Morphology of Barite Synthesized by In-Situ Mixing of Na 2 SO 4 and BaCl 2 Solutions at 200 ◦ C

: Barite is an abundant sulfate mineral in nature. Especially, the variety of morphologies of barite is often driven by the mixing of Ba-bearing hydrothermal ﬂuid and sulfate-bearing seawater around hydrothermal chimneys. In order to better understand the factors affecting the morphology and precipitation mechanism(s) of barite in seaﬂoor hydrothermal systems, we synthesized barite by a new method of in-situ mixing of BaCl 2 and Na 2 SO 4 solutions at 200 ◦ C while varying Ba concentrations and ratios of Ba 2+ /SO 42 − , and at room temperature for comparison. The results show that barite synthesized by in-situ mixing of BaCl 2 and Na 2 SO 4 solutions at 200 ◦ C forms a variety of morphologies, including rod-shaped, granular, plate-shaped, dendritic, X-shaped, and T-shaped morphologies, while room temperature barites display relatively simple, granular, or leaf-like morphologies. Thus, temperature affects barite morphology. Moreover, dendritic barite crystals only occurred at conditions where Ba 2+ is in excess of SO 42 − at the experimental concentrations. The dendritic morphology of barite may be an important typomorphic feature of barite formed in high-temperature ﬂuids directly mixing with excess Ba 2+ relative to SO 42 − .


Introduction
Barite (BaSO 4 ) is a ubiquitous mineral in the earth's crust [1,2]. Early Earth's marine environments were anoxic and sulfate deposits prevailed, resulting in barite as the dominant or the only sulfate mineral within bedded sulfate deposits older than 2.4 Ga [3][4][5][6][7]. Due to its high density and strong resistance to chemical weathering, barite is present throughout Earth's history and has the potential to preserve a record of conditions attending formation, which could be useful for interpretations of Earth's ancient rocks and paleoenvironments [8].
There are many ways of forming natural barite, including magmatic, biogenic, and hydrothermal genesis. Magmatic barite has been found in the Mianning rare earth deposit, Sichuan, Southwestern China [9]. In this deposit, barite is believed to have formed prior to or at the same time as massive quartz and fluorite, in the late stages of magma crystallization [10]. Biogenic barite occurs in various sedimentary environments, generally by combining barium with the skeletons of plankton [11,12]. Hydrothermal barite forms by the reaction of ore-forming hydrothermal fluids with surrounding rock [13,14]. Moreover, hydrothermal barite is the most abundant mineral in seafloor sulfate-sulfide deposits formed in back-arc basins, such as Kuroko, Mariana, and Okinawa deposits [15]. In-depth studies of the deposition mechanisms of sulfate and sulfide minerals in these deposits show that barite is formed by the rapid mixing of Ba 2+ -containing hydrothermal fluid with the surrounding SO 4 2− -rich seawater [16][17][18][19]. For example, barite in the hydrothermal vent of the Endeavour Segment, Juan de Fuca Ridge has a variety of morphologies, including plate, leaf, needle, or radiating cone shapes, and dendritic crystals [19].
On the other hand, many experimental studies on the precipitation of barite from aqueous solution have been carried out to investigate the precipitation kinetics of barite in hydrothermal environments [15,[20][21][22][23][24][25]. The results show that the morphology of barite varies greatly, depending on many factors. However, in previous experiments, bariumbearing solutions and sulfate-bearing solutions were mixed at room temperature, before heating to the desired temperature for crystal growth. This process is quite different from the process of barite precipitation from hydrothermal fluids since barite is believed to be formed by mixing a Ba 2+ -bearing solution with an SO 4 2− -bearing solution at high temperatures in natural hydrothermal systems [19]. Therefore, the previous experimental results may not be directly applicable to the precipitation of barite in hydrothermal vents.
In order to better simulate the formation process of barite, and better understand the factors affecting the morphology and precipitation mechanisms of barite in seafloor hydrothermal solutions, we synthesized barite by in-situ mixing of barium-bearing and sulfate-bearing solutions of variable concentrations and mixing ratios of Ba 2+ /SO 4 2− at 200 • C and discuss the effects of different mixing ratios of barium and sulfate solutions on the morphology of barite. We also compare the morphologies of barites synthesized by in-situ mixing of Na 2 SO 4 and BaCl 2 solutions at 200 • C with those grown at room temperature.

Materials
The reagents, anhydrous sodium sulfate (Na 2 SO 4 , 99.99%, metals basis) and barium chloride dihydrate (BaCl 2 ·2H 2 O, 99.99%, metals basis), used in the experiments are all produced by Shanghai Aladdin Biochemical Technology Co., Ltd. Solutions were prepared with ultra-pure water. An SO 4 2− concentration similar to that of seawater (0.03 mol/kg) was chosen. In addition, to investigate the effect of concentration on barite precipitation, a higher concentration of SO 4 2− (0.1 mol/kg) was also prepared. Ba 2+ solutions of similar concentration (0.03 mol/kg and 0.1 mol/kg) were also prepared.

Experimental Methods
Hydrothermal synthesis is a common method of synthesis [26][27][28]. Rather than mixing solutions at room temperature and then heating them to high temperatures for crystal growth, this study synthesized barite by a new method of in-situ mixing of BaCl 2 and Na 2 SO 4 solutions at 200 • C. The experiments utilized conventional stainless steel autoclaves with two Teflon bottle linings (Figure 1a). The height of the stainless steel autoclave is 70 mm and the diameter is 45 mm, while the heights of the outer Teflon bottle and the inner bottle are 55 mm and 30 mm, with diameters of 32 mm and 15 mm, respectively. The filling degree of the whole equipment is 47%. The sodium sulfate solution was first added to the inner Teflon bottle, and the barium chloride solution was added to the outer Teflon bottle, then the Teflon lining with these separated solutions was sealed within the stainless steel autoclave. The stainless steel autoclave with these starting solutions was maintained vertically while in the electric furnace to avoid solutions mixing at temperatures lower than 200 • C. After the temperature was raised to 200 • C and maintained for one hour, the electric furnace was rotated from vertical to horizontal, allowing the solutions in the inner and outer bottles to mix in-situ at 200 • C ( Figure 1b). All experiments were kept at 200 • C for 7 days. At the end of the experiment, the power was cut off and the run product was quenched. Note that our pre-experiment results showed that no barite was observed in the solutions in the inner and outer bottles when keeping the temperature at 200 • C for 7 days without rotation. Therefore, the barite produced in the high-temperature rotation experiment was formed by in-situ mixing of solutions at high temperatures. Teflon bottle, then the Teflon lining with these separated solutions was sealed within the stainless steel autoclave. The stainless steel autoclave with these starting solutions was maintained vertically while in the electric furnace to avoid solutions mixing at temperatures lower than 200 °C. After the temperature was raised to 200 °C and maintained for one hour, the electric furnace was rotated from vertical to horizontal, allowing the solutions in the inner and outer bottles to mix in-situ at 200 °C ( Figure 1b). All experiments were kept at 200 °C for 7 days. At the end of the experiment, the power was cut off and the run product was quenched. Note that our pre-experiment results showed that no barite was observed in the solutions in the inner and outer bottles when keeping the temperature at 200 °C for 7 days without rotation. Therefore, the barite produced in the hightemperature rotation experiment was formed by in-situ mixing of solutions at high temperatures. After quenching and cooling, the solution and products in the Teflon bottle were poured into the centrifugal tube and centrifuged to extract the precipitate. Then the precipitate was washed three times alternately with deionized water and anhydrous ethanol. Finally, the white precipitate was dried at 60 °C to obtain a white powder.
For the mixing experiments at room temperature, different mixing ratios of sodium sulfate solution and barium chloride solution were added into the Teflon bottle and stirred evenly to produce a white precipitate, and then the mixed solution was kept at room temperature for 7 days. The purification of the product used the same procedure as that of the in-situ mixing experiments at 200 °C.
We carried out 10 experiments of in-situ mixing of Na2SO4 and BaCl2 solutions at 200 °C and 10 experiments of mixing solutions at room temperature with 5 solution mixing ratios at two initial concentrations. The experimental conditions are shown in Table 1. After quenching and cooling, the solution and products in the Teflon bottle were poured into the centrifugal tube and centrifuged to extract the precipitate. Then the precipitate was washed three times alternately with deionized water and anhydrous ethanol. Finally, the white precipitate was dried at 60 • C to obtain a white powder.
For the mixing experiments at room temperature, different mixing ratios of sodium sulfate solution and barium chloride solution were added into the Teflon bottle and stirred evenly to produce a white precipitate, and then the mixed solution was kept at room temperature for 7 days. The purification of the product used the same procedure as that of the in-situ mixing experiments at 200 • C.
We carried out 10 experiments of in-situ mixing of Na 2 SO 4 and BaCl 2 solutions at 200 • C and 10 experiments of mixing solutions at room temperature with 5 solution mixing ratios at two initial concentrations. The experimental conditions are shown in Table 1.

Analytical Methods
A Renishaw inVia Reflex spectrometer system equipped with a standard confocal microscope was used for Raman spectral analysis at the Institute of Geochemistry, Chinese Academy of Sciences. A Renishaw diode-pumped solid-state laser provided 532 nm laser excitation with 5 mW power at the sample. An 1800 grooves/mm grating was used giving a spectral resolution of 1.2 cm −1 . Depolarized Raman spectra were obtained using 10 s integration times with 5 accumulations and a 50× Leica long working distance microscope objective, which focused the beam to a spot size of 1.6 µm. Wavenumber calibration was carried out using a silicon standard.
The scanning electron microscope (SEM) images were obtained using an FEI-Scanning Electron Microscope with Scios Dual Beam System at the Institute of Geochemistry, Chinese Academy of Sciences. The instrument was operated at an acceleration voltage of 5-20 kV, a beam current of 0.2-0.4 nA, and a working distance of 7-10 mm. The sample was dispersed in alcohol, dropped onto conductive glue, coated with gold, and then analyzed by a scanning electron microscope.
Powder X-ray diffraction analyses of barite were carried out using an Empyrean diffractometer (Malvern Panalytical) at the Institute of Geochemistry, Chinese Academy of Sciences. The maximum power of the cermet X-ray tube (Cu target) was 2.2 kW; the minimum step of the goniometer was 0.0001 • , and the working current was 40 mA, scanning speed was 10 • /min, and the scanning range was 10-70 • . The data obtained were analyzed with Jade software, and unit cell parameters were obtained.

Morphology of Barite Synthesized by In-Situ Mixing of Solutions at 200 • C
As shown in Figure 2, barite synthesized using 5 mixing ratios of Na 2 SO 4 and BaCl 2 solutions by in-situ mixing at 200 • C produced six different morphologies, including rodshaped, granular, plate-shaped, dendritic, X-shaped, and T-shaped crystals. The specific morphologies appearing in the mixed solution of different proportions are shown in Table 2.  Table  2.    At concentrations of Na 2 SO 4 and BaCl 2 solutions of 0.03 mol/kg, rod-shaped, granular, X-shaped, and T-shaped crystals appeared in all mixing ratios, while plate-shaped crystals appeared in the mixes of 1:1, 3:7, and 2:8 and dendritic shapes only appeared in mixes of 2:8. In addition, at concentrations of Na 2 SO 4 and BaCl 2 of 0.1 mol/kg, rod-shaped, granular, X-shaped, and T-shaped crystals appeared in all 5 mixing proportions, while plate-shaped crystals appeared in the mixes of 1:1, 3:7, and 2:8, and dendritic appeared in the mixes of 3:7 and 2:8.
The histograms of the barite particle size synthesized in mixes of 0.03 mol/kg Na 2 SO 4 and BaCl 2 with a ratio of 2:8 at 200 • C are shown in Figure 3. The plate-shaped barite with a particle size of 4.0-6.0 µm accounts for about 42% of the total, the rod-shaped barite with a particle size of 1.0-1.5 µm accounts for about 18% of the total, the granular barite with a particle size of 2.0-2.5 µm accounts for about 39% of the total, the dendritic barites with a particle size in the range of 1.5-2.0 µm and 2.0-2.5 µm account for about 26% of the total, respectively, the T-shaped barite with a particle size in the range of 3.0-4.0 µm and 4.0-5.0 µm account for about 33% of the total, respectively, and the X-shaped barite with a particle size of 5.0-6.0 µm accounts for about 33% of the total.
EER REVIEW 6 of 16 particle size in the range of 1.5-2.0 μm and 2.0-2.5 μm account for about 26% of the total, respectively, the T-shaped barite with a particle size in the range of 3.0-4.0 μm and 4.0-5.0 μm account for about 33% of the total, respectively, and the X-shaped barite with a particle size of 5.0-6.0 μm accounts for about 33% of the total.

Morphology of Barite Synthesized at Room Temperature
When the initial ion concentration is constant, the morphologies of the barite synthesized at room temperature with different mixing ratios are the same, and the change of the mixing ratio does not change barite morphology (Figure 4). The morphology of barite is leaf-like when the initial concentration of Na2SO4 and BaCl2 solution is 0.03 mol/kg; the morphology of barite consists of nanospheres when the initial concentration of Na2SO4 and BaCl2 solution is 0.1 mol/kg. And the particle size distribution of barite synthesized by mixed solutions at room temperature shows a normal distribution ( Figure 5).

Morphology of Barite Synthesized at Room Temperature
When the initial ion concentration is constant, the morphologies of the barite synthesized at room temperature with different mixing ratios are the same, and the change of the mixing ratio does not change barite morphology (Figure 4). The morphology of barite is leaf-like when the initial concentration of Na 2 SO 4 and BaCl 2 solution is 0.03 mol/kg; the morphology of barite consists of nanospheres when the initial concentration of Na 2 SO 4 and BaCl 2 solution is 0.1 mol/kg. And the particle size distribution of barite synthesized by mixed solutions at room temperature shows a normal distribution ( Figure 5). Crystals 2021, 11, x FOR PEER REVIEW 7 of 16

Raman Spectra
The Raman spectra of barite synthesized at room temperature and 200 °C are shown in Figure 6. The structure of barite is orthorhombic, of the Pbnm space group [29]. Each S atom coordinates with four oxygen atoms to form SO4 tetrahedra while each Ba 2+ coordinates with 12 oxygen atoms. The SO4 tetrahedra have Cs site group symmetry which theoretically has 9 degrees of vibrational freedom [30] (i.e., one nondegenerate (ν1), one doubly degenerate (ν2), and two triply degenerate modes (ν3 and ν4)). Moreover, one additional mode at 1104 cm −1 caused by SO4 tetrahedral distortion remained unassigned in this study. The Raman peaks below 400 cm −1 are classified as Ba-O12 vibrations. The observed patterns are consistent with those reported previously [31,32] and are listed in Table 3.
The Raman spectra of barite synthesized in these experiments are all similar. Moreover, changes in initial ion concentrations, mixing ratios, and temperature do not change from the positions of characteristic peaks of barite at room temperature ( Figure 6). The characteristic Raman peak positions of all synthesized barites are basically consistent with the characteristic peak positions of the pure barite Raman spectrum, and there are no other miscellaneous/unknown peaks. Therefore, Raman hasn't detected any impurities.

Raman Spectra
The Raman spectra of barite synthesized at room temperature and 200 °C are shown in Figure 6. The structure of barite is orthorhombic, of the Pbnm space group [29]. Each S atom coordinates with four oxygen atoms to form SO4 tetrahedra while each Ba 2+ coordinates with 12 oxygen atoms. The SO4 tetrahedra have Cs site group symmetry which theoretically has 9 degrees of vibrational freedom [30] (i.e., one nondegenerate (ν1), one doubly degenerate (ν2), and two triply degenerate modes (ν3 and ν4)). Moreover, one additional mode at 1104 cm −1 caused by SO4 tetrahedral distortion remained unassigned in this study. The Raman peaks below 400 cm −1 are classified as Ba-O12 vibrations. The observed patterns are consistent with those reported previously [31,32] and are listed in Table 3.
The Raman spectra of barite synthesized in these experiments are all similar. Moreover, changes in initial ion concentrations, mixing ratios, and temperature do not change from the positions of characteristic peaks of barite at room temperature ( Figure 6). The characteristic Raman peak positions of all synthesized barites are basically consistent with the characteristic peak positions of the pure barite Raman spectrum, and there are no other miscellaneous/unknown peaks. Therefore, Raman hasn't detected any impurities.

Raman Spectra
The Raman spectra of barite synthesized at room temperature and 200 • C are shown in Figure 6. The structure of barite is orthorhombic, of the Pbnm space group [29]. Each S atom coordinates with four oxygen atoms to form SO 4 tetrahedra while each Ba 2+ coordinates with 12 oxygen atoms. The SO 4 tetrahedra have Cs site group symmetry which theoretically has 9 degrees of vibrational freedom [30] (i.e., one nondegenerate (ν 1 ), one doubly degenerate (ν 2 ), and two triply degenerate modes (ν 3 and ν 4 )). Moreover, one additional mode at 1104 cm −1 caused by SO 4 tetrahedral distortion remained unassigned in this study. The Raman peaks below 400 cm −1 are classified as Ba-O 12 vibrations. The observed patterns are consistent with those reported previously [31,32] and are listed in Table 3.    The Raman spectra of barite synthesized in these experiments are all similar. Moreover, changes in initial ion concentrations, mixing ratios, and temperature do not change from the positions of characteristic peaks of barite at room temperature ( Figure 6). The characteristic Raman peak positions of all synthesized barites are basically consistent with the characteristic peak positions of the pure barite Raman spectrum, and there are no other miscellaneous/unknown peaks. Therefore, Raman hasn't detected any impurities.

Powder XRD
Although the temperatures, concentrations, and mixing ratios are different, the 2θ of the main peak of barite synthesized in our experiments are basically the same as those of the standard barite.

The Effect of Temperature on the Morphology of Barite
Temperature is the key factor affecting the morphology of barite. At el peratures, with all other reaction conditions the same, the morphology of the barites is different. In this study, the morphology of barite synthesized by m tions at room temperature is relatively simple, which is granular or leaf-like When the concentration of Na2SO4 and BaCl2 solution is 0.03 mol/kg, barite

The Effect of Temperature on the Morphology of Barite
Temperature is the key factor affecting the morphology of barite. At elevated temperatures, with all other reaction conditions the same, the morphology of the synthesized barites is different. In this study, the morphology of barite synthesized by mixing solutions at room temperature is relatively simple, which is granular or leaf-like (Figure 4). When the concentration of Na 2 SO 4 and BaCl 2 solution is 0.03 mol/kg, barite is leaf-like, and when the concentrations of Na 2 SO 4 and BaCl 2 solution are 0.1 mol/kg, barite crystals are small and granular. The morphology of the barite synthesized at 25 • C in a previous study [33] is similar to that of leafy barite synthesized at room temperature in this study. Barites synthesized at 150 • C by using barium chloride solutions and native sulfur have the morphologies of rod-shaped and X-shaped [10]. Barites synthesized in this study by in-situ mixing of Na 2 SO 4 and BaCl 2 solutions at 200 • C show six different morphologies, including rod, granular, plate, dendritic, X-shaped, T-shaped crystals, while barites synthesized by mixing barium solutions and sulfate solutions at room temperature and subsequent heating to 200 • C, show near-equiaxed granular crystals of well-developed crystal forms [34]. Therefore, the morphologies of barites change from small granular and leaf-like at 25 • C to rod-shaped and X-shaped at 150 • C, and rod, granular, plate, dendritic, X-shaped, T-shaped crystals at 200 • C, which shows the key control of the morphology of barite is temperature.

Effect of Solution Mixing Ratio on the Morphology of Barite
Precipitation of barite occurs when the product of concentrations of Ba 2+ and SO 4 2− solutions exceed the solubility constant [35]. Varying ionic ratios might be an effective and rather simple way of achieving a degree of crystal morphology control [33,36].
In this study, different solution mixing ratios change the dominant morphology of barite (Table 4). When the concentration of Na 2 SO 4 and BaCl 2 solution is 0.03 mol/kg, the dominant morphology of barite with mixing ratios of 8:2 and 2:8 is rod-shaped, the dominant morphology of barite with a mixing ratio of 7:3 is T-shaped, and the dominant morphology of barite with mixing ratios of 1:1 and 3:7 is granular. However, when the initial concentration of Na 2 SO 4 and BaCl 2 in solution increases from 0.03 mol/kg to 0.1 mol/kg, the dominant morphology of barite is rod-shaped regardless of the mixing ratio. In addition, when the mixing ratios of Ba 2+ to SO 4 2− solutions with the same concentration is greater than 7:3, the average particle size is increased (Figure 8). example, at higher initial concentrations of Na2SO4 and BaCl2, the dominant morphology of barite at the mixing ratio of 1:1 and 3:7 changes from granular to rod-like crystal (Table  4). Furthermore, at initial concentrations of Na2SO4 and BaCl2 of 0.03 mol/kg, dendritic barite only appears at mixing ratios of 2:8. However, dendritic barite appears when the concentration of Na2SO4 and BaCl2 increases to 0.1 mol/kg and the mixing ratio is 3:7 (Table 2), indicating that increasing Ba 2+ concentration favors the formation of dendritic barite.

Morphological Development of Barite
The morphological development of barite growing in solution is a complicated process. Previous studies indicate that the final morphology of barite depends on the competition of nucleation and crystal growth at different supersaturation ratios [37]. Moreover, the growth of crystals mainly depends on the growth conditions [40,41]. At lower reagent concentrations, homogeneous nucleation generates numerous nanoparticles, while the For barite synthesized by mixing Na 2 SO 4 and BaCl 2 solutions with the same concentration at room temperature, the morphology does not change with the initial solution mixing ratio (Figure 4).

Effect of Supersaturation on the Morphology of Barite
In many kinetic studies of precipitation, it was found that one of the key parameters for characterizing reacting systems is the supersaturation, S. In the case of barite, the supersaturation can be written as [37]: where a(Ba 2+ ) and a(SO 4 2− ) are the activities of Ba 2+ and SO 4 2− , respectively, K sp is the solubility product of barite. For barite, K sp is 9.82 × 10 −11 at 25 • C [8]. Previous work has demonstrated that at high reagent supersaturation ratios (the natural logarithm of supersaturation ratio, lnS = 16.97), a large number of very small spherical nanoparticles will be formed. At low reagent concentrations (lnS = 13.75), the leaf-like products will be formed [37]. Our experimental results are consistent with the previous research results. In our experiments, when the concentrations of Na 2 SO 4 and BaCl 2 are 0.03 mol/kg, barite is leaf-like, and at concentrations of Na 2 SO 4 and BaCl 2 are 0.1 mol/kg, barite is small and granular.
At 200 • C, the solubility product of barite is 4 × 10 −10 [38], which is larger than that at room temperature. Further, the degree of supersaturation with respect to barite can be changed by varying the barium chloride or sodium sulfate solution concentrations [33]. Dendritic crystals with rough surfaces formed from solutions of high barium chloride concentrations [39]. In our research, dendritic barite was also synthesized when the concentration of barium chloride was high. Figure 8 shows that although the synthesis temperature and the initial ratios are the same, the particle size of the synthesized barite varies with the initial concentration of Na 2 SO 4 and BaCl 2 in the solution. With increasing ion concentration, barite particle sizes increase, indicating a positive correlation between the initial concentration of Na 2 SO 4 and BaCl 2 solution and the particle size of the synthesized barite. At 0.1 mol/kg Na 2 SO 4 and BaCl 2 in solution, the average particle size of rod-shaped and T-shaped barite crystals increase, while the largest granular and X-shaped barite crystals, form in mixing ratios of 1:1. As plate-shaped and dendritic morphologies only form in mixtures of Na 2 SO 4 :BaCl 2 = 1:1 and 2:8, there is no comparison here. Table 4 shows the dominant morphology of barite changing when other reaction conditions are held constant (except for concentration). For example, at higher initial concentrations of Na 2 SO 4 and BaCl 2 , the dominant morphology of barite at the mixing ratio of 1:1 and 3:7 changes from granular to rod-like crystal (Table 4). Furthermore, at initial concentrations of Na 2 SO 4 and BaCl 2 of 0.03 mol/kg, dendritic barite only appears at mixing ratios of 2:8. However, dendritic barite appears when the concentration of Na 2 SO 4 and BaCl 2 increases to 0.1 mol/kg and the mixing ratio is 3:7 ( Table 2), indicating that increasing Ba 2+ concentration favors the formation of dendritic barite.

Morphological Development of Barite
The morphological development of barite growing in solution is a complicated process. Previous studies indicate that the final morphology of barite depends on the competition of nucleation and crystal growth at different supersaturation ratios [37]. Moreover, the growth of crystals mainly depends on the growth conditions [40,41]. At lower reagent concentrations, homogeneous nucleation generates numerous nanoparticles, while the growth and heterogeneous nucleation of the particles forms bridges to connect the particles, forming leaf-like crystals [37]. However, at high reagent supersaturation ratios, a short nucleation burst leads to the production of abundant spherical nanoparticles, which exhausts most of the reagent and limits the further growth of particles [42]. The morphological development of barite synthesized at room temperature in this study is consistent with this scenario (Figure 4).
However, the development of the morphology of barite formed by in-situ mixing BaCl 2 and Na 2 SO 4 solutions at 200 • C is more complicated. When mixing in situ at high temperatures, the nucleation and growth of barite occur simultaneously. There is no time to produce perfect crystal nuclei, and supersaturation in the environment around the crystal nucleus is high, so crystal nuclei tend to grow randomly, forming abundant defects. At these defect points, the crystal lattice is disturbed, and new layers begin growing [43]. Moreover, at higher temperatures the fluid state is unstable, and the atomic thermal vibration is strong, so Ba 2+ and SO 4 2− ions can obtain enough energy to overcome the fluid resistance and migrate rapidly. As a result, a wide variety of shapes form, including rod-shaped, granular, plate-shaped, dendritic, X-shaped, and T-shaped.
Moreover, the dendritic barite synthesized by in-situ mixing of 0.03 mol/kg Na 2 SO 4 and BaCl 2 at mixing ratios of 2:8 and 0.1 mol/kg Na 2 SO 4 and BaCl 2 at mixing ratios of 3:7 and 2:8 at 200 • C is very special. The diffusion-limited aggregation model is usually used to explain the formation of such dendritic crystals [40,41]. The core idea of the diffusionlimited aggregation model is that crystal growth is mainly limited by diffusion. During crystal growth, the surrounding growth points diffuse toward the center and "adhere" to the growth center, and all the particles that can diffuse to the crystal nucleus can grow continuously, thus forming a branching epitaxy and dendritic crystal [44]. According to the diffusion-limited aggregation model, the formation process of dendritic barite by in-situ mixing of BaCl 2 and Na 2 SO 4 solutions at mixing ratios of 3:7 and 2:8 at 200 • C can be briefly described as follows (Figure 9). crystal growth, the surrounding growth points diffuse toward the center and "adhere" to the growth center, and all the particles that can diffuse to the crystal nucleus can grow continuously, thus forming a branching epitaxy and dendritic crystal [44]. According to the diffusion-limited aggregation model, the formation process of dendritic barite by insitu mixing of BaCl2 and Na2SO4 solutions at mixing ratios of 3:7 and 2:8 at 200 °C can be briefly described as follows (Figure 9). It is well known that dendritic growth occurs under non-equilibrium conditions leading to morphologies that are not necessarily the most stable in terms of surface energy [45,46]. Due to the high ambient temperature of the crystal nucleus, the unstable fluid It is well known that dendritic growth occurs under non-equilibrium conditions leading to morphologies that are not necessarily the most stable in terms of surface energy [45,46]. Due to the high ambient temperature of the crystal nucleus, the unstable fluid state, the strong thermal vibration of atoms, and the high Ba 2+ concentrations, some crystal nuclei may preferentially grow. Therefore, granular crystals first develop into rod-like crystals (Figure 9b,c). With the progress of growth, on the one hand, the rod-shaped crystals grow along the long axis of the column, however, agglomeration points due to the high local concentration will be formed at some positions on the column surface [47]. Then agglomeration points on the column surface of the rod-shaped barite gradually uplift ( Figure 9d). As the nanocrystals continue to gather and grow, new growth fronts are created. Consequently, overgrowth occurs in the new growth front and branches gradually develop on the sides (Figure 9e). Finally, in a manner similar to growth patterns of trees, overgrowth occurs at some point on the branch, resulting in the formation of dendritic barite (Figure 9f).

Geological Implications
The formation and evolution of seafloor hydrothermal vents are often accompanied by barite precipitation [19]. On the seafloor, heat generated by deep magmatic activity drives seawater circulation along mid-ocean ridges, island arcs, and back-arc basins [48]. When the fluids circulate, heated fluids react with the surrounding rock, and vent onto the seafloor, forming hydrothermal chimneys and mounds. Where hydrothermal fluids are discharged, the mixing of hydrothermal fluids with the surrounding seawater drives mineral precipitation [49]. The outer wall of this porous chimney contains a typical series of low-temperature minerals (sphalerite, barite, etc.), which is formed by hydrothermal fluid cooling and/or mixing with seawater [50]. The precipitation of barite is formed by the mixture of Ba 2+ in hydrothermal fluid and SO 4 2− in seawater [35]. Various forms of barite form in hydrothermal vents at Endeavour, from well-formed tabular and bladed crystals to dendritic crystals [19]. Dendritic barite forms where the outer wall of the nozzle is in contact with the seawater, which is very similar to the barite synthesized by in-situ mixing of BaCl 2 and Na 2 SO 4 solutions at 200 • C in this study. Temperature measurements of fluid inclusions in barite from hydrothermal vents show formation temperatures of pressure-corrected inclusions are 180-240 • C, which is also close to the experimental temperature (200 • C) in this study. Combined with field observations and our experimental results, it is concluded that dendritic structures may be an important morphological feature of barite formed by in-situ mixing of seafloor hydrothermal solutions at high temperatures.
However, the mixing ratio of Na 2 SO 4 and BaCl 2 solutions for dendritic barite formation in our experiment are not exactly the same as that for dendritic barite formation in the hydrothermal vent at Endeavour. The dendritic barite is believed to be formed by mixing of Ba 2− -bearing hydrothermal fluids with the SO 4 2− -bearing surrounding seawater with a mixing ratio of 4:6 at 180-240 • C in hydrothermal vents at Endeavour [19]. In our experiments, the dendritic barite only occurred under the conditions of excess Ba 2+ relative to SO 4 2− . If we regard Na 2 SO 4 and BaCl 2 solutions at 200 • C as SO 4 2− rich hot seawater and Ba 2+ -containing hydrothermal fluid, respectively, more than 80% of the hydrothermal components are needed to form dendritic crystals. This hydrothermal component is much larger than that (40%) estimated in the hydrothermal vent of Endeavor [19], which means that the formation of dendritic barite in natural hydrothermal vents may be affected by other species in the fluids and further researches are needed.

Conclusions
Barite of a range of morphologies was synthesized at 200 • C and room temperature by in-situ mixing of BaCl 2 and Na 2 SO 4 solutions. The synthesized barite was analyzed by SEM, XRD, and Raman spectroscopy. The main conclusions of this study are as follows: The temperature has an obvious effect on the morphology of synthesized barite. Barite synthesized by in-situ mixing of BaCl 2 and Na 2 SO 4 solutions at 200 • C has rich morphologies, such as rod-shaped, granular, plate-shaped, dendritic, X-shaped, and T-shaped crystals, while the morphology of barite synthesized at room temperature is relatively simple; granular or leaf-like.
When barite was synthesized by in-situ mixing of BaCl 2 and Na 2 SO 4 solutions at 200 • C, the morphology of barite is affected by the initial mixing ratio. Dendritic barites will appear when Ba 2+ is in excess of SO 4 2− . In this study, dendritic barite only appeared at Ba 2+ /SO 4 2− molar ratios of 8:2 with initial concentrations of Na 2 SO 4 and BaCl 2 of 0.03 mol/kg or at Ba 2+ /SO 4 2− molar ratios of 8:2 and 7:3 and initial concentrations of Na 2 SO 4 and BaCl 2 of 0.1 mol/kg. Dendritic textures may be an important typomorphic feature of barite formed under the conditions of high-temperature fluid mixing, such as in seafloor hydrothermal systems. Dendritic barite is formed at the contact between the outer wall of the seafloor hydrothermal vent and seawater, which is very similar to the barite synthesized by in-situ mixing of BaCl 2 and Na 2 SO 4 solutions at 200 • C in this study.