Mineralogy and Magnetic Behavior of Yellow to Red Xuanhua-Type Agate and Its Indication to the Forming Condition

: Iron oxides/hydroxides are important magnetic minerals to provide information about changes in the forming environment. However, the magnetic behavior in agate has been rarely investigated. In this study, the magnetic behavior of the Xuanhua-type agate with intense yellow to red colors from the Xuanhua District (China) was investigated by temperature dependence of magnetic susceptibility, hysteresis loop, isothermal remanent magnetization and the analysis of remanent coercivity components from the gradient acquisition plot. Yellow goethite and red hematite can be quantitatively identiﬁed by XRD and Raman spectroscopy due to their relatively higher content. Results showed that the red, yellow and orange Xuanhua-type agate had different magnetic behavior, and magnetite existed in the yellow and orange ones. Fluid inclusions in such agate had the homogenization temperature of ~168 ◦ C to 264 ◦ C. All results suggested that the dehydration of goethite to form hematite was the main reason for the high remnant coercivity (above 1000 mT) of hematite in the red agate. The co-existence of magnetite and goethite in the yellow and orange agate reﬂects the transformation from Fe 2+ to Fe 3+ , indicating the change in the redox property of the environment. Unique patterns mainly formed by hematite and goethite make it a popular gem-material with high research value.


Introduction
Agate, formed by hydrothermal fluid filling the pores or cavities in igneous rocks, is mainly composed of α-quartz with minor impurities. Among its multiple colors, the range from yellow to red is usually caused by iron oxides/hydroxides (especially hematite and goethite) [1,2] which show weak magnetic properties at ambient conditions. Iron oxides/hydroxides are widely distributed in nature and common in soil and rocks, among which goethite and hematite are more thermodynamically stable at ambient temperature and are often the end members of many transformations [3]. The magnetic characteristics of these iron oxides/hydroxides have been used to provide information on the change in the environment and climate, genesis and trace mineral identification [4][5][6][7][8][9][10][11][12]. However, most studies concerning the rock magnetic properties of iron oxides/hydroxides focus on natural single mineral aggregates, synthesized minerals, or the minerals sieved and selected from soil and sediments [13,14]. There lacks investigation on the magnetic behavior of iron oxides and hydroxides in agate. However, because the main mineral component is diamagnetic quartz and a few kinds of Fe-bearing minerals, yellow to red agate can be regarded as an excellent material for studying the magnetic behavior of iron oxides/hydroxides formed in the hydrothermal environment. However, due to their low content under the detection limit of XRD, iron oxides/hydroxides in most agate are usually identified by a laser Raman spectrometer which cannot provide quantitative data of their content. Therefore, there is a need for more investigation to find the relationship between magnetic behavior, the variety and the content of iron oxides/hydroxides in such an agate.
Nevertheless, a new variety of agate with intense yellow to red colors was found in the Xuanhua District (in Zhangjiakou, Hebei province, China) in recent years and is commercially called Xuanhua agate in the Chinese jewelry market. Its intense color indicates a high content of iron oxides/hydroxides, making such agate an ideal sample for discussing the magnetic behavior of yellow to red agate. A few studies of Xuanhua agate have been performed and have confirmed the presence of hematite and goethite. However, these investigations mainly focused on the gemological properties, microstructure features and geological characteristics [2,[15][16][17][18]. The magnetic behavior of iron oxides and hydroxides in Xuanhua agate and its indication to the formation condition need further discussion.
In this study, magnetic behavior of yellow to red agate was investigated by the temperature dependence of magnetic susceptibility (χ-T curves), hysteresis loop, isothermal remnant magnetization (IRM) and the analysis of remnant coercivity components (based on IRM curves). Based on the information of mineral composition and the content obtained from the Raman spectra and X-ray diffraction patterns as well as the temperature measuring of fluid inclusions, the indication of these magnetic characteristics to the forming condition of yellow to red agate from Xuanhua District was further discussed.

Materials and Methods
All investigated samples were collected from the Xuanhua agate deposit. The area of this deposit is about 20 km 2 , located near Dishuiya Village, Xuanhua District, Zhangjiakou, Hebei Province, China, and belongs to the northern margin of the North China Craton. The subduction of the palaeo-Pacific plate to the Eurasian plate was intense from the Late Jurassic to Early Cretaceous period, and a series of intermediate-acid igneous rocks were formed by volcanic eruptions on the surface [19][20][21]. The strata of the Xuanhua agate mining area mainly consists of the Jurassic Tiaojishan formation (J 2 t 1~J 2 t 5 from bottom to top), the Jurassic Houcheng formation (J 3 h), the Cretaceous Zhangjiakou formation (J 3 z), and the Cenozoic Quaternary loess (Q 4 ) ( Figure 1). Xuanhua agate occurs as an amygdaloid, ellipsoidal or vein shape in the pores or fractures of trachyte of the Jurassic Tiaojishan formation [17,18]. The agate-bearing terrane is about 3.5 km in length of EW, 3.3 km in width of SN and several meters deep ( Figure 2a).
Compared with other agate, the intense colors (mainly red, yellow and orange) and low transparency (translucent to opaque) are the outstanding feature of the Xuanhua agate ( Figure 2b). In this investigation, five representative samples numbered XH-05 (yellow), XH-02-Y (intense yellow), XH-02-R (orange), XH-D-01 (red) and XH-D-02 (intense red) were selected from a large collection of Xuanhua agate samples. These samples were shaped into different forms for different experiments: a cube with the length of 1 cm for the measurement of hysteresis loop and IRM, powder with the grain size under 74µm for the experiments of χ-T curves and X-ray diffraction, and some pieces of slice with the thickness about 30µm for microscopic observation.
Mineral components were determined through Raman spectroscopy and X-ray diffraction. The Raman spectroscopy study was performed by a Renishaw-inVia microscope at room temperature, with a laser source of 532 nm at 10 mW in mode laser power at 100%, the Raman shift range from 100 to 2000 cm −1 , and testing spot size of 1µm. X-ray diffraction patterns were obtained by Bruker D8 Advance X-ray power diffractometer, with the angle range from 10 • to 65 • with a step length of 0.02 • /step and scanning speed of 0.25 • /min. The mineral contents were calculated by the relative intensity ratio of diffraction peaks of different phases and the detection limit of this diffractometer is 0.5 wt.%. Microphotographs of magnetic minerals were captured by OLYMPUS BX51 polarized microscope with magnification of 500×.  Temperature dependence of magnetic susceptibility (χ-T curve) was performed in an Ar atmosphere using KLY-3S Kappa bridge with the CS-3 temperature control system. Samples were heated from 25 • C to 700 • C and cooled back to 25 • C at a rate of 12 • C/min. The heating and cooling curves can provide information on the mineral alternation of the samples.
Hysteresis loops, acquisition curves of isothermal remnant magnetization (IRM) and their back-field demagnetizations were measured with MicroMag Model 3900 Vibrating Sample Magnetometer (VSM, sensitivity = 0.5 × 10 −9 Am 2 ) up to a maximum field of 1.5 T with a field increment of 2 mT and averaging time of 200 ms. The saturation magnetization (Ms), the saturation remnant magnetization (Mrs) and coercivity (Hc) were acquired from hysteresis loops after the correction for the paramagnetic slope (at 70% of the maximum field). The remnant coercivity (Hcr) was obtained from IRM curves which can be transferred to the linear acquisition plot(LAP) and the gradient acquisition plot(GAP) to offer information about remnant coercivity components.
Fluid inclusion measurements were completed with the Linkam THMSG600 heatingfreezing stage attached to an Olympus BX51 microscope equipped with 5×, 10×, 50× and 100× long working distance Olympus objectives. The stage was capable of measurements in the range of −196 • C to 600 • C. The temperature increased at the rate of 0.2 • C/min until total homogenization after the gas in the fluid inclusions turn smaller and the results were within the error of ±0.2 • C. Prior to carrying heating and cooling measurements, the stage was calibrated with synthetic H 2 O and international pure H 2 O inclusion standard. The homogenization temperature was recorded after each experiment.

Major Mineral Components
Under polarized microscope, yellow to red minerals are in dot shape with submicron to micron size (Figure 3a-c). Raman spectra show that the agate samples mainly consist of α-quartz. In addition, the red, yellow and orange colors are separately caused by the presence of hematite, goethite and the mixture of hematite and goethite (Figure 3d

Hysteresis Loop
Red samples XH-D-01 and XH-D-02 (Figure 4f,g) which contain hematite, do not achieve the saturation in the field up to 1.5 T and show the coercivity values of 324 mT and 437.8 mT, respectively. The coercivity values and the shape of the hysteresis loop are similar to those of natural hematite in soil and sediments [14].
In contrast, yellow samples XH-05 and XH-02-Y (Figure 4h,i) which contain goethite, reach the saturation before 300 mT and have the coercivity values of 3.017 mT and 3.281 mT, respectively. The coercivity values and the shape of the hysteresis loop are different from the high coercivity of goethite but similar to those of superparamagnetic magnetite [22].
However, the orange sample XH-02-R (Figure 4j) has the wasp-waisted hysteresis loop with the coercivity of 148.9 mT, which results from the mixture of high and low coercivity components [23].
The coercivities of magnetic minerals were found to be linked with the grain size of magnetic particles [14,24]. In this study, the coercivity values of two red samples are 324 mT (sample XH-D-01) and 437.8 mT (sample XH-D-02) measured at room temperature, indicating their average grain size of hematite is likely to be in the range of 0.3~3 µm [14], which is consistent with the grain size from microscopic observation (Figure 3a-c).

Temperature Dependence of Magnetic Susceptibility (χ-T Curves)
For red samples XH-D-01 and XH-D-02 (Figure 4k,l), their heating curves between 580 • C and 700 • C show obvious decreasing trends, indicating there is magnetism transformation when it is approximately 700 • C. The Néel temperature of hematite is around 685 • C [25,26] so it can be inferred that there exists some hematite after 580 • C, and this hematite reaches its Néel point when the temperature is approximately 700 • C. However, the cooling curves remain nearly unchanged with lower susceptibility at this range but show the Hopkinson effect and the Curie point of magnetite. These changes suggest that hematite exists before the heating process and its transformation into magnetite during the heating process results in the increase in magnetic susceptibility after experiment at room temperature.
Compared with hematite, the magnetism of goethite is much weaker and the Néel temperature of goethite is usually between 60 • C and 130 • C [27,28]. Due to the low content, it is difficult to find the sign of the Néel point of goethite in both yellow samples XH-05/XH-02-Y and the orange sample XH-02-R from their χ-T curves (Figure 4m-o). However, the different values of susceptibility before and after heat treatment are much higher in yellow samples than those in red samples. The Hopkinson effect is also obvious in yellow and orange samples.
Goethite dehydrates to hematite at 200 • C~400 • C [3] and it was verified that hematite can be transformed into magnetite when heated in an environment with an oxygen fugacity under 10 −18 [29,30]. This experiment is performed in an argon atmosphere with low oxygen fugacity which can be regarded as a reducing atmosphere [31], making it possible that hematite and goethite in agate can be converted into magnetite. The color change in investigated samples from yellow/red (common color of goethite and hematite, respectively) to black (common color of magnetite) also verifies this process.
In addition, there is a relationship between the variation degree of susceptibility (the difference value of susceptibility of unit mass samples before and after the χ-T experiment) and the content of hematite and/or goethite (obtained by XRD analysis). Due to the transformation of goethite during the heating process, the variation degree of susceptibility before and after the χ-T experiment among unit mass agate samples with different colors has the following feature ( Figure 5): yellow agate (goethite) > orange agate (hematite + goethite) > red agate (hematite). For samples with the same color, the variation degree shows positive correlation with the detected content of hematite and goethite.

Isothermal Remnant Magnetization and Remnant Coercivity Components
For the red samples (Figure 6a,b), the acquisition curves of isothermal remnant magnetization (IRM) increase continuously and does not saturate before 1.5 T, indicative of the contribution of high-coercivity magnetic minerals. However, the IRM acquisition curves of two yellow samples (Figure 6c,d) increase rapidly in the low field and almost saturates before 100 mT, which suggests the existence of low-coercivity magnetic minerals. As for the orange sample (Figure 6e), the curve can be divided into two stages due to the contribution of low-and high-coercivity magnetic minerals: (1) the rapid increase before 100 mT reflects the contribution of low-coercivity magnetic minerals; (2) increase in IRM after 100 mT without saturation at 1.5 T indicates the presence of high-coercivity magnetic minerals. The IRM acquisition curves can be unmixed into magnetic components using the package developed by Kruiver et al. [32]. Figure 6 shows the linear acquisition plot (LAP) (Figure 6f-j) transformed from IRM acquisition curves and the gradient acquisition plot (GAP) (Figure 6k-o) which is the first-order derivation of LAP. The unmixing results clearly suggest that the red samples contain only one magnetic component with high remnant coercivity over 700 mT (Figure 6k,l), while the yellow samples contain one magnetic component with low remnant coercivity below 100 mT (Figure 6m,n). However, the orange sample shows the mixture of a high remnant coercivity component and a low one (Figure 4o), which can confirm the analysis result of its hysteresis loop.

Formation Temperature of Xuanhua-Type Agate
Fluid inclusions in minerals can provide information of its forming condition. It is hard to find fluid inclusions in quartz fibers of agate. However, the quartz druse in the core of agate forms during the last stage, and its fluid inclusions can offer information about the lower limit of agate formation temperature. Based on observation, inclusions in the quartz druse of Xuanhua agate are generally small and the gas-liquid inclusions are relatively rare compared with pure liquid inclusions. In this study, gas-liquid inclusions easy to observe were selected for homogenization temperature measurement ( Figure 7). As is shown in Table 1, the homogenization temperature falls in between 168.8 • C and 264.0 • C, relatively higher than that of most agate [33][34][35].  gas-liquid 6 × 7 10 178.8 Figure 7c gas-liquid 12 × 17 10 216.3 Figure 7d gas-liquid 3 × 4 20 264.0

Discussion
For red Xuanhua agate samples whose color are caused by the presence of hematite, the Néel point of χ-T curve near 700 • C and the characteristics of the hysteresis loop are similar to natural hematite. Their IRM curves and gradient acquisition plots also show the presence of only one magnetic mineral component with high remanent coercivity. Hematite can directly form in the aqueous solution containing iron ions at room temperature or be produced by the transformation of other iron oxides/hydroxides as temperature increases [3]. However, the homogenization temperature measurement of gas-fluid inclusions in this study indicates the formation temperature of these red Xuanhua agate samples is likely to be higher than the temperature range of 168.8 • C~264.0 • C.
The remanence coercivity has been reported to be usually under 1000 mT for natural hematite (with the size of <0.25~10 µm) formed in soil and sediments [36]. In this study, for the red and orange agate samples, the remnant coercivity values obtained from the hysteresis loops indicates the grain size of their hematite is likely to be in the range of 0.3~3 µm. Besides, the remanence coercivity of such agate is approximate to (B1/2 = 707.9 mT for XH-D-02) or much higher (B1/2 = 1584.9 mT for XH-D-01; B1/2 = 1000 mT for XH-02-R) than that of natural hematite with a similar grain size (usually under 1000 mT). It was confirmed that the remanence coercivity of fine-grained hematite derived from goethite (dehydration of goethite between 200 • C~400 • C) often has very high remnant coercivity which is usually above 1 T [3,13,14,24]. Therefore, it can be inferred that the hematite with a high remanence coercivity (above 1000 mT) in red agate XH-D-01 and orange agate XH-02-R is likely to be mainly formed by dehydration from goethite, indicating the temperature of the fluid forming such agate probably once reached above 200 • C which can be verified by the relatively higher homogenization temperature of fluid inclusions in Xuanhua agate.
Yellow and orange agates show no obvious magnetic characteristics of goethite, due to its low content and the weak magnetism. However, from the behavior of hysteresis loop, IRM acquisition curves and the analysis of coercivity components, there is a magnetic mineral with low coercivity (e.g., magnetite or maghemite) [37,38] and extremely low content (under the detection limit of XRD) in such agate. In view of the formation of agate which usually occurs in the closed cavity or pores of surrounding rocks filled by hydrothermal fluid, iron oxides/hydroxides are regarded to be formed during the formation of quartz fibers of agate [2]. For maghemite, it is usually formed by the transformation of other iron oxides/hydroxides above 200 • C but the phase transformation of goethite also occurs above 200 • C. However, magnetite can be formed with goethite simultaneously at an ambient temperature [39,40]. Besides, magnetite was found in the host rocks of such agate samples ( Figure 8). Therefore, it can be inferred that the magnetic mineral with low coercivity is magnetite, and its existence confirms the transformation of Fe 2+ to Fe 3+ during the formation of agate. This transformation indicates the redox property of the forming environment changed during agate formation, which produces various magnetic minerals [3,40]. Based on the unique characteristics, the yellow to red agate from the Xuanhua District can be categorized as the Xuanhua-type agate. It is noted that the Xuanhua-type agate is a very popular gem-material. It may have a very bright color in red and yellow, and the two colors are various, occasionally occupying the entire view. Most cut pieces have a certain texture formed by varied components of red hematite and/or yellow goethite, such as dendritic goethite, banded hematite and goethite, displaying a contrasting, changeable and charming landscape, with animal-like and other patterns (Figure 9). The conditions for the formation of iron oxides and hydroxides are determined mainly by variations in the redox potential and pH of the medium [3]. These patterns within the host agates were likely formed by different temperature, pressure, f O 2 and pH values, and compositions of the parent fluids for formation of each certain individual agate, making the Xuanhua-type agate high in research value in gemology.

Conclusions
The magnetic behavior of iron oxides/hydroxides with a relatively higher content in yellow to red Xuanhua-type agate can provide some information about the formation condition, which along with the simple mineral composition makes such agate an excellent material for the study of a hydrothermal environment. The hysteresis loop, isothermal remnant magnetization and analysis of remanent coercivity components offer more nondestructive methods to discuss the genesis of agate and hydrothermal environments rich in iron oxides. All results suggested that the dehydration of goethite to form hematite was the main reason for the high remnant coercivity (above 1000 mT) of hematite in the red agate. The co-existence of magnetite and goethite in the yellow and orange agate reflects the transformation from Fe 2+ to Fe 3+ , indicating the change in the redox property of the environment. The indication of the magnetic characteristics of iron oxides in agate to the formation condition not only develops the application of rock magnetism to gemology, but also provides more methods to probe into the genesis of different hydrothermal environments. The relationship between the formation environment and the content and magnetic characteristics of iron oxides/hydroxides was preliminarily discussed in this study. However, more samples with similar features from different deposits are required to enrich the conclusion. Changing the forming condition influenced by the external environment produces various and charming patterns mainly displayed by red hematite and yellow goethite aggregates. These patterns make the Xuanhua-type agate a very popular gem-material with high research value. Data Availability Statement: Data available on request due to restrictions privacy. The data provided in this study can be obtained at the request of the corresponding author. As the data needs further research, the data is currently not publicly available.