Individual Mineral Grains Research Articles

Characterisation of phosphorus deportment in ferromanganese oxide ores – Implications for P removal

Characterisation of six supergene ferromanganese ore samples revealed that P was hosted by several different Fe and Mn oxides. Goethite had the highest (up to >1% or higher) and highest mean (up to 0.32%) P contents, then cryptomelane-hollandite (mean up to 0.12% P) and nsutite-pyrolusite (mean up to 0.14% P). When present in the same sample, nsutite had a higher mean P content than pyrolusite. Phosphate minerals were not detected in the samples. P deportment was variable with deposit, deposit type and within individual mineral grains. EPMA showed that P content is elevated in cryptomelane with higher Mn/(Fe + Al + Si) ratios and in todorokite with lower Mn/(Fe + Al + Si) ratios. The results show that higher phosphorus supergene ferromanganese ores with considerable cryptomelane and nsutite contents are likely to have multi-phase Fe-P and Mn-P deportment. The propensity for very fine-scale intergrowths of goethite and Mn oxides ± aluminosilicates in some ore types means that reliable characterisation of P deportment is critical in informing potential P-reduction pathways for ferroalloy feedstocks. P was most probably incorporated into Mn oxides via near-surface adsorption via complexation with cations such as Ca or Mg in todorokite, but the presence of structural P cannot be precluded in cryptomelane and nsutite.

Дослідження впливу різних наповнювачів на атмосферостійкість асфальтобетонів

Introduction. The properties of bituminous-mineral materials are determined by the bonds formed between individual mineral grains and depend on the characteristics of the bitumen, the thickness of the bitumen layer covering the mineral grains, as well as the interaction processes between the mineral materials and bitumen at their joint interface. To ensure a strong and stable bond between the grains, the bitumen must evenly cover the surface of the mineral materials with a thin layer. To improve the strength of road pavement made from bituminous-mineral material, it is necessary for the maximum amount of bitumen to be adsorbed by the mineral material, minimizing the content of free bitumen. Problem Statement. Depending on the nature of the mineral material and the chemical composition of the bitumen, the properties of its thin layers change in various ways. On an active mineral surface, the strength of the bitumen in a thin layer increases as the thickness decreases, while on an inactive surface, the strength of the bitumen remains largely unchanged with a decrease in layer thickness.

Further investigation of spatially resolved single grain quartz OSL and TL signals

The use of luminescence signals from single mineral grains for optical dating has become a valuable and frequently utilised tool in Quaternary Geochronology. Single grain luminescence dating is particularly beneficial in complex depositional settings, however the ability to measure single grain signals also offers the opportunity to assess intrinsic luminescence properties of individual mineral grains. The use of spatially resolved luminescence technologies such as an electron multiplier charge coupled device is of benefit when making luminescence measurements at single grain scales because they allow stimulation with light emitting diodes, and this offers a number of key benefits related to stimulation power when it comes to the assessment of characteristics such as optically stimulated luminescence (OSL) decay rate and the calculation of parameters such as the fast ratio and photo ionisation cross-sections. In this paper, the intra- and inter-sample variability of sensitised single grain thermoluminescence (TL) and OSL signals is considered. A comparison between TL and OSL signals is undertaken, as well as calculation of the fast ratio, OSL component photo ionisation cross-sections, thermal stability, and characteristic dose for a suite of quartz samples from a range of geographic locations and depositional settings. For these heated signals, key findings include the lack of relationship between OSL signal intensity and dominance of the fast component, the fitting of two components (a fast and slow component) is the most common fit for single grain OSL signals, characteristic doses from fast dominated signals suggesting saturation at c. 150 Gy, and the identification of the ultrafast OSL component. Intra-sample variability across all measured parameters is observed, suggesting that for this suite of samples, variability is the norm rather than the exception, and that the intrinsic luminescence characteristics of quartz are variable and diverse.

Insights on the origin of oldhamite in enstatite meteorites from Ca stable isotopes

In order to understand the origin of oldhamite (CaS) in enstatite meteorites, we report Ca isotopic compositions (δ44/40Ca) of oldhamite (obtained from water leachate of bulk chondrites and aubrites and mineral separates from the Norton County aubrite) and silicate minerals from different types of enstatite chondrites and aubrite. The δ44/40Ca of the bulk enstatite chondrites range from 1.05 ‰ to 1.24 ‰, with an average of 1.13 ± 0.12 ‰, higher than that of the estimate of the bulk silicate earth (∼0.94 ‰). Major and trace element analyses show that the water leachates of enstatite chondrites are mainly composed of oldhamite, and they take over 20.6–68.5 % Ca of the bulk meteorite Ca budget. The Ca isotope fractionation between oldhamite and residual silicate (Δ44/40Caoldhamite-silicate) for the studied enstatite chondrites is minimum (−0.44 ‰) for Abee (impact-melt breccia) and maximum (+0.16) for St.Marks (EH5). The Ca isotope fractionation between oldhamite (individual mineral grains and leachate) and silicates in Norton County varies from −0.47 ‰ to −0.31 ‰ with an average of −0.41 ‰. These Δ44/40Caoldhamite-silicate correlate well with previous theoretical calculation and suggests that the oldhamites in Norton County are in isotopic equilibrium with co-existing silicates, and therefore were formed during magmatic processes. However, in enstatite chondrites, the large variation on Δ44/40Caoldhamite-silicate and its negative correlation with metamorphic temperature reflects the redistribution and equilibration of Ca isotopes during metamorphism. The variable Δ44/40Caoldhamite-silicate found in unequilibrated chondrites reflect kinetic Ca isotope fractionation between oldhamite and nebular gas and therefore is evidence for the formation of oldhamite by condensation in the solar nebula.

Modes of occurrence of critical metals (Nb-Ta-Zr-Hf-REY-Ga) in altered volcanic ashes in the Xuanwei Formation, eastern Yunnan Province, SW China: A quantitative evaluation based on sequential chemical extraction

Altered volcanic ash with highly-elevated concentrations of Nb(Ta), Zr(Hf), Ga, and rare earth elements and yttrium (REY) is an untraditional type of critical metal deposit that has been discovered in Yunnan Province, southwest China. Previous studies have briefly described the modes of occurrence of Nb(Ta)-Zr(Hf)-REY-Ga in the ore deposit, showing that Zr and Hf are mostly hosted in zircon; light rare earth elements (LREE) mainly occur in REY-bearing carbonates and phosphates; Nb and Ta are primarily hosted in the former Fe-Ti oxide phases and zircon; and Ga is predominantly associated with clay minerals. However, previous studies of the modes of occurrence of critical metal elements (Nb-Ta-Zr-Hf-REY-Ga) have been limited to qualitative or semi-quantitative studies of individual mineral grains, and quantitative research of the modes of occurrence of these elements in whole rocks is absent. This paper quantifies the modes of occurrence of critical elements (Nb-Ta-Zr-Hf-REY-Ga) using a sequential chemical extraction method. The results show that about 65.3–86.7% Nb is associated with hydrofluoric acid (HF)-soluble aluminosilicates (clays) and 13.3–34.7% in the insoluble phases such as Nb-rutile. Approximately 87.6–90.2% Zr is the primary hosted in zircon and 9.5–12.0% Zr appears to be associated with HF-soluble aluminosilicates in the altered volcanic ash. In felsic altered volcanic ashes (Al2O3/TiO2 ratio between 21 and 70), the REY are predominantly in the authigenic minerals such as REY-bearing aluminum phosphates and sulfates of the alunite supergroup (APS minerals), carbonates or fluorocarbonates. About 74.0–89.4% LREE, 26.5–34.2% middle rare earth elements (MREY), and 22.1–24.8% heavy rare earth elements (HREE) occur in HF-soluble aluminosilicates. In intermediate altered volcanic ash samples (Al2O3/TiO2 ratio between 8 and 21), 93.0% LREE are associated with insoluble phases such as monazite and zircon. Gallium is predominantly associated with HF-soluble aluminosilicates (73.3–80.5%) in all altered volcanic ash samples.

Morphological analysis of mineral grains from different sedimentary environments using automated static image analysis

The properties of sediment grain shape provide valuable information about the transport mechanisms in different sedimentary and geomorphological environments. With the emergence of new, high-resolution analytical techniques, it has become possible to quickly examine the grain shape properties of a large number of individual mineral grains. In this study, we used automated image analysis (Malvern Morphologi G3SE-ID) to investigate mineral particles of four sediment types from different depositional environments (sand sheet (1), floodplain (2) and fluvial channel deposits (3), Pleistocene infilling material of sand wedges (4), n = 20) in the Carpathian Basin (Central Europe). Our primary objective was to identify quantitative key variables that can help objectively distinguish certain geomorphological environments located in the Carpathian Basin. In our analysis and data processing (which included techniques such as hierarchical cluster analysis, Wilks' λ, Kruskal–Wallis, multivariate analysis of variance and principal component analysis) we focused on four variables related to grain shape: circularity (form), convexity (surface texture), solidity (roundness) and elongation (form). The form of sedimentary grains depends largely on the physical properties of their source area, while the roundness depends on the energy of the transport medium and the distance of transportation. Surface texture or convexity can change in a relatively short time in a fluvial environment.The study revealed that distinguishing geomorphological environments can be achieved by analyzing the circularity, convexity and solidity parameters of the sediment grains. Based on the established grouping, the analyses carried out with hundred repetitions showed that high sensitivity circularity, convexity and solidity variables were the most effective attributes regarding Kruskal–Wallis test statistics that provided significant (p < 0.001) results between the analyzed sedimentary environments, while the elongation was not able to provide significant results between the grouped samples. Statistical analyses of the MANOVA test with hundredfold repetitions showed significant differences between the derived groups. Wilks' λ test statistics and PCA showed that convexity and high sensitivity circularity discriminate the groups. Separate analyses of aeolian and fluvial sediments have been carried out. The Kruskal–Wallis analysis of variance showed the significant differences considering all four variables, and differences were also significant in the case of the MANOVA test. Wilks' λ test statistics and PCA showed that convexity, high sensitivity circularity and solidity discriminate the groups.According to our results, the circularity parameter can provide information about the transport distance, while the solidity parameter can indicate the transport energy. The convexity parameter can serve as an indicator of both transport distance, as well as post-depositional processes. Some infilling materials underwent multiple transport processes, including high energy aqueous, wind transport mechanisms, and the post-depositional alteration process (frost weathering), while others originated from sand-sheet covered areas (active during Pleistocene glacials). The solidity parameter proved effective in separating sediments with similarly high convexity values (smooth surface), which were, in our case, from recent aeolian and fluvial environments. This result was due to the investigated fluvial sediments that inherited their form and low level of roundness from their source area. Our research supports that aeolian transport is more effective in rounding the grains than the aqueous environment.Using automated static image analysis producing statistically stable results with hundreds of analyzed mineral grains provides useful indicators for paleogeographical reconstruction studies by investigating paleo and recent sediments.

ДОСЛІДЖЕННЯ ВПЛИВУ НАПОВНЮВАЧІВ РІЗНОГО ПОХОДЖЕННЯ НА ВЛАСТИВОСТІ АСФАЛЬТОБЕТОНІВ

Introduction. The properties of a bituminous material are determined by the peculiarities of the bonds that arise between individual mineral grains and depend on the properties of bitumen, the thickness of the bitumen layer covering the mineral grains, as well as on the processes of interaction between mineral materials and bitumen on their common interface. To ensure a strong and stable bond between the grains, bitumen should evenly cover the surface of the mineral materials with a thin layer. To increase the strength of the pavement constructed from bituminous material, it is necessary that the maximum amount of bitumen is adsorbed by the mineral material, and the content of free bitumen is reduced to a minimum value. Problem Ыtatement. Depending on the nature of the mineral material and the chemical composition of the bitumen, the properties of its thin layers change in different ways. The strength of bitumen in a thin layer on an active mineral surface increases with decreasing thickness, while on an inactive surface the strength of bitumen changes little with decreasing layer thickness. Objective. The present study was devoted to determining the effect of aggregates of different origins on the volumetric properties of asphalt concrete and the mechanical properties of asphalt concrete, as well as on their weather resistance. Materials and methods. Mineral filler from limestone, granite, quartzite, and fly ash was used for the study. Results. The results of the research showed that the type of aggregate has virtually no effect on changes in the volumetric and mechanical properties of asphalt concrete. However, at the same time, asphalt concrete with different aggregates differs significantly in water and frost resistance.

Three-dimensional imaging of high-velocity-impact induced crack growth in carbonaceous meteorites

The material strength of meteorites provides useful information on the make-up and history of asteroids. However, the unique determination of the material strength of a meteorite is difficult because of the wide range of strengths many meteorites exhibit. Even within a single sample, complicated textures and mineral granular compositions make measurements difficult. Michikami et al. (2019) investigated the impact-induced crack growth in ordinary (L5) chondrites and indicated that crack growth is largely affected by the strength of individual mineral grains (and/or chondrules). In this study, we examine the strengths of mineral grains in carbonaceous meteorites qualitatively. To this end, we use X-ray microtomography to investigate how chondrules are affected by impact-induced crack growth in carbonaceous meteorites. Spherical alumina projectiles with a diameter of 1.0 mm were fired into the surfaces of seven Allende (CV) meteorite target samples with sizes of ∼1 to 2 cm at a nominal impact velocity of 2.0 km/s. In addition, spherical glass projectiles with a diameter 0.8 mm were fired into the target surfaces of two Murchison (CM) and two Aguas Zarcas (CM) meteorite target samples with sizes of ∼2 cm at a nominal impact velocity of 4.0 km/s. The results show that most cracks in CV chondrites tend to grow along the boundary surfaces of the chondrules, while most chondrule-related cracks in CM samples grow regardless of the boundary surfaces of the chondrules. This suggests that crack growth is largely affected by the chondrules' strength as indicated by Michikami et al. (2019). The weaker the strength of chondrules, the more likely crack growth tends to occur regardless of chondrule boundaries. We found that the mesostasis of chondrules in CM meteorite Murchison (and likely Aguas Zarcas) has experienced aqueous alteration and the chondrules have become structurally weak as a whole. This indicates that impact-induced crack propagation in CM chondrites differs from thermal-fatigue induced crack propagation inferred from previous studies. As the sample material to be returned from asteroid Bennu is considered to be related to CM chondrites, we propose that observation of the cracks in chondrules in Bennu samples might tell us whether those cracks are impact- or thermal-fatigue-induced.

Trace-element analysis of mineral grains in Ryugu rock fragment sections by synchrotron-based confocal X-ray fluorescence

A fundamental parameter-based quantification scheme for confocal XRF was applied to sub-micron synchrotron radiation X-ray fluorescence (SR-XRF) data obtained at the beamline P06 of the Deutsches Elektronen-Synchrotron (DESY, Hamburg, Germany) from two sections C0033-01 and C0033-04 that were wet cut from rock fragment C0033 collected from Cb-type asteroid (162173) Ryugu by JAXA’s Hayabusa2 mission. Trace-element quantifications show that C0033 bulk matrix is CI-like, whereas individual mineral grains (i.e., magnetite, pyrrhotite, dolomite, apatite and breunnerite) show, depending on the respective phase, minor to strong deviations. The non-destructive nature of SR-XRF coupled with a new PyMca (a Python toolkit for XRF data analysis)-based quantification approach, performed in parallel with the synchrotron experiments, proves to be an attractive tool for the initial analysis of samples from return missions, such as Hayabusa2 and OSIRIS-REx, the latter returning material from a B-type asteroid (101955) Bennu in 2023.Graphical

Beta dose heterogeneity in sediment samples measured using a Timepix pixelated detector and its implications for optical dating of individual mineral grains

Single-grain optical dating has been widely used to estimate depositional ages for Quaternary sediments. An understanding of the sources influencing the dispersion of equivalent dose (D e ) estimates from individual grain measurements is essential for accurate age determination. Beta microdosimetry (i.e., beta dose deposition at the sub-millimetre scale) is a known cause of spread in single-grain D e values, so the detection and assessment of beta dose rate ( D ˙ β ) variation is important to properly interpret these data. Here we demonstrate the application of Timepix, a pixelated detector that directly measures in situ beta emissions based on cluster analysis, to determine the beta microdosimetry of natural sediment samples from a cave deposit in Russia and a sand dune in Australia. We describe a Timepix measurement and data processing procedure for natural sediments, and establish sample-specific calibration curves using associated sample radioelement concentrations to convert Timepix count rates into estimates of D ˙ β . On the basis of the Timepix analysis, a 2D ‘heat map’ of D ˙ β at sub-millimetre resolution was obtained for each sample. Our results show that the D ˙ β estimates are heterogeneous and their non-uniformity gives rise to 9 ± 4 to 26 ± 5% overdispersion in the single-grain D e distributions for the samples examined here. We discuss the likely sources of D ˙ β heterogeneity, based on micromorphological investigations of these sediment samples, which include a variety of materials present in natural deposits associated with D ˙ β ‘hot’ and ‘cold’ spots. A comparison of the D ˙ β dispersion with that of the corresponding single-grain D e values shows that the scatter among the latter can be fully or partly explained by beta microdosimetry and other known sources of overdispersion. • Timepix pixelated detector used to measure beta dose rates of natural sediments. • Measurement and data analysis procedure proposed and tested on four samples. • Spatially-resolved ‘heat map’ of beta dose rates obtained at sub-millimetre resolution. • Beta heterogeneity gives rise to ∼9–26% overdispersion of single-grain D e values. • Beta heterogeneity can fully or partly explain overdispersion of single-grain D e values.

Mineral-scale variation in the trace metal and sulfur isotope composition of pyrite: implications for metal and sulfur sources in mafic VMS deposits

The link between metal enrichment and the addition of a magmatic volatile phase in volcanogenic massive sulfide deposits and actively forming seafloor massive sulfide deposits remains poorly characterized. This is especially true when considering how metal, sulfur and fluid flux change with time. In this study, we combine in situ sulfur isotope (δ34S; n = 31) measurements with trace metal chemistry of pyrite (n = 143) from the Mala VMS deposit, Troodos, Cyprus. The aim of our study is to assess the links between volatile influx and metal enrichment and establish how, or indeed if, this is preserved at the scale of individual mineral grains. We classify pyrite based on texture into colloform, granular, disseminated and massive varieties. The trace metal content of different pyrite textures is highly variable and relates to fluid temperature and secondary reworking that are influenced by the location of the sample within the mound. The sulfur isotope composition of pyrite at Mala ranges from − 17.1 to 7.5‰ (n = 31), with a range of − 10.9 to 2.5‰ within a single pyrite crystal. This variation is attributed to changes in the relative proportion of sulfur sourced from (i) SO2 disproportionation, (ii) thermochemical sulfate reduction, (iii) the leaching of igneous sulfur/sulfide and (iv) bacterial sulfate reduction. Our data shows that there is no correlation between δ34S values and the concentration of volatile elements (Te, Se) and Au in pyrite at Mala indicating that remobilization of trace metals occurred within the mound.

Raman Spectroscopy Coupled with Reflectance Spectroscopy as a Tool for the Characterization of Key Hydrothermal Alteration Minerals in Epithermal Au-Ag Systems: Utility and Implications for Mineral Exploration.

Raman spectroscopy of fine-grained hydrothermal alteration minerals, and phyllosilicates in particular, presents certain challenges. However, given the increasingly widespread recognition of field portable visible–near infrared–shortwave infrared (Vis-NIR-SWIR) spectroscopy as a valuable tool in the mineral exploration industry, Raman microspectroscopy has promise as an approach for developing detailed complementary information on hydrothermal alteration phases in ore-forming systems. Here we present exemplar high-quality Raman and Vis-NIR-SWIR spectra of four key hydrothermal alteration minerals (pyrophyllite, white mica, chlorite, and alunite) that are common in precious metal epithermal systems, from deposits on the island of Newfoundland, Canada. The results reported here demonstrate that Raman microspectroscopy can accurately characterize pyrophyllite, white mica, chlorite, and alunite and provide details on their compositional variation at the microscale. In particular, spectral differences in the 1000–1150 cm−1 white mica Raman band allows the distinction between low-Tschermak phases (muscovite, paragonite) and phases with higher degrees of Tschermak substitution (phengitic white mica composition). The peak position of the main chlorite Raman band shifts between 683 cm−1 for Mg-rich chlorite and 665 cm−1 for Fe-rich chlorite and can be therefore used for semiquantitative estimation of the Fe2+ content in chlorite. Furthermore, while Vis-NIR-SWIR macrospectroscopy allows the rapid identification of the overall composition of the most abundant hydrothermal alteration mineral in a given sample, Raman microspectroscopy provides an in-depth spectral and chemical characterization of individual mineral grains, preserving the spatial and paragenetic context of each mineral and allowing for the distinction of chemical variation between (and within) different mineral grains. This is particularly useful in the case of alunite, white mica, and chlorite, minerals with extensive solid solution, where microscale characterization can provide information on the alteration zonation useful for mineral exploration and provide insight into mineral deposit genesis.

Establishing a pIRIR procedure for De determination of composite mineral grains from volcanic terranes: A case study of sediments from Liang Bua, Indonesia

Volcanic sediments are generally difficult to date using luminescence dating methods, but many important archaeological and palaeoanthropological sites are located in volcanic regions. Here we present an improved dating procedure for grains with composite mineralogies deposited at Liang Bua, the type locality of Homo floresiensis in Indonesia, using the post-infrared infrared stimulated luminescence (pIRIR) signal. Individual mineral grains that emit detectable pIRIR signals are composed of a range of feldspar varieties, quartz, clay minerals, heavy minerals and volcanic glass, rendering the isolation of individual potassium-rich feldspar grains infeasible. We investigate the luminescence behaviour of these composite mineral grains in detail, including their thermal stability, anomalous fading and dose-response characteristics. A standardised growth curve is developed to enable more time-efficient measurements, together with a ‘micro-aliquot’ approach in which each hole on a disc contains approximately 8–10 grains. Less than 1% of grains yield detectable pIRIR signals when measured individually, so the use of micro-aliquots provides, in effect, a means of estimating the equivalent dose (D e ) at single-grain resolution. Our results show that the pIRIR signal measured at 275 °C is suitable for estimating D e values of these composite grains, without the need for residual dose or fading corrections. • Improved pIRIR procedure to date Liang Bua, type locality of Homo floresiensis. • Analysed composite mineral grains in this volcanic region at single-grain resolution. • Established SGC parameters for the time-efficient measurement of equivalent dose. • Number of accepted data points increased six-fold using new preparation methods.

Trace element geochemistry of coarse-grained angrites from Northwest Africa: Implications for their petrogenesis on the angrite parent body.

The angrites are a class of achondrites that encompass a wide range of igneous textures from quenched, volcanic, and subvolcanic stones to slowly cooled, plutonic rocks. The compositions of the various geochemical reservoirs generating this variety of rocks have not been investigated fully because historically the numbers and masses of angrites available for study have been quite small. However, the rapid increase in meteorites from Northwest Africa (NWA) has enabled a renewed opportunity for such an investigation. In particular, three samples, NWA 2999, 4590, and 4801, have provided a new window into our understanding of the origin and petrogenesis of the coarse-grained (plutonic) angrites. We report here the trace element abundances in individual mineral grains of pyroxene, plagioclase, olivine/kirschsteinite, and phosphate and in the whole-rock samples. We utilize these data to constrain the petrogenetic history of each of these samples on the angrite parent body (e.g., parental melt compositions and oxygen fugacity conditions) and assess genetic relationships to previously investigated angrites. The trace element abundances in each of the three coarse-grained angrites studied here indicate a unique history for each. The observed trace element abundances and patterns in NWA 2999 show similarities with previously studied fine-grained, volcanic angrites and potentially indicate a common geochemical source reservoir, even though NWA 2999 is temporally distinct from the volcanic angrites. In contrast, NWA 4590 has trace element characteristics analogous to other coarse-grained angrites (e.g., Lewis Cliff [LEW] 86010), suggesting that these samples originated from geochemically similar source reservoirs. The third angrite, NWA 4801, exhibits geochemical characteristics most similar to the plutonic, coarse-grained angrites, but also appears to have some affinities in its trace element characteristics to the volcanic, fine-grained angrites. This suggests that NWA 4801 may represent a petrogenetic link between two distinct geochemical reservoirs on the angrite parent body. In aggregate, the trace element distributions in these three plutonic angrites suggest that while they may have originated up to several million years after the fine-grained angrites, they sampled a range of source reservoirs on the angrite parent body. Some of these source reservoirs were likely similar to those of the fine-grained angrites, but others had distinct geochemical characteristics.

The physics of fault friction: insights from experiments on simulated gouges at low shearing velocities

Abstract. The strength properties of fault rocks at shearing rates spanning the transition from crystal–plastic flow to frictional slip play a central role in determining the distribution of crustal stress, strain, and seismicity in tectonically active regions. We review experimental and microphysical modelling work, which is aimed at elucidating the processes that control the transition from pervasive ductile flow of fault rock to rate-and-state-dependent frictional (RSF) slip and to runaway rupture, carried out at Utrecht University in the past 2 decades or so. We address shear experiments on simulated gouges composed of calcite, halite–phyllosilicate mixtures, and phyllosilicate–quartz mixtures performed under laboratory conditions spanning the brittle–ductile transition. With increasing shear rate (or decreasing temperature), the results consistently show transitions from (1) stable velocity-strengthening (v-strengthening) behaviour, to potentially unstable v-weakening behaviour, and (2) back to v strengthening. Sample microstructures show that the first transition seen at low shear rates and/or high temperatures represents a switch from pervasive, fully ductile deformation to frictional sliding involving dilatant granular flow in localized shear bands where intergranular slip is incompletely accommodated by creep of individual mineral grains. A recent microphysical model, which treats fault rock deformation as controlled by competition between rate-sensitive (diffusional or crystal–plastic) deformation of individual grains and rate-insensitive sliding interactions between grains (granular flow), predicts both transitions well. Unlike classical RSF approaches, this model quantitatively reproduces a wide range of (transient) frictional behaviours using input parameters with direct physical meaning, with the latest progress focusing on incorporation of dynamic weakening processes characterizing co-seismic fault rupture. When implemented in numerical codes for crustal fault slip, the model offers a single unified framework for understanding slip patch nucleation and growth to critical (seismogenic) dimensions, as well as for simulating the entire seismic cycle.

Characterization of sedimentary environment, provenance and distribution of heavy minerals along the Tamil Nadu shoreline, East coast of India

Placer mineral deposits and their limited occurrence have gathered worldwide attention for researchers and geologists to exploit new areas of deposition. This research aims to know the transportation and depositional environment of sediments, provenance and distribution of heavy minerals between Besant Nagar and Sathurangapattinam in the coastal region of Tamil Nadu (India). The sediment samples are collected for textural analysis, magnetic susceptibility (MS), petrographical and mineralogical (XRD) studies of sediment samples, including the heavy minerals studies. Grain size analysis showed fine to medium-grained sandy sediment with very well to moderately sorting indicate reworking of beach edges accompanied by unimodal to the trimodal distribution of sediment from the riverine and marine environment. Grains are mostly symmetrically skewed with platykurtic to leptokurtic nature, indicating low- to high-energy environment prevailing in the region where the deposition and erosion are equally balanced in the area. Bivariate and one percentile–median (CM) pattern plots indicated that most of the sediment is deposited by beach processes transporting sediment by bottom suspension and rolling. The amounts of magnetic or iron-bearing minerals are high in locations no. 1 (Besant Nagar beach), 5 (Injambakkam beach) and 12 (Tiger cave beach) interpreted using MS studies, whereas the heavy mineral weight percentage is higher in locations no. 11 (Pattipulam beach) and 12 (Tiger cave beach). The correlation plot of Heavy wt% vs MS showed positive correlation and in general plots deciphered that the locations with high wt% (Besant Nagar, Injambakkam and Tiger cave beach) showed higher MS indicating the presence of magnetic minerals and the correlation plot between individual mineral grain count% vs MS explicated that the locations were rich in magnetic minerals like ilmenite, magnetite (opaques) and epidote (medium magnetic). The petrographical study of heavy mineral and XRD analysis revealed that the study area contained a high percentage of opaques, garnet, zircon, rutile, chlorite, sillimanite, hypersthene, hornblende, tourmaline, possibly derived from charnockites, granulite gneiss, migmatites and basic dykes. The overall investigation concludes that the sediments are distributed at various locations by marine processes depositing sediment rich in magnetic and heavy minerals derived from various rock type sources transported by rivers and later driven by longshore currents of oceans flowing from S to N, N to S during different months and SW–NE monsoon play a vital role in the distribution of beach placers.

Lazed and Diffused: Untangling Noble Gas Thermochronometry Data for Exhumation Rates

Thermochronometric data can record the thermal history of rocks as they cool from high temperatures at depth to lower temperatures at the surface. This provides a unique perspective on the tectonic processes that form topography and the erosional processes that destroy it. However, quantitatively interpreting such data is a challenge because multiple models can do an equally good job at reproducing the data. In this article, we describe how inverse modeling can be used to improve quantitative interpretations of noble gas thermochronometric data on a variety of scales, ranging from mountain belts to individual mineral grains.

Validation of the LnTn method for De determination in optical dating of K-feldspar and quartz

Abstract In optical dating, equivalent dose (De) values for a sample are commonly obtained from measurement of the luminescence signals of individual mineral grains or aliquots and projection of the natural signals onto the corresponding regenerative-dose response curves. A final De estimate is calculated by using a statistical model to combine the individual De values. This method can be problematic for samples that contain large numbers of ‘saturated’ grains, which may result in underestimation of the final De value due to truncation of the full De distribution at high doses. To circumvent this problem, Li et al. (2017) proposed a new method—the so-called LnTn method—in which the re-normalised sensitivity-corrected natural signals (Ln/Tn) are analysed for all measured grains from a particular sample, and the weighted mean re-normalised Ln/Tn value is projected onto a standardised growth curve to estimate the final sample De. As no grains or aliquots are rejected because they are ‘saturated’, a full (untruncated) distribution of re-normalised Ln/Tn ratios is obtained. In this study, we use numerical simulations of ‘samples’ with a variety of assigned burial dose (palaeodose) distributions to systematically investigate the shape of the resulting Ln/Tn distributions. We test application of the central age, minimum age and finite mixture models to Ln/Tn ratios (rather than to De values) for well-bleached, insufficiently bleached and post-depositionally mixed samples. Our results show that application of statistical models to Ln/Tn ratios from single grains or aliquots is reliable, and that the LnTn method can be used to date samples with De values up to 4 times the characteristic saturation dose (D0), which is well above the conservative upper limit of 2D0.

Influence of Thermal and Mechanical Loading on Development of Microcracks in Granite

Fire safety is one of the most important concerns in underground engineering projects. Exposure to high temperature can weaken rocks and potentially cause catastrophic failure of the rock mass, either during the fire, or after the fire. In this study, Brazilian tensile tests were conducted on unheated and heat-treated Bukit Timah Granite. The microcrack length distribution and orientation were then recorded and analyzed through the fluorescent dye impregnation technique. The amount of microcracks generated was found to be proportional to the heating temperature. However, other factors such as the mineralogy and the texture of the rock were also found to influence the length and orientation of the microcracks. The modal microcrack lengths of the specimens were found to be similar despite different thermal and mechanical treatments of the specimens, while the orientation of the microcracks was found to be influenced by the type of minerals. Besides the average grain size, another factor which played a key role in determining the microcrack characteristics is the size and shape of the individual mineral grains.

Difference in structural chemistry of non-coking and coking coal using acid treatment demineralization technique

ABSTRACT Recently, the structural information about non-coking and coking coal has attracted pronounced interest for sustainable utilization of poor coking coal in different industries specifically for steel industry like Tata Steel. In the present work, demineralization techniques via acid treatment (HCl-HF) are effective to remove mineral phase from coal. The difference in carbon chemistry of non-coking (CDPCI), weakly coking (Curragh SS), and prime coking coals (Illawarra and Jamadoba) has been studying through advanced characterization methods like petrography, TGA-DTG, FTIR spectroscopy, X-ray diffraction, Raman, 13C solid-state NMR, FESEM, and TEM. These advanced characterization techniques offer comparative structural information of different coal through the distribution of individual coal maceral and mineral grains with their rank (petrographic study), functional group study along aliphatic-aromatic structural relationship measurement (FTIR), crystalline/graphitic nature of carbon (XRD & Raman) with oxygenated carbon skeletal structural information (Solid State 13C NMR), aromatic-aliphatic cross-linking behavior (FESEM) and their orientation toward lattice fringe structure (TEM).