**1. Introduction**

Red beds can be classified as continental or oceanic red beds according to the depositional environment in which they formed. At present, most are continental; oceanic ones are few due to their special formation conditions [1–3]. Continental red beds are not lithologically different from ordinary clastic sedimentary rocks, but they are of interest to geologists and geomorphologists because of their bright colors and unique landform types. According to erosion characteristics especially in China, red bed landforms are divided into Danxia landforms, red bed mountains, red bed hills, and red bed plateaus or platforms [4].

Research on continental red beds can be traced back to the 19th century [5–7]. Geologists have conducted a wide range of research regarding geomorphology, paleomagnetism, paleontology, energy, and minerals [8–15]. However, debates on the formation mechanism of their color still exist. Early geologists held two views on the origin of red beds. Some researchers believed that red sandstone formed in desert environments and was analogous to the red sand dunes of some modern tropical desert conditions, whereby the color was inherited from the fine-grained hematite film wrapped around the periphery of the sand particles [16], while other researchers suggested that red beds originated from lateritic weathering and formed in warm and humid environments where weathering of the nearby parent rocks produced the color originating from the residual red clay rich in iron oxides [17–21]. Thus, for a long time, the formation of continental red beds was assigned some paleoclimatic significance. However, geologists subsequently found that red beds formed in both arid and humid tropical climates [22–24]. The color of the red beds was formed during diagenesis, and the pigment hematite was derived from in situ alterations of iron-bearing mineral fragments or iron oxyhydroxide dihydroxylation [25]. Paleoclimatic conditions were not critical for the formation of the red color [7].

**Citation:** He, W.; Yang, Z.; Du, H.; Hu, J.; Zhang, K.; Hou, W.; Li, H. Micro-Mechanisms and Implications of Continental Red Beds. *Minerals* **2022**, *12*, 934. https://doi.org/ 10.3390/min12080934

Academic Editor: Luca Aldega

Received: 29 June 2022 Accepted: 22 July 2022 Published: 25 July 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Geologists have confirmed that fine-grained hematite is the main cause of the red color through study of secondary reduction zones and the extraction of pigmentary minerals [21,26,27]. Confusion remains as to its source, existence, and formation process. In recent years, the genesis of red beds has once again become of interest to geoscientists, because the presence of hematite could be indicative of the iron cycle, water storage, and the existence of life on Mars [28–30]. Therefore, this paper attempts to review the progress of research on issues related to the continental red beds, summarize the mechanisms of its color formation, and explore future development directions.

#### **2. Red Substance**

#### *2.1. Color-Rendering Characteristics*

The color of red beds depends mainly on its inherited color and oxidized color, with the inherited color being the color exhibited by the clastic minerals themselves in the rock, while the oxidized color is usually caused by iron oxides or oxyhydroxides contained in its matrix and cement. It is generally accepted that the red color in the red beds originates from hematite, but there is some fluctuation in the hue of the red layer from different places. If the Munsell color system is used to describe the red beds, the color of most red beds usually varies between 5YR and 5 RP [16,31–33]. This is mainly because the red beds also include other iron oxides or oxyhydroxides such as goethite, ferrihydrite, and lepidocrocite. Table 1 shows the color variation values of eight synthetic samples measured by Scheinost and Schwertmann (1999) using the Munsell color system [34]. Hematite has the most reddish hue on average and goethite shows a yellowish-brown color, while the remaining iron oxides or oxyhydroxides are in between, which would explain the reddish-brown color of many red beds. However, different iron oxides differ greatly in tinting strength and hiding power. For example, the tinting strength of hematite is much greater than that of goethite at the same particle size and distribution, even though all iron oxides have strong tinting strength [33]. Even a small percentage of hematite mixed with goethite can mask the color of goethite [35]. Moreover, hematite has a greater covering power than goethite (30–60 m<sup>2</sup> ·kg−<sup>1</sup> and 15–20 m<sup>2</sup> ·kg−<sup>1</sup> , respectively) [36]. Therefore, it is necessary to study hematite in the red beds and, thus, its color genesis.


**Table 1.** Munsell colors of the Fe oxides (median and range) (Reprinted with permission from Ref. [34]. 1999, John Wiley and Sons).

Note: *N* = number of samples.

The shape and size, distribution, aggregation, and cation substitution of hematite particles can also lead to changes in color. Franz (1981) showed that acicular hematite has higher reflectance and scattering ability in the long-wavelength region of the spectrum than more symmetrical hematite particles and, therefore, tends to have a more yellowish hue [26]. The tinting strength of hematite powder increases with decreasing particle size, and the strongest coloring ability of hematite is achieved when the particle size decreases to the optimal particle size, i.e., the particle size with the largest scattering cross-section (about 1 µm) [33]. Therefore, even in very brightly colored red beds, hematite is often difficult to detect by XRD. In contrast, large crystals or dense aggregates of hematite are usually dark brown or black, due to much greater absorption than scattered throughout the visible region. The arrangement of hematite aggregates likewise affects the color variation. In the Devonian red beds of Scotland, Turner and Archer (1977) observed oriented aggregates of platelet-like hematite crystals epitaxially growing on altered biotite, and the combination of such small lamellar hematite crystals into oriented aggregates causes a color shift to purple [27,37]. In addition, other metal substitutions of some Fe in the crystals can also modify the color of hematite. For example, manganese- and titanium-substituted hematite is black, while, in the case of aluminum substitution, it shifts to a more reddish hue due to the reduction in particle size [38,39]. particles can also lead to changes in color. Franz (1981) showed that acicular hematite has higher reflectance and scattering ability in the long-wavelength region of the spectrum than more symmetrical hematite particles and, therefore, tends to have a more yellowish hue [26]. The tinting strength of hematite powder increases with decreasing particle size, and the strongest coloring ability of hematite is achieved when the particle size decreases to the optimal particle size, i.e., the particle size with the largest scattering cross-section (about 1 μm) [33]. Therefore, even in very brightly colored red beds, hematite is often difficult to detect by XRD. In contrast, large crystals or dense aggregates of hematite are usually dark brown or black, due to much greater absorption than scattered throughout the visible region. The arrangement of hematite aggregates likewise affects the color variation. In the Devonian red beds of Scotland, Turner and Archer (1977) observed oriented aggregates of platelet-like hematite crystals epitaxially growing on altered biotite, and the combination of such small lamellar hematite crystals into oriented aggregates causes a color shift to purple [27,37]. In addition, other metal substitutions of some Fe in the crystals can also modify the color of hematite. For example, manganese- and titanium-substituted hematite is black, while, in the case of aluminum substitution, it shifts to a more reddish hue due to the reduction in particle size [38,39]. *2.2. Distribution and Existence of Hematite*

The shape and size, distribution, aggregation, and cation substitution of hematite

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#### *2.2. Distribution and Existence of Hematite* There are two main forms of authigenic hematite in red beds. One is in the form of

There are two main forms of authigenic hematite in red beds. One is in the form of single or polycrystalline aggregates coexisting with other authigenic minerals around the clastic particles, forming along the cleavage planes of mica (mainly biotite) or filling the pores between the particles; a small number of hematite particles are also present in fractures [40–43]. The crystal form of this hematite ranges from semi-hedral to euhedral, usually with relatively small quantities but large particles, mainly on the micrometer scale. There are also some submicron crystals. For example, Eren et al. (2013) discovered rhombohedral hematite crystal cross-sections with a grain size of about 0.3 mm in the red sandstones of the Early Cambrian Hüdai Formation in the Aydıncık (Mersin) Zone, central Taurides, southern Turkey (Figure 1a) [44]. Rasmussen and Muhling (2019) found radiolarian aggregates (Figure 1c) and slatted hematite filling granular voids (Figure 1d) in metamorphic red sandstones and shales in the Stirling Formation of Western Australia [45]. single or polycrystalline aggregates coexisting with other authigenic minerals around the clastic particles, forming along the cleavage planes of mica (mainly biotite) or filling the pores between the particles; a small number of hematite particles are also present in fractures [40–43]. The crystal form of this hematite ranges from semi-hedral to euhedral, usually with relatively small quantities but large particles, mainly on the micrometer scale. There are also some submicron crystals. For example, Eren et al. (2013) discovered rhombohedral hematite crystal cross-sections with a grain size of about 0.3 mm in the red sandstones of the Early Cambrian Hüdai Formation in the Aydıncık (Mersin) Zone, central Taurides, southern Turkey (Figure 1a) [44]. Rasmussen and Muhling (2019) found radiolarian aggregates (Figure 1c) and slatted hematite filling granular voids (Figure 1d) in metamorphic red sandstones and shales in the Stirling Formation of Western Australia [45].

**Figure 1.** (**a**) Hematite in a thin section illustrating a euhedral hexagonal crystal form; (**b**) hematite outgrowths aligned in the fabric defined by strain fringes on quartz grains; (**c**) hematite plates partly filling a grain-shaped cavity; (**d**) hematite grain with large radiating blades. (**a**) is reprinted with **Figure 1.** (**a**) Hematite in a thin section illustrating a euhedral hexagonal crystal form; (**b**) hematite outgrowths aligned in the fabric defined by strain fringes on quartz grains; (**c**) hematite plates partly filling a grain-shaped cavity; (**d**) hematite grain with large radiating blades. (**a**) is reprinted with permission from Ref. [44]. 2013, Turkish Journal of Earth Sciences; (**b**–**d**) are reprinted with permission from Ref. [45]. 2019, Elsevier.

1

Another type of hematite is distributed as microfine grained crystals mixed with clay minerals in clusters disseminated in the matrix and cement of the red beds [46], partially forming a thin film of iron-bearing clay around individual clastic particles; clay films are present on clastic particles in almost all red beds. Films are sometimes developed at the contacts with detrital grains and are covered by quartz secondary overgrowth or wrapped by carbonate cement; clay-containing films formed after quartz overgrowth also exist [42]. This hematite is distributed microscopically and diffusely on montmorillonite and illite/smectite wafers or filled among the micropores between the illite/smectite layers (Figure 2a,b). Most of the hematite is in tiny crystal agglomerates or in individual microcrystals form in the pores of the montmorillonite and exhibit a variety of shapes (Figure 2d) [47]. There are spherical, rod-shaped, leaf-shaped, and well-developed hexagonal flake hematite single crystals, as well as fibrous to lath-shaped, rosette clusters and spherical hematite/goethite aggregates (Figure 2c–f) [47,48]. These morphological features are closely related to the relative abundance of the iron-bearing mineral precursors in the rocks. Hematite particles dispersed in clay minerals are usually small but very abundant and are the main source of coloring of the red beds. The hematite mainly constitutes submicron or even nanoscale crystals; hence, it is difficult to identify it using conventional detection methods [49]. minerals in clusters disseminated in the matrix and cement of the red beds [46], partially forming a thin film of iron-bearing clay around individual clastic particles; clay films are present on clastic particles in almost all red beds. Films are sometimes developed at the contacts with detrital grains and are covered by quartz secondary overgrowth or wrapped by carbonate cement; clay-containing films formed after quartz overgrowth also exist [42]. This hematite is distributed microscopically and diffusely on montmorillonite and illite/smectite wafers or filled among the micropores between the illite/smectite layers (Figure 2a,b). Most of the hematite is in tiny crystal agglomerates or in individual microcrystals form in the pores of the montmorillonite and exhibit a variety of shapes (Figure 2d) [47]. There are spherical, rod-shaped, leaf-shaped, and well-developed hexagonal flake hematite single crystals, as well as fibrous to lath-shaped, rosette clusters and spherical hematite/goethite aggregates (Figure 2c–f) [47,48]. These morphological features are closely related to the relative abundance of the iron-bearing mineral precursors in the rocks. Hematite particles dispersed in clay minerals are usually small but very abundant and are the main source of coloring of the red beds. The hematite mainly constitutes submicron or even nanoscale crystals; hence, it is difficult to identify it using conventional detection methods [49].

permission from Ref. [44]. 2013, Turkish Journal of Earth Sciences; (**b**–**d**) are reprinted with permis-

Another type of hematite is distributed as microfine grained crystals mixed with clay

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sion from Ref. [45]. 2019, Elsevier.

**Figure 2.** (**a**) Hematite scattered over illite/smectite layers; (**b**) fine and dispersed hematite distributed among clay minerals; (**c**) hematite pigment showing a patchy coating on detrital grains; (**d**) plated hexagonal hematite crystals distributed on the surface of a detrital mineral; (**e**) spherical hematite particles; (**f**) rod-shaped (or bacillus-shaped) hematite particles disseminated on the grain surface. (**a**,**b**,**d**) are reprinted with permission from Ref. [47]. 2020, Geoscience; (**c**,**e**,**f**) are reprinted with permission from Ref. [44]. 2013, Turkish Journal of Earth Sciences. **Figure 2.** (**a**) Hematite scattered over illite/smectite layers; (**b**) fine and dispersed hematite distributed among clay minerals; (**c**) hematite pigment showing a patchy coating on detrital grains; (**d**) plated hexagonal hematite crystals distributed on the surface of a detrital mineral; (**e**) spherical hematite particles; (**f**) rod-shaped (or bacillus-shaped) hematite particles disseminated on the grain surface. (**a**,**b**,**d**) are reprinted with permission from Ref. [47]. 2020, Geoscience; (**c**,**e**,**f**) are reprinted with permission from Ref. [44]. 2013, Turkish Journal of Earth Sciences.

The form of authigenic hematite in the red beds seems to correspond to the age of the red beds. Compared with the hematite in the form of single or polycrystalline aggregates, the latter (mixed with clay minerals) is more widely distributed in time and space, The form of authigenic hematite in the red beds seems to correspond to the age of the red beds. Compared with the hematite in the form of single or polycrystalline aggregates, the latter (mixed with clay minerals) is more widely distributed in time and space, appearing in red beds of all ages. In Mesozoic and Cenozoic red beds, hematite is almost only adsorbed on the surface and/or in the voids of clay minerals [41,48,50]. In older red beds, hematite can be depleted to varying degrees due to later tectonics, groundwater penetration, or the participation of hydrothermal fluids, and then replaced by

euhedral to semi-hedral authigenic hematite grains formed in situ and secondary pores or aggregates [42,43]. Residues at the grain contacts or the edges of some reddish iron-bearing clays are surrounded by quartz secondary enlarged edges. Older red beds have a greater proportion of granular hematite particles and a darker overall color.

#### **3. The Source and Formation of Hematite**
