biotite is a possible explanation for the high Li content of the geothermal waters in the study area and possibly elsewhere. The main Li-bearing minerals of the Battonya Complex are biotite and muscovite. Chloritisation and subordinate sericitisation result in significant lithium loss. Chlorite can incorporate Li in its crystal lattice but to a lesser degree than the other sheet silicates, like biotite and muscovite.

Early cretaceous ridge subduction in the Shandong Peninsula, Eastern China, indicated by Laoshan A-type granite

Early Cretaceous A-type granites are widespread in the Shandong Peninsula, which can be used to elucidate the tectonic evolution of the eastern China and the destruction of the North China Craton. However, their genesis is still controversial. Several competing models, ranging from slab break-off, postorogenic extension, foundering of the lower crust and ridge subduction, were proposed. Here, we report zircon U–Pb ages, whole-rock and apatite geochemical compositions of the Laoshan granite and discuss its tectonic implications. The Laoshan granite has typical characteristics of A-type granite with high FeOT/(FeOT + MgO) ratios (0.90–0.97) and 10000*Ga/Al ratios (2.70–3.36) and high total alkali (Na2O + K2O: 7.95–8.70 wt%) contents and Zr+Nb+Ce+Y (most &amp;gt;350 ppm) concentrations. The Laoshan granite is further classified as A1-type based on the Yb/Ta-Y/Nb and Ce/Nb-Y/Nb diagrams and the Nb-Y-3Ga and Nb-Y-Ce triangular discriminant diagrams. Zircon U–Pb dating of two Laoshan granite samples yielded emplacement ages of 117.8 ± 1.0 Ma and 120.1 ± 1.3 Ma, respectively. The oxygen fugacity of the Laoshan granite magma is low, as indicated by zircon Ce4+/Ce3+ ratios (most &amp;lt;300). The crystallization temperature of zircon varies significantly, ranging from 652 to 830°C. The apatite compositions show that the Laoshan granite has high F (2.09–2.72 wt%) and low Cl (0.01–0.09 wt%) contents, consistent with influence by fluid released from the decomposition of phengite. Apatite rare earth elements show that mantle sources are also involved in Laoshan A-type granite. Combined previous studies of A-type granitic plutons in the Shandong Province and the Lower Yangtze River belt with the drifting history of the Pacific plate, we propose that the flat subduction of the spreading ridge between the Pacific and the Izanagi plates was responsible for the formation of Laoshan A-type granite.

Influence of thermal cycling in the mild temperature range on the physical properties of cultural stones

The thermoclasty of building stones caused by insolation is an increasingly crucial problem. Moreover, how changes in surface temperature and accumulated thermal cycles would affect cultural stones, such as stone monuments, stone-clad historical buildings, statues, reliefs on stone faces, remains unknown. As limited information is available regarding the thermoclasty of stone materials owing to insolation, this study analyzes experimentally whether repeated heating and cooling degrades or makes common forms of stone material more durable. The samples of granite, marble, and sandstone which are used as building stones of cultural heritages were repeatedly subjected to temperature changes from 4 °C to 84 °C at a change rate of ±2 °C/min for 144 cycles. Furthermore, physical changes were assessed via the measurements of Leeb rebound hardness, ultrasonic pulse velocity, and acoustic emissions at certain intervals. Changes in the pore size distribution owing to thermal cycling were also assessed using mercury intrusion porosimetry. The results showed a slight decrease in the rebound hardness of all samples, however, the pulse velocity in the granite and marble drastically reduced during the initial thermal cycle (≤ 4 cycles) and then recovered (> 4 cycles). The formation of new microcracks at these boundaries and inside the minerals can be attributed to the initial cycling, which resulted in thermal stress at the interfaces. Moreover, after four cycles, re-strengthening may occur due to the evaporation of adsorbed water at the boundaries and anisotropic mineral expansion, instead of the adsorption of mineral surface. In the sandstone, there was a slight change in physical properties after the thermal cycling, suggesting that the thermal strain generated by the thermal cycling was absorbed by large pores. The stone response to repeated thermal disturbances, with phases of both damage and healing, results in a more complicated internal change. Our findings indicate that the three various stone types demonstrated different physical changes in response to thermal cycles of a mild temperature.

NOVOUKRAINSKYI MASSIF: SOURCE OF ORIGINAL MAGMAS AND TIME OF FORMATION

NOVOUKRAINSKYI MASSIF: SOURCE OF ORIGINAL MAGMAS AND TIME OF FORMATION

Study on the Constitutive Model and the Identification of Parameters of Granite Based on the Crack Closure Ratio

A large number of cracks exist inside brittle rocks, and the evolution and expansion of cracks inside rocks have adverse effects on the deformation and strength of rocks, leading to the reduction of mechanical properties of rocks and endangering the stability and safety of rock engineering. Taking the granite samples taken from underground plant of Yebatan hydropower station as the research object, conventional triaxial compression tests were carried out. The characteristics of crack evolution and expansion in granite is investigated, and the parameters of granite are analysed and identified for different mechanical states by using stress-crack strain evolution constitutive model. The stress-crack strain evolution constitutive model was embedded into FLAC3D for triaxial loading tests through secondary development. The results show that the stress-crack strain evolution constitutive model is reasonable and accurate enough to be applied in numerical simulation of underground power-house of Yebatan hydropower station.

Identification of fracturing behavior in thermally cracked granite using the frequency spectral characteristics of acoustic emission

The frequency characteristics of acoustic emission (AE) during triaxial compression of thermally cracked and unheated (“fresh”) granite samples were investigated with the aim of understanding the influence of pre-existing cracks on precursor information regarding macroscopic failure. The peak frequency during the damage process was the same for thermally cracked and fresh granites. Analysis of AE signals showed that signals with low peak frequency appeared before failure of the sample, implying the initiation of macrocracks with progressive growth of cracks. The peak amplitude of the frequency spectrum recorded in the thermally cracked samples was much lower than that in the fresh samples. This result suggests two reasons for the difference in peak amplitude: reduction in shear modulus and the attenuation filtering phenomenon caused by thermal cracks (i.e., the differences in overall bulk properties). In particular, the maximum value of peak amplitude in the low-frequency band for the thermally cracked samples was smaller than that for fresh samples. This characteristic can be related to the properties of source of AE signals, such as the stress drop and crack size. Assuming that pre-existing thermal cracks grow during the pre-failure stage, the events with low peak frequency and low peak amplitude in the heat-treated samples are interpreted as exhibiting a low stress drop for individual events. Therefore, although AE signals with low frequency can be considered as precursors to rock failure, cracking behavior suggested by events with low frequency depends on the initial damage condition of the rock sample.

Impact of the urea nitrate content in the leaching solution on the granite weight decrease and the fluoride ion concentration

The paper presents the results of the study on the impact of the initial concentration of urea nitrate and fluoride ion in the leaching solution on the weight decrease of granite mined at Prydniprovsk Specialized Quarry and the change in the fluoride ion concentration in the solution during intermittent and continuous agitation. During the experiments, we used urea nitrate with weight fraction of nitrate acid 41.5%, urea - 44.3%, water - 14.2%, and ammonium fluoride-bifluoride (fluorine content was 61.2%). Fluoride ion concentration was measured using a fluoride-selective electrode relatively a saturated silver chloride half-cell while the acidity of solutions was measured by titrimetry. Weight decrease was studied for a granite sample, grain fraction -2.0 + 1.0 mm, during intermittent stirring depending on the initial concentration of urea nitrate 0.25 g/dm3, 0.5 g/dm3, and 1.0 g/dm3 and fluoride ion 2.81×10-2 g/dm3 to 3.20×10-1 g/dm3 with intermittent stirring at a temperature of 20±2 °C for 7 days. The data on the change in sample weight depending on the initial concentration of fluoride ion and addition of urea nitrate were received. It was revealed that the decrease in the fluoride ion concentration ambiguously depended on the initial concentration of both urea nitrate and fluoride ion. In our opinion, the fluoride ion concentration decrease occurred with adding urea nitrate due to the fact that nitric acid urea could form adducts with both sulfuric and fluoric acids resulting in the decrease of their reactivity. We have studied the decrease of the granite sample weight and the change in the fluoride ion concentration under continuous stirring and at a temperature of 30±2 °C for 4 days. It was found that the change in the sample weight and concentration of fluoride ion during continuous stirring differed from the similar data obtained during intermittent stirring due to more intensive formation of urea nitrate adducts with sulfuric and fluoride acids. The initial dissolution rate was calculated for a granite sample, its value was 2.384×10-7 s-1 after adding urea nitrate and 2.299×10-7 s-1 without its addition.

Partial melting of subducted continental crust during the exhumation: Insights from Palaeozoic granitic rocks in South Altyn, western China

The early Palaeozoic continental subduction–collisional belt in South Altyn (SA) western China distributes a large amount of (ultra)-high-pressure ((U)HP) metamorphic and granitic rocks associated with subduction, collision, and exhumation. In this study, four granitic rocks were collected from Bashenwake located in eastern SA. The ages of the garnet-bearing alkali-feldspar granite (GAFG), garnet-bearing granitic gneiss (GGG), alkali-feldspar granite (AFG), and syenogranite (SG) are 458.0 ± 4.1, 483.0 ± 4.1, 450.0 ± 2.4, and 482.0 ± 4.9 Ma, respectively. Whole-rock major and trace elements and Sr–Nd–Pb isotope, and zircon Hf isotope compositions for all samples exhibit characteristics of a continental crustal source. Based on partial melting modeling, the source temperature of the GGG, GAFG, AFG and SG was concluded as 750–780 °C, 730–760 °C, 730–770 °C and ca. 800 °C, respectively. The source pressure was estimated as ca. 0.8 GPa for the GGG, GAFG and AFG, and ca.1.2 GPa for SG. Combined with studies on (U)HP metamorphic rocks in SA, this study reveals that GGG is formed by metamorphism while SG is formed from partial melting of upper crustal rocks heated by upwelling mantle during the initial stage of exhumation (ca. 480 Ma) of subducted continental crust in SA. During further stage of exhumation of the subducted continental crust at ca. 450 Ma, the GAFG and AFG were formed from partial melting of GGG as a result of decreased pressure and further mantle upwelling. Consequently, the exhumation age of subducted continental crust in eastern SA is ca. 30 Myr earlier than that in western SA. Thus, the geochronology and petrogenesis of four granitic samples in this study exhibit one of very few evidence for partial melting of subducted continental crust during the exhumation and also provide magmatism exhibits evidence for the differential exhumation model of eastern SA in Palaeozoic.

A first attempt at a provenance study in the Jadar block (Serbia) by means of U-Pb zircon geochronology

U-Pb geochronology of zircon grains retrieved from magmatic rocks intruding the Jadar block terrane in the central Balkans is used here to add new constraints on the terrane accretion processes and the provenance of crustal sources of this potentially exotic crustal block. Using an unorthodox approach, we analyzed zircons extracted from the products of Cenozoic (Cer and Boranja granitoid massifs) and Triassic magmatism (Bobija andesitic tuff - Pietra Verde). In fourteen samples of granites and epiclastites, we analyzed about 600 grains, and of these, about 30-40% were derived from the basement and were used further for the geological interpretation. Most samples show a similar Precambrian and Paleozoic age spectrum, including ubiquitous Neoproterozoic and well-defined Silurian-Ordovician populations. Only a few older zircons are present, composing minor populations at c. 1.2 Ga and 3.2 Ga. The younger zircons represent a ubiquitous Triassic population that is the strongest in all samples. This age population is most likely associated with local Permo-Triassic magmatism generated due to the opening of the Neotethys. In contrast to the magmatic rocks of Boranja and Bobija, the zircon age spectrum of the Cer polyphase pluton shows a strong Carboniferous peak, indicating a potentially important link to the Variscan margin of Eurasia. This supports opposing interpretations that either this part of the Jadar block terrane represents a southern continuation of the ?Bukkium? and Sana-Una terranes comprising displaced fragments of the southern European Variscan foreland, or, more likely, that it has an Adria affinity and that these zircons are derived from Cretaceous sediments of the Sava Zone, i.e., the suture that separates European and Adriatic domains, which were assimilated during the intrusion of the Cer granitic magmas.

Geochemistry of granites in and around Chityala, Cuddapah Basin

The granites of the Chityala region, which is close to Hyderabad, have been chosen for an organised research. Granites are the subject of petrographic research, which also include modal analysis. These investigations suggest that the subsequent petrogenetic processes have entirely destroyed the original magmatic structures. Furthermore, perthite and myrmekite frequently exhibit replacement structures and textural intergrowth. Six representative granite samples were examined for trace elements and seventeen representative granite samples were subjected to major elemental analysis. The data collected and shown in various binary and ternary diagrams allow for the observation of trends in the major oxides and trace elements. Geochemical variation graphs for the granites in the analysed area do not show any clear origin.

Britholite-(Ce) from the metaluminous granite of SW Egypt

Abstract Optical microscopy and scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analyses were used to characterize britholite-(Ce) of the metaluminous granite (Proterozoic) intruded into the Nubian Formation in southwestern Egypt and having a relatively high content of radioactive elements (potassium = 4.4 wt%, thorium = 52 ppm, uranium = 10 ppm). The britholite-(Ce) studied here incorporates light rare earth elements (LREE) including lanthanum, cerium, praseodymium, and neodymium as well as thorium. The mineral forms as tabular crystals in a fine-grained quartz-K-feldspar-plagioclase matrix, coexisting with other accessory minerals including biotite, zircon, clinochlore, titanite, and magnetite. Primary britholite-(Ce) usually includes titanite, zircon, and magnetite and is occasionally included in biotite. The inclusion of britholite-(Ce) in biotite suggests that the two minerals crystallized approximately coevally. Petrographical and SEM-EDS data indicate breakdown of the primary magmatic britholite-(Ce) in the samples. This study constitutes the first report for a sole presence of britholite-(Ce) as LREE phase in metaluminous granite samples from Egypt.

Soil Water Content Regression Analysis of Measurement Data from Hyperspectral Camera in Weathered Granite Soils

Soil water content is one of the most common physical parameters that cause landslides or debris flow. Therefore, it is of very importance to determine or predict the water content quickly and non-destructively. This study investigates the hyperspectral information in the visible near-infrared regions (VNIR) of different samples of granite soils possessing varying water content. Totally 162 granite samples were taken from each mountain area. The samples with different water contents were examined using a hyperspectral radiometer operating in the 400~1000nm range by obtaining the spectral curves. It was found that the variation of VNIR is consistent with the water content variation of weathered granite soils, and the hyperspectral camera was able to detect it. A Partial Least Squares Regression (PLSR) analysis was applied to develop a calibration-model. The PLSR model resulted in a good correlation between the water content and VNIR profile. The results demonstrate that the hyperspectral camera combined with the PLSR model can be a useful and non-destructive tool for determining soil water contents in the weathered granite soils.

Weakening of tensile strength of granitic rock by HV-HF-AC actuation of piezoelectric properties of Quartz: a 3D numerical study

High-voltage and high-frequency alternating current (HV-HF-AC) excitation of piezoelectric properties of Quartz is a potential method to induce cracks in granite. This was recently shown in a numerical feasibility study [6], where cracking was induced on cylindrical rock samples made of granite by sinusoidal AC excitation at the frequency of ~100 kHz and the amplitude of ~10 kV. However, this study did not investigate the weakening effect due to this cracking on the tensile strength of the sample. The present study addresses this topic numerically. For this end, a numerical method based on 3D embedded discontinuity finite elements for rock fracture and an explicit time stepping scheme to solve the coupled piezoelectro-mechanical problem is adopted. Rock heterogeneity and anisotropy are accounted for at the mineral mesotructure level. A preliminary numerical simulation demonstrates that the HV-HF-AC treatment reduces the tensile strength of a cylindrical granite sample by 12 %, making it thus a potential non-conventional pre-treatment method in comminution and excavation of Quartz bearing rocks and ores.

Thermally Induced Microcracks in Granite and Their Effect on the Macroscale Mechanical Behavior

AbstractUnderstanding the thermal effects on rock is critical for the exploration of geothermal resources and the temperature‐driven evolution of Earth. The macroscale mechanical behavior of granite under thermal loading can be complicated due to microcracking and the heterogeneity of minerals. In this study, the thermally induced microcrack propagation of granite was first observed in real time by an ultrahigh‐temperature instrument on an optical microscope. The previous investigation believed that the α–β quartz transition at approximately 573°C leads to a sharp decrease in macroscale mechanical behavior. However, the present experimental results revealed that microcracks initiate at 300°C and coalesce between 400 and 600°C, which is the main reason for the sharp decrease in macroscale mechanical properties of granite. Additionally, an accurate grain‐based model based on the mineralogical morphology of granite samples adopted mechanical parameters of minerals obtained from microscale rock mechanical techniques. It is able to well reproduce the process of microcracking and the strength evolution of granite during heating. The numerical results show that the heterogeneity of thermal expansion and mechanical properties of minerals produce local thermal stress concentrations in granite. The uneven tensile‐compressive stress between feldspar and quartz and the shear stress around biotite result in the initiation and propagation of microcracks. The present work is potentially useful for understanding and predicting the mechanical behavior of granites under thermal loading with different mineral compositions and microstructures.