Vibrational Investigation of Pressure-Induced Phase Transitions of Hydroxycarbonate Malachite Cu 2 (CO 3 )(OH) 2

: Malachite Cu 2 (CO 3 )(OH) 2 is a common hydroxycarbonate that contains about 15.3 wt % H 2 O. Its structural chemistry sheds light on other hydroxyl minerals that play a role in the water recycling of our planet. Here using Raman and infrared spectroscopy measurements, we studied the vibrational characteristics and structural evolution of malachite in a diamond anvil cell at room temperature (25 ◦ C) up to ~29 GPa. Three types of vibrations were analyzed including Cu–O vibrations (300–600 cm − 1 ), [CO 3 ] 2 − vibrations (700–1600 cm − 1 ), and O–H stretches (3200–3500 cm − 1 ). We present novel observations of mode discontinuities at pressures of ~7, ~15, and ~23 GPa, suggesting three phase transitions, respectively. First, pressure has a great e ﬀ ect on the degree of deformation of the [CuO 6 ] octahedron, as is manifested by the various shifting slopes of the Cu–O modes. [CuO 6 ] deformation results in a rotation of the structural unit and accordingly a phase transition at ~7 GPa. Upon compression to ~15 GPa, the O–H bands redshift progressively with signiﬁcant broadness, indicative of an enhancement of the hydrogen bonding, a shortening of the O ··· O distance, and possibly somewhat of a desymmetrization of the O–H ··· O bond. O–H mode hardening is identiﬁed above ~15 GPa coupled with a growth in the amplitude of the lower-energy bands. These observations can be interpreted as some reorientation or reordering of the hydrogen bonding. A further increment of pressure leads to a change in the overall compression mechanism of the structure at ~23 GPa, which is characterized by the blueshift of the O–H stretches and the softening of the O–C–O in-plane bending bands. The hydrogen bonding weakens due to a substantial enhancement of the Cu–H repulsion e ﬀ ect, and the O ··· O bond length shows no further shortening. In addition, the change in the local geometry of hydrogen is also induced by the softening of the [CO 3 ] 2 − units. In this regard we may expect malachite and other analogous hydroxyl minerals as capable of transporting water downward towards the Earth’s transition zone (~23 GPa). Our results furnish our knowledge on the chemistry of hydrogen bonding at mantle conditions and open a new window in understanding the synergistic relations of water and carbon recycling in the deep Earth.


Introduction
The circulation of water plays an important role in geological processes in the evolution of our planet. In descending slabs, the crustal water is recycled into the Earth's mantle through hydrogen-bearing phases (e.g., serpentine Mg 3 (Si 4 O 10 )(OH) 2 ) and nominally anhydrous minerals (e.g., olivine (Mg,Fe) 2 (SiO4)) [1]. At various depths in subduction regions, the presence of water is probably the origin of the observed seismic anomalies such as low velocities and high shear wave Minerals 2020, 10, 277; doi:10.3390/min10030277 www.mdpi.com/journal/minerals splitting [2]. Meanwhile, slabs dehydrating progressively with depth and releasing water provide a potential mechanism for rehydration of the overlaying mantle, exerting profound influence on mantle convection and the resulting volatile recycling and heterogeneity [1,3]. Thus, for insights into water recycling into the Earth's interior, as well as its effects on the physicochemical conditions and dynamics of the mantle, a primary method is to study the thermodynamic stabilities of hydrogen-bearing minerals under high pressure conditions. A succession of dense hydrous magnesium silicates (e.g., phase B: Mg 10 Si 3 O 14 (OH) 4 , phase D: MgSi 2 O 4 (OH) 2 , phase E: Mg 2 SiO 2 (OH) 4 , phase H: MgSiO 2 (OH) 2 and super-hydrous phase B) has been demonstrated as stable within the subducting lithosphere to lower mantle pressures [4][5][6][7]. Yet these Mg end-members decompose at temperatures lower than those of the mantle geotherms, and thus they cannot be long-term water reservoirs in the deep Earth. Substitution of Mg with Al stabilizes the hydrous phases to higher temperatures [8][9][10], but their occurrences have been scarcely reported in nature. A pressing issue emerges regarding whether other hydrogen-bearing phases exist that could account for the estimated water content inside the Earth [11][12][13].
Here we focus on a hydroxycarbonate malachite Cu 2 (CO 3 )(OH) 2 which contains about 15.3 wt % H 2 O. Malachite is an important index mineral for hydrous environments enriched with carbonate ions [14], which provide crucial clues for sequestration and circulation of water on the Earth. Malachite has a relatively simple chemistry whose high-pressure behavior will increase our understanding of the hydrogen-carriers in the deep Earth. Malachite also merits our preferential considerations because of its superior bonding of hydrogen with [CuO 6 ] octahedron due to the strong and partially covalent effect of Cu 2+ [15,16]. At atmospheric pressure, the thermodynamic properties of malachite have been studied in terms of heat capacity and enthalpy change in the temperature range of 20-500 • C [17]. Above 380 • C malachite breaks down releasing water and CO 2 simultaneously [18]. The application of pressure, however, counteracts the thermal decomposition effect, shifts the thermodynamic stability toward higher temperatures, and even realizes an over-hydrated state for some hydrous minerals (e.g., zeolites; [19]); but the effect of pressure on the properties of malachite is confused considerably. Measurements on the behaviors of malachite under pressure have been confined to an X-ray diffraction study under 10 GPa [20] and a density functional theory research to a maximum pressure of 5.72 GPa [21]. It has been reported that a phase transition of malachite occurs at 6-7 GPa driven by a deformation effect of [CuO 6 ] octahedron. The higher-pressure phase, rosasite, is much higher in density and atomic interactions are dramatically enhanced. The behavior of hydrogen during structural change is more crucial in water preservation, yet it is the least understood. Therefore, the present study takes advantage of in-situ Raman and infrared spectroscopy to examine the vibrational characteristics of malachite with contributions from all types of modes. With the aim of revealing the physical processes involving the response of hydrogen to pressure, the synergistic relations of hydrogen and the coordinated [CuO 6 ] octahedron and [CO 3 ] 2− units are discussed in detail at the molecular level. Our results will add to our knowledge of the structural chemistry of hydrogen agents in the deep Earth.
A symmetrical-type diamond anvil cell [22] with type-II diamond anvils (culet size 300 µm) was utilized for generating pressure. A rhenium gasket with an initial thickness of 250 µm, was pre-indented to 50 µm. A hole of 150 µm in diameter was drilled in the center of the indentation by electro-spark erosion, used as a sample chamber. A malachite chip was loaded into the chamber filled with silicone oil as a pressure transmitting medium in Raman spectroscopic measurements. For infrared spectroscopy, a thin malachite chip 10 µm in thickness was sandwiched between dried KBr layers that served as an infrared window and pressure medium. Pressures were measured according to the shifts of ruby fluorescence lines [23]. Both vibrational measurements were conducted at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing, China.
In Raman spectroscopy experiments, a 488 nm solid-state continuous-wave laser equipped with WITec alpha 300R was employed as an excitation source. A piece of single-crystal silicon was used to calibrate the wave numbers of the Raman shifts. The spectra of malachite were recorded through a 10× magnification objective lens with a 0.55 numerical aperture in backscattering geometry to the sample. After scattered Raman light passed through a pinhole of 20 µm, the light was dispersed on a grating of 300 g/mm, resulting in a spectral resolution of 1.105 cm −1 in the spectral region of 0-4500 cm −1 . The laser power was 20 mW and the acquisition time for each spectrum was 10 s with an accumulation of two times.
Infrared spectroscopy is particularly sensitive to rearrangements of hydrogen atoms in structures. High-pressure Fourier transform infrared spectroscopy (FTIR) measurements were carried out on a Bruker VERTEX 70V instrument (Bruker Optik GmbH, Ettlingen, Germany). A Bruker Hyperion 2000 microscope (Bruker Optics, Billerica, MA, USA) was used to record spectra in a 20 × 20 µm 2 aperture in a region of 650-5000 cm −1 , encompassing the vibrational characteristics of both O-H stretches and [CO 3 ] 2− modes. A KBr beam-splitter (Interspectrum, Tartumaa, Estonia) and a liquid-nitrogen cooled MCT-A (mercury-cadmium-telluride) detector (Block Engineering Technology, Southborough, MA, USA) were applied. A total of 640 scans were accumulated in each spectrum, and the acquisition time for each spectrum was 520 s, to improve the signal-to-noise ratio at a 4 cm −1 resolution. For individual pressure increments, we recorded a background spectrum and automatically subtracted it from the pattern of the sample.
When collecting a spectrum with new features under pressure, we check the spectra of different positions to verify if the anomalies are general characteristics that are indicative of structural changes or just due to local stress or defects. After competition of the experiments, the spectral manipulation and analyses were undertaken by Jandel Peakfit software package (v4.12, Systat Software, San Jose, CA, USA). A Voight function was used for peak fitting with the minimum components used. The fitting was performed until reproducible results were obtained with squared correlations R 2 ≥ 0.995.

Vibrational Characteristics of Malachite at Atmospheric Pressure and Room Temperature
A representative Raman spectrum of malachite at atmospheric pressure and room temperature (25 • C) is presented in Figure 1. The spectrum is divided into three sections depending on the type of vibration. In the spectral region of 100-600 cm   Figure 2). In common with the Raman spectrum, the O-C-O symmetrical vibration is resolved by two peaks, a progressive one at 1097.22 and a weak companion at 1117.96 cm −1 ; the in-plane mode splits at 711.54 and 747.58 cm −1 , but the asymmetrical stretches are characterized with two broad bands over 1250-1600 cm −1 spectral range which is best deconvoluted as five components. Infrared modes at 815.92, 828.10, and 876.28 cm −1 are derived from the out-of-plane bending (ν2, Raman spectra suffer a lack of signal). There is a mixed band at ~1043.4 cm −1 related to the in-plane and out-of-plane Cu-OH bending. The two O-H stretching peaks are observed with the most absorbance at 3334.72 and 3408.95 cm −1 . Despite the intensity of hydrogen bonding, it is not expected to affect the vibrations of the [CO3] 2− units significantly as the O-C-O bands remain sharp and do not shift much with respect to those in carbonates. Infrared weak modes located in the frequency range of 2850-3000 cm −1 are due to the C-H stretching vibrations from the silicone oil (pressure medium). The above assignments and interpretations are in agreement with those in previous studies [28,29].  The infrared spectrum of malachite encompasses the [CO 3 ] 2− internal modes and O-H stretches ( Figure 2). In common with the Raman spectrum, the O-C-O symmetrical vibration is resolved by two peaks, a progressive one at 1097.22 and a weak companion at 1117.96 cm −1 ; the in-plane mode splits at 711.54 and 747.58 cm −1 , but the asymmetrical stretches are characterized with two broad bands over 1250-1600 cm −1 spectral range which is best deconvoluted as five components. Infrared modes at 815.92, 828.10, and 876.28 cm −1 are derived from the out-of-plane bending (ν 2 , Raman spectra suffer a lack of signal). There is a mixed band at~1043.4 cm −1 related to the in-plane and out-of-plane Cu-OH bending. The two O-H stretching peaks are observed with the most absorbance at 3334.72 and 3408.95 cm −1 . Despite the intensity of hydrogen bonding, it is not expected to affect the vibrations of the [CO 3 ] 2− units significantly as the O-C-O bands remain sharp and do not shift much with respect to those in carbonates. Infrared weak modes located in the frequency range of 2850-3000 cm −1 are due to the C-H stretching vibrations from the silicone oil (pressure medium). The above assignments and interpretations are in agreement with those in previous studies [28,29].  Figure 2). In common with the Raman spectrum, the O-C-O symmetrical vibration is resolved by two peaks, a progressive one at 1097.22 and a weak companion at 1117.96 cm −1 ; the in-plane mode splits at 711.54 and 747.58 cm −1 , but the asymmetrical stretches are characterized with two broad bands over 1250-1600 cm −1 spectral range which is best deconvoluted as five components. Infrared modes at 815.92, 828.10, and 876.28 cm −1 are derived from the out-of-plane bending (ν2, Raman spectra suffer a lack of signal). There is a mixed band at ~1043.4 cm −1 related to the in-plane and out-of-plane Cu-OH bending. The two O-H stretching peaks are observed with the most absorbance at 3334.72 and 3408.95 cm −1 . Despite the intensity of hydrogen bonding, it is not expected to affect the vibrations of the [CO3] 2− units significantly as the O-C-O bands remain sharp and do not shift much with respect to those in carbonates. Infrared weak modes located in the frequency range of 2850-3000 cm −1 are due to the C-H stretching vibrations from the silicone oil (pressure medium). The above assignments and interpretations are in agreement with those in previous studies [28,29].

Vibrational Changes of Malachite under Pressure
Malachite was compressed at room temperature up to 28.9 and 29.2 GPa, respectively, in Raman and infrared spectroscopic measurements, and monitored on both compression and decompression processes. Selected spectra at elevated pressures are displayed in Figures 3 and 4. In addition to the modes observed at atmospheric pressure, two more bands appear in the Raman spectra in the range of 2850-3000 cm −1 which are assignable to the C-H stretches of the pressure medium. In additin, more O-H stretching bands are resolved in the infrared spectra due to the high sensitivity of hydrogen bonding to pressure. Mode frequencies derived from the high-pressure Raman and infrared data are listed in Tables 1 and 2. We selected Cu-O bands and O-C-O stretches from the Raman data for pressure effect analyses, while the modes representative of O-C-O bending and O-H stretches are measured from the infrared spectra. The change in frequency for the selected modes as a function of pressure are plotted in Figure 5 with the fitting results are listed in Table 3.
Three dramatic changes in the vibrational spectra of malachite are evident as pressure grows tõ 7,~15, and~23 GPa. First, upon compression to~7 GPa, the Cu-O modes in the lower-energy range of 300-400 cm −1 redshift whereas the higher-energy (400-600 cm −1 ) blueshift with a more pronounced pressure dependence (d υi /d P ) T (υi is the frequency of a mode under a certain pressure and P is pressure; (d υi /d P ) T means the slope of mode shifting with pressure at room temperature). This observation suggests an increasing degree of the deformation of the [CuO 6 ] octahedron. It is noted that the pressure-induced redshift of the lower-energy modes provides a clue to the instability of malachite under higher pressure. Thus, at~7 GPa, the band initially at 354.56 cm −1 (0.6 GPa) splits into 347.80 and 358.99 cm −1 (Figure 5a). In addition, a sharp increase of ∆ν = 10.5 cm −1 occurs in the frequency of an O-C-O in-plane band 710.17 cm −1 (0.8 GPa) at~7 GPa coupled with a change in the pressure dependence from 0.56(8) cm −1 ·GPa −1 to −1.48(4) cm −1 ·GPa −1 . An out-of-bending band 891.52 cm −1 first redshifts at a slope of −3.60(4) cm −1 ·GPa −1 , but across~7 GPa it turns to blueshift with pressure at 0.92(8) cm −1 ·GPa −1 (Figure 5c). These anomalies are indicative of the onset of a phase transition. The transition pressure determined here is consistent with the results in previous studies [20,21]. As pressure grows to~7 GPa, the [CO 3 ] 2− stretches shift to higher frequencies at a constant slope of 2.12 (3) [25,36]. In this process significant broadness is observed for all O-H stretches.  At ~15 GPa, we notice a merging of double peaks at ~370 cm −1 , a mode split at ~1000 cm −1 , and dramatic changes in the mode intensities of all types. These changes are along with the development of bands at ~250, ~450, ~800, ~1150, ~1480, and ~2800 cm −1 . Further compression enhances the bands ~450 and ~1150 cm −1 , which is suggestive of an increase in the additional Cu-O interactions. The   At ~15 GPa, we notice a merging of double peaks at ~370 cm −1 , a mode split at ~1000 cm −1 , and dramatic changes in the mode intensities of all types. These changes are along with the development of bands at ~250, ~450, ~800, ~1150, ~1480, and ~2800 cm −1 . Further compression enhances the bands ~450 and ~1150 cm −1 , which is suggestive of an increase in the additional Cu-O interactions. The At~15 GPa, we notice a merging of double peaks at~370 cm −1 , a mode split at~1000 cm −1 , and dramatic changes in the mode intensities of all types. These changes are along with the development of bands at~250,~450,~800,~1150,~1480, and~2800 cm −1 . Further compression enhances the bands~450 and~1150 cm −1 , which is suggestive of an increase in the additional Cu-O interactions. The systematical redshifting of the lower-energy Cu-O modes slow down and display zero pressure slope above~15 GPa, yet the higher-energy modes blueshift as usual (Figure 5a). The two bands correspond to the [CO 3 ] 2− internal modes, one at~800 cm −1 increases in amplitude and the other at 1480 cm −1 remains at a similar amplitude under pressure. The most striking change is the mode hardening of the O-H stretches; for example, the band 3251.55 cm −1 redshifts sharply from −12.58(4) cm −1 ·GPa −1 to −1.03(2) cm −1 ·GPa −1 (Figure 5d). The comparatively weaker pressure slope is also observed in the shift of the band appearing at~950 cm −1 at~7 GPa, which confirms that this band contains a contribution from the O-H vibrations. As pressure increases, the two higher-energy bands 3485.88 and 3410.79 cm −1 give rise to a lower-energy band 3251.55 cm −1 , the latter becoming prominent above~15 GPa. A new band emerges at~2800 cm −1 .
As pressure further increases to~23 GPa, the O-C-O stretching modes go on blueshift but with smaller pressure slopes (Figure 5b). The slope reduces from 2.12(3) cm −1 ·GPa −1 to 1.69 (6) (Figure 5c). In addition, a further decrease of ∆ν =~66 cm −1 is observed in the frequency of the O-H band 3410.79 cm −1 over the 20.6-23 GPa span, followed by a rapid reduction in the amplitude (Figure 5d). The other two bands 3485.88 and 3251.55 cm −1 , contrary to the redshift nature, begin blueshift trends at~23 GPa. In this process, no appreciable broadness of the O-H modes is observed.  The vibrational data upon decompression show negligible hysteresis. As seen in Figure 5a,b, the external Cu-O modes and O-C-O stretching modes, shifting to higher frequencies under pressure, are located at a little higher frequencies under similar pressures upon decompression. The redshift O-H stretching modes shift back to higher frequencies during decompression but with lower frequencies under comparable pressures (Figure 5d). When pressure is released completely, the spectra characteristics of the pure malachite are recovered. This indicates that the malachite structure experienced elastic deformations under compression. During the following decompression process, the modes show a slightly delayed response to pressure, which should be accounted for by the hysteresis. The hysteresis is due likely to the polycrystalline sample used here that develops microscopic or local stress at grain boundaries, leading to a slightly delayed relaxation. Four discrete pressure regions with color contrast represent different relations. Linear regression is used for most data analysis except for an O-C-O asymmetrical band ν3-1 in (b-1) and an out-of-bending band ν2-2 in (c-1), both fitted by parabolic curves. The pressure slopes (dυi/dP)T derived from the fitting analyses are listed in Table 3. Table 3. Pressure-dependences of modes (dυi/dP)T (cm −1 ·GPa −1 ) of three types in Cu2(CO3)(OH)2 in different pressure regions.   Four discrete pressure regions with color contrast represent different relations. Linear regression is used for most data analysis except for an O-C-O asymmetrical band ν 3-1 in (b-1) and an out-of-bending band ν 2-2 in (c-1), both fitted by parabolic curves. The pressure slopes (d υi /d P ) T derived from the fitting analyses are listed in Table 3.

Structural Transitions of Malachite under Pressure
We observed three phase transitions of malachite at pressures of~7,~15, and~23 GPa, respectively. A progressive degree of [CuO 6 ] deformation can be identified by the various pressure dependencies of the Cu-O modes. In light of the transition of malachite described in previous studies [20,21], the present results provide vibrational evidence for the notion that the pressurized deformation of the [CuO 6 ] octahedron is responsible for the rotation of the cell unit, resulting in a different orientation of the crystal cell in the higher-pressure phase.
A further increase in pressure (above~15 GPa) leads to the appearance of more modes, which implies that at least the number of formula units in the unit cell changed. Considering the vibrations of [CO 3 ] 2− in carbonates under high pressure conditions [30][31][32][33][34], we attribute the development of the [CO 3 ] 2− internal modes to a decreased symmetry of the local geometry. It is also noted that upon compression from~7 to~23 GPa, the separation between the two O-C-O in-plane bands increases from~25 to~50 cm −1 . Likewise, the pressure-enhanced splitting of the asymmetrical stretches are also notable. The increased splitting originates from an increased asymmetry of the local geometry of [CO 3 ] 2− or more likely, the distortion of [CO 3 ] 2− itself [37]. One can expect a structural rearrangement gradually occurring in this pressure range, involving a tilt effect of the [CO 3 ] 2− units or a conversion from a [CO 3 ] 2− triangle to [CO 4 ] tetrahedron. We suggest that a possible trigger for this change should be that the approach of the triangle O 2− towards the central C 4+ under pressure gets to its limit and the [CO 3 ] 2− approaches as non-planar. As a consequence, the overall compression mechanism of the structure changes. Above~23 GPa, the two principle O-C-O stretching bands slow down in their blueshift process, resulting in lower frequencies of the stretching modes under higher pressures in comparison with the values extrapolated from the lower-pressure data. Since the frequency of the symmetric stretch is predominantly determined by the O-C-O bond stretching force constant, it is a very sensitive probe of the C-O bond strength [37]. The lowered frequencies of the stretches under higher pressures suggest that the C-O bonds become a little bit weaker than those at lower pressures. Such a decreased bond strength may stem from a lengthening of the C-O bond, a shift in the effective charges of C 4+ /O 2− and/or a strengthening of the Cu-O bonds [34]. Here we propose that the combination effect of an elongated C-O bond and a reinforced Cu-O bond is responsible for the weakening of the C-O bonds. The idea that the C-O bond grows longer under pressure has been reported in carbonates magnesite MgCO 3 at 30 GPa, which is probably induced by a progressive rotation of the [MgO 6 ] octahedron [38,39]. Likewise, in malachite the lengthening of the C-O bond may likely be the result of a continuous rotation of the [CuO 6 ] octahedron (as discussed above). Giving rise to the three phase transitions, a growth of pressure has a negligible effect on the intensity of most bands. This implies that the main character of the C-O bonds remains unchanged. The rigidness of the [CO 3 ] 2− units to pressure is very common in [CO 3 ] 2− -bearing minerals. For comparison, a series of [SiO 4 ]-phases tend to amorphize under comparable pressure-temperature conditions [40,41]. Thus, the stability of [CO 3 ] 2− is superior from a crystal chemical standpoint. In addition, the observed phase transitions seem to not involve the broken Cu-O, C-O, or H-O bonds as the transitions are readily reversible.
The chemistry of hydrogen bonding in malachite during compression is complex: it shows a mode disappearance (3232.14 cm −1 , 0.8 GPa) and mode emerging (~950 cm −1 ) at~7 GPa, coupled with a decrease in the frequency of ∆ν =~48 cm −1 (3410.79 cm −1 , 0.8 GPa) over 5.1-7 GPa pressure span. At~15 GPa it exhibits a systematical mode hardening, and over the 20.6-23 GPa interval, there is a further decrease in the frequency of ∆ν =~66 cm −1 (3410.79 cm −1 ). These changes hint at the presence of O-H bonds in many types, with the evolution of each type of bond showing its own trend. The phenomenon that the O-H modes generally shift to lower frequencies reflects an enhancement in the strength of the hydrogen bonding involving the shortening of the O-H bond. The observation of a pressurized significant broadness in the band width indicates that the O-H···O bonds may become more asymmetric [42] or there may be unresolved band splitting. According to the correlation function between O-H stretching frequencies and O···O bond length [43], we calculated the distance of O···O as 2.72, 2.80, and 2.87 Å, respectively, for 3251.55, 3410.79, and 3485.88 cm −1 bands at 0.8 GPa. When pressure increased to 15.8 GPa, the O···O length decreased successively as 2.66, 2.73, and 2.79 Å; but above 15.8 GPa, the O···O length corresponding to the 3251.55 cm −1 band remains almost unchanged, whereas the other two shorten to 2.69 and 2.77 Å at 23.0 GPa, respectively. Above~23 GPa, a blueshift upon pressure is noted which, to the best of our knowledge, is not common in the case of O-H stretches. It has been suggested that due to H-H repulsion and O-H bond elongation, the O-H stretches will blueshift with pressure increasing [44][45][46][47]. In consideration of the sites and coordination mode of hydrogen in malachite, we propose that the blueshift of the O-H stretches may possibly result from some re-organization of the hydrogen due to the compression of the coordinated [CuO 6 ] octahedron and a substantially increasing repulsion effect from Cu 2+ . Further, the O···O distance connected to the hydrogen shows no further shortening in the pressure range of 23.0-29.2 GPa. As the pressure dependence of the O-H stretches changes at~23 GPa, matching closely the pressure at which the O-C-O bending starts to soften, we speculate that the evolution of the [CO 3 ] 2− units which connect with [CuO 6 ], exerts a significant impact on the anisotropic compression of the [CuO 6 ] octahedron, and further on the local geometry of hydrogen. In the entire compression process, the main change in the O-H spectra is the evolution of the complex contour, involving two mixed bands dominantly at 3410.79 and 3485.88 cm −1 in a somewhat blurred contour at 0.8 GPa, which varies to a more blurred contour involving three components over a frequency span of 3000-3400 cm −1 prominently at 3391.76 cm −1 above~15 GPa, grows in amplitude at 3051.26 cm −1 above~23 GPa, and preserves this shape until~29 GPa. This evolution confirms a gradual reorientation or reordering of hydrogen under pressure. In spite of a little blueshift of several modes (including Cu-O, O-C-O, and O-H), no considerable redshift was noted for the majority of the vibrations. This eliminates the possibility of an overhydrated state of malachite at the present experiment's conditions, but malachite has an easily pressurized deformable structure with rather large interspaces between the adjacent [CuO 6 ] layers, which is anticipated to be capable of accommodating additional amounts of water. This hypothesis calls for further verification by high-pressure and temperature experiments using a water medium.

Expectations on Hydroxycarbonates in the Earth's Mantle
At the molecular level, the site and stability of hydrogen in hydroxycarbonates are dependent on bond strength and local geometry; both are constrained directly by the coordinated polyhedron. In addition, the existence of the [CO 3 ] 2− units has an indirect, but important influence on the geometry of hydrogen. For example, in malachite the mode softening of O-C-O bending at~23 GPa is related to the weakening of the strength of the hydrogen bonding. In fact, as the hydrogen bond acceptor, the strength of the C-O bond is also greatly modified [15,16]. Their synergistic relations have already been documented by the experimental observation that the water and CO 2 will be released simultaneously above 380 • C at atmospheric pressure [18]. We speculate that some interrelations in the stability of water and carbon within hydroxycarbonate exist at high-pressure and temperature conditions. Our attempts to the high-pressure vibrations of malachite emphasize that the stability of the hydrogen bonding can be extended extensively by pressures up to~23 GPa. In this regard, we may expect an increased capacity in the water storage of malachite, associated with carbon storage, approaching, at most, the bottom of the Earth's transition zone. The stability of hydrogen in malachite may hold true for other hydroxycarbonates. In nature, more and more hydrogen-bearing and hydrous minerals (e.g., a phase Egg [48]; a water-bearing ringwoodite [13]; or ferropericlase [49]) have been discovered as inclusions in diamonds originated from the transition zone and the topmost of the lower mantle. These findings strongly support the view that not only is the mantle at least locally hydrated, but also the water recycling in the hydrous mantle associates closely with the carbon circulation.
The properties of water under mantle conditions have been the subject of petrologic and geophysical interest for decades. The water storage capacity of malachite, relative to dense hydrous magnesium silicates, following a relatively cold temperature path, is expected to be at analogous levels but with superior stabilities. The present spectroscopic data provide a manifestation and pressure range of the stability of hydrogen in malachite, which could have relevance for the high-pressure behaviors of other hydroxycarbonates (M-CO 2 -H 2 O systems, M = Cu, Zn, Mg, etc.). The stability field of hydroxycarbonate should be further characterized by careful inspection using X-ray diffraction and vibrational measurements at simultaneously high-pressure and temperature conditions.

Conclusions
The present study explored the high-pressure vibrations of a hydroxycarbonate, malachite Cu 2 (CO 3 )(OH) 2 , by in-situ Raman and infrared spectroscopic measurements at room temperature up tõ 29 GPa. Substantial changes in the mode intensities and discontinuities disclose three phase transitions at pressures of~7,~15, and~23 GPa. Upon compression, the Cu-O bands shift variously with pressure, suggesting an increasing degree of deformation of the [CuO 6 ] octahedron. The O-H stretching modes redshift with considerable pressure slopes up to~15 GPa, above which a mode hardening occurs which reflects some reorientation or reordering of the hydrogen bonding or a transition of the hydrogen sites. These changes indicate the occurrence of a new phase which is characterized with additional modes pointing to a different number of formula units in the unit cell. At further increments in pressure abovẽ 23 GPa, the O-C-O in-plane modes soften and the O-H stretches start to shift to higher frequencies.
These anomalies imply a weakening of the hydrogen bonding due at least partly to the evolution of the [CO 3 ] 2− units. Our results thus suggest an increasing capacity of water storage of malachite up tõ 23 GPa. The evolution of hydrogen bonding in malachite varying with pressure may hold true for other analogous hydroxyl minerals. Moreover, we highlight that in hydroxycarbonates the stability of hydrogen seems to be associated with that of carbon.
Author Contributions: Conceptualization, methodology, formal analysis, resources, data curation, project administration, funding acquisition, supervision, writing-original draft: J.G.; investigation, software, validation, visualization, writing-review and editing: J.G. and X.Y. All authors have read and agreed to the published version of the manuscript.