Gemological Characteristics and Origin of the Zhanguohong Agate from Beipiao, Liaoning Province, China: A Combined Microscopic, X-ray Di ﬀ raction, and Raman Spectroscopic Study

: The Zhanguohong agate from Beipiao (Liaoning province, China), which occurs in the intermediate–felsic volcanic breccias of the Early Cretaceous Yixian Formation, generally shows massive and banded structures, with red, yellow, and / or white layers or zones. Little research has been done on its mineralogical and gemological characteristics or its genesis. In this study, we present petrographic and spectroscopic constraints on the mineral composition and micro-texture of the silica matrix, as well as the ferruginous inclusions within the agates, in order to deduce the origin of the Zhanguohong agate. According to the microscopic observations, sandwich-like interlayered micro-granular quartz, ﬁbrous chalcedony, and jigsaw quartz bands are common in the banded agates. X-ray di ﬀ raction (XRD) and Raman spectroscopic analyses revealed that all of the samples were mainly composed of α -quartz and moganite, with minor hematite and goethite. The moganite content (17–54 wt%) of the silica matrix decreases by varying degrees from the outermost to the innermost part of the banded agates. The crystal defects and ferric iron in the microcrystalline silica grains probably contributed to the moganite crystallization. The red, yellow, and orange zones are rich in hematite, goethite, and their mixtures, respectively. The ore-forming ﬂuids ﬂuctuated between acidic and alkaline within a temperature range of 100–200 ◦ C and at a sustained positive Eh. Combined with the ﬁeld observations, these results suggest that the multiperiod precipitation of the agates probably resulted from the episodic volcanic activity during the Early Cretaceous lithospheric extension in eastern China. ﬀ raction spectroscopic analysis, this presents detailed information on the gemological characteristics of the in the Beipiao district. ﬁeld investigations, we try to provide an insight into the origin of the

Agates mainly exhibit microcrystalline fibrous, granular, and jigsaw textures with different contents of moganite [1,3,12] which belongs to the monoclinic group. The structure of moganite has been described as the alternate stacking of layers of left-and right-handed α-quartz corresponding to a  [35,36]); (b) geological map of the Cunzhuyingzi agate deposit and its adjacent areas (modified after [35,36]  An ore-bearing zone has been found in the Yixian Formation near Cunzhuyingzi village, which generally strikes E-W and dips gently (30°) to the N (Figures 1 and 2a). It extends for up to 4 km with various widths (<150 m) [34].   [35,36]); (b) geological map of the Cunzhuyingzi agate deposit and its adjacent areas (modified after [35,36]    The XRD analysis was carried out by a Rigaku D/max-rA type 12 kW rotating target diffractometer with a Cu-Kα radiation (40 kV, 100 mA) and a graphite monochromator at the Beida Micro-Structure Analytical Lab of Beijing, China. Measurements were conducted in the range between 10-40° 2θ with a scan rate of 2°/min. Raman spectra of the silica matrix and inclusions were acquired between 100 and 1500 cm -1 using the Horiba LabRAM HR Evolution equipped with a Peltier-cooled charged-coupled device (CCD) detector, edge filters, and a Nd-YAG laser (100 mW, 532 nm) at the School of Gemology, China University of Geosciences, Beijing, China. The confocal hole was fixed at 100 μm, and the diffraction grating was 600 grooves/mm. The laser focusing and sample viewing were operated through 100×, 50×, and 10× objective lens with a polaroid. With this configuration, the spatial resolution was <1 μm, and the spectral resolution was determined to be ~1 cm -1 . The spectrometer

Materials and Methods
Fifteen agate samples (Z-1 to Z-15) with different shapes and colors were collected and studied, especially irregularly-shaped examples ( Figure 3). Featuring red, yellow, and/or white colors, the majority of the samples show banded structures (Figures 2c-e and 3), while the others exhibit massive structures (Figures 2f and 3). Macrocrystalline quartz grains are common at the center of the banded agates (Figures 2d and 3) and sometimes act as middle layers (Figures 2e and 3).
The XRD analysis was carried out by a Rigaku D/max-rA type 12 kW rotating target diffractometer with a Cu-Kα radiation (40 kV, 100 mA) and a graphite monochromator at the Beida Micro-Structure Analytical Lab of Beijing, China. Measurements were conducted in the range between 10-40 • 2θ with a scan rate of 2 • /min.
Raman spectra of the silica matrix and inclusions were acquired between 100 and 1500 cm −1 using the Horiba LabRAM HR Evolution equipped with a Peltier-cooled charged-coupled device (CCD) detector, edge filters, and a Nd-YAG laser (100 mW, 532 nm) at the School of Gemology, China University of Geosciences, Beijing, China. The confocal hole was fixed at 100 µm, and the diffraction grating was 600 grooves/mm. The laser focusing and sample viewing were operated through 100×, 50×, and 10× objective lens with a polaroid. With this configuration, the spatial resolution was <1 µm, and the spectral resolution was determined to be~1 cm −1 . The spectrometer calibration was set using the 520.5 cm −1 band of a silicon wafer. The spectra were recorded at a laser power of 50 mW with 3 s acquisition time and twice accumulation.
LabSpec software (Version LabSpec 6, Horiba Ltd., Kyoto, Japan) and were normalized by the strongest band (465 cm -1 ) intensity normalization method [61]. The curve-fitting algorithm in the OriginPro sofware (Version OriginPro 2018 SR1, OriginLab, Northampton, UK) was used to fit the pre-processed spectra in the 400-600 cm -1 range. The spectra were deconvoluted with Fourier Self-Deconvolution (Pro), and only two peaks were found at 502 cm -1 (moganite) and 465 cm -1 (quartz). The peak parameters (e.g., position, width, and area) were obtained by fitting the spectra with Lorentzian functions constrained by a constant minimum baseline.

Microscopic Observations
The Zhanguohong agates in the Beipiao district are mainly composed of microcrystalline silica phases with different micro-textures (i.e., micro-granular quartz, fibrous chalcedony, and jigsaw quartz) and macro-quartz, with minor Fe compounds (Figures 4 and 5). Two distinct types of agates with banded and massive structures were observed.
Concentric and rhythmic bands resulting from changes in composition and/or texture are common in the banded agates, which are emphasized by the Fe compounds (Figure 4a-g). Here, the terms micro-granular quartz, fibrous chalcedony, jigsaw quartz, and macro-quartz bands are used to describe the microscopic characteristics of the banded agates. The macro-quartz (~0.05-1 mm in size) bands are preferentially located in the center of the banded agates (Figure 4a,b,f,g) and occasionally occur as interlayers among the microcrystalline silica bands (Figure 4e). Sandwich-like interlayered micro-granular quartz (~5-20 μm in size) and fibrous chalcedony bands comprise the majority of the concentric and rhythmic bands (Figure 4c-g), with a few jigsaw quartz band interlayers ( Figure  4c,d,f,g). However, some banded agates are predominant with micro-granular quartz bands or fibrous chalcedony bands (Figure 4a,b). Near the boundary between the volcanic wall rock and the agate, banded and/or blocky macro-quartz and micro-granular quartz aggregates were observed ( Figure 4b). The complex changes in texture (e.g., size and shape) (Figure 4a-g) suggest that the formation of the banded agates may have been a multi-stage process. Spherical and radiated structures are locally present in some of the banded agates (Figure 4f,g). The former, which is thought to be the result of nucleation and growth starting in the innermost part of a free space [6], is The spectra of the silica phases were background corrected by the polynomial method using the LabSpec software (Version LabSpec 6, Horiba Ltd., Kyoto, Japan) and were normalized by the strongest band (465 cm −1 ) intensity normalization method [61]. The curve-fitting algorithm in the OriginPro sofware (Version OriginPro 2018 SR1, OriginLab, Northampton, UK) was used to fit the pre-processed spectra in the 400-600 cm −1 range. The spectra were deconvoluted with Fourier Self-Deconvolution (Pro), and only two peaks were found at 502 cm −1 (moganite) and 465 cm −1 (quartz). The peak parameters (e.g., position, width, and area) were obtained by fitting the spectra with Lorentzian functions constrained by a constant minimum baseline.

Microscopic Observations
The Zhanguohong agates in the Beipiao district are mainly composed of microcrystalline silica phases with different micro-textures (i.e., micro-granular quartz, fibrous chalcedony, and jigsaw quartz) and macro-quartz, with minor Fe compounds (Figures 4 and 5). Two distinct types of agates with banded and massive structures were observed.
Concentric and rhythmic bands resulting from changes in composition and/or texture are common in the banded agates, which are emphasized by the Fe compounds (Figure 4a-g). Here, the terms micro-granular quartz, fibrous chalcedony, jigsaw quartz, and macro-quartz bands are used to describe the microscopic characteristics of the banded agates. The macro-quartz (~0.05-1 mm in size) bands are preferentially located in the center of the banded agates (Figure 4a,b,f,g) and occasionally occur as interlayers among the microcrystalline silica bands (Figure 4e). Sandwich-like interlayered micro-granular quartz (~5-20 µm in size) and fibrous chalcedony bands comprise the majority of the concentric and rhythmic bands (Figure 4c-g), with a few jigsaw quartz band interlayers (Figure 4c,d,f,g). However, some banded agates are predominant with micro-granular quartz bands or fibrous chalcedony bands (Figure 4a,b). Near the boundary between the volcanic wall rock and the agate, banded and/or blocky macro-quartz and micro-granular quartz aggregates were observed (Figure 4b). The complex changes in texture (e.g., size and shape) (Figure 4a-g) suggest that the formation of the banded agates may have been a multi-stage process. Spherical and radiated structures are locally present in some of the banded agates (Figure 4f,g). The former, which is thought to be the result of nucleation and growth starting in the innermost part of a free space [6], is composed of micro-granular quartz grains (Figure 4f), and the latter of microcrystalline silica aggregates (i.e., fibrous chalcedony and jigsaw-like  (Figure 4g). At least two periods of silica precipitation occurred, based on the occurrence of microcrystalline silica veins which cross-cut the concentric and rhythmic bands (Figure 4a,c). However, without concentric and rhythmic bands, massive agates are predominantly comprised of micro-granular quartz grains and/or fibrous chalcedony aggregates (Figure 4h). Fe-hydroxides, which were identified using Raman spectroscopic analysis (as discussed later in section 4.3.2). The Fe compounds are generally present as short prismatic (Figure 5a), spherical (Figure 5b), and granular ( Figure 5c) inclusions distributed perpendicular to the growth direction of the agate. The majority of these inclusions form Fe-rich layers (Figure 5a,c), while the others are scattered randomly in the silica matrix ( Figure 5b). However, in some of the red and yellow zones, the ferruginous particles are too small to identify under the optical microscope ( Figure 5b). In addition, the transmission of Fe compounds is common in agates (Figures 4d and 5d).

XRD Analysis
Two samples (Z-4 and Z-9) were chosen for XRD analysis ( Figure 6). Their moganite contents were significantly different. It was low in sample Z-9, which was demonstrated by the negligible moganite peaks; while it was relatively high in sample Z-4, but was still low compared with the α-quartz content. The moganite contents of samples Z-4 (11%) and Z-9 (7%) were estimated by the peak area ratio method [16]. Hematite and albite were also present as minor phases in sample Z-4. The existence of the latter phase was probably due to the entrainment of pieces of wall rocks (Figures 3 and 4a). Pseudocrystalline and amorphous silica phases were not detected. None was goethite.  Red and yellow zones and bands in the agates are highly impregnated with Fe-oxides and Fe-hydroxides, which were identified using Raman spectroscopic analysis (as discussed later in

XRD Analysis
Two samples (Z-4 and Z-9) were chosen for XRD analysis ( Figure 6). Their moganite contents were significantly different. It was low in sample Z-9, which was demonstrated by the negligible moganite peaks; while it was relatively high in sample Z-4, but was still low compared with the α-quartz content. The moganite contents of samples Z-4 (11%) and Z-9 (7%) were estimated by the peak area ratio method [16]. Hematite and albite were also present as minor phases in sample Z-4. The existence of the latter phase was probably due to the entrainment of pieces of wall rocks (Figures 3  and 4a). Pseudocrystalline and amorphous silica phases were not detected. None was goethite.

Raman Spectroscopic Analysis
Compared with XRD analysis, in-situ Raman spectroscopy is a more effective method of distinguishing between different silica phases at the micron-scale [62,63]. Both the silica matrix and the ferruginous inclusions in the Zhanguohong agates were analyzed using Raman spectroscopy. The relative Raman calibration model proposed by Götze et al. (1998) [3] was used to evaluate the moganite content of the silica matrix. moganite peaks; while it was relatively high in sample Z-4, but was still low compared with the α-quartz content. The moganite contents of samples Z-4 (11%) and Z-9 (7%) were estimated by the peak area ratio method [16]. Hematite and albite were also present as minor phases in sample Z-4. The existence of the latter phase was probably due to the entrainment of pieces of wall rocks (Figures 3 and 4a). Pseudocrystalline and amorphous silica phases were not detected. None was goethite.

Silica Matrix
The silica matrix consists of α-quartz and moganite. However, hematite and goethite also appear in the Raman spectra, which is attributed to the presence of fine-grained Fe-bearing particles in the matrix (Figure 7). Pseudocrystalline and amorphous silica phases were not detected in any of the samples. Marker bands were assigned for the silica phases (i.e., α-quartz and moganite) and Fe compounds (i.e., hematite and goethite), and are listed in Table 1. Figures 8-10 illustrate the spatial distribution of the moganite content, which ranges from 17 to 54 wt%, in the Zhanguohong agates (samples Z-2, -4, -6, -8, and -12). Detailed microscopic observations were conducted before the Raman spectroscopic analysis to ensure that the Fe-bearing zones were excluded as much as possible. The micro-granular quartz contains up to 54 wt% moganite near the wall rock ( Figure 8, point 1), whereas the moganite content is much lower among the micro-granular quartz aggregates, which have a spherical structure (Figure 10b,d). In general, the majority of the moganite contents are between 20 and 45 wt% (Figures 8-10). With the consistent precipitation of silica phases, the moganite content fluctuates and decreases with different degrees in the banded agates (Figures 8 and 9). Moreover, this variation in moganite content is found in microcrystalline silica grains formed in different periods as well (Figure 10c,d), which may reflect the physicochemical differences in their parent fluids [2].   Figures 8-10 illustrate the spatial distribution of the moganite content, which ranges from 17 to 54 wt%, in the Zhanguohong agates (samples Z-2, -4, -6, -8, and -12). Detailed microscopic observations were conducted before the Raman spectroscopic analysis to ensure that the Fe-bearing zones were excluded as much as possible. The micro-granular quartz contains up to 54 wt% moganite near the wall rock ( Figure 8, point 1), whereas the moganite content is much lower among the micro-granular quartz aggregates, which have a spherical structure (Figure 10b,d). In general, the majority of the moganite contents are between 20 and 45 wt% (Figures 8-10). With the consistent precipitation of silica phases, the moganite content fluctuates and decreases with different degrees in the banded agates (Figures 8 and 9). Moreover, this variation in moganite content is found in microcrystalline silica grains formed in different periods as well (Figure 10c,d), which may reflect the physicochemical differences in their parent fluids [2].      In the matrix, hematite is evidenced by marker bands at 225, 246, 411, 499, 610, and 1316 cm -1 , while goethite is evidenced by maker bands at 298 and 394 cm -1 (Figure 7). Several studies have suggested that ferric iron acts as an agent in the formation of moganite [2][3][4]. In order to determine whether there is a correlation between ferric iron and moganite, this study qualitatively analyzed the intensity of the band at ~1316 cm -1 (hematite) in the normalized spectra (Figures 8d, 9c, and  10d). This band was chosen due to its relatively high intensity and lack of overlap with other bands. Because of the superimposition of the ferruginous inclusions (faintly red, orange, and/or yellow fine grains under the microscope) underlying the sample surface, some of the Raman spectroscopic analysis points (i.e., the open circles in Figure 11) were omitted when we determined the correlation. After this omission, a positive correlation was obtained between the intensity of Raman peak ~1316 cm -1 and the moganite content of the silica matrix ( Figure 11).  In the matrix, hematite is evidenced by marker bands at 225, 246, 411, 499, 610, and 1316 cm −1 , while goethite is evidenced by maker bands at 298 and 394 cm −1 (Figure 7). Several studies have suggested that ferric iron acts as an agent in the formation of moganite [2][3][4]. In order to determine whether there is a correlation between ferric iron and moganite, this study qualitatively analyzed the intensity of the band at~1316 cm −1 (hematite) in the normalized spectra (Figure 8d, Figure 9c, and Figure 10d). This band was chosen due to its relatively high intensity and lack of overlap with other bands. Because of the superimposition of the ferruginous inclusions (faintly red, orange, and/or yellow fine grains under the microscope) underlying the sample surface, some of the Raman spectroscopic analysis points (i.e., the open circles in Figure 11) were omitted when we determined the correlation. After this omission, a positive correlation was obtained between the intensity of Raman peak~1316 cm −1 and the moganite content of the silica matrix ( Figure 11). In the matrix, hematite is evidenced by marker bands at 225, 246, 411, 499, 610, and 1316 cm -1 , while goethite is evidenced by maker bands at 298 and 394 cm -1 (Figure 7). Several studies have suggested that ferric iron acts as an agent in the formation of moganite [2][3][4]. In order to determine whether there is a correlation between ferric iron and moganite, this study qualitatively analyzed the intensity of the band at ~1316 cm -1 (hematite) in the normalized spectra (Figures 8d, 9c, and  10d). This band was chosen due to its relatively high intensity and lack of overlap with other bands. Because of the superimposition of the ferruginous inclusions (faintly red, orange, and/or yellow fine grains under the microscope) underlying the sample surface, some of the Raman spectroscopic analysis points (i.e., the open circles in Figure 11) were omitted when we determined the correlation. After this omission, a positive correlation was obtained between the intensity of Raman peak ~1316 cm -1 and the moganite content of the silica matrix ( Figure 11).

Moganite in the Zhanguohong Agate
The Raman spectroscopic analysis revealed that the moganite contents of the Zhanguohong agates in the Beipiao district vary significantly (17-54 wt%, Figures 8-10), which is in agreement with the findings that the moganite contents are significantly different for the different samples, and even for different zones of the same sample collected from other places [20,62,68,69]. In general, the moganite content decreases and fluctuates with different degrees in the banded agates during the progressive precipitation of silica (Figures 8 and 9). Moganite crystallization appears to be favorable in the initial stage (Figures 8 and 9), which indicates that moganite formation may be attributed to the high structural defects generated by rapid silica growth from a strongly supersaturated solution [70][71][72]. With progressive precipitation, the degree of silica supersaturation and the crystallization rate of silica gradually decrease [70]. As a consequence, the density of the defects decreases, which results in the decreasing moganite contents observed in the Zhanguohong agates (Figures 8 and 9). However, the moganite contents also exhibit significant fluctuations (Figures 8 and 9), which are likely caused by local oscillations in the degree of silica supersaturation [73,74]. The dark red inclusions consist mainly of hematite with minor α-quartz ( Figure 12, points 1 and 2), and the presence of goethite makes their color more orangish (Figure 12, points 3 and 4). Furthermore, the high goethite content of these inclusions leads to a yellow appearance ( Figure 12, point 5).

Moganite in the Zhanguohong Agate
The Raman spectroscopic analysis revealed that the moganite contents of the Zhanguohong agates in the Beipiao district vary significantly (17-54 wt%, Figures 8-10), which is in agreement with the findings that the moganite contents are significantly different for the different samples, and even for different zones of the same sample collected from other places [20,62,68,69]. In general, the moganite content decreases and fluctuates with different degrees in the banded agates during the progressive precipitation of silica (Figures 8 and 9). Moganite crystallization appears to be favorable in the initial stage (Figures 8 and 9), which indicates that moganite formation may be attributed to the high structural defects generated by rapid silica growth from a strongly supersaturated solution [70][71][72]. With progressive precipitation, the degree of silica supersaturation and the crystallization rate of silica gradually decrease [70]. As a consequence, the density of the defects decreases, which results in the decreasing moganite contents observed in the Zhanguohong agates (Figures 8 and 9). However, the moganite contents also exhibit significant fluctuations (Figures 8 and 9), which are likely caused by local oscillations in the degree of silica supersaturation [73,74].
The relatively low moganite content (Figure 9, point 1) at the very beginning of stage I can be ascribed to the recrystallization of the pre-existing silica phases [6] next to the wall rock where alteration is easier. Alternatively, it can be attributed to the mixing of the micro-granular quartz with the underlying macro-quartz grains. In addition, the moganite content deviates slightly from the trendline at the end of stage I (e.g., points 5-7 in Figure 8, and points 5 and 6 in Figure 9), and sometimes it is slightly lower than that in stage II (Figures 8 and 9). This low moganite content may be due to alteration or recrystallization after the initial precipitation of the microcrystalline silica grains, which results in their jigsaw-like micro-textures with irregularly shaped boundaries, particularly along the direction of silica growth (Figures 8a and 9a).
Compared with the Raman spectroscopic results, the XRD analysis indicated much lower moganite contents (~10 wt%) for the Zhanguohong agates ( Figure 6, Figure 8, Figure 9, and Figure 10). This difference can be explained by the occurrence of nano-sized moganite, which makes a limited contribution to the Bragg reflection [3]. In addition, the mixture of macro-quartz grains and wall rock in XRD analysis further reduces the moganite content (Figures 2 and 6).
It has been suggested that moganite crystallization is favored by the alkaline fluid combined with high activities of ferric iron [2,4,5]. The positive correlation between the moganite content and the intensity of~1316 cm −1 peak ( Figure 11) provides evidence that ferric iron probably plays an important role in moganite crystallization.
The statistics for the moganite contents of microcrystalline silica samples (i.e., agate, chalcedony, chert, and flint) determined using XRD and Raman spectroscopy in the literature [2,10,16,23,30,31,68,69,75,76] and in this study ( Figure 13) reveal that the proportion of moganite increases as the volcanic wall rocks become younger, that is, from Devonian to Neogene, based on the change in the maximum moganite content for each period. This is consistent with the findings of Moxon and his coworkers [10,16,75]. It should be noted that the ages of the Phanerozoic rocks are approximate ages. For example, if the volcanic wall rocks formed in the Cretaceous (145.5-65.5 Ma), the median value (105.5 Ma) was taken as the approximate age. It has been suggested that moganite is thermodynamically less stable [15] and recrystallizes readily to α-quartz over millions of years [2] or in the presence of water (e.g., hydrothermal activity and weathering) [16,17,20]. The decay rate (~0.218 wt%/Ma) of moganite could be evaluated. We suppose that the wide variation in the moganite contents of the Zhanguohong agates is more likely due to the initial difference rather than to heterogeneous recrystallization or alteration after its precipitation. The relatively low moganite content (Figure 9, point 1) at the very beginning of stage Ⅰ can be ascribed to the recrystallization of the pre-existing silica phases [6] next to the wall rock where alteration is easier. Alternatively, it can be attributed to the mixing of the micro-granular quartz with the underlying macro-quartz grains. In addition, the moganite content deviates slightly from the trendline at the end of stage Ⅰ (e.g., points 5-7 in Figure 8, and points 5 and 6 in Figure 9), and sometimes it is slightly lower than that in stage Ⅱ (Figures 8 and 9). This low moganite content may be due to alteration or recrystallization after the initial precipitation of the microcrystalline silica grains, which results in their jigsaw-like micro-textures with irregularly shaped boundaries, particularly along the direction of silica growth (Figures 8a and 9a).
Compared with the Raman spectroscopic results, the XRD analysis indicated much lower moganite contents (~10 wt%) for the Zhanguohong agates (Figures 6,8-10). This difference can be explained by the occurrence of nano-sized moganite, which makes a limited contribution to the Bragg reflection [3]. In addition, the mixture of macro-quartz grains and wall rock in XRD analysis further reduces the moganite content (Figures 2 and 6).
It has been suggested that moganite crystallization is favored by the alkaline fluid combined with high activities of ferric iron [2,4,5]. The positive correlation between the moganite content and the intensity of ~1316 cm -1 peak ( Figure 11) provides evidence that ferric iron probably plays an important role in moganite crystallization. The statistics for the moganite contents of microcrystalline silica samples (i.e., agate, chalcedony, chert, and flint) determined using XRD and Raman spectroscopy in the literature [2,10,16,23,30,31,68,69,75,76] and in this study ( Figure 13) reveal that the proportion of moganite increases as the volcanic wall rocks become younger, that is, from Devonian to Neogene, based on

Coloration Mechanism
Yellow, red, and orange bands or zones are common in the Zhanguohong agates. These colored zones are rich in red and/or yellow, relatively coarse (µm-scale) inclusions with short prismatic (Figure 5a), spherical (Figure 5b), and granular (Figure 5c) shapes, or contain extremely fine colored grains. The Raman spectroscopic analysis reveals that these inclusions are mixtures of Fe compounds (i.e., hematite and goethite) and silica phases (Figure 12), which suggests that these Fe-bearing minerals and silica phases simultaneously precipitated from the solution. In general, the red and yellow zones consist largely of hematite-and goethite-bearing particles (i.e., coarse inclusions and fine grains), respectively. To some extent, when they mix together, the color will turn orange ( Figure 12). In other words, as the hematite content increases and the goethite content decreases, the color of the agate changes from yellow, to orange, to red.
Fe compounds were also detected by the XRD analysis ( Figure 6). The absence of goethite in the XRD patterns may be ascribed to its low content, which was below the detection limit. Low as their contents are, hematite and goethite have high tinting strengths [77], which results in the abundance of colored zones in the Zhanguohong agates.
Some of the goethite-and hematite-bearing inclusions comprise several colored (yellow, orange, and red) bands parallel to the sandwich-like interlayers composed of microcrystalline silica with different micro-textures (Figure 5a,c), which could result from the self-purification process during silica crystallization [4,6,7]. The presence of spherical inclusions (Figure 5b) indicates that the fluids were locally gelatinous. It has been suggested that the Fe compounds in agates may originate from the weathering products of primary Fe-bearing phases [6,78]. Some of the Fe-bearing particles in the Zhanguohong agates are large and irregular (Figures 4c and 5d), which imply an allogenic origin. Overlain by the Yixian Formation, the Archean crystalline basement rocks are rich of iron deposits. It is reasonable to infer that these rocks act as the provenance of the Fe compounds in the agates. However, to ascertain the origin of these Fe compounds, further more detailed research (e.g., trace element and isotope geochemistry studies) needs to be conducted on the Fe-bearing minerals hosted in both the Archean crystalline basement rocks and the agates.

Origin of the Zhanguohong Agates
The formation temperature of agates hosted in volcanic rocks is still controversial [9,78]. A temperature range of 50-200 • C (probably >150 • C) was reported based on fluid inclusion and oxygen isotope studies [7,78,79]. With a high moganite content, agates are thought to be formed at a relatively high temperature (100-200 • C) ( [72] and references therein). Considering their high moganite contents (17-54 wt%) (Figure 11), Zhanguohong agates in the Beipiao district are more likely to have been precipitated from ore-forming fluids at temperatures of 100-200 • C.
It was suggested that ferruginous layered silicates (i.e., nontronite and lembergite) prefer to crystallize from Si-and Fe-bearing solutions with a negative Eh [80][81][82]. The absence of ferruginous layered silicates in the Zhanguohong agates indicates that the ore-forming fluids were oxidizing. Furthermore, the extensive occurrence of ferric minerals (hematite and goethite) in the agates confirms this oxidizing condition.
Experimental studies on synthetic Fe-bearing minerals reveal that the formation of hematite and goethite is strongly pH dependent in the temperature range of 100-200 • C at low pressures [83]. Hematite, goethite, and their mixtures can be precipitated in acidic, alkaline, and alkaline-neutral conditions, respectively [83,84]. With hematite, goethite and their mixtures respectively, the red, yellow, and orange layers in banded agates imply pH fluctuations between acidic and alkaline during the formation of the agates.
The agates in Beipiao generally exhibit concentric and rhythmic bands, and sometimes these bands are cut by microcrystalline silica veins (Figure 4a,c), which indicates multiple periods of fluid activity during the formation of the agates. This is also confirmed by the existence of interlayered macro-quartz bands at hand specimen and microscopic scales (Figures 2e and 4e), which are thought to have been formed during the final stage of silica precipitation in each period. Field investigations reveal that intense episodic volcanic activity occurred in Beipiao during the Early Cretaceous lithospheric extension in eastern China. The corresponding magmatic fluids probably greatly contributed to the multiperiod precipitation of the agates. During the intermittent crevasse eruptions of these volcanoes in the Cretaceous rift basins near Beipiao, a series of intermediate-felsic volcanic breccias were formed, which contained plenty of irregular cavities and syn-kinematic fractures. These cavities and fractures provided sufficient space for the migration of the fluids and the precipitation of the agates.
The Si-rich fluids derived from the syn-or post-magmatic activity migrated upward from a deep level and may have extracted Fe-bearing minerals from the Archean crystalline basement wall rocks. When the fluids flowed into the intermediate-felsic volcanic breccias in the Yixian Formation, which were at or near the Earth's surface, the confining pressure dropped significantly and the temperature decreased to less than or equal to 200 • C. Consequently, the ore-forming fluids became highly supersaturated with respect to silica [1], and microcrystalline silica precipitated rapidly on the walls of the cavities and fractures. With progressive precipitation, the degree of silica supersaturation, the crystallization rate of silica, and the density of the defects in the silica matrix decreased, which resulted in the decreasing moganite content and the increasing size of the quartz grains, from microcrystalline (micro-granular, fibrous, and/or jigsaw quartz) to macrocrystalline in the banded agates ( Figure 14). However, local oscillations in silica supersaturation [73,74] induced fluctuations in the moganite content ( Figure 14a). In addition, Fe-bearing inclusions were precipitated along some bands parallel to the sandwich-like interlayers composed of microcrystalline silica grains with different textures under sustained oxidizing conditions, which resulted from the self-purification process during silica crystallization ( Figure 14b). As the pH conditions of the fluids fluctuated between acidic and alkaline, the distinct red, yellow, and white bands in the Zhanguohong agates were formed ( Figure 14). These cavities and fractures provided sufficient space for the migration of the fluids and the precipitation of the agates. The Si-rich fluids derived from the syn-or post-magmatic activity migrated upward from a deep level and may have extracted Fe-bearing minerals from the Archean crystalline basement wall rocks. When the fluids flowed into the intermediate-felsic volcanic breccias in the Yixian Formation, which were at or near the Earth's surface, the confining pressure dropped significantly and the temperature decreased to less than or equal to 200 °C. Consequently, the ore-forming fluids became highly supersaturated with respect to silica [1], and microcrystalline silica precipitated rapidly on the walls of the cavities and fractures. With progressive precipitation, the degree of silica supersaturation, the crystallization rate of silica, and the density of the defects in the silica matrix decreased, which resulted in the decreasing moganite content and the increasing size of the quartz grains, from microcrystalline (micro-granular, fibrous, and/or jigsaw quartz) to macrocrystalline in the banded agates ( Figure 14). However, local oscillations in silica supersaturation [73,74] induced fluctuations in the moganite content ( Figure 14a). In addition, Fe-bearing inclusions were precipitated along some bands parallel to the sandwich-like interlayers composed of microcrystalline silica grains with different textures under sustained oxidizing conditions, which resulted from the self-purification process during silica crystallization ( Figure 14b). As the pH conditions of the fluids fluctuated between acidic and alkaline, the distinct red, yellow, and white bands in the Zhanguohong agates were formed ( Figure 14).

Conclusions
We conclude that the Zhanguohong agates occurring in the intermediate-felsic volcanic breccias of the Early Cretaceous Yixian Formation in the Beipiao district exhibit banded and massive structures. Featuring red, yellow and/or white colors, they are predominantly composed of α-quartz and moganite, with minor hematite and goethite.

Conclusions
We conclude that the Zhanguohong agates occurring in the intermediate-felsic volcanic breccias of the Early Cretaceous Yixian Formation in the Beipiao district exhibit banded and massive structures. Featuring red, yellow and/or white colors, they are predominantly composed of α-quartz and moganite, with minor hematite and goethite.
The moganite content (17-54 wt%) of the silica matrix fluctuated and decreased with varying degrees during the progressive precipitation of silica. The crystal defects and ferric iron in the microcrystalline silica grains probably contributed to the formation of moganite.
The ferruginous inclusions are mixtures of hematite, goethite, and silica phases. The red, yellow, and orange zones are rich in hematite, goethite, and their mixtures, respectively.
Also considering the results of previous studies, we infer that the Zhanguohong agates probably precipitated under sustained oxidizing conditions at temperatures of 100-200 • C and that the pH condition of the ore-forming fluids fluctuated between acidic and alkaline. The multiperiod precipitation of the agates probably resulted from the episodic volcanic activity during the intense Early Cretaceous lithospheric extension in eastern China.