**Nanomineralogy**

Printed Edition of the Special Issue Published in *Minerals* Yiwen Ju, Quan Wan and Michael F. Hochella, Jr. Edited by

www.mdpi.com/journal/minerals

**Nanomineralogy**

**Nanomineralogy**

Special Issue Editors

**Yiwen Ju Quan Wan Michael F. Hochella, Jr.**

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*Special Issue Editors* Yiwen Ju University of Chinese Academy of Sciences China

Quan Wan Institute of Geochemistry, Chinese Academy of Sciences China

Michael F. Hochella, Jr. Virginia Polytechnic Institute and State University USA

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This is a reprint of articles from the Special Issue published online in the open access journal *Minerals* (ISSN 2075-163X) (available at: https://www.mdpi.com/journal/minerals/special issues/ Nanomineralogy).

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### **Contents**



### **About the Special Issue Editors**

**Yiwen Ju** was born in Anhui, China, in 1963. He is a professor of Geology and a doctoral supervisor at the University of the Chinese Academy of Sciences. He obtained his B.S. from Anhui University of Science and Technology in 1985. From 1985 to 2000, he served as a senior engineer for Huaibei Mining (Group) Co. Ltd., China. He received his M.S. and Ph.D. from the China University of Mining and Technology in 1994 and 2003, respectively. He became a postdoctoral researcher in 2003 and a full professor in 2010 at the University of the Chinese Academy of Sciences and worked as a senior visiting scholar at Virginia Tech, USA, in 2010–2011. His main research interests are structural geology and energy basin geology, unconventional energy geology, coal and gas geology, and nanogeology. Currently, he serves as the director of the Nanogeology Specialized Committee of the Geological Society of China and as the vice president of the Chinese Sub-society for Soft Rock Engineering & Deep Disaster Control. He serves as an associate editor of the Journal of Nanoscience and Nanotechnology, a guest editor for a Special Issue of Minerals, and an editorial board member of the China Geology, Journal of China Coal Society and Journal of Natural Gas Geoscience, among other publications. He has played the role of principal leader for more than 20 national scientific research projects in China, including Key Programs for the National Natural Science Foundation of China, National Major Science and Technology Projects of China, National Basic Research Programs of China (973 Program), and a Strategic Priority Research Program of the Chinese Academy of Sciences. He has authored more than 200 publications, including more than 90 SCI-indexed scientific papers and 3 books, and has won seven scientific and technological awards at the provincial and national levels. In addition to maintaining academic communication and cooperation with many international universities and research agencies in the United States, Australia, France, Japan, South Africa, and Nepal, he has organized and chaired more than 20 conferences, workshops, and symposia and has given more than 20 talks at scientific conferences. He was the proposer and executive chairman of the Chinese Xiangshan Science Conferences on "Deep Coal Mine Gas Disasters and Coalbed Methane Development" and "Frontier Science Problems of Nanogeology and Accumulation of Metallogenic" in 2012 and 2013, respectively. He served as session chairman of the shale gas branch of the AAPG Annual Meeting, the unconventional oil and gas branch of the Euro–Asia Economic Forum, and the coal and carbon chemistry symposium in China and Japan in 2013; as an organizing committee member and session chairman of the International Geological Congress in Nepal in 2015 and 2016; as the chairman of the International Symposium on Nanogeoscience in 2016 and 2018; and as a member of the technical committee of the International Chinese Shale Gas Conference in 2019.

**Quan Wan** is a professor at the Institute of Geochemistry, Chinese Academy of Sciences (CAS). He was born in Sichuan, China, in 1971. He received his B.S. in Chemistry in 1994 and his M.S. in Polymer Science and Engineering in 1997 from Peking University, Beijing, China. In 2002, he received his Ph.D. in Materials Science and Engineering from the State University of New York at Stony Brook, USA. From 2005 to 2010, he worked in the Center for Bioengineering and Biomaterials at Temple University, Philadelphia, USA, as a postdoctoral researcher and then as an associate scientist. In 2010, he joined the Institute of Geochemistry, CAS, as a professor through the CAS Hundred Talent Program. He has broad scientific interests in the fields of nanogeoscience and mineralogy and currently serves as vice director of the Specialized Committee on Nanogeology of the Geological Society of China and the Specialized Committee on Environmental Mineralogy of the Chinese Society of Mineralogy, Petrology and Geochemistry. Ongoing research topics in his group include sorption behavior and occurrence state of gold (nanoparticles) on pyrite/arsenian pyrite/clay minerals, nanoconfinement effects of silica and mineral micro/mesopores, and pore characteristics and gas enrichment in shale gas reservoirs. By combining laboratory simulation and field sample characterization, he and his collaborators have enhanced mechanistic understandings of the origin, special properties, and interfacial reactivity of nanoparticulate and nanoporous materials, as well as the associated implications for natural resources (such as Carlin-type gold ore and shale gas reserves) and the environment. In 2018, he joined his colleagues in organizing the first international conference dedicated to nanogeoscience, for which he served as local host and secretary general.

**Michael F. Hochella, Jr.** was born in Yokohama, Japan, in 1953 to American parents. He is currently a University Distinguished Professor (Emeritus) at Virginia Tech in Blacksburg, Virginia, USA, and also a Laboratory Fellow at the Pacific Northwest National Laboratory in Richland, Washington, USA. He is a specialist in nanoscience and nanotechnology related to geological, biological, and environmental science at local, regional, and global levels. He is the Founder and Former Director of NanoEarth, the National Center for Earth and Environmental Nanotechnology, funded by the U.S. National Science Foundation and located at Virginia Tech. Hochella received his B.S. and M.S. degrees from Virginia Tech in 1975 and 1977, respectively, and his Ph.D. from Stanford University in 1981. He has been a professor, first at Stanford and then at Virginia Tech, for a total of 31 years. He has been a Fulbright Scholar and has won a Humboldt Award, the Brindley Lecture Award, the Dana Medal, the Virginia Scientist of the Year Award, and the Virginia Outstanding Faculty Award. He has been elected Fellow in nine international scientific societies and laboratories, including the American Association of the Advancement of Science (AAAS) and the American Geophysical Union (AGU). He has held the Presidencies of both the Geochemical Society and the Mineralogical Society of America. He has also won the Distinguished Service Medal of the Geochemical Society. He has served on high-level advisory boards to the National Science Foundation and the Department of Energy in the United States. He has a total of 203 professional peer-reviewed publications, including five papers in the journal Science, and has been cited over 16,500 times in the professional literature, a tally that is currently increasing at a rate of 1400 new citations per year (Google Scholar). He has given hundreds of invited lectures in 17 countries (Europe, Far East, Middle East, and Africa). Professor Hochella has taught a total of 11 different courses at Stanford and Virginia Tech, from introductory freshman classes to advanced classes for Ph.D. students. He has developed brand new courses and also new curricula at the undergraduate and graduate levels. He has also closely advised and mentored over 100 students, each for multiple years, and moved them towards the specific careers of their choice. Twenty-four of his former graduate students and postdocs have been professors at universities in five countries, while many others hold prominent positions in national laboratories, industry, and scientific publishing.

### **Preface to "Nanomineralogy"**

Over several hundred years of development, Earth science has made great advances at the largest of scales (e.g., tectonics) and the smallest of scales (e.g., molecular geochemistry). In the 21st century, the latest technological revolutions (e.g. molecular biology and precise GPS measurement) continue to add important components of how we understand Earth. Even more recently, these latest advancements also have included nanogeoscience, which has clearly become another international frontier now ushering in revolutionary developments. If Earth science is to continue to see breakthroughs at ultrasmall scales, then geoscience problems and phenomena must continue to be studied at the nanoscale in order to reveal and better understand the mechanisms and processes at the larger scales of Earth. Nanomineralogy, an important aspect of nanogeoscience, focuses on the composition, structure, and physical and chemical properties of nanoscale minerals and their interrelations with other Earth critical components. Nanomineralogy is characterized by high-resolution transmission electron microscopy (HRTEM), scanning probe microscopy (SPM), and other high-resolution microscopies, several of them synchrotron-based. In combination with an array of spectroscopic and gas adsorption methods, nanomineralogy reveals the mesoscopic structure, morphology, interface relationships, and formation mechanism of minerals at the nanoscale.

Nanomineralogy allows the study of a series of interfacial interactions of nanominerals, mineral nanoparticles, and nanopore structures, including the growth, dissolution, transformation, interaction, and evolution of fluids, organisms, and minerals. In addition, exploration and application of the physicochemical properties of nanomineral resources can help broaden mineralogy's scope for development and prospects for practical applications.

In 2018, the International Symposium on Nanogeoscience was held in Guiyang, China. Scholars from around the globe gathered to discuss recent progress and development trends in various aspects of nanogeoscience, including nanomineralogy. To give a sampling of the latest progress in nanomineralogy and related fields, we offer this Special Issue, whose 20 papers describe a full range of recent nanomineralogic achievements relating to everything from nanominerals and geochemistry, mineral nanostructures, and nanomineral deformation, to nanopores in oil and gas reservoirs, nanomineral deposits, and nanomineral material. More specifically, this issue includes the following:

(1) Nanominerals and geochemistry: The morphology and structural phase transition of various natural nanominerals are described, as are the experimental synthesis of nanomineral crystals; the adsorption, percolation, dissolution, metasomatism of nanominerals; and the environmental conditions of nanomineral composition transformation.

(2) Structural and seismic geology: The mechanism of the movement of nanominerals is added to the ductile shear zone theory. The seismogenic fault zone is revealed to be a viscoelastic friction belt of nanoparticles, with the macroscopic system's instability possibly caused by the release of energy from microscopic particles.

(3) Unconventional energy geology: The structure and composition of nanominerals in shale or tight reservoirs and the change of nanopores and their fractal dimensions are described, and structural characteristics such as the size, shape, and connectivity of nanopores in unconventional oil and gas reservoirs, along with their evolutionary laws and control factors, are preliminarily clarified.

(4) Ore deposits and mineral materials: Genesis of Carlin-type gold deposits is analyzed at the nanoscale; the significance of using metallic nanoparticles to search for concealed deposits is discussed; and the preparation, characterization, and physicochemical properties of nanomineral materials are explored.

Today, nanomineralogy faces a new strategic opportunity as well as a revolutionary challenge. We look forward to future developments of nanomineralogy made with the aid of cutting-edge research methods, based on the experience and achievements made possible by nanoscience and geoscience through the study of the application and interaction of mineralogy research techniques and nanotechnological methods in the Earth's lithosphere, atmosphere, hydrosphere, and biosphere. We also anticipate detailed interpretations of geological evolution and information about Earth's development on the nanoscale, as well as further clarification of the basic connotations and main research directions of nanomineralogy. These include the following topics and more: nanostructure and surface effects of minerals and genesis of rock, evolutionary mechanisms of nanoparticles and nanopores in natural geological bodies, characteristics and formation mechanisms of nanodeformation, nanogeochemical processes and their behaviors, nanometer effects of hydrocarbon accumulation and their dynamic mechanisms, nanometallogenesis and associated patterns, nanominerals and new composite materials, and nanomineralogy and disaster/environmental problems. We intend to make full use of the advantages offered by multidisciplinary study, cooperating extensively with global geoscientists while promoting research into nanomineralogy and its associated resources, such as through environment- and disaster-related science and technology projects. In doing so, we aim to spread the global word about the latest research achievements in nanomineralogy while identifying key scientific issues in the field and enriching and developing the theory and methods of nanogeoscience. In this way, we will be able to provide an important theoretical basis for use of minerals and materials, exploration and development of energy resources and mineral resources, prevention of disasters, and protection of the environment.

We thus present this special nanomineralogy-focused issue of Minerals with the aim of encouraging our colleagues to familiarize themselves with current developments, trends, and directions in nanomineralogy, enabling an understanding of the potential of the field as a whole. We look forward to developing further scientific research and cooperation in nanomineralogy, hoping thereby to attract and guide young scholars to participate in this field. We thank the Editor-in-Chief of Minerals for his encouragement and support, and we are also grateful to the other editors for their help with English-language revision. We thank Ziyuan Ouyang, Youwei Du, Jinxing Dai, Jishun Ren, Guowei Zhang, Yaolin Shi, Zhijun Jin, Zhenqi Song, Caineng Zou, Yanxin Wang, et al., the academicians of the Chinese Academy of Sciences, for their guidance. We are also particularly grateful to the scientific research institutions, universities, and state-owned enterprises in China that extended their support and assistance. Finally, we thank the authors and reviewers for their efforts and contributions. Without their help and support, it would have been impossible to produce this invaluable Special Issue.

> **Yiwen Ju, Quan Wan, Michael F. Hochella, Jr.** *Special Issue Editors*

### *Editorial* **Editorial for Special Issue "Nanomineralogy"**

#### **Yiwen Ju 1,\*, Quan Wan <sup>2</sup> and Michael F. Hochella, Jr. <sup>3</sup>**


Received: 28 May 2020; Accepted: 3 June 2020; Published: 5 June 2020

Nanoscience and nanotechnology study the properties of materials within the range 0.1~100 nm. Materials at the nanoscale have special physicochemical properties owing to their surface and interface, small size, quantum tunneling, and quantum size effects. Nanotechnology has crossed over into many areas of Earth science, driving important advances in mineralogy, petrology, geochemistry, structural geology, energy geology, mineral deposits, geologic hazards, and environmental geology [1–7]. Studying problems and phenomena at the nanometer scale, which reveals mechanisms as well as processes, is the inevitable future of developments in Earth science. Nanogeosciences are the global frontier for the contemporary cross-development of geosciences and nanotechnology, and the rise of nanoscience is set to bring about a revolutionary leap in the development of Earth science in the 21st century, driving breakthroughs in Earth science on the ultra-micro scale [1–3].

Nanomineralogy, an important aspect of nanogeoscience, is characterized by high-resolution transmission electron microscopy (HRTEM); scanning probe microscopy (SPM); and other high-resolution microscopies, several of them synchrotron-based, in combination with an array of spectroscopic and gas adsorption methods. It focuses on the composition, structure, and physical and chemical properties of nanoscale minerals and their interrelations with other Earth critical components [2–4], involving everything from nanominerals and geochemistry, mineral nanostructures, and nanomineral deformation to nanopores in oil and gas reservoirs, nanomineral deposits, and nanomineral materials. The exploration and application of the physical and chemical properties of nanomineral resources and environment broaden mineralogy's scope for the development of practical applications as well as the prospects of such applications.

Nanominerals have both resource and environmental attributes [8–11] and offer an extensive record of nanoscale substances and geological processes. With recent developments in nanoscience, research into nanomineral particles, nanomineral solids, nanomineral structures, and resources and the environment have also deepened [1–13]. In many cases, nanominerals or minerals with nanostructures appear in aggregate form. These include mineral particles with a grain size at the nanoscale, minerals of one-dimensional nanostructure (such as halloysite of nanotube structure), and layered minerals of two-dimensional nanostructure (such as clay minerals). Studies of actual minerals by HRTEM, STM, and AFM have demonstrated the existence of nanoparticles and nanostructures, with such nanophenomena more common in crystal surfaces and interfaces. The special nanostructure and composition of a crystal's surface determine its surface properties, which are significant in discussing the physical and chemical environment for the formation and genesis of minerals. Between two mineral phase transitions, not only do nanoscale particles exist, but also nanoparticle aggregates appear under particular physical and chemical conditions—including clay minerals, zeolites, colloidal minerals, volcanic glass, meteorite glass, fusion crust, and tectonite—and fall primarily into four categories: (1) clay minerals, (2) quasicrystal nanostructures, (3) nanostructures in colloids, and (4) nanostructures

in crystalline rocks. Minerals with grain size larger than 1 μm can provide information about minerals' late growth stage, whereas those with grain size between 0.1 and 100 nm can provide records about their initial crystallization. The polymorph, polytype, multi-body, and micro-intergrowth of minerals at the nanoscale are the actual minimum bodies capable of reflecting geological information at present. Accordingly, this kind of information must be integrated to more fully reflect the physical and chemical environment in which minerals were formed. At present, the active exploration of techniques and methods for developing and using nanoscale minerals, together with the quest for breakthroughs in both theory and practice, is heightening mineralogists' ability to understand and transform nature [2,4].

Some of the papers [14–20] in this issue deal with nanominerals and geochemistry. They describe the morphology and structural phase transition of various natural nanominerals, as well as the experimental synthesis of nanomineral crystals; the adsorption, percolation, dissolution, and metasomatism of nanominerals; and the environmental transformation conditions of nanomineral composition. Hong and coauthors [14] have reported two structural phase transitions and metallization for nanocrystalline rutile using a diamond anvil cell at ~7.2, ~12.3, and ~14.5 GPa. During compression, a structural phase transition from rutile to baddeleyite occurs, with the high electrical conductivity value offering a crucial clue regarding metallization, as confirmed by temperature-dependent electrical conductivity measurements. On decompression, a structural phase transformation from baddeleyite to columbite occurs. HRTEM observations regarding the starting and recovered samples indicate that phase transformations from rutile to baddeleyite to columbite are irreversible under high pressure. Bjorn von der Heyden and coauthors [15] investigated analytical approaches to characterizing natural Fe nanoparticles before detailing a dedicated synchrotron-based X-ray spectromicroscopy investigation into the speciation of suspended Fe nanoparticles collected from fluvial, marine, and lacustrine surface waters. They identified ferrous, ferric, and magnetite classes of Fe nanoparticles (10–100 nm), all of which exhibited a high degree of heterogeneity in the local bonding environment around the Fe center. The results provide an important baseline for natural nanoparticle speciation in pristine aquatic systems, highlighting the degree of interparticle variability. Nie and coauthors [16] employ such an experimental approach to study the facile hydrothermal synthesis of nanocubic pyrite crystals using greigite (Fe3S4) and thiourea (NH2CSNFH2) as precursors. Nanocubic pyrite (FeS2) crystals with exposed (100) crystal faces and sizes of 100–200 nm were successfully synthesized via a facile hydrothermal method using greigite as the iron precursor and thiourea as the sulfur source. These results demonstrate the critical influence of sulfur source on pyrite morphology. These findings also provide new insights into the formation environments and pathways of nanocubic pyrite under hydrothermal conditions. Gu and coauthors [17] explore the variability in rare earth elements (REEs) and trace metal concentrations in dolostone samples of the Cryogenian Nantuo Formation of the Neoproterozoic Era, taken from a drill core in South China. Petrological evidence suggests that the dolostone in the Nantuo Formation was formed in nearshore waters. All the examined dolostone samples featured a significant enrichment of manganese and middle rare earth elements (MREEs), suggesting that the dolostone samples were deposited from suboxic to iron-enriched and anoxic waters. The MREE-enriched dolostone were transported by colloids and nanoparticles in meltwaters. Liu and coauthors [18] report the results of the geochemical alteration and mineralogy of coals under the influence of fault motion. Five coal samples were carefully collected from a reverse fault zone in Qi'nan colliery, China. Systematic detection methods were employed to analyze the different chemical and physical characteristics of the fault-related coal samples. The frictional heat and strong ductile deformation generated by the fault motion led to the dissociation of phenol and carboxyl groups in coal molecules, which sharply decreased the concentrations of elements Co and Mo bound to these functional groups. The disruption and delamination of laminar clay minerals by strong compression–shear stress significantly increased the adsorption sites for related elements, especially heavy rare earth elements (HREE) and MREE. Nanoscale clay minerals resulting from stress-induced scaly exfoliation also enhanced the retention capability of REE. Zhang and coauthors [19] present a model for the metal-bearing nanoparticles observed in soils and fault gouges over the Shenjiayao gold

deposit and their significance. Their findings indicate that ore-forming elements in soils can come only from deep-seated ore bodies and that they occur in nanoparticles, and an obvious relationship between metal nanoparticles in fault gouges and soils, with the metallic nanoparticles in fault gouges representing a transitional phase along the whole vertical migration process. Pyrite is the most common authigenic mineral preserved in many ancient sedimentary rocks. Liu and coauthors [20] demonstrate pyrite morphology to be an indicator of paleoredox conditions and shale gas content in the Longmaxi and Wufeng shales in the Middle Yangtze area, South China. Based on the formation mechanism, pyrites in the Longmaxi and Wufeng shales can be divided into syngenetic pyrites, early diagenetic pyrites, and late diagenetic pyrites. The size of the pyrite framboid microcrystals is used to distinguish syngenetic pyrite and diagenetic pyrite, owing to their different formation mechanisms. The sedimentary environment of the Longmaxi and Wufeng shales is mainly a euxinic environment with good reduction conditions, but some parts of the Longmaxi Formation reflect an oxic–dysoxic environment. The process of thermochemical sulfate reduction (TSR), accompanied by the formation of diagenetic pyrite, results in oxidation of organic matter (OM) and depletion of the H bond of OM, which influences the amounts of alkane gas produced by organic matter during thermal evolution. Consequently, the size of pyrite framboids and microcrystals could be widely used to rapidly evaluate paleoredox conditions and the gas content in shales.

Verberne and coauthors [21] deal with structural geology, including the mechanism of nanomineral movement in ductile shear zone theory. Principal slip zones (PSZs) are narrow (<10-cm) bands of localized shear deformation that occur in the cores of upper-crustal fault zones, where they accommodate the bulk of fault displacement. Natural and experimentally formed PSZs consistently show the presence of nanocrystallites in the <100-nm size range. The physical properties of nanocrystalline materials may profoundly affect fault rheology and the structural characteristics of nanocrystalline (NC) PSZs observed in natural faults and in experiments. Numerous reports in the literature show that such zones form in a wide range of faulted rock types, under a wide range of conditions pertaining to seismic and aseismic upper-crustal fault slip, as well as that they frequently show an internal crystallographic-preferred orientation (CPO) and partial amorphization and form glossy or "mirrorlike" slip surfaces. It is suggested that the widespread occurrence of NC PSZs in upper-crustal faults is of general significance. Specifically, the generally high rates of (diffusion) creep in nanocrystalline fault rock may play a key role in controlling the depth limits of the seismogenic zone.

Several papers [22–27] in this special issue focus on energy geology, addressing the structure and composition of coal and shale nanominerals as well as changes in nanopores and their fractal dimensions. They also describe structural characteristics such as the size, shape, and connectivity of nanopores in unconventional oil and gas reservoirs, along with their evolutionary laws and control factors. Chen and coauthors [22] focus on an experimental methodological issue involving total porosity measured for shale gas reservoir samples from the Lower Silurian Longmaxi formation in southeast Chongqing, China. Measuring the total porosity in shale gas reservoir samples remains a challenge because of their fine-grained texture, low porosity, ultra-low permeability, and high content of organic matter (OM) and clay mineral. The composition content porosimetry method, which is a new method for evaluating the porosity of shale samples, was used in the study to measure the total porosity of shale gas reservoir samples, based on bulk and grain density values. The findings indicated that the composition content porosimetry porosity generally increases with increasing OM and clay content and decreases with increasing quartz and feldspar content. Huang and coauthors [23] evaluated the pore structure characteristics of Lower Silurian Longmaxi shale gas reservoirs in Well LD1 of the Laifeng–Xianfeng Block, Upper Yangtze region in South China. N2 adsorption and helium ion microscope (HIM) were used to investigate various pore features, including pore volume, pore surface area, and pore size distribution. The calculated results show a good hydrocarbon storage capacity and development potential for the shale samples. Meanwhile, the reservoir space and migration pathways may be affected by the small pore size. The authors found that large pores usually have large values of fractal dimensions and identified a strong positive correlation between the fractal dimensions and pore

volume, as well as pore surface area. Fractal dimensions can be taken as a visual indicator of the degree of development of a pore structure in shale. Zhu and coauthors [24] studied tectonic and thermal controls on the nano–micro structural characteristics of a Cambrian organic-rich shale. Naturally deformed samples from the main source rocks are selected from the Lower Cambrian Lujiaping Formation in the Dabashan Thrust–Fold Belt to investigate nanometer to micrometer-sized structures. Their findings indicated that fracture-related pores predominate over mineral-hosted pores, as well as that these two pore types account for 90% of total pore space. Overall, Lujiaping deformed over-mature samples have abundant nano to micro-sized inorganic pores. High-resolution SEM images provide direct evidence of the formation of nano- and microsized structures such as OM–clay aggregates and silica nanograins. The characteristics of nanopore structure in shale play a crucial role in methane adsorption and determination of the occurrence and migration of shale gas. Using an integrated approach of X-ray diffraction (XRD), N2 adsorption, and field emission scanning electron microscopy (FE-SEM), Yu and coauthors [25] systematically focused on eight drilling samples of marine Taiyuan shale from well ZK1 in southern North China to study the characteristics and heterogeneity of their nanopore structure. Their findings indicated that different sedimentary environments may control the precipitation of clay and quartz between transitional shale and marine shale, leading to different organic matter (OM)–clay relationships and different correlations between total organic carbon (TOC) and mineral content. Taiyuan shale of higher heterogeneity is highly fractal, and its fractal dimensions are principally related to micropores. Li and coauthors [26] present a thorough investigation of the impacts of matrix compositions on the nanopore structure and fractal characteristics of lacustrine shales from the Changling Fault Depression (CFD), Southern Songliao Basin. The Lower Cretaceous Shahezi shales are targets for lacustrine shale gas exploration. The Shahezi shales were investigated to explore the impacts of rock composition, including organic matter and minerals, on pore structure and fractal characteristics. Seven lithofacies can be identified on a mineralogy-based shale classification scheme. Inorganic mineral–hosted pores are the most abundant pore type, whereas relatively few organic matter (OM) pores are observed in FE-SEM images of the Shahezi shales. The primary controlling factors for pore structure in Shahezi shales are clay minerals rather than OM. Clay content is the most significant factor controlling the fractal dimensions of the lacustrine Shahezi shale. Pore connectivity of lacustrine shales has been inadequately documented in previous papers. Li and coauthors [27] examine findings relating to lacustrine shales from the lower Cretaceous Shahezi Formation in the Changling Fault Depression (CFD) based on investigations using field emission scanning electron microscopy (FE-SEM), mercury intrusion capillary pressure (MICP), low-pressure gas (CO2 and N2) sorption (LPGA), and spontaneous fluid imbibition (SFI) experiments. Their findings indicate that pores observed from FE-SEM images are primarily interparticle (interP) pores in clay minerals and organic matter (OM) pores. Pore connectivity proceeds in the calcareous shale > argillaceous shale > siliceous shale. The connected pores of Shahezi shales are affected mainly by the abundance and coexistence of OM pores and clay carbonate mineral host pores.

The other articles [28–32] in this special issue are devoted to ore deposits and mineral materials. The genesis of Carlin-type gold deposits is analyzed at the nano scale; the significance of using metallic nanoparticles to search for concealed deposits is discussed; and the preparation, characterization, and physicochemical properties of nanomineral materials are explored. Tan and coauthors [28] present two hydrothermal events at the Shuiyindong Carlin-type gold deposit in southwestern China from Sm–Nd dating of fluorite and calcite. The Shuiyindong Gold Mine hosts one of the largest and highest-grade strata-bound Carlin-type gold deposits discovered to date in Southwestern China. The ore is hosted mainly in Upper Permian bioclastic limestone near the axis of an anticline. The gold is hosted mainly in arsenian pyrite and arsenopyrite, primarily as crystal lattice gold, submicroscopic particles, and nanoparticles. Fluorite commonly occurs at the vicinity of an unconformity between the Middle–Upper Permian formations, proposed as the structural conduit that fed the ore fluids. Calcite commonly fills fractures at the periphery of decarbonated rocks, which contain high-grade orebodies. Their study was intended to verify the occurrence of two distinct hydrothermal events at the Shuiyindong, based on

Sm–Nd isotope dating of the fluorite and calcite, which contains considerable concentrations of REE, and exhibit variable Sm/Nd ratios, facilitating the direct dating of associated hydrothermal events. Two groups of Sm–Nd isochron ages suggest two episodes of hydrothermal events in Shuiyindong. The age of the calcite likely represents the late stage of the gold mineralization period. Initial Nd isotopic compositions indicate that the Nd in the fluorite and calcite was likely derived from mixtures of basaltic volcanic tuff and bioclastic limestone of the Permian formations. Lu and coauthors [29] contribute to a better understanding of the green preparation and performance and mechanism of nanoporous pyrrhotite by thermal treatment of pyrite as an effective Hg(II) adsorbent. The removal of Hg(II) from aqueous solutions using pyrrhotite derived from the thermal activation of natural pyrite was explored by batch experiments. The adsorption isotherms demonstrated that the sorption of Hg(II) by modified pyrite (MPy) can be fitted well by the Langmuir model. The removal of Hg(II) by MPy was attributed mainly to a chemical reaction that resulted in cinnabar formation as well as electrostatic attraction between the negative charges in MPy and positive charges of Hg(II). The results of this work suggest that the thermal activation of natural pyrite is highly important for effective use of ore resources to remove Hg(II). Ca-bentonite was used as the feedstock material for the synthesis of hydroxysodalite because of its high Al, Si content, good chemical reactivity, and natural abundance. Liu and coauthors [30] report the results of one-step synthesis of hydroxysodalite using natural bentonite at moderate temperatures. The Na/Si molar ratio and reaction temperature both played important roles in controlling the degree of crystallinity of the synthetic hydroxysodalite. Although clays are widely used as sorbents for heavy metals due to their high specific surface areas, low cost, and ubiquity in most soil and sediment environments, they still face inherent limitations owing to the low loading capacity of heavy metals. Yao and coauthors [31] studied enhanced potential toxic metal removal using a novel hierarchical SiO2–Mg(OH)2 nanocomposite derived from sepiolite. The structural characteristics of the resulting modified samples were characterized by a wide range of techniques, including field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and nitrogen physisorption analysis. The results show that a hierarchical nanocomposite constructed by loading Mg(OH)2 nanosheets onto amorphous SiO2 nanotubes can be successfully prepared, with the nanocomposite having a high surface area (377.3 m2/g) and pore volume (0.96 cm3/g). Batch removal experiments indicate that the nanocomposite exhibits high removal efficiency toward Gd(III), Pb(II), and Cd(II). Zhou and coauthors [32] address the characterization and amoxicillin adsorption activity of mesopore CaCO3 microparticles prepared using rape flower pollen. CaCO3 adsorbent was characterized using X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The equilibrium adsorption data on amoxicillin were explained using the Langmuir, Freundlich, and Temkin adsorption isotherm models, whereas equilibrium adsorption of as-prepared CaCO3 was better depicted using the Langmuir adsorption model, indicating the favorable adsorption of amoxicillin.

The understanding and cognition of the Earth directly affect human beings' ability to survive and develop. The exploration and development of energy and mineral deposits, disaster and environmental prediction and management, and the like are all urgent problems in Earth science and relate to its resources, disasters, and environment. Using the nanoscale to re-understand and analyze these problems will open new perspectives and directions for researchers, whose object in conducting nanomineralogy is related chiefly to minerals and their pores in different spheres of the Earth system.

As a geoscientific frontier, nanomineralogy faces novel opportunities and challenges. Inevitably, some results of prior research, and the knowledge based on them, will be overturned. Nanomineralogy is still in its infancy, however, and although certain achievements have already been made, understanding is still lacking concerning the formation mechanism of mineral elements; the migration process, structural form, and disaster-related and environmental effects of nanoparticles; and the mineralization of nanopores. Taking full advantage of the advantages of multidisciplinary cross-development; promoting international cooperation among scientists; and systematically researching nanomineralogy, including by seeking concentrated solutions to key scientific problems, will enrich and spur the

development of nanomineralogic theory and methods. Such an approach will provide an important theoretical basis for use of new mineral- and carbon-based materials, the exploration and exploitation of energy and mineral resources, environmental protection, and disaster prediction.

**Acknowledgments:** The authors would like to express their deep appreciation for having been invited to serve as guest editors for the Nanomineralogy special issue of *Minerals*. Special thanks also go to the article authors and their sponsoring organizations. The authors thank the Editors-in-Chief, assistant editors, editors, and reviewers for their informative comments and constructive suggestions, which helped the contributing authors improve the quality of their manuscripts.

**Conflicts of Interest:** The authors declare no conflicts of interest.

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