Phase Transitions in Natural Vanadinite at High Pressures

: The structural stability of vanadinite, Pb 5 [VO 4 ] 3 Cl, is reported by high-pressure experiments using synchrotron radiation X-ray diffraction (XRD) and Raman spectroscopy. XRD experiments were performed up to 44.6 GPa and 700 K using an externally-heated diamond anvil cell (EHDAC), and Raman spectroscopy measurements were performed up to 26.8 GPa at room temperature. XRD experiments revealed a reversible phase transition of vanadinite at 23 GPa and 600 K, which is accompanied by a discontinuous volume reduction and color change of the mineral from transparent to reddish during compression. The high-pressure Raman spectra of vanadinite show apparent changes between 18.0 and 22.8 GPa and ﬁnally become amorphous at 26.8 GPa, suggesting structural transitions of this mineral upon compression. The structural changes can be distinguished by the emergence of a new vibrational mode that can be attributed to the distortion of [VO 4 ] and the larger distortion of the V–O bonds, respectively. The [VO 4 ] internal modes in vanadinite give isothermal mode Grüneisen parameters varying from 0.149 to 0.286, yielding an average VO 4 internal mode Grüneisen parameters of 0.202.


Introduction
Apatite group minerals with the general formula A 5 (BO 4 ) 3 X allow numerous substitutions of cations or anions in their crystal structures and have widespread industrial applications, i.e., as metal sequestration agents for water treatment and contaminated soil remediation [1][2][3], photo-catalytic materials [4,5], and nuclear waste storage materials [6][7][8]. Moreover, apatite group minerals play important roles in many geological settings and serve as potential phosphorus-bearing phase in the Earth's deep mantle through subduction and sensitive tracers for magma derivation/evolution [9][10][11][12]. It is crucial to understand the stability and properties of apatite-group minerals at high pressures, especially during the slab subduction. Thus, their high-pressure and high-temperature behaviors have been widely investigated [13][14][15][16][17][18][19].
Vanadinite Pb 5 [VO 4 ] 3 Cl is a vanadate apatite group mineral, with a structure related to that of hexagonal apatite Ca 5 [PO 4 ] 3 (F,Cl,OH) (space group P6 3 /m). It is often associated with other accessory minerals such as mimetite and pyromorphite. One of the most important countries of origin for vanadinite is Morocco [20] and Mibladén is one of the main vanadinite localities where there are carbonate-hosted stratabound lead deposits. Vanadinite occurs as an alteration product of galena in close association with barite, cerussite and anglesite [21]. As an apatite-group mineral with the general formula of A 4 I A 6 II [BO 4 ] 6 X 2 (A I and A II = Ca 2+ , Pb 2+ , Cd 2+ . . . , B = P 5+ , V 5+ , As 5+ . . . , X = F − , Cl − , Br − , OH − ), vanadinite adopts a "hexagonal network" [22], composed of AO n polyhedra and isolated BO 4 tetrahedra and forming large [1]-channels, where the X anions locate [13].

Materials and Methods
Natural gem-quality, transparent and red-orange single crystals of vanadinite from Mibladén in Morocco were used in this study. The microanalysis of its chemical composition was carried out at Beijing Research Institute of Uranium Geology (Beijing, China) by JXA-8100 Electron Probe Micro-analyzer (EPMA), with 20 kV accelerating voltage, 1 × 10 −8 A beam current, and 40 • exit angle.
In situ HPHT single-crystal XRD experiments were carried out at beamline 13-BMC with the wavelength of 0.43409 Å at Advanced Photon Source (APS), Argonne National Laboratory (ANL) [32]. A BX90-type diamond anvil cell with a resistive micro-heater [33] was employed to achieve high pressure and high temperature conditions. A rhenium (Re) foil with an initial thickness of 260 µm was used as the gasket. The gasket was pre-indented to a thickness of about 30 µm, and a hole of 120 µm in diameter was drilled at the center to serve as sample chamber. Single crystal vanadinite sample, ruby spheres, and gold were loaded into the sample chamber. Neon was loaded using the COMPRES/GSECARS gas-loading system [34] and serves as the pressure transmitting medium [35]. Gold was used as the primary pressure scale [36], with which pressures were determined before and after each XRD measurement. In order to find evidence of phase transition in vanadinite, several rounds of temperature/pressure changing were carried out. The corresponding PT path of HPHT experiments were illustrated in Figure 1. Each condition was lasted Minerals 2021, 11, 1217 3 of 13 about 30 min. At first temperature was increased from 300 K to 700 K at constant 40 PSI condition. Then, pressure was increased from 50 PSI to 110 PSI at the same temperature 700 K. Subsequently, temperature was decreased from 700 K to 300 K at constant 110 PSI condition. After that the cell was compressed from 110 PSI to 190 PSI at 300 K. These steps were repeated in turn and the experiments was ended at 44.6 GPa and 400 K because of an abrupt gas leaking. The XRD experiments were only performed during compression controlled by a membrane system in 13-BMC, and no XRD data were collected during decompression. DIOPTAS software was used to integrate the XRD patterns [37].
Minerals 2021, 11, x FOR PEER REVIEW 3 of 14 transition in vanadinite, several rounds of temperature/pressure changing were carried out. The corresponding PT path of HPHT experiments were illustrated in Figure 1. Each condition was lasted about 30 min. At first temperature was increased from 300 K to 700 K at constant 40 PSI condition. Then, pressure was increased from 50 PSI to 110 PSI at the same temperature 700 K. Subsequently, temperature was decreased from 700 K to 300 K at constant 110 PSI condition. After that the cell was compressed from 110 PSI to 190 PSI at 300 K. These steps were repeated in turn and the experiments was ended at 44.6 GPa and 400 K because of an abrupt gas leaking. The XRD experiments were only performed during compression controlled by a membrane system in 13-BMC, and no XRD data were collected during decompression. DIOPTAS software was used to integrate the XRD patterns [37]. By using a symmetry-type DAC with a pair of 300 µm culet size diamond anvils, which was prepared in a similar manner as XRD experiments, Raman spectra of vanadinite were collected at room temperature using a Dilor micro-Raman system at University of Hawaii at Manoa with a 532 nm wavelength laser and a spectrometer with a liquid nitrogen cooled CCD detector. The experimental pressures were determined by the ruby fluorescence method [38]. The typical data collection time was 120 s for each spectrum after waiting for at least 10 min after each compression.

EPMA Data of the Vanadinite Sample
The chemical composition of the vanadinite sample was determined from the average of three analyses of the sample by EPMA. The V2O5, PbO, P2O5, and Cl contents vary from 18.99% to 19.59%, 79.35% to 79.70%, 0.19% to 0.64%, and 0.15% to 0.24%, with the average 19.29%, 79.55%, 0.35%, and 0.19% content respectively. Only one analysis detected 0.05% Na2O content. Therefore, the calculated standard deviations of these contents are 0.24, 0.15, 0.20, 0.04 and 0.02 (for Na2O). The chemical formula of vanadinite was calculated by assuming the number of O atoms being 12, which is determined as Pb5.14Na0.01(V2.83P0.07O12)Cl0.07(F,OH)0.72. As fluorine (F) and hydroxyl (OH − ) cannot be detected, their contents were calculated by valence balance methods. It is also noted that there is little isomorphic substitution between P and V, Na and Pb elements, the similar phenomena were also be detected by Frost et al. [39]. By using a symmetry-type DAC with a pair of 300 µm culet size diamond anvils, which was prepared in a similar manner as XRD experiments, Raman spectra of vanadinite were collected at room temperature using a Dilor micro-Raman system at University of Hawaii at Manoa with a 532 nm wavelength laser and a spectrometer with a liquid nitrogen cooled CCD detector. The experimental pressures were determined by the ruby fluorescence method [38]. The typical data collection time was 120 s for each spectrum after waiting for at least 10 min after each compression.

EPMA Data of the Vanadinite Sample
The chemical composition of the vanadinite sample was determined from the average of three analyses of the sample by EPMA. The V 2 O 5 , PbO, P 2 O 5 , and Cl contents vary from 18.99% to 19.59%, 79.35% to 79.70%, 0.19% to 0.64%, and 0.15% to 0.24%, with the average 19.29%, 79.55%, 0.35%, and 0.19% content respectively. Only one analysis detected 0.05% Na 2 O content. Therefore, the calculated standard deviations of these contents are 0.24, 0.15, 0.20, 0.04 and 0.02 (for Na 2 O). The chemical formula of vanadinite was calculated by assuming the number of O atoms being 12, which is determined as Pb 5.14 Na 0.01 (V 2.83 P 0.07 O 12 )Cl 0.07 (F,OH) 0.72 . As fluorine (F) and hydroxyl (OH − ) cannot be detected, their contents were calculated by valence balance methods. It is also noted that there is little isomorphic substitution between P and V, Na and Pb elements, the similar phenomena were also be detected by Frost et al. [39].

XRD Results of Vanadinite at HPHT Conditions
Previous experiments suggested that phosphate minerals, such as vanadinite [13], memitite [28,29], pyromorphite [28,29], Pb10[(PO4)6-x(VO4)x]F2 [40], fluorapatite [26], alforsite [18], and tuite [11], remain stable at least up to about 7 GPa, which is consistent with our HPHT XRD data for natural vanadinite collected from 7.1 to 44.6 GPa and at temperatures from 300 to 700 K. Representative XRD patterns collected at HPHT are shown in Figure 2, in which the splittings of Bragg's peaks emerge at 23.1 GPa and 600 K (the inset in Figure 2). Furthermore, the diffraction patterns became more complex at 25.8 GPa and upon heating to 500 K (Figure 2d), indicative of a displacive phase transition [41]. The opening angle of the DAC, 39°, is not large enough to allow us to process the single crystal XRD data to obtain the detailed structural information. However, the integrated diffraction patterns could be indexed using the PDindexer software with the embedded AMCSD database [42,43]. New peak appears (2θ = 4.72°, d = 5.27 Å) at 23.1 GPa and 600 K (Figure 3a). At 25.8 GPa and 500 K, the indistinct (101) peak appeared (2θ = 4.82°, d = 5.16 Å) (Figure 3a), and the 102 and 210 peaks merge into one at the same condition. All these phenomena are in accordance with the presumed displacive phase transition. Although the experiments of Zhai et al. [44] showed that apatite decomposed to tuite after 24 h heating at 15 GPa and 1800 K in a 1000-tons Kawai-type multi-anvil apparatus, the three strongest (110, 015 and 205) peaks of rhombohedral tuite [43] are not consistent with what were observed in our HPHT XRD patterns ( Figure 3a). Therefore, the more complex diffraction patterns here indicate an unknown lower symmetry structure. Figure  4 illustrates the relationship between pressure and volume of the vanadinite sample. An obvious volume reduction is observed at ~23 GPa, signaling a possible phase transition. The opening angle of the DAC, 39 • , is not large enough to allow us to process the single crystal XRD data to obtain the detailed structural information. However, the integrated diffraction patterns could be indexed using the PDindexer software with the embedded AMCSD database [42,43]. New peak appears (2θ = 4.72 • , d = 5.27 Å) at 23.1 GPa and 600 K (Figure 3a). At 25.8 GPa and 500 K, the indistinct (101) peak appeared (2θ = 4.82 • , d = 5.16 Å) (Figure 3a), and the 102 and 210 peaks merge into one at the same condition. All these phenomena are in accordance with the presumed displacive phase transition. Although the experiments of Zhai et al. [44] showed that apatite decomposed to tuite after 24 h heating at 15 GPa and 1800 K in a 1000-tons Kawai-type multi-anvil apparatus, the three strongest (110, 015 and 205) peaks of rhombohedral tuite [43] are not consistent with what were observed in our HPHT XRD patterns (Figure 3a). Therefore, the more complex diffraction patterns here indicate an unknown lower symmetry structure. Figure 4 illustrates the relationship between pressure and volume of the vanadinite sample. An obvious volume reduction is observed at~23 GPa, signaling a possible phase transition.
Under the optical microscope, an obvious change from transparent to opaque was observed at around 26.2 GPa and 600 K, and the sample remained opaque to the highest pressures. This was verified by a drastic color change from reddish (reddish for naked eye but orange in the photo taken by Leica microscope with Sony camera due to the whitebalance setting) to transparent during decompression after the experiments (Figure 3b), indicative a reversible phase transition at~26.2 GPa. Similar phenomena were observed in many compounds upon compression, i.e., KTb(MoO 4 ) 2 [45], siderite [46], and manganotantalite [47]. Jayaraman et al. [45] attributed the color change of KTb(MoO 4 ) 2 to 4f -5d transition in Tb initiated by the structural transition, and consequent intervalence charge transfer between Tb and Mo. Müeller et al. [46] associated the transparent-green color change of siderite to the spin transition. Liu et al. [47] explained the phenomenon by the alteration of electronic structure properties and a narrowing of band gap in Mn(Ta,Nb) 2 O 6 . It is anticipated that the transparent-reddish color change of vanadinite observed in this study likely results from the energy level splitting of transition element V. Future experiments and theoretical calculations are necessary to fully understand the character of reversible pressure-induced color changes in vanadinite.    Under the optical microscope, an obvious change from transparent to opaque was observed at around 26.2 GPa and 600 K, and the sample remained opaque to the highest pressures. This was verified by a drastic color change from reddish (reddish for naked eye but orange in the photo taken by Leica microscope with Sony camera due to the whitebalance setting) to transparent during decompression after the experiments (Figure 3b), indicative a reversible phase transition at ~26.2 GPa. Similar phenomena were observed in many compounds upon compression, i.e., KTb(MoO4)2 [45], siderite [46], and manganotantalite [47]. Jayaraman et al. [45] attributed the color change of KTb(MoO4)2 to 4f-5d transition in Tb initiated by the structural transition, and consequent intervalence charge transfer between Tb and Mo. Müeller et al. [46] associated the transparent-green color change of siderite to the spin transition. Liu et al. [47] explained the phenomenon by the alteration of electronic structure properties and a narrowing of band gap in Mn(Ta,Nb)2O6. It is anticipated that the transparent-reddish color change of vanadinite observed in this study likely results from the energy level splitting of transition element V. Future experiments and theoretical calculations are necessary to fully understand the character of reversible pressure-induced color changes in vanadinite.

Raman Spectroscopy Studies of Vanadinite at Ambient Conditions and High Pressures
Raman spectra of free VO4 3− and PO4 3− ions, as well as of pure vanadinite and pyromorphite have been investigated by many authors [18,39,[48][49][50]. In general, four bands resulting from vibration of the undistorted free VO4 3− molecule can be observed [39,48]. Only three bands are observed in synthetic pyromorphite-vanadinite solid solutions due to the distorted VO4 3− ions [50]. Bartholomai et al. [48] observed the υ1 band at 825 cm −1 , υ3 at 792 and 718 cm −1 , υ2 at 322 and 291 cm −1 and υ4 at 414, 354 and 322 cm −1 . Frost et al. [39] have studied the Raman spectra of [Pb5(VO4)3Cl] specimens from three localities, and they assigned the intense band at 827 cm −1 to the υ1 symmetric stretching vibration, the two splitting bands 815 and 790 cm −1 to be the υ3 antisymmetric stretching modes, the 414 and 395 cm −1 bands to be υ4 modes, the complex bands at 366, 355, 323 and 290 cm −1 to be the υ2 vibration.
The Raman spectrum of our vanadinite sample was collected at ambient conditions ( Figure 5). A total of nine bands are observed, which is less than the predicted internal and external modes in some apatite-structured phosphate [18,[50][51][52], but agrees with other vanadinite specimens [39,50]. The strongest single band observed at 848 cm −1 is assigned to the ν 1 symmetric stretching mode [50] and three bands at 802, 791 and 760 cm −1 are attributed to ν 3 antisymmetric stretching modes, respectively. The relatively broad 420 cm −1 band is attributed to the ν 4 OVO bending vibration [50]. The other four overlapping ν 2 symmetric bending modes are located narrowly at 367, 357, 336 and 295 cm −1 , which are caused by the lowering of the VO 4 3− symmetry within the vanadinite structure [50]. The Raman active modes of vanadinite are in good agreement with previous studies, in which it was found that the ν 1 symmetric stretching band in mimetite-arsenian pyromorphite [39] and pyromorphite-vanadinite solid solutions [50] shifts toward lower wavenumber with the increase of the atomic mass in [BO 4 ] tetrahedra (B = P, V, As). This can be anticipated, because the frequency of a Raman band is dependent on lattice vibrations, the masses of the atoms/ions, and the strength between the atoms/ions [17,53].
The Raman active modes of vanadinite are in good agreement with previous studies, in which it was found that the ν1 symmetric stretching band in mimetite-arsenian pyromorphite [39] and pyromorphite-vanadinite solid solutions [50] shifts toward lower wavenumber with the increase of the atomic mass in [BO4] tetrahedra (B = P, V, As). This can be anticipated, because the frequency of a Raman band is dependent on lattice vibrations, the masses of the atoms/ions, and the strength between the atoms/ions [17,53]. Ten HP Raman spectra were collected from 7.3 GPa and up to 26.8 GPa. Seven Raman spectra of decompression were also measured. Figure 6 presents the selected Raman spectra, which have typical changes at different pressures. Peak shifts at HP are summarized and illustrated in Table 1 and Figure 7. Due to the presence of a fluorescent background in the spectra recorded from the diamond anvil, the quality of Raman spectra obtained during compression is not so good as that collected in open air at ambient conditions, such as the intensities of ν3-2 band around 791 cm −1 and ν2-4 band around 295 cm −1 . Their intensities become lower and even unresolvable at higher pressures. It is shown in Figure 6 that all peaks display significant blue-shift upon compression to higher pressures. This is likely caused by the shorter Pb-O and V-O bond lengths which is response to the increasing pressure and stronger bonds, larger force constant and consequently higher vibrational frequency according to Hooke's law. Ten HP Raman spectra were collected from 7.3 GPa and up to 26.8 GPa. Seven Raman spectra of decompression were also measured. Figure 6 presents the selected Raman spectra, which have typical changes at different pressures. Peak shifts at HP are summarized and illustrated in Table 1 and Figure 7. Due to the presence of a fluorescent background in the spectra recorded from the diamond anvil, the quality of Raman spectra obtained during compression is not so good as that collected in open air at ambient conditions, such as the intensities of ν 3-2 band around 791 cm −1 and ν 2-4 band around 295 cm −1 . Their intensities become lower and even unresolvable at higher pressures. It is shown in Figure 6 that all peaks display significant blue-shift upon compression to higher pressures. This is likely caused by the shorter Pb-O and V-O bond lengths which is response to the increasing pressure and stronger bonds, larger force constant and consequently higher vibrational frequency according to Hooke's law.
The relatively smooth evolution of the Raman modes pressures up to 18.0 GPa may be attributed by relative displacements of cations, and perhaps coupled with slight distortions of the VO 4 tetrahedra [54]. At 18.0 GPa, a new vibrational band observed in 903 cm −1 emerges (red arrows in Figure 6b). Similarly, a new ν 1 Raman mode of alforsite Ba 5 (PO 4 ) 3 Cl was observed to occur at 24.1 GPa and was attributed to the new splitting of the PO 4 tetrahedra internal modes under high pressure and indicating the intensification of the PO 4 unit distortion [18]. It is contrary to the lessening of the PO 4 tetrahedra distortion in other fluorapatite high-pressure Raman studies [27,31]. Meanwhile, the ν 4 band becomes substantial at 18.0 GPa (black arrows in Figure 6a), which indicates the stronger bending vibration between OVO atoms. The relatively smooth evolution of the Raman modes pressures up to 18.0 GPa may be attributed by relative displacements of cations, and perhaps coupled with slight distortions of the VO4 tetrahedra [54]. At 18.0 GPa, a new vibrational band observed in 903 cm −1 emerges (red arrows in Figure 6b). Similarly, a new ν1 Raman mode of alforsite Ba5(PO4)3Cl was observed to occur at 24.1 GPa and was attributed to the new splitting of the PO4 tetrahedra internal modes under high pressure and indicating the intensification of the PO4 unit distortion [18]. It is contrary to the lessening of the PO4 tetrahedra distortion in other fluorapatite high-pressure Raman studies [27,31]. Meanwhile, the ν4 band becomes substantial at 18.0 GPa (black arrows in Figure 6a), which indicates the stronger bending vibration between OVO atoms.
When pressure reaches 20.5 GPa, substantial changes are observed in the Raman spectra. The new bands around 903 cm −1 become stronger (Figure 6b). Other five new bands (red arrows in Figure 6) appeared on both sides of ν1 mode (around 860 cm −1 ) and lower frequency around 478 cm −1 , accompanying the obvious red-shift of ν1 and ν3 bands (detailed shifts shown in Table 1). The similar new bands with higher energy and opposite shift in Raman peaks with applied pressure were also observed for other apatite group minerals, such as Sr5(PO4)3F [17], Ba5(PO4)3Cl [18], Ba3(VO4)2 [54], and Ca9R(VO4)7 (R = La, Nd, Gd) [55]. Zhai et al. [17,18] attributed these new bands to the new splitting of PO4 internal Raman modes under high pressures. Grzechnik and McMillan [54] proposed that adjacent distorted VO4 group in Ba3(VO4)2 may be forced together via edge-or cornersharing, which attributed to the condensations under compression and caused the significant changes of Raman modes. Kosyl et al. [55] observed opposite shift in Ca9R(VO4)7 Raman peaks at pressure of ~9-11 GPa and attributed them to phase transitions. Although the structure of vanadate and vanadinite are different from each other, their Raman spectra have two same main groups of bands corresponding to the V-O stretching modes (750-850 cm −1 ) and O-V-O bending modes (300-400 cm −1 ), respectively. Therefore, the corresponding new bands and red-shifts observed in vanadinite may be attributed to an intensified distortion of the V-O bonds and phase transition [54,55].  (Figure 6b). Meanwhile, the ν3-2 803 cm −1 band continued shifting to lower wavenumbers 799 cm −1 with distinct pressure dependence in comparison to lowpressure data (Figure 7). The different pressure dependence of Raman modes may be ex-  When pressure reaches 20.5 GPa, substantial changes are observed in the Raman spectra. The new bands around 903 cm −1 become stronger (Figure 6b). Other five new bands (red arrows in Figure 6) appeared on both sides of ν1 mode (around 860 cm −1 ) and lower frequency around 478 cm −1 , accompanying the obvious red-shift of ν 1 and ν 3 bands (detailed shifts shown in Table 1). The similar new bands with higher energy and opposite shift in Raman peaks with applied pressure were also observed for other apatite group minerals, such as Sr 5 (PO 4 ) 3 F [17], Ba 5 (PO 4 ) 3 Cl [18], Ba 3 (VO 4 ) 2 [54], and Ca 9 R(VO 4 ) 7 (R = La, Nd, Gd) [55]. Zhai et al. [17,18] attributed these new bands to the new splitting of PO 4 internal Raman modes under high pressures. Grzechnik and McMillan [54] proposed that adjacent distorted VO 4 group in Ba 3 (VO 4 ) 2 may be forced together via edge-or corner-sharing, which attributed to the condensations under compression and caused the significant changes of Raman modes. Kosyl et al. [55] observed opposite shift in Ca 9 R(VO 4 ) 7 Raman peaks at pressure of~9-11 GPa and attributed them to phase transitions. Although the structure of vanadate and vanadinite are different from each other, their Raman spectra have two same main groups of bands corresponding to the V-O stretching modes (750-850 cm −1 ) and O-V-O bending modes (300-400 cm −1 ), respectively. Therefore, the corresponding new bands and red-shifts observed in vanadinite may be attributed to an intensified distortion of the V-O bonds and phase transition [54,55].
Upon further compression to 22.8 GPa, the 903 cm −1 and the new 885, 876 cm −1 double bands disappeared (Figure 6b). Meanwhile, the ν 3-2 803 cm −1 band continued shifting to lower wavenumbers 799 cm −1 with distinct pressure dependence in comparison to low-pressure data (Figure 7). The different pressure dependence of Raman modes may be explained by that certain P-O bond length decreases and then increases upon compression, or vice versa in the case of the tetrahedral angle variance [27] even though the average P-O bond length of fluorapatite decreases with increasing pressure. This is also in agreement with the displacive phase transition as indicated in the HPHT XRD experiments. The ν 2-1 , ν 2-2 , and ν 2-3 bands were observed to merged into one because of their differing pressure shifts (Figure 6), which was also observed in Ca 5 (PO 4 ) 3 F stronadelphite [31]. The lower frequency ν 2-4 band cannot be resolved at 22.8 GPa.
The Raman spectrum collected at 26.3 GPa is dominated by a visible broadening peak near 850 cm −1 , with other peaks almost disappeared (Figure 6b). This is likely due to the transition to an amorphous phase [55]. At 26.8 GPa, the vanadinite totally transfers to amorphous phase ( Figure 6). The proposed condensation occurs continuously along a structural coordinate, in which adjacent distorted [VO 4 ] tetrahedra may be forced together via edge-or corner-sharing, because it is not associated with any significant hysteresis during decompression (Figures 6 and 7) [54]. The spectrum of the room pressure phase is recovered below approximately 18.8 GPa, which means the observed distortions are reversible.
The detailed wavenumbers of the Raman bands varying with pressure below 18.0 GPa are listed in Table 1 and displayed in Figure 7. It is apparent that the modes with the same origin exhibit similar dependences; for example, the bands arisen from the symmetric and antisymmetric stretching vibration of the V-O bond blue-shift at slopes of 2.57-3.81 cm −1 ·GPa −1 , which is more than twice times the rate of the others (1.19-1.27 cm −1 ·GPa −1 ) for OVO bonding vibrations except the ν 2-4 . The largest pressure dependence value of ν 2-4 may be due to the limited data and the difficulty to locate peak positions precisely above 8.9 GPa. The high frequency modes exhibit a steeper pressure dependence for wavenumber as pressure increases. Figures 6 and 7 indicate that vanadinite may undergo phase transitions between 18.0 and 22.8 GPa.
Mode Grüneisen parameters (γ) are calculated using γ i = (B 0 /ν i )(d νi /d p ) T , where ν i is the wavenumber of the ith mode and B 0 is the bulk modulus at room temperature. We used the isothermal bulk modulus Kv 0 of 57(1) GPa [13] for the calculation. The mode Grüneisen parameters for each of the measured bands are given in Table 2. The γ value of the modes vary from 0.149 to 0.286, which yields an average VO 4 internal mode Grüneisen parameters of 0.202. The biggest mode Grüneisen parameters of ν 2-4 is not taken into account. This average mode Grüneisen parameter is smaller than 0.314 of [PO 4 ] in alforsite [18], 0.332 in stronadelphite [17], 0.361 in tuite [11], 0.446 in fluorapatite [27], and 1.084 of [SiO 4 ] in kyanite [56]. Compared with the tetrahedral modes in other phosphate or silicate minerals, [VO 4 ] internal modes in vanadinite show lower average isothermal Grüneisen parameters, which is reasonable, as [VO 4 ] tetrahedra in phosphates are more compressible than [PO 4 ] in other phosphates and [SiO 4 ] in silicates. Furthermore, the behavior of Pb-O(Cl) polyhedra is much less rigid than that of Sr-O or Ca-O polyhedral in phosphate, which results in the different local crystal field surrounding the VO 4 tetrahedra in vanadinite. Note: a initial wavenumbers (cm −1 ) of a vibrational mode at atmospheric pressure; b pressure dependence of a mode at room temperature in cm −1 ·GPa −1 ; c Grüneisen parameters calculated using 7 i = (B 0 /ν i )(dvi/dp) T .

Conclusions
We have performed in situ XRD and Raman experiments for vanadinite up to 44.6 GPa, 700 K and 26.8 GPa at room temperature, respectively. The HPHT XRD results demonstrate that vanadinite undergoes a phase transition at 23.1 GPa and 600 K, and the new phase may be stable up to 44.6 GPa and 400 K, which proposed vanadinite is one of the hosts for REEs and ions during the subduction of lithosphere to the transition zone.
HP Raman experiment suggests that vanadinite experiences phase transitions between 18.0 and 22.8 GPa, which are distinguished by the pressure dependences of the vibrational modes, the emergence of new modes, and opposite shift in Raman peaks. The results from compression at 18.0 GPa may be attributed to the increasing distortions of the VO 4 tetrahedra. Upon compression to 20.5 GPa, the corresponding new bands and red-shifts may be attributed to a large distortion of the V-O bonds. At higher pressure, vanadinite was observed to transform to an amorphous phase. The spectrum of the room pressure phase is recovered below 18.8 GPa, which means the observed distortion are reversible. The mode Grüneisen parameters verified that the [VO 4 ] tetrahedra in vanadinite is more compressible than those in other apatite-group minerals.