The Effect of Different Outer Cations on the Stability of Fluorotitanium Complex

Abstract

Fluoride-rich fluid is believed to be able to activate and migrate Ti and other high field-strength elements to the greatest extent. The stability of F-rich titanium complexes can ensure their migration in the fluid, but is inseparable from the physical and chemical properties of the fluid, such as concentration, temperature and pH value—important factors affecting the stability of the complexes. In this study, the influence of the outer cationic complex fluid on the stability of the fluorine titanium complex was studied. Studies were based on different kinds of fluorine titanium complex (K₂TiF₆, Na₂TiF₆, (NH₄)₂TiF₆ and H₂TiF₆) in 100 MPa pressure. Under the condition of 200~500 °C temperature, we found that as the temperature rises, the hydrolysis of F-rich titanium complexes is violent. We compared the stability of four F-titanium complexes with different outer cations according to the hydrolysis rate and the cumulative hydrolysis equilibrium constant. We compared the F-titanium complexes with alkali metal as the outer cations that are more stable, such as K₂TiF₆ and Na₂TiF₆. However, the F-rich titanium complex in an acidic fluid is relatively unstable, which is not conducive to the migration of Ti elements. Due to the water–rock reactions that occur in hydrothermal fluid migration, mixing and alteration, once in the hydrothermal system, the fluid composition, pH value and temperature change. Thus, the F-titanium complex becomes extremely unstable, leading to the precipitation of titanium from the hydrothermal fluid and the growth of Ti-rich minerals.

1. Introduction

In hydrothermal mineralization, the existence and migration of ore-forming elements have always been of interest. This geological process is very conducive to the existence of elements in the form of complexes due to the high-temperature and high-pressure conditions, complex material components, high concentration of volatile components, high density and other characteristics [1]. Metallic elements mainly exist and migrate in the form of coordination and polyacid complexes in most geological processes dominated by fluid [1,2,3,4,5,6]. In the process of element migration, metal elements precipitate and accumulate if the external environmental changes (e.g., temperature, pressure and pH) affect the stability of the complexes [7,8,9]. Therefore, it is of great significance to study the stability and hydrolysis behavior of complexes under high temperature and pressure to guide the migration and enrichment of elements in deep geological processes and improve metallogenic theory.

The activity and circulation of Ti and other high field strength elements (HFSE) in deep geological processes are often limited by the solubility of mineral carriers, such as rutile [10,11,12,13,14,15]. However, HFSE is generally considered to be the most inert metal element in the fluid, which has been confirmed by a large number of rutile solubility experiments. For example, the solubility of rutile in deionized water is less than 100 μg/mL; although it is significantly improved in the system with NaCl or silicate, it is also less than 5000 μg/mL. Ti and other HFSEs in fluorine-containing fluids or melts are prone to forming coordination complexes of the fluorine-containing acid group and cause large-scale migration with the formation of coordination [7,13,16,17]. Some geological phenomena also support this view, such as the existence of daughter minerals rich in La, Ce, Zr, Ti and Nb [18]. At the same time, rutile particles are also common in natural high-pressure veins and often coexist with F-rich minerals (such as fluorapatite, fluorite, etc.) [19]. Many studies show that rutile easily dissolves in F-rich fluid in different environments, forming fluorine-containing acid coordination complexes, and HFSE in rutile enters the fluid with the dissolution of rutile [7,13,16,17,19,20]. However, it is not clear whether the different outer cations have an influence on the stability of the coordination complexes with the fluoric acid group and on the migration and activation of elements. The dissolution and migration of sedimentary elements is a dynamic process [21], and the influencing factors are various. Therefore, we take advantage of hydrolysis experiments of potassium fluotitanate (K₂TiF₆), sodium fluotitanate, fluotitanate acid and amine fluotitanate under high temperature and pressure to study their hydrolysis rules and compare and analyze the effect of different outer cations on the fluotitanate complex’s stability. Thereupon, we can have a more comprehensive understanding of Ti migration in the fluid of geological processes.

2. Experiment and Analysis Method

2.1. Experiment Principle

Under high temperature and pressure, we use K₂TiF₆, Na₂TiF₆, (NH₄)₂TiF₆ and H₂TiF₆ as the initial experiment by observing the degree of hydrolysis in the hydrothermal solution to evaluate the migration behavior of Ti in natural geological processes. The four compounds all have a fluoric complex anion-[TiF₆]²⁻ and the coordination structure of six fluorine; this is the most stable fluorotitanium inorganic complex with fluorinated coordination [20,22] in which ionization and hydrolysis take place in water step by step. The whole process is summarized by the following reaction formula:

[TiF₆]²⁻ ⇆ [TiF₅]⁻ + F

(1)

[TiF₆]²⁻ + H₂O ⇆ [Ti(OH)F₅]²⁻ + HF

(2)

[Ti(OH)F₅]²⁻ + H₂O ⇆ [Ti(OH)₂F₄]²⁻ + HF

(3)

[Ti(OH)₂F₄]²⁻ ⇆ [Ti(OH)₂F₃]⁻ + F

(4)

[Ti(OH)₂F₃]⁻ ⇆ [Ti(OH)₂F₂]⁰ + F

(5)

[Ti(OH)₂F₂]⁰ + H₂O ⇆ [Ti(OH)₃F]⁰ + HF

(6)

[Ti(OH)₃F]⁰ + H₂O ⇆ [Ti(OH)₄]⁰ + HF

(7)

[Ti(OH)₄]⁰ ⇆ TiO₂↓ + 2H₂O

(8)

Under certain conditions, these complexes coexist stably and have a dynamic equilibrium of chemical reactions. However, for the fluotitanate complex, the ultimate product is the precipitate. When Ti is separated from fluid in a large precipitate, it is difficult for the precipitate to redissolve at certain temperatures and pressures [7]. The corresponding complex of fluotitanate remains in fluid; hence, the hydrolysis degree of the fluotitanate complex can be expressed by the precipitate amount or surplus quantity. Moreover, the precipitation capacity or surplus quantity of the fluotitanate complex is the powerful outcome of the migration and enrichment of Ti.

We study the stability change process of titanium complexes in high temperature and pressure conditions by observing the hydrolysis degree of X₂TiF₆ (X represents different cations), as the hydrolysis degree of X₂TiF₆ is positively correlated with temperature and negatively correlated with the initial concentration of X₂TiF₆, but not sensitive to pressure. Therefore, we conduct the high temperature and pressure reaction by controlling the concentration of the initial reactant to 0.02 mol/L (the concentration of Ti is 960 μg/mL) at 100 MPa. By controlling the temperature, the pattern of stability is obtained, and we can draw the corresponding conclusions. According to the reaction processes of (1)–(8), we can synthesize the reaction into an ideal reaction equation:

X₂TiF₆ + 2H₂O → TiO₂↓ + 4HF + 2XF

(9)

With the increase in reaction temperature, X₂TiF₆ undergoes significant hydrolysis and produces a large number of TiO₂ precipitates within a certain limit. Therefore, we can determine the stability of the fluorotitanium complex by measuring the reaction degree under the set condition of the special temperature gradient we set at first.

2.2. Experimental Procedures

The research work was completed in the hydrothermal Laboratory of the high temperature and high-pressure experimental platform of Guangzhou Institute of geochemistry, Chinese Academy of Sciences, Guangzhou, China. Firstly, we prepared the initial substance of 0.02 mol/L K₂TiF₆ and (NH₄)₂TiF₆ solution directly by adding solid matter up to theoretical standards dissolved in deionized water under normal temperature and pressure with no obvious precipitation. As for Na₂TiF₆ solution, we added HF solution to the liquor to effectively increase the solubility of solid substances since the Na₂TiF₆ solid is not completely soluble in deionized water when preparing 0.02 mol/L low concentration Na₂TiF₆ solution. Therefore, the Na₂TiF₆ initial solution was made by dissolving 0.416 g Na₂TiF₆ solid into mixed liquor of 97 g HF with 3 g deionized water. The 0.02 mol/L H₂TiF₆ solution was prepared by adding 0.5467 g 60% H₂TiF₆ solution into deionized water to 100 g. When all the initial substances did not precipitate obviously at normal temperature and pressure, which laid a foundation for the following experiments, the initial solution was welded and sealed in the gold tube with a diameter of 4 mm and a length of 2–3 mm. The sealed gold tube was kept in an oven at 110 °C for two hours. If there is no mass loss, it means that the gold tube is well welded and sealed, which can be used for the next high-temperature and high-pressure experiment.

The equipment used in the diagenetic and metallogenic simulation system at the high temperature and pressure experiment stage is a kind of device in which the temperature can reach 500 MPa, and the temperature can reach 950 °C. The device includes six vertical Tuttlle-type cold sealed hydrothermal autoclaves with water as the pressure transmission medium. We put the sample tube into autoclaves and filled the remaining space with nickel rod, then used the inflator to set the pressure established by experimentation and used the furnace to raise the temperature of the autoclave to the temperature of the experiment. The conditions of the experiment formulated by temperature and pressure (temperature: 200–500 °C, pressure: 100 MPa) were within the effective range of the experimental device, and the simulations were effective; therefore, it is feasible to carry out this experimental research with this experimental device.

After the experiment, we stopped the heating and then used the ice bucket to quench the autoclave quickly; this can prevent the possibility of tube explosion and reverse dissolution. After that, we took out the sample tube, used a disposable needle tube to extract the upper clear liquid and centrifuged it at 4000 rpm for 15 min, then diluted the solution 1000 times to be tested.

The diluted solution was sent to the test center of Sun Yat-sen University (Guangzhou, China) to use the Thermo Fisher iCAP Q type inductively coupled plasma mass spectrometer to test and analyze Ti element concentration.

2.3. Experimental Result

We can realize the stability of the fluorotitanium complex under the objective condition by testing the reaction degree under the setting condition with a special temperature gradient. The main characterization parameters are hydrolysis rate and cumulative hydrolysis constant. The calculation of hydrolysis rate is as follows:

Hydrolysis rate = (initial Ti concentration − Ti concentration after reaction)/initial Ti concentration × 100%

The cumulative hydrolysis equilibrium constant can be the ideal parameter of the token stability of the complex [7,23,24]. As the concentration of each substance is relatively low in the reaction, and we do not have the activity coefficient data under the high temperature and pressure conditions, the activity coefficient of solution is approximately equal to 1. Therefore, the cumulative hydrolysis equilibrium constant is the constant of reaction (9), where we replace the activity degree with the concentration of substance mass:

$$\mathrm{K}=\frac{{\mathrm{C}}_{\mathrm{HF}}^4\times {\mathrm{C}}_{{\mathrm{F}}^{-}}^2}{{\mathrm{C}}_{{\mathrm{TF}}_6^{2-}}}$$

(10)

The cumulative hydrolysis equilibrium constant K is only controlled by the kinds of complex, temperature and pressure. Temperature plays a decisive role, and according to previous hydrolysis experiments, the effect of pressure on the hydrolysis constants of these complexes is minimal [24,25,26]. Accordingly, there is a linear relationship between −lnK and 1/T [7]:

$$-\mathrm{lnK}=\frac{{\mathsf{\Delta }}_{\mathrm{r}}{\mathrm{H}}_{\mathrm{m}}^{\mathsf{\theta }}}{\mathrm{RT}}-\frac{{\mathsf{\Delta }}_{\mathrm{r}}{\mathrm{S}}_{\mathrm{m}}^{\mathsf{\theta }}}{\mathrm{R}}$$

(11)

where R is the ideal gas constant, about 8.314 kJ/(mol∙K). Due to the above quantification process, we can quantify the stability of the titanium complex of several initial reactions in the experiments. We calculated the hydrolysis rate and cumulate hydrolysis equilibrium constant of each substance in the corresponding temperature gradient by processing the reaction data of each substance measured in the test center of Sun Yat-sen University, Guangzhou, China. The experimental result can be seen in

Table 1. Hydrolysis results of different F-Ti complexes for various temperatures at 100 MPa (>12 h).

Sample Type	Initial Concentration	Temperature	Pressure	Post Hydrolytic Concentration	Hydrolysis Rate	(−lnK)
	(Ti, μg/mL)	(°C)		(Ti, μg/mL)	(%)
K2TiF6	960	200 °C	100 MPa	370	61.5	14.6
		250 °C		206	78.5	12.54
		300 °C		162	83.1	11.96
		400 °C		30	96.8	9.35
		500 °C		8	99.1	7.89
Na2TiF6	960	200 °C	100 MPa	265	72.4	-
		250 °C		365	61.98	-
		300 °C		330	65.63	-
		400 °C		230.5	75.99	-
		500 °C		210.75	78.13	-
H2TiF6	960	200 °C	100 MPa	21.65	97.74	5.15
		250 °C		33.5	96.61	5.67
		300 °C		1.965	99.8	2.63
		400 °C		2.235	99.77	2.76
		500 °C		3.5	99.64	3.21
(NH4)2TiF6	960	200 °C	100 MPa	194	79.79	12.38
		250 °C		31	96.77	9.39
		300 °C		20.875	97.83	8.93
		400 °C		5.725	99.4	7.54
		500 °C		4.525	99.53	7.30

.

3. Hydrolysis Rate

3.1. The Calculation of Hydrolysis Rate

From Table 1, we see that the hydrolysis degree of four fluorotitanium complexes, K₂TiF₆, Na₂TiF₆, (NH₄)₂TiF₆ and H₂TiF₆, increased with rising temperature. Taking K₂TiF₆ as an example, with the initial concentration of 960 μg/mL, when the temperature increased from 200 °C to 500 °C, the hydrolysis rate increased from 61.5% to 99.1%, and the hydrolysis degree became more violent in the higher temperature environment. In other words, the stability of K₂TiF₆ decreased with the increase in temperature, and the content of Ti in the solution decreased as well. The other three fluorotitanium also showed the same trend with K₂TiF₆ when the temperature increased under the set temperature gradient generally. While Na₂TiF₆ fluctuated in high temperature and pressure reactions, as the hydrolysis of Na₂TiF₆ was up to 72.4% when the temperature condition of the reaction was 200 °C, the hydrolysis rate was much higher than that of 250 °C and 300 °C. The reason for this abnormality may be that the supernatant was not completely extracted from the reaction solution, which led to a higher result than normal. However, following the four gradient conditions of temperature, the hydrolysis rate of the complex increased with temperature, from 61.98% to 78.12%, and the hydrolysis behavior of K₂TiF₆ changed when the temperature rose. The hydrolysis of the H₂TiF₆ complex was at a high level from the beginning of the experiment, and the hydrolysis rate reached 97.74% when it was 200 °C and reached complete hydrolysis when it rose from 200 °C to 500 °C. Due to the high degree of hydrolysis, the linear relationship of the hydrolysis rate value fluctuates slightly when the temperature rises to 500 °C, which is the normal range. According to the experimental data of H₂TiF₆, the high hydrolysis rate of H₂TiF₆ under high temperature and pressure conditions with poor stability shows that it is hard for Ti to migrate on a large scale in the H₂TiF₆ complex. It is noteworthy that at 200 °C, (NH₄)₂TiF₆ has a hydrolysis rate of 79.79%, while others are similar to H₂TiF₆, with a high degree of hydrolysis, all over 96%. Owning to the narrow temperature gradient, we do not involve much lower temperature conditions and can not come to a conclusion whether the hydrolysis rate of (NH₄)₂TiF₆ complex is lower or if its stability is better at a lower temperature when we eliminate the interference of experiment error. Thus, we need further research. However, under the condition simulated in this paper, the (NH₄)₂TiF₆ complex has poor stability at relatively high temperatures, so it cannot be used as an effective form of Ti element migration and enrichment.

The diversity of hydrolysis degree of different fluorotitanium complexes at the same temperature can be seen in Figure 1. The results demonstrate that hydrolysis rate is Na₂TiF₆ < K₂TiF₆ < (NH₄)₂TiF₆ ≈ H₂TiF₆. For example, when the temperature was 300 °C, the hydrolysis rate of Na₂TiF₆ was 65.36%, the hydrolysis rate of K₂TiF₆ was 83.1%, the (NH₄)₂TiF₆ was 97.83% and the H₂TiF₆ was 99.80%. The hydrolysis rate of Na₂TiF₆ and K₂TiF₆ are apparently lower than (NH₄)₂TiF₆ and H₂TiF₆. However, K₂TiF₆ with the same outer cation as alkali metal changes more obviously with temperature than Na₂TiF₆, and its hydrolysis rate is also relatively higher. The solubility of (NH₄)₂TiF₆ and H₂TiF₆ is generally over 96% under the temperature and pressure condition involved in the experiment, which shows that they have poor stability. Thus, it is hard for Ti to realize general migration and enrichment in the form of (NH₄)₂TiF₆ and H₂TiF₆.

3.2. Cumulative Hydrolysis Equilibrium Constant

The stability of the complex is often affected by many factors such as temperature, pressure and the pH of solution. In order to assess the stability of the complex more effectively, the paper used the method provided by He et al. [7] to calculate the hydrolysis constant of the complex, which uses a better parameter to characterize the stability of the complex, since the hydrolysis constant is only relative to the complex type, temperature and pressure. According to the initial concentration and post-reaction concentration of each complex, the concentration of each substance in the reaction equilibrium solution can be calculated, followed by the equilibrium constant of each reaction according to (10). The calculation results are shown in Table 1, and according to the calculation results, the cumulative hydrolysis constant and temperature of each complex are fitted, and the fitting results are shown in Figure 2a,b.

Since plenty of HF was added when making up the Na₂TiF₆ solution (to promote the dissolution of solids), the concentration of each substance in the solution cannot be accurately determined when the reaction reaches equilibrium. Therefore, the hydrolysis equilibrium constant of Na₂TiF₆ can not be calculated. Furthermore, because the cation of H₂TiF₆ is H⁺, the calculation formula of the hydrolysis equilibrium constant is different from other complexes, which is K = ${\mathrm{C}}_{\mathrm{HF}}^6/{\mathrm{C}}_{{\mathrm{TiF}}_6^{2-}}$. Therefore, H₂TiF₆ can not be directly compared with the cumulative hydrolysis equilibrium constant of other complexes.

As shown in Figure 2a, the fitting relation between temperature and hydrolysis constant of the fluorotitanium complex, K₂TiF₆ and (NH₄)₂TiF₆ have a better fitting effect, −lnK and 1/T can be considered a linear correlation, and the fitting equation of K₂TiF₆ is:

−lnK = 8072.3/T − 2.534

(12)

With the fitting coefficient R² = 0.989, Δ_rH_m^ɵ/R = 8072.3, Δ_rS^ɵ/R = 2.534 can be obtained. Then, we can get the following thermodynamic parameters Δ_rH_m^ɵ = 67.113 kJ/mol, Δ_rS_m^ɵ = 21.068 J/(mol∙K), the fitting result can be seen from the Figure 2a. Similarly, the fitting equation of (NH₄)₂TiF₆ can be obtained:

−lnK = 5782.1/T − 0.7756

(13)

With the fitting coefficient R² = 0.8649, we can come to Δ_rH_m^ɵ = 48.072 kJ/mol, ΔrS_m^ɵ = 6.448 J/(mol∙K), and the fitting result is shown in Figure 2a.

As shown in Figure 2b, the fitting equation R² = 0.5333 for H₂TiF₆ has a poor fitting effect, and the points in the figure are obviously scattered. The experimental data of H₂TiF₆ show that the hydrolysis rate of H₂TiF₆ under high pressure and temperature is extremely high. The hydrolysis degree of some neighboring temperature ranges has little difference; hence it has less of a fitting effect, and the hydrolysis equilibrium constant is distinct from other complexes. Therefore, the stability of H₂TiF₆ cannot make simple comparisons with other clathrates by the accumulated hydrolysis equilibrium constant.

From Figure 2a, we can see the relative size of the cumulative hydrolysis equilibrium constant between K₂TiF₆ and (NH₄)₂TiF₆. K₂TiF₆ at the top of the figure has a lower cumulative hydrolysis equilibrium constant K, i.e., the more stable the complex is. In term of stability, K₂TiF₆ > (NH₄)₂TiF₆. In addition, we can get the influence of temperature on the cumulative hydrolysis equilibrium constant of a free kind of complex from the slope of a straight line. The larger the slope is, the greater the influence of temperature on the corresponding complex. Figure 2a,b reflect that the influence of temperature on stability is K₂TiF₆ > (NH₄)₂TiF₆ > H₂TiF₆, and the degree of influence on the stability of K₂TiF₆ has little difference from (NH₄)₂TiF₆.

4. Discussion

4.1. The Stability Diversity of Fluorotitanium Complex with Different Outer Cation

Some phenomena of nature and results from previous high temperature and pressure experiments show that elemental F contained in fluid can activate and migrate Ti [7,13,16,17,19,20]. However, geological fluid is a complex system. In addition to concentration, temperature and pressure, the factor which affects the migration and enrichment of an element in complex systems cannot be ignored. Therefore, through the hydrolysis experiment of potassium fluotitanate, sodium fluotitanate, fluotitanate and ammonium fluotitanate under high temperature and pressure, we analyze and compare the influence of different outer cations on the stability of the fluotitanate complex so as to have a clear understanding on the stability of the titanium complex in hydrothermal fluid.

From the hydrolysis rate and the calculation of the cumulative hydrolysis equilibrium constant, it can be seen that the stability of two kinds of Ti-F complexes is better. In the former experiment on the solubility of rutile, the solubility in the deionized water was below 100 μg/mL. However, when it was in a NaCl or silicate system, the solubility of rutile significantly increased to 5000 μg/mL. It is believed that with the increase in Na/Al ratio and Na-Al silicate (albite) content in the solution, the solubility of rutile will continue to increase, which may be due to the formation of Na-Ti complexes when Ti dissolves [12,27,28,29]. At the same time, the dissolution of rutile might be connected to the formation of an alkali metal titanate complex. Therefore, we conclude the reason that K₂TiF₆ and (NH₄)₂TiF₆ are more stable is due to the formation of the alkali metal titanate complex. Therefore, it can be recognized that the fluorotitanium complex in the fluid that enriches K⁺ and Na⁺ is more stable and easy to migrate in proper conditions of temperature and pressure. For example, the profound study on phenocryst structure and fluid inclusions in several typical porphyry copper ore bodies in China [30] found that the main components of fluid inclusions are H₂O, CO₂, K⁺ and Na. Other scholars found that the economic potential of rutile deposits associated with porphyry copper deposits is huge [31]. It demonstrates that the fluorotitanium complex can migrate on a certain scale in fluids that enrich K⁺ and Na⁺, and once the condition changes, the stability of the fluorotitanium complex will decrease and lead to the separating of Ti from fluid with the form of TiO₂.

As for why Na₂TiF₆ is more stable than K₂TiF₆, it may be due to the difference in chemical bond strength, which is caused by discrepant ionic radius; it is also possible that the initial Na₂TiF₆ can inhibit the reaction (9) by adding excessive HF.

The hydrolysis level of H₂TiF₆ and (NH₄)₂TiF₆ is close, and the degree of both is relatively high, which may be due to the fact that HF is a weak acid and:

HF ⇌ H⁺ + F

(14)

That is to say, HF is partially ionized into F⁻ and H⁺, while H⁺ is introduced by H₂TiF₆ through the hydrolysis of H⁺ and NH₄⁺, which will reverse the equation (9). Thus, the F⁻ content in the fluid is reduced, and it will also reduce TiF₆²⁻ and make Ti separate from fluid in the form of TiO₂. For this reason, the stability of H₂TiF₆ and (NH₄)₂TiF₆ is poor. The phenomenon also shows the influence of pH in solution on the stability of the fluorotitanium complex, which will decrease with the increase of acidity of the fluid. The fluorotitanium complex is easy to decompose into TiO₂, which gradually makes Ti separate from the fluid and settle down in a corresponding geological environment.

4.2. The Activity of Titanium in Nature Fluid

Fluorine-rich fluid can activate and migrate Ti and other elements of HFSE [7,13,16,17,19,20]; similarly, the solubility of rutile in fluorine-rich fluid is much greater than that of other fluids [13,32,33]. However, the dissolution, migration and precipitation of an element is a continuous process of a dynamic cycle, affected by concentration, temperature, pressure, pH and others [21]. The migration scale of Ti in fluid is determined by the stability in the process of migration [21,23]. In the provenance of rutile and others that concentrate Ti and other HFSEs, the concentrations are dissolved to their maximum solubility. When the element begins to migrate, the solution equilibrium of provenance will be destroyed, and the content of Ti that can migrate in the fluid will be significantly reduced. According to previous calculations, with the increase of rutile solubility, the amount of Ti migration will also increase, and for subduction fluid, the maximum amount of Ti migration may not exceed 6700 μg/mL [7].

The influence of pressure on the fluorotitanium complex is weak; whether it is at low- or high-pressure conditions, the fluorotitanium complex always follows a certain hydrolysis rule at a certain temperature. The major factors that affect the motility of fluorotitanium complex are still the concentration, temperature and pH. It illustrates that Ti dissolved from the provenance is hydrolyzed to a certain extent with the migration of the fluid, and the remaining Ti can continue to migrate effectively in the form of a complex. This is also confirmed by the fact that the eclogite approaches a vein body, which is obviously influenced by fluid, can enrich rutile in high-pressure or ultrahigh-pressure rocks [19,34,35,36]. The influence of temperature on the hydrolysis of the complex is apparent. From the experimental results in this paper, with the increase in temperature, the higher the degree of hydrolysis of the complex, the weaker the stability of the complex. The point can be explained by the Δ_rH_m^θ > 0 of the hydrolysis reaction, where the hydrolysis of the complex is an endothermic reaction; the increasing temperature will promote the hydrolysis reaction to move in a positive direction. However, according to the previous research results, rutile also increases with temperature in different solutions [13,32,33,37]. The two phenomena seem to contradict each other. In fact, the influence trend is ultimately determined by the pH of the fluid. According to the research of He et al. [7], under the condition of constant pH in the fluid, the stability of the flourotitanium complex in the low pH range decrease with the increase of temperature, while in the high pH range, it is opposite. In addition, from the experimental results of this paper, in the basic solution containing K⁺ and Na⁺, the stability of the fluorotitanium complex in the fluid is obviously better than in the acid solution. Therefore, the effect of pH on the reactive fluorotitanium complex can not be ignored.

The chemical stability of hydrothermal solution should be guaranteed if the fluorotitanium has long-distance migration. Most of the fluids in nature are mainly Cl, C or S-containing solvents [5,38,39,40,41,42], which is why Ti and other HFSEs in the majority of cases are not active; but for subduction fluid, in the subduction zone, the polysiliceous muscovite subducted to the deeper part undergoes pyrolysis or dehydration, resulting in a large amount of F entering the fluid [7,43,44,45,46], and realizing the migration of Ti and other elements. Moreover, for the magmatic-hydrothermal system, F forms fluorine-rich fluid by preconcentration in magma and then mass distribution into the late-stage fluid [7,47,48,49,50] to realize the migration of Ti and other elements. Therefore, if there are foreign metal cations such as Ca, they combine with F in the fluid by forming CaF₂ and then lead to the precipitation of minerals of Ti and other HFSEs. The fluid in the Bayun Obo rare earth metal deposit enters the carbonate rock, which leads to the precipitation and mineralization of rare metals in the fluid, accompanied by a large number of fluorite precipitation [51]. In the same way, the change of composition, pH and temperature of hydrothermal solution caused the instability of the fluorotitanium complex, which led to the precipitation and growth of Ti from the hydrothermal fluid into minerals.

5. Conclusions

In this paper, we studied the stability of different titanium complexes (K₂TiF₆, Na₂TiF₆, (NH₄)₂TiF₆ and H₂TiF₆) under the pressure of 100 MPa and the temperatures of 200~500 °C, with the reaction time of more than 12 h, and we may draw some conclusions:

(1) Under hydrothermal conditions, the hydrolysis of the fluorotitanium complex is proportional to temperature.

(2) Comparing the stability of four kinds of fluorotitanium complexes, which differ in outer cations, by using hydrolysis rate and cumulative hydrolysis equilibrium constant, the fluorotitanium complexes with alkali metals as outer cations (K₂TiF₆ and Na₂TiF₆) have higher stability, which lays a foundation for Ti migration in the fluid. The reason may be related to the formation of the alkali metal titanate complex.

(3) The dissolution, migration and precipitation of elements are a continuous process of a dynamic cycle, which can be affected by temperature, concentration, pressure, pH and other factors. The influence of pH on the stability of the fluorotitanium complex is related to the change of temperature in acid fluid; the stability of the fluorotitanium complex is relatively poor, which is not conducive to the migration of Ti. A decrease in the pH of the fluid causes Ti to precipitate from the fluid.