Large Quartz Grains Research Articles

Ancient Aeolian Reservoirs of the East Siberia Craton

Fine-grained strata deposited on the Eastern Siberian craton are predominantly considered to mainly consist of Neoproterozoic sandstones. Clastic rocks near the unconformity border of the Ediacaran and the Riphean are represented by sandstone and siltstone layers with thicknesses of several tens of meters, belonging to the Nepa, Tira, and Byuk horizons in the Nepa–Botuoba region. These Neoproterozoic sandstones have features characteristic of aeolianites formed under the action of high wind velocity in the Ediacaran period. Sandstone samples near the Riphean–Ediacaran boundary were collected from five deep wells and characterized for granulometry and mineral composition using optical microscopy, XRD, SEM, and ICP-MS techniques. These sandstones have a high proportion of quartz (60–98%) with minor amounts of feldspars, carbonate, and sulfate cements. Thin sections of the sandy rocks feature bimodal distributions of the grains throughout many sections, with large well-rounded quartz grains being several orders of magnitude greater than the silt matrix grains. The monomineralic quartz rocks have an overgrowth of quartz grains. These rocks can be petroleum reservoirs with good porosity and permeability, but in most of the studied intervals, a high content of anhydrite and dolomite interstitial cement significantly reduces both. The porosity of the rocks is low, while the permeability is very low, which may be associated with a significant amount of clay and cement material. Aeolianites normally contain large amounts of bimodal quartz (due to its high stability and resistance to weathering) and possess the presence of heavy minerals.

Investigation of the Quartz Distribution in Electro-Porcelain Materials

Analysis of the microstructure of electro-porcelain is important to better understand its influence on mechanical, electrical and aging behavior. The microstructures of two electro-porcelain materials with low and high quartz contents were analyzed with respect to the distribution of quartz. Using an adequate evaluation of EDS (energy dispersive spectroscopy) mapping data, a reproducible analysis of the size distribution and quantity of quartz was achieved. The method allows the analysis of large areas (12.5 mm2). Therefore, the probability of occurrence for a few large quartz grains could be determined. Independent of the overall amount of quartz in the materials, a wide distribution of the grain size was observed. The size of the large detected quartz grains in both materials was very similar. Around the large quartz particles, microcrack systems with lengths of several 100 µm were observed. They are linearly correlated with the equivalent circle diameter of the quartz grains. The evaluation of the cracks allowed us to determine the critical size below which no cracks around the quartz particles are formed. This size is approximately 10 µm.

Technical note: On the reliability of laboratory beta-source calibration for luminescence dating

Abstract. The dose rate of the 90Sr / 90Y beta source used in most luminescence readers is a laboratory key parameter. There is a well-established body of knowledge about parameters controlling accuracy and precision of the calibration value but some hard-to-explain inconsistencies still exist. Here, we have investigated the impact of grain size, aliquot size and irradiation geometry on the resulting calibration value through experiments and simulations. The resulting data indicate that the dose rate of an individual beta source results from the interplay of a number of parameters, most of which are well established by previous studies. Our study provides evidence for the impact of aliquot size on the absorbed dose in particular for grain sizes of 50–200 µm. For this grain-size fraction, the absorbed dose is enhanced by ∼ 10 %–20 % as aliquot size decreases due to the radial increase of dose rate towards the centre of the aliquot. The enhancement is most variable for 50–100 µm grains mounted as aliquots of < 8 mm size. The enhancement is reversed when large grains are mounted as small aliquots due to the edge effect by which the dose induced by backscattered electrons is reduced. While the build-up of charge dictates the increase of absorbed dose with the increase of grain size, this principle becomes more variable with changing irradiation geometry. We conclude that future calibration samples should consist of subsamples composed of small, medium, large and very large quartz grains, each obtaining several gamma doses. The calibration value measured with small, medium and large aliquots is then obtained from the inverse slope of the fitted line, not from a single data point. In this way, all possible irradiation geometries of an individual beta source are covered, and the precision of the calibration is improved.

Early Palaeoproterozoic granulite-facies metamorphism and partial melting of eclogite-facies rocks in the Salma association, eastern Fennoscandian Shield, Russia

The Salma-type Archaean eclogites exposed along the northwestern boundary of the Belomorian Eclogite Province in the eastern Fennoscandian Shield formed as a result of the Mesoarchaean–Neoarchaean subduction and collision. The common protoliths of the Salma-type subduction-related eclogites were oceanic layered gabbro and volcanic-sedimentary assemblage. The eclogite-facies pillow lavas and associated alumina-siliceous sediments that fill interpillow space and intercalate with lava flows are the main objects of our work. The kyanite-garnet–phengite–quartz rocks formed after alumina-siliceous sediments contain fluid inclusions trapped in large relic quartz grains. The fluid inclusions yielded an isochore that corresponds to PT-conditions of a beginning of the Salma oceanic rock subduction from the seafloor level that generally confirms the sedimentary provenance of these rocks. The alumina-siliceous sediments underwent the eclogite-facies metamorphism at pressure no lower than 21 kbar and temperatures of 650–750 °C and transformed into kyanite-garnet–phengite–quartz rocks. During exhumation under granulite-facies conditions at temperatures up to 900 °C and pressure down to 9 kbar, eclogite facies metasediments underwent partial melting accompanied by disequilibrium breakdown of phengite + quartz association with formation complex polymineralic pseudomorphs consisting of feldspars, biotite, muscovite, kyanite, corundum, and dumortierite. U-Pb dating of Th-rich igneous zircon from melted metasedimentary and mafic rocks using the LA-ICP-MS and TIMS methods yielded the time of granulite facies event accompanied by partial melting processes at ~2.45 Ga. After this, zircon underwent fluid-induced alteration, causing partial dissolution followed by precipitation of new Th–poor zircon and zircon rims around ancient grains at ~1.9 Ga ago

Preservation of erniettomorph fossils in clay-rich siliciclastic deposits from the Ediacaran Wood Canyon Formation, Nevada: Ediacaran preservation in sandstone beds

Author(s): Hall, JG; Smith, EF; Tamura, N; Fakra, SC; Bosak, T | Abstract: Three-dimensionally preserved Ediacaran fossils occur globally within sandstone beds. Sandy siliciclastic deposits of the Ediacaran Wood Canyon Formation (WCF) in the Montgomery Mountains, Nevada, contain two fossil morphologies with similar shapes and sizes: one exhibits mm-scale ridges and a distinct lower boundary and the other is devoid of these diagnostic features. We interpret these as taphomorphs of erniettomorphs, soft-bodied organisms with uncertain taxonomic affinities. We explore the cast-and-mould preservation of both taphomorphs by petrography, Raman spectroscopy, X-ray fluorescence microprobe and X-ray diffraction. All fossils and the surrounding sedimentary matrix contain quartz grains, iron-rich chlorite and muscovite. The ridged fossils contain about 70% larger quartz grains compared to the ridgeless taphomorph, indicating a lower abundance of clay minerals in the ridged fossil. Chlorite and muscovite likely originated from smectite and kaolinite precursors that underwent lower greenschist facies metamorphism. Kaolinite and smectite are inferred to have been abundant in sediments around the ridged fossil, which enabled the preservation of a continuous, distinct, clay- and kerogen-rich bottom boundary. The prevalence of quartz in the ridged fossils of the WCF and in erniettomorphs from other localities also suggests a role for this mineral in three-dimensional preservation of erniettomorphs in sandstone and siltstone deposits.

Water saturation effects on dynamic behavior and microstructure damage of sandstone: Phenomena and mechanisms

Underground rock structures are frequently subjected to water erosion and dynamic disturbance simultaneously. Herein, in order to study the coupled effects of water and high strain rate on the mechanical behavior and microstructures of sandstone, we conducted a series of dynamic unconfined compressive tests on oven-dried and water-saturated sandstone core samples using a split Hopkinson pressure bar. Test results revealed that, three macroscopic final patterns, namely unbroken, axial split and pulverization, were observed. At given strain rates, the presence of water weakens the dynamic peak stress and the dissipated energy density (the ratio of energy dissipation to residual axial strain) of rock sample but enhances the elastic modulus regardless of the failure pattern. Additionally, the saturated sample owns a higher rate dependence of dynamic strength compared to the dry one under the explored range of strain rate (43.9–156.7 s−1), which indicates that water-weakening on rock strength gradually attenuates with the increase of strain rate. Interestingly, thin sections analysis microscopically showed that the failure of dry samples is characterized by the intra-granular fracturing in larger quartz grains while that of saturated samples by the inter-granular fracturing. The underlying mechanisms were interpreted with two micro-mechanical damage models.

Analyzing the causes of crack formation in porcelain and the ways to eliminate them

The studies deal with the porcelain products produced by casting finely-dispersed slips into gypsum molds. The causes of the occurrence of cracks on the edges of products after bisque and glazed firing were established and the ways of their elimination were proposed. As a result of the comprehensive research into the basic ceramic mass of the assigned composition and products from it, a series of the technological factors that cause cracking were determined. Determining the fineness of grinding of production slips of various batches showed that the residue on sieve No. 0063 ranged from 0.7 to 3.5 %. The direct correlation between the occurrence of cracks in products and an elevated indicator of the residue that contributed to the stratification of a ceramic slip in the process of settling in gypsum molds was established. This led to the emergence of internal stresses in the structure of a potsherd. It was established that another factor that caused cracking of products was the existence in the composition of the ceramic mass of large quartz grains, which in the heating-cooling process are capable of modification transformations. In addition, the low temperature of bisque firing did not contribute to the completion of the process of dehydration of clay and mica minerals, which strengthened the internal stresses in a potsherd. The research revealed that to ensure qualitative indicators of ceramic products, it is necessary not only to control the residue on a sieve but also to take into consideration the distribution of the fractional composition of a ceramic slip, while the content of the quartz component of 30–63 µm should not exceed 12 % by weight. This contributes to the formation of a dense homogeneous potsherd with a high content of a mullite phase. As a result of the research, it was also proposed to change the temperature of bisque firing from 660 to 800 °C. It is at this temperature that the processes of dehydration of layered silicates are completed and the shrinkage processes are stabilized. The obtained results can be applied at the typical production of household products from low-temperature porcelain

Preservation of erniettomorph fossils in clay-rich siliciclastic deposits from the Ediacaran Wood Canyon Formation, Nevada.

Three-dimensionally preserved Ediacaran fossils occur globally within sandstone beds. Sandy siliciclastic deposits of the Ediacaran Wood Canyon Formation (WCF) in the Montgomery Mountains, Nevada, contain two fossil morphologies with similar shapes and sizes: one exhibits mm-scale ridges and a distinct lower boundary and the other is devoid of these diagnostic features. We interpret these as taphomorphs of erniettomorphs, soft-bodied organisms with uncertain taxonomic affinities. We explore the cast-and-mould preservation of both taphomorphs by petrography, Raman spectroscopy, X-ray fluorescence microprobe and X-ray diffraction. All fossils and the surrounding sedimentary matrix contain quartz grains, iron-rich chlorite and muscovite. The ridged fossils contain about 70% larger quartz grains compared to the ridgeless taphomorph, indicating a lower abundance of clay minerals in the ridged fossil. Chlorite and muscovite likely originated from smectite and kaolinite precursors that underwent lower greenschist facies metamorphism. Kaolinite and smectite are inferred to have been abundant in sediments around the ridged fossil, which enabled the preservation of a continuous, distinct, clay- and kerogen-rich bottom boundary. The prevalence of quartz in the ridged fossils of the WCF and in erniettomorphs from other localities also suggests a role for this mineral in three-dimensional preservation of erniettomorphs in sandstone and siltstone deposits.

Islamic glazed wares from ancient Termez (southern Uzbekistan). Raw materials and techniques

Ancient Termez, located on the southern border of Transoxiana/Mawarannahr, was an important pottery production centre during the Islamic period. Recent archaeological and archaeometric research carried out by the Spanish-Uzbek team evidenced the manufacture of glazed and unglazed vessels at the workshops found in the lower city (shahristan) and its suburbs (rabad). Glazed local products, mainly dated between the 9th and 16th/17th centuries, comprise slip-painted, underglaze and inglaze painted wares, splashed sgraffiato, and monochrome wares. The present study focuses on the chemical, mineralogical and petrographic examination of different types of glazed ceramics recovered at Termez excavations in order to identify the microstructure and composition of the glazes, the technological processes involved in their manufacture, and their evolution over the centuries. Thin polished sections were prepared and slips and glazes were analysed by optical microscopy (OM) and scanning electron microscopy (SEM). The results reveal that all the local/regional ceramics have a slip and a transparent glaze, with one exception. Colour decorations were applied over the slip, forming thinner or thicker layers depending on the desired final colour. Green (copper), red (iron) and brown/black (iron and in some cases iron plus manganese) pigments were used for the decorations. A high lead glaze was used in the ceramics found in the alluvial plain dating between the 9th and the 11th century, while an alkaline glaze is associated with the majority of ceramics collected in the shahristan dating between the 12th and the 17th century. An alumina rich clay mixed with lead oxide was used in the slip from the alluvial plain ceramics while the slip contained large quartz grains in the ceramics found in shahristan. Three imports from the Iraqi regions, a monochrome lustreware bowl and two white opaque glazed dishes dating from the 9th–10th centuries, were also analysed. They have the characteristic tin-opacified mixed lead-alkali glazes and fine calcareous pastes.

Determination of rock type and moisture content in flysch using TLS intensity in the Elerji quarry (south-west Slovenia)

Terrestrial laser scanning (TLS) was performed in tectonically undisturbed flysch in the Elerji quarry (south-west Slovenia), which represents heterogeneous rock mass with alternation of marlstone and sandstone beds. A 9.3-m-thick lithological profile was measured in detail and then compared to TLS values of reflected intensity and mineralogical composition of general marlstones and sandstones. The results showed distinguishable differences between the intensity values of marlstones and sandstones, as these two groups were found to be statistically different. It was also possible to discriminate these units with regard to their moisture content, as different values were observed for combinations of lithology and moisture content. Correlations of intensity and RGB (red, green, blue) values for the mineralogical composition of marlstones and sandstones revealed a good correlation of intensity with combined calcite and quartz content; however, such correlation is valid only for sandstones due to their bigger grain size. Larger quartz grains can scatter the laser beam and lower the correlation. The results showed that acquiring data from heterogeneous rock mass using TLS could be used for the geotechnical engineering classification of distant, inaccessible outcrops and excavated faces, based on the well-established RMR (Rock Mass Rating) and GSI (Geological Strength Index) rock mass classification systems. Both RMR and GSI classification systems classify rock mass based on rock composition, structure and surface conditions of discontinuities. Lithological segmentation of heterogeneous rocks presents, therefore, an important input parameter for their further classification.

Application of nanoindentation for uncertainty assessment of elastic properties in mudrocks from micro- to well-log scales

Uncertainty in estimates of elastic properties of soft mudrock components, such as clay minerals and kerogen, can influence well-log-based evaluation of effective elastic properties in organic-rich mudrocks. Existing methods, such as effective medium models for well-log-based assessment of elastic properties, assume a constant stiffness and an idealized shape for rock components. However, these characteristics might vary depending on the distribution and size of that particular component, as well as its adjacent components. Furthermore, there is a significant uncertainty in elastic properties of kerogen in the case of organic-rich mudrocks. The uncertainty associated with the aforementioned parameters on effective elastic properties of rocks has not been investigated in existing publications. In this paper, we quantified the variability in elastic properties of individual mudrock components caused by their spatial distribution, size, and rock fabric at the microscale and their impacts on well-log-based evaluation of effective elastic properties. We performed nanoindentation mechanical tests on samples from the Haynesville and the lower Eagle Ford Formations, to measure Young’s modulus and hardness at targeted locations. Then, we quantified the variability of Young’s modulus in the microscale and its impact on effective elastic properties at the micro- and well-log scales. Results reveal significant uncertainties in measurements of elastic properties of soft rock components, associated with their location and size. Young’s moduli of individual clay components are higher when located adjacent to stiff rock components, such as large quartz and calcite grains. Results reveal that 25% and 12% uncertainties in measured elastic properties of clay minerals affect well-log-based estimates of effective elastic stiffness coefficients up to 29% and 11% in the Haynesville and the lower Eagle Ford Formations, respectively. These uncertainties can be more significant in cases with a higher concentration of clay minerals and kerogen.

A microstructural study of the fault gouge in the granite, Yangbuk, Gyeongju, southeastern Korea, with implications for multiple faulting

Microstructures developed in the fault gouge zone, Yangbuk, Gyeongju city, Korea, were characterized with emphasis on distribution of remaining grains. Based on XRD analysis, the fault gouges consist of quartz, feldspar and clay minerals composed of abundant illite and chlorite, with vein minerals including laumontite and pyrite. As the fractal dimension (D-value) of grains gradually increases, the pore space in the fault gouge becomes smaller, causing the change in microstructures. Quartz and feldspar exerted a different influence on the development of microstructures in the fault gouge zone, derived from essential reactivity inherent between the two minerals during multiple faulting. Quartz grains intensively suffered grain size reduction while the content and size of coarse grains decreased. Feldspar grains tend to influence the formation of clay minerals made up for the matrix. Although large quartz grains highly fractured with abrasions caused the grain size reduction in the gouge zone during early fault activities, fine-grained quartz in the clay-rich matrix easily remained due to the reduction in friction resulted from clay minerals. On the other hand, feldspar grains underwent more abrasion and dissolution, leading to an increase in the amount of clay minerals newly formed during fault activities. Changes in microstructures were accompanied by precipitation of the secondary minerals during multiple fault activities in connection with fracturing of main rock-forming minerals such as quartz and feldspar derived from the host rock.

Grain boundary diffusion of titanium in polycrystalline quartz and its implications for titanium in quartz (TitaniQ) geothermobarometry

We have performed a series of experiments to measure diffusivity of Ti in polycrystalline quartz under high pressure/temperature, nominally anhydrous conditions. Resulting diffusion profiles reveal operation of both slow lattice diffusion and faster grain boundary diffusion. Over the temperature range investigated, 1000–1400°C, grain boundary diffusion of Ti is between 3 and 4 orders of magnitude faster than lattice diffusion and can be expressed by the following Arrhenius relationship:D(m2/s)=2.00±0.08×107exp(-195±7kJ.mol-1/RT).Grain boundary diffusion is expected to have a considerable influence on Ti mobility in the crust in Si-rich rocks under fluid-absent conditions, especially in fine-grained rocks, with grain boundaries acting as fast conduits for transporting Ti. This has important consequences for the application of Ti in quartz geothermobarometry (TitaniQ). Grain boundary diffusion is a viable mechanism for re-equilibrating Ti contents in quartz-rich rocks to lower values, for example during dynamic recrystallization. This implies that TitaniQ can be applied to relatively low temperatures (below 600°C) although zonation of Ti contents in larger quartz grains is expected due to the relative sluggishness of lattice diffusion under these conditions and because fast diffusion in grain boundary regions effectively inhibits growth entrapment. Grain boundary diffusion for Ti also has implications for the activity of Ti in quartz-rich rocks and application of the TitaniQ geothermobarometer.

T–M coupled effects on cracking behaviors and reliability analysis of double-notched crustal rocks

Experiments on crack initiation and propagation of double-notched rocks caused by thermal–mechanical loading were carried out by using in-situ SEM observation technique. The results indicated that thermal-induced or thermal–mechanical-induced cracking mechanisms depend strongly on thermal deformation mismatch between the mineral compositions. Micro cracks initiation occurred mainly in the harder and larger quartz grains, but the failure strength of rock did not adequately degrade when surface micro crack density is less than 40(mm−2). At last, a reliability analysis of 3-parameter Weibull distribution model is presented to discuss the effect of the thermal-induced micro crack on failure strength of rock. Threshold value by thermal-induced micro crack density to influence on the failure strength of crustal rock can be estimated at about 50–60(mm−2).

Characterization of clays from Mezafe and Mengono (Ne-Libreville, Gabon) for potential uses in fired products

Clay materials may contribute to promote the sustainable development of population accommodations in a developing country as Gabon. They are low cost versatile raw materials for the building industry. A mineral survey led to the discovery of clay occurrences along the highway Libreville-Cocoa-beach from Mezafe to Mengono (NE Libreville). These clays were characterized by chemical analyses, grain size distribution (sieving, sedimentometry and Fraunhofer laser light scattering), plasticity, X-ray diffraction on crude powder and oriented fine fractions (natural, glycolated or heated at 550°C), Fourier transform infrared spectroscopy, adsorption–desorption of nitrogen, cation exchange capacity, and scanning electron microscopy. Samples were fired from 900 to 1150°C and a few properties such as water absorption, linear shrinkage and bending strength were determined. Mineralogically, raw clay samples are made up mainly of quartz, kaolinite, illite and chlorite. Accessories are orthoclase, anatase, ilmenite, magnetite, goethite, lepidocrocite, Fe–Al sulfate, La–Ce rich crandallite or a mineral likely related to Ce-belovite, pyrite, Cu–Fe rich silicate, carbonates, bitumen and other organic matters. The decrease of SiO2 contents and the increase of Al2O3 contents of fractions under 125μm suggest the presence of large size quartz grains and fine kaolinite particles. The sum kaolinite+illite+chlorite increases from 25–42% in crude samples to 37–48% in clay fractions (135μm) with an increment rate of 15–40% at Mezafe and above 50% at Mengono. Thus, screening only can yield clay-rich raw materials similar to some European commercial kaolins. Results of firing from 900 to 1150°C show that water absorption, linear shrinkage and bending strength values are in the norm of tiles and fired earth. Only one sample's properties need to be enhanced for terra-cotta production.

Rare eclogite‐mafic granulite in felsic granulite in Blanský les: precursor of intermediate granulite in the Bohemian Massif?

AbstractMafic granulite, generated from eclogite, occurs in felsic granulite at Kleť, Blanský les, in the Bohemian Massif. This is significant because such eclogite is very rare within the felsic granulite massifs. Moreover, at this locality, strong interaction has occurred between the mafic granulite and the adjacent felsic granulite producing intermediate granulite, such intermediate granulite being of enigmatic origin elsewhere. The mafic granulite involves garnet from the original eclogite, containing large idiomorphic inclusions of omphacite, plagioclase and quartz, as well as rutile. The edge of the garnet is replaced by a plagioclase corona, with the garnet zoned towards the corona and also the inclusions. The original omphacite–quartz–?plagioclase matrix has recrystallized to coarse‐grained polygonal (‘equilibrium’‐textured) plagioclase‐diopsidic clinopyroxene–orthopyroxene also with brown amphibole commonly in the vicinity of garnet. Somewhat larger quartz grains are embedded in this matrix, along with minor ilmenite, rutile and zircon. Combining the core garnet composition with core inclusion compositions gives a pressure of the order of 18 kbar from assemblage and isopleths on a P−T pseudosection, with temperature poorly constrained, but most likely >900 °C. From this P−T pseudosection, the recrystallization of the matrix took place at ~12 kbar, and from Zr‐in‐rutile thermometry, at relatively hot conditions of 900–950 °C. It is largely at these conditions that the eclogite/mafic granulite interacted with the felsic granulite to make intermediate granulite (see next paper).

EBSD study of Quartz c-axis orientations in the silicified host rocks of the Kasuga gold deposit, Southwest Japan

The Kasuga High Sulfidation gold deposit is hosted in the silicified Late Miocene andesite of Nansatsu Volcanic Rocks in the southern part of the Satsuma Peninsula, Southwest Japan. Optical microscopic observations and X-ray fluorescence spectrometry (Horiba XGT-5000) analysis of two silicified andesite samples from the Kasuga ore body revealed that the rocks underwent different degrees of hydrothermal alteration and no tectonic deformation afterward. Crystallographic orientations of quartz composing the silicified andesite samples studied using Electron Backscatter Diffraction, EBSD (EDAX GE-1, TSL-OIM) displayed different trends in the c-axis orientations between the samples. The c-axes of quartz grains are mainly oriented sub-horizontal or plunging at low angles in the less-silicified andesite. The highly silicified sample has larger quartz grains with c-axes plunging at high angles. The difference in c-axis orientations is provably related to the silicification processes in the earlier and later stages of hydrothermal alteration. The results of this study suggest that quartz c-axis orientations of rocks deformed under low strain regimes are potentially inherited from initial undeformed state, for example crystallization from hydrothermal solutions.

P.3.025 An fMRI study of anticipation, near-miss and winning outcomes in pathological gambling and cocaine-dependence during slot-machine gaming

We have performed a series of experiments to measure diffusivity of Ti in polycrystalline quartz under high pressure/temperature, nominally anhydrous conditions. Resulting diffusion profiles reveal operation of both slow lattice diffusion and faster grain boundary diffusion. Over the temperature range investigated, 1000–1400 °C, grain boundary diffusion of Ti is between 3 and 4 orders of magnitude faster than lattice diffusion and can be expressed by the following Arrhenius relationship:Grain boundary diffusion is expected to have a considerable influence on Ti mobility in the crust in Si-rich rocks under fluid-absent conditions, especially in fine-grained rocks, with grain boundaries acting as fast conduits for transporting Ti. This has important consequences for the application of Ti in quartz geothermobarometry (TitaniQ). Grain boundary diffusion is a viable mechanism for re-equilibrating Ti contents in quartz-rich rocks to lower values, for example during dynamic recrystallization. This implies that TitaniQ can be applied to relatively low temperatures (below 600 °C) although zonation of Ti contents in larger quartz grains is expected due to the relative sluggishness of lattice diffusion under these conditions and because fast diffusion in grain boundary regions effectively inhibits growth entrapment. Grain boundary diffusion for Ti also has implications for the activity of Ti in quartz-rich rocks and application of the TitaniQ geothermobarometer.

Nanoscale porosity in SAFOD core samples (San Andreas Fault)

With transmission electron microscopy (TEM) we observed nanometer-sized pores in four ultracataclastic and fractured core samples recovered from different depths of the main bore hole of the San Andreas Fault Observatory at Depth (SAFOD). Cutting of foils with a focused ion beam technique (FIB) allowed identifying porosity down to the nm scale. Between 40 and 50% of all pores could be identified as in-situ pores without any damage related to sample preparation. The total porosity estimated from TEM micrographs (1–5%) is comparable to the connected fault rock porosity (2.8–6.7%) estimated by pressure-induced injection of mercury. Permeability estimates for cataclastic fault rocks are 10−21–10−19 m2 and 10−17 m2 for the fractured fault rock. Porosity and permeability are independent of sample depth. TEM images reveal that the porosity is intimately linked to fault rock composition and associated with deformation. The TEM-estimated porosity of the samples increases with increasing clay content. The highest porosity was estimated in the vicinity of an active fault trace. The largest pores with an equivalent radius>200nm occur around large quartz and feldspar grains or grain-fragments while the smallest pores (equivalent radius<50nm) are typically observed in the extremely fine-grained matrix (grain size<1μm). Based on pore morphology we distinguish different pore types varying with fault rock fabric and alteration. The pores were probably filled with formation water and/or hydrothermal fluids at elevated pore fluid pressure, preventing pore collapse. The pore geometry derived from TEM observations and BET (Brunauer, Emmett and Teller) gas adsorption/desorption hysteresis curves indicates pore blocking effects in the fine-grained matrix. Observations of isolated pores in TEM micrographs and high pore body to pore throat ratios inferred from mercury injection suggest elevated pore fluid pressure in the low permeability cataclasites, reducing shear strength of the fault.

Failure of Layered Sandstone under Brazilian Test Conditions: Effect of Micro-Scale Parameters on Macro-Scale Behaviour

The experimental research in this paper focuses on the effect of micro-scale parameters on macro-scale behaviour of samples tested under Brazilian test conditions for one particular layered sandstone, i.e. sandstone from Modave in the South of Belgium. Five (visually) similar blocks are studied; however, they are different at micro-scale. Their differences on microscopic level (such as grain size, mineral contents and number of layer boundaries) affect the failure process. Fracture length parallel to the layers increases if more layer boundaries (e.g. per cm) are present and more weak minerals. The Brazilian tensile strength of studied layered Modave sandstone is larger for larger quartz grains, while fracture length parallel to the layers is smaller. Quartz grains in the studied layered sandstone are classified as fine sand.