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Left: detail of 2D slice from medium resolution CT scan (voxel size: 9 µm); right: 2D slice from high
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When dealing with the characterization of the structure and composition of natural stones, problems of representativeness and choice of analysis technique almost always occur. Since feature-sizes are typically spread over the nanometer to centimeter range, there is never one single technique that allows a rapid and complete characterization. Over t...
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Focused ion beam scanning electron microscopy (FIB-SEM) is a commonly used three-dimensional (3D) pore-network reconstruction method for shales due to its unique capability in imaging nano-scale pores. However, it has been found that for pore space of lacustrine shales with strongly heterogeneous pore structures, the conventional FIB-SEM 3D models...
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... Backscatter electron analysis from scanning electron microscopy (BSE-SEM) produces high-resolution image data of both rock samples and thin-sections, showing detailed structural and geochemical (mineralogical) information (De Boever et al., 2015;Reed, 2005). This allows an in-depth exploration of the rock microstructures and the coupled chemical characteristics in the BSE-SEM image to be made using image processing techniques. ...
Backscatter electron analysis from scanning electron microscopes (BSE-SEM) produces high-resolution image data of both rock samples and thin-sections, showing detailed structural and geochemical (mineralogical) information. This allows an in-depth exploration of the rock microstructures and the coupled chemical characteristics in the BSE-SEM image to be made using image processing techniques. Although image processing is a powerful tool for revealing the more subtle data “hidden” in a picture, it is not a commonly employed method in geoscientific microstructural analysis. Here, we briefly introduce the general principles of image processing, and further discuss its application in studying rock microstructures using BSE-SEM image data.
... However, discrepancies arise when comparing results from different techniques, which are often complementary as they analyse different pore size ranges (Anovitz and Cole 2015;Coletti et al. 2016). Combined approaches are necessary for a realistic reconstruction of the pore system in both natural and artificial stones (Barbera et al. 2014;De Boever et al. 2015;Coletti et al. 2016). ...
The presence and distribution of pores in natural stones affect their durability and aesthetic value, especially when exposed to weathering agents like salt crystallization and freeze–thaw cycles. In this study, a multi-analytical approach was used to analyse the pore structure of twelve carbonate rocks, including different types of limestone and the Carrara marble. Mercury intrusion porosimetry, digital imaging analysis on backscattered electron images taken at the scanning electron microscope, and micro-computed tomography were used to overcome the limitations of each technique and create a more accurate reconstruction of the pore structure. This approach can aid in predicting the deterioration processes stones in heritage structures.
... location, b. hydrography of the area before the late 19th century, c. photograph of the 1970 profile by Molnár with his names of the lithological members, d. geological cross-section of the site and view of the outcrop during early spring e. detailed lithology of the outcrop with the position of samples under study marked)quantitative information on density differences observed between rock-forming textural components in a material. These can be thus assessed using various statistical approaches(De Boever et al., 2015). The CT measurements were performed on 4 samples representing all members of the carbonate sequence of Csólyospálos (Cs-1, Cs-2, Cs-3, Cs-4, and Cs-5), using the Siemens Emotion 6 medical scanner at the Department of Radiology, University of Pécs, Hungary. ...
This research focuses on the study of sedimentary rocks, in particular freshwater carbonates, as a paleoenvironmental and paleoclimatological record. Computer tomography (CT) analysis provides quantitative data based on rock-forming component (RFC) density differences, which allows insight into sedimentary environmental changes over time. The CT data are consistent with the results of geochemical and palaeoecological studies, providing a useful tool for understanding carbonate heterogeneity and refining rock nomenclature.
The study applied statistical and geostatistical tools to evaluate and visualize the heterogeneity of carbonate block samples based on density values obtained from CT images expressed in HU. Density values obtained from CT data correlate with the density of different rock-forming components present in the carbonates, and EM mixture analysis was used to successfully segment and identify different RFCs using CT data. In contrast to the rock component estimates based on thin sections, the CT data provide better estimates of the total sample volume, suggesting that the rock nomenclature needs to be reconsidered based on CT analysis.
The study also investigates the formation of freshwater carbonates in the Great Plain and their relationship with hydrological and climatic conditions. The study evaluates the temporal variation of carbonate rock-forming components quantified by CT on a millennial scale in terms of hydrological and climatic factors controlling the formation of carbonates, thus their potential use as a paleoclimatic and paleohydrological proxy. We also investigated spatial variations in the void spaces of freshwater carbonate samples, to explore how changes in void volume caused by water infiltration into the rock and then flow through pores, dissolution, freeze-thaw and weathering affect the malleability of rocks in areas more exposed to weathering, for use as building and ornamental materials, and to assess their vulnerability. This information will help to identify rock surfaces requiring increased insulative properties for construction and decoration.
The importance of quantifying non-destructively cavity spaces using CT and exploring potential water flow paths in freshwater carbonate rocks is emphasised. CT analysis, combined with modern geotechnical measurements, contributes to models for a better understanding of weathering effects on building rocks. The results suggest the use of conservation treatments, such as water repellents, to protect the preserved rocks from corrosion by water and weathering in certain parts of the rock surface. Mapping the distribution, connectivity and orientation of
megapores in the rock will allow the identification of surfaces and regions less susceptible to water infiltration. This analysis helps to select the optimum orientation of slabs and tiles and identifies areas that may require increased waterproofing during construction to prevent possible structural changes.
Overall, the research provides new insights into the freshwater carbonates of the Great Plain, including their rock-forming components, paleoenvironmental and paleohydrological setting, petrophysical properties and controlling factors. The application of CT analysis is valuable in understanding the micro-scale complexity of carbonate systems and in promoting effective conservation measures (Pini and Madonna, 2016).
The study of quaternary freshwater carbonate from the Great Hungarian Plain has produced important new results concerning the freshwater carbonate in the hypersaline lake in GHP, the rock-forming components, the palaeoclimatic and palaeohydrological conditions during the late Pleistocene and Holocene, in addition to petrophysical properties such as void space larger than 200 microns, and the freshwater carbonate controlling factors:
Defining rock-forming components of Holocene freshwater carbonates via univariate
statistical and mixture analysis of computer tomography data.
1- The research conducted on four carbonate samples from Csolyospálos utilized
computed tomography (CT) scanning to investigate and record the heterogeneity and textural characteristics of the carbonates. The study found that CT analysis offers a more objective and direct approach compared to traditional thin-section analysis. The quantitative data obtained through CT analysis in three dimensions enables a comprehensive evaluation of the entire sample, including detailed micro-scale resolution. In contrast, thin sections provide limited information as they rely on
selectively chosen subsamples of small (couple of square mms) parts and being also
subjective estimations.
2- By using CT data we investigated lithified and unlithified freshwater carbonate
samples. We applied an EM-mixture analysis algorithm to determine the HU range of
each rock forming component known from the textural analysis of previous studies.
For the lithified samples the following intervals were defined for each RFC: empty
pores < 850 HU, filled or partially filled pores between 600 - 2500 HU, calcium carbonate matrix between 2000 – 3100 HU, high magnesium carbonate matrix between 2500 – 3200 HU, high-density matrix components and limonite between 2800 – 3400 HU. On the other hand, RFCs of the non-lithified samples had somewhat different ranges: the empty pores < 550 HU, filled or partially filled pores between 300 - 1900 HU, calcium carbonate matrix between 1300 – 2300 HU, high magnesium carbonate matrix between 1600 – 3100 HU, high-density matrix components 2500 – 3200 HU.
3- We calculated the relative percentage of each component, and the average percentage of the lithified samples were as follows: empty pores ~ 0.13%, filled or partially filled pores 18.37%, calcium carbonate matrix 62.0 %, high magnesium carbonate matrix 14.3%, high-density matrix components and limonite 5.2%. For non-lithified samples the following values were received: empty pores ~ 1.65 %, filled or partially filled pores 40.5%, calcium carbonate matrix ~55.35%, high magnesium carbonate matrix 1.25%, high-density matrix components 1.25%. To provide an example, the characteristics of the minerals that make up rocks were matched with descriptions of samples given by Molnár (1991).
Palaeoenvironmental and palaeoclimatic inferences based on X-ray computer
tomography: a case study of alkaline lake deposits in Hungary.
1- X-ray computer tomography (CT) allows for quantitative data based on density
differences of RFCs, providing insights into sediment formation and environmental
properties through time at high resolution. Density changes in the matrix, as captured by medical CT, correspond to climatic oscillations and precipitation patterns in carbonate formations at high resolution. Changes in evaporation rates, temperature, and rainfall influence the type of carbonate minerals precipitated, leading to variations in densities. The CT method, combined with statistical analysis, proves effective in revealing millennial and centennial sedimentary cycles and past paleoenvironmental changes.
2- Analysing computer tomography data, colder conditions in the North Atlantic from 10.3 to 9.3 thousand years before the present (kilo-years Cal BP) had a notable impact on freshwater carbonate formation in the GHP. These conditions led to increased humidity, resulting in a rise in the groundwater table and the precipitation of dominantly calcite from pore waters instead of high-magnesium calcite. The negative shifts in CT density values in the dated rock samples depicted these changes clearly and showed a good correlation with isotope geochemical and paleoecological data of previous studies (molnar 1996).
The use of CT analysis in revealing structural heterogeneity of freshwater carbonate
decoration and construction stone.
1- Zones are defined based on the major trend changes in the percentage of void space and the mean value; therefore, some directions could contain only two zones such as (ZYX sample I), and others involved seven zones like (XZY sample II).
2- Macro-CT scanned data revealed voids which are greater than voxel size. Major
trending was vertical with horizontal connected from the front and back sides once the water reaches the interior parts, the good connectivity and vertically oriented pores facilitate upward by capillary action and downward flow by gravity.
3- The analysis of Sample I revealed that the front and back of the brick had higher
porosity and greater susceptibility to water absorption compared to the left and right sides of the central part. This indicates the need for effective insulation in the front and back positions to prevent water ingress into the central areas.
4- The lateral areas of the brick displayed negligible lengths and proportions of potential water pathways (PWPs), making them less prone to water uptake. Consequently, minimal insulation can be deemed acceptable for these regions, reducing the risk of water infiltration.
... And the percentage of dolomite dropped the most and only accounted for 9.3% after reaction, which proved that the main reaction was the dissolution of dolomite. Dolomite was the main carbonate mineral reacting with hydrochloric acid (Bello et al., 2022;De Boever et al., 2015;Kong et al., 2019). Fig. 15 shows the changes in the microscopic morphology in S1 before and after reaction with hydrochloric acid. ...
... Despite continuous technological and computational advances (Wang & Miller, 2020), most applications in mineral characterization are rather limited to the 3D segmentation of major phases, i.e., pores, low-density phases, and high-density phases (Guntoro et al., 2019a). Therefore, recent work in lCT focuses on the development of image postprocessing procedures (Becker et al., 2016;Guntoro et al., 2019b), whether or not together with complementary microscopic techniques (De Boever et al., 2015;Laforce et al., 2016;Reyes et al., 2017;Warlo et al., 2021), to differentiate between complex intergrown mineral phases. In future, the integration of machine learning and artificial intelligence is considered to be crucial for the generation of mineralogical information from standard lCT data (Guntoro et al., 2019a). ...
Advanced chemical and mineralogical techniques are necessary to further our understanding of ore deposits and their genesis. Using X-ray micro-computed tomography (µCT) and an automated mineralogy (AM) system based on scanning electron microscopy with an energy-dispersive X-ray spectrometer (SEM–EDX), we investigated the internal mineralogy of Sn–Nb–Ta pegmatites. This paper presents a comprehensive methodology to quantify and visualize the mineral relationships of ore samples in three-dimensional space at the microscopic scale. A list of all possible minerals present, a so-called mineral library, was deduced with a SEM-based AM system and served as the ground truth for the interpretation of µCT data. A reconstructed attenuation coefficient (µrec) was calculated for mineral phases that have been identified and provided a most correct guidance to differentiate between minerals for a given experimental µCT setup. Despite some limitation in sample size and mineral identification, these complementary techniques enabled the differentiation of a Fe–Li mica from biotite based on the chemical attribution of lithium to µrec. Using statistical descriptors, we quantified the general orientation of individual mineral phases and their spatial correlation to comply with the needs of processing large datasets at a low computational expense. Applying this comprehensive methodology to a case study demonstrates the possibilities of combining a SEM-based AM system with µCT analysis to investigate ore samples at the microscopic scale.
... Sub-resolution pores can also be detected by subtracting images before and after contrasting agents such as cesium chloride (CsCl) or potassium iodide (KI) have been added 131,132 . However, disadvantages of this method include (1) Characterizing samples with sub-resolution pores is challenging: ignoring these pores results in an underestimation of the porosity and pore connectivity, but including them requires either assumptions and generalizations, and/or data from other methods such as SEM or MIP 133 . A reliable correlation of these different datasets is also far from straightforward. ...
... Application of μCT enables the non-destructive, 3D visualization of internal phases hosted within a rock sample. Geological samples have been studied using this technique, including igneous rocks (Baker et al., 2012;Boone et al., 2011;Callow et al., 2018;Cid et al., 2017;Jardine et al., 2018), sedimentary rocks/sediment cores (Christe et al., 2011;De Boever et al., 2015;Emmanouilidis et al., 2020;Lai et al., 2017;Munawar et al., 2018;Zhang et al., 2019), peat and soils (Le Bayon et al., 2021;Liernur et al., 2017;Turberg et al., 2014), metamorphic rocks (Aerden and Ruiz-Fuentes, 2020;Corti et al., 2019;Sayab et al., 2021;Sayab et al., 2017;Zucali et al., 2014), ore/drill cores (Godel, 2013;Kyle and Ketcham, 2015;Reyes et al., 2017;Sahiström et al., 2019;Warlo et al., 2021) and meteorites (Friedrich et al., 2008;Porfido et al., 2020;Tsuchiyama et al., 2002;Uesugi et al., 2010). To our knowledge ophiolitic rocks have not been extensively examined using μCT applications, although there are a few cases reported concerning ophiolitic chromitites (Prichard et al., 2017;Prichard et al., 2018;Prichard et al., 2015), as well as serpentinized and carbonatized ultramafic rocks (Beinlich et al., 2020;Noël et al., 2018). ...
... μXRF can also be regarded as an additional technique or even a non-destructive alternative for EM to determine the mineral's elemental composition when required (Bergqvist et al., 2019;Porfido et al., 2020;Suuronen and Sayab, 2018;Wildenschild and Sheppard, 2013). Although Method 4 possess many advantages, the deployment of EM along with μCT is strongly suggested as their integration is beneficial for investigating rock's mineral constituents (De Boever et al., 2015). Similar conclusions have been drawn by Warlo et al. (2021) after combining EM with X-ray computed tomography although this concerns ore characterization. ...
X-ray micro-computed tomography (μCT) was applied upon selected ophiolitic rock samples from various localities of the Vardar ophiolite outcrops in North Greece. Effectiveness of the μCT application was evaluated through this case study by comparing results with other state-of-the-art techniques (e.g., optical microscopy, mineral chemistry microanalyses, XRD and QEMSCAN) to provide suggestive methodologies for optimum characterization, geological modelling, and visualization of ophiolitic rocks. The research outcomes provide an innovative approach for accurate modal composition calculations, crystal structure and mineral distribution in a 3D perspective, by combining μCT results with mineral chemical analyses. The information obtained is critical for investigating ophiolitic rocks to resolve complex petrogenetic and post-magmatic phenomena, to identify fabrics related to deformation, and furthermore results can also be used for applied research purposes.
The obtained μCT results suggest that distributions of mineral’s grayscale values strongly rely on three key factors: i) participation of mineral phases with distinct attenuation coefficient and/or density properties, ii) coexistence of different mafic minerals or mafic with non-mafic phases, iii) variability in their mineral chemistry. The ability to analyze and visualize the internal mineral constituents of ophiolitic rocks samples, through the combination of μCT and Energy-Dispersive X-ray spectroscopy (EDS), can lead to advanced 3D stereological rock fabric analyses, which is advantageous compared to 2D methodologies. The μCT allowed to perform rock fabric calculations (best-fit ellipsoids and with volume) upon specified grain size distributions to identify and characterize the 3D morphological properties of the participating crystals and their preferable orientation.
... As micro-CT cannot provide chemical information, prior knowledge about the sample composition is needed on which different absorption coefficients for the individual minerals can be calculated (Cnudde and Boone, 2013). Therefore, it is often used in combination with other methods such as SEM-EDS when characterizing geological materials (Godel et al., 2010;Chetty et al., 2012;De Boever et al., 2015;Reyes et al., 2017). In this work we use sp-CT in combination with micro-CT as an additional method to provide chemical information. ...
The Mt. Rudnaya MSS-ISS (monosulfide and intermediate solid solution) fine-grained ores from a NE termination of Norilsk 1 deposit were analyzed using a combination of X-ray computed micro tomography, spectral X-ray computed micro tomography and scanning electron microscopy to achieve both, 2D and 3D data. The ores consist of ISS composed of tiny lamellar intergrowths of cubanite and chalcopyrite solid solutions, which form up to 4-mm distinct globules surrounded by an ISS-MSS matrix. Our X-ray computed micro tomography results may provide 3D textural evidence of a possible natural sulfide-sulfide liquid immiscibility between Cu-rich and Cu-poor sulfide liquids that occurred before MSS and ISS were crystallized. The platinum group minerals (PGM) distribution shows that 20.6 vol% of all PGM occur in the ISS-MSS matrix and 79.4 vol% in the ISS globules. We suggest that this distributional behavior is due to the fact that the platinum group elements (PGE) cannot be dissolved in ISS, which led to the formation of the large PGM grains, which are up to 120 μm on their longest axis. The initial enrichment of ISS in PGE was controlled by differences in the partition coefficients of platinum and palladium between Cu-poor and Cu-rich liquids.
... The attenuation coefficient depends on the density of each material, its atomic number and is function of the X-ray beam energy, especially for polychromatic beams used in lab-based devices. In a lab-based XCT system, X-rays are generated in a tube by emitting electrons that are accelerated and directed to strike a target (anode) made of tungsten, platinum or molybdenum (Brabant, 2013;De Boever et al., 2015). The energy released generates X-rays according to two processes that are the deceleration of incident electrons (bremsstrahlung effect) and the change of energy shell position of the electrons of the anode (Wildenschild et al., 2002). ...
... Otherwise, lab-based XCT can be easily combined with other techniques, such as energy dispersive X-ray spectroscopy (EDS), to determine the chemical nature of rock constituents. This 2D-3D registration method has been commonly used in studies related to the characterization of sandstone and limestone rocks (Golab et al., 2010(Golab et al., , 2013De Boever et al., 2015;Lai et al., 2015;Ellis and Peters, 2016) but also in metamorphic studies (Macente et al., 2017). Two-dimensional grayscale backscattered electron images (BSE) from scanning electron microscopy (SEM) can be registered with the XCT grayscale images and may help in the identification of phases (Latham et al., 2008;Sok et al., 2010). ...
The study of the microstructure of rocks is essential for our contemporary and future challenges in energy, engineering and construction. Furthermore, this study allows us to characterize the geological deformation processes that led to the current state of geological formations. Fine-grained clastic rocks, commonly called "shales", represent about two-thirds of all sedimentary rocks. 3D data concerning silt-sized grains or clasts embedded in the porous clay-rich matrix of this type of rock are relatively scarce despite the fact that these data are crucial to understand the anisotropic properties of these rocks at the macroscale but also to evaluate the deformation state of the rock matrix. A better understanding of the microstructure of these rocks would allow us to predict their mechanical or physical properties, which are essential for applications in the energy sector, among others. X-ray computed tomography (XCT) is a non-destructive technique providing a 3D image of the microstructure of any object. A direct geometric characterization of the constituents of fine-grained clastic rocks is possible with this technique. Based on XCT images, this thesis aims first to develop methodological aspects to study the 3D shape fabric of silt particles and their spatial distribution. The moments of inertia of segmented grains from 3D digital images are used for this development. We then present applications on fine-grained rocks with a sedimentary fabric and on deformed fine-grained rocks with a tectonic fabric. The first application part of the thesis focuses on the same lithologic unit having experienced different amounts of deformation. Samples from the South Pyrenean Basin and samples from a historical outcrop in the Central Appalachians were collected. We provide new data on the evolution of the 3D shape of grains and pores at the micrometer scale and their arrangement in the rock matrix with respect to the deformation intensity. The obtained data allow discussing the deformation mechanisms at the grain scale of the different mineralogical phases. However, the limited size of the imaged samples by means of XCT (≤ 2 mm diameter) raises the question of the representativeness of these analyses. On the South Pyrenean site, some samples are studied in more detail to evaluate the homogeneity of the results. We show that the XCT data complement the indirect petrophysical measurements by providing access to localized sub-fabrics that are integrated in a bulk measurement of the rock fabric. The limits are reached when the characteristic length of the deformation structures are on the order of the sample size imaged by XCT. In the second application part, samples from turbiditic systems of the South Pyrenean basin are analyzed. These systems, when deformed in compressive tectonic settings, record the same amount of shortening differently expressed in the various siliciclastic matrices. The results obtained from the shape data of the clasts are compared to our bulk magnetic fabric measurements and show a good consistency. The methodology presented in this work can be extended to other types of porous and granular media for a better understanding of the influence of fabric anisotropy on their macroscopic properties and mechanical behavior.
... Unresolved porosity is visible in these images as zones with gray values that are intermediate between solid and void, due to their intermediate density (Cnudde & Boone, 2013). These regions can then be imaged by higher-resolution techniques, that are then correlated back to the lower-resolution micro-CT scan (De Boever et al., 2015;Devarapalli et al., 2017;Lin et al., 2019). The unresolved pores, which we will by definition refer to as the microporosity, can play a crucial role in the sample's multiphase flow behavior (Bultreys, Stappen, et al., 2016;Mehmani et al., 2020). ...
Image‐based pore‐scale modeling is an important method to study multiphase flow in permeable rocks. However, in many rocks, the pore size distribution is so wide that it cannot be resolved in a single pore‐space image, typically acquired using micro‐computed tomography (micro‐CT). Recent multi‐scale models therefore incorporate sub‐voxel porosity maps, created by differential micro‐CT imaging of a contrast fluid in the pores. These maps delineate different microporous flow zones in the model, which must be assigned petrophysical properties as input. The uncertainty on the pore scale physics in these models is therefore heightened by uncertainties on the representation of unresolved pores, also called sub‐rock typing. Here, we address this by validating a multi‐scale pore network model using a drainage experiment imaged with differential micro‐CT on an Estaillades limestone sample. We find that porosity map‐based sub‐rock typing was unable to match the micrometer‐scale experimental fluid distributions. To investigate why, we introduce a novel baseline sub‐rock typing method, based on a 3D map of the experimental capillary pressure function. By incorporating this data, we successfully remove most of the sub‐rock typing uncertainty from the model, obtaining a close fit to the experimental fluid distributions. Comparison between the two methods shows that in this sample, the porosity map is poorly correlated to the multiphase flow behavior of microporosity. The method introduced in this paper can help to constrain the sources of uncertainties in multi‐scale models in reference cases, facilitating the development of simulations in complex reservoir rocks important for for example, geological storage of CO2.
