Thermal Annealing and Phase Transformation of Serpentine-Like Garnierite

This study is focused on the vibrational and microstructural aspects of the thermally induced transformation of serpentine-like garnierite into quartz, forsterite, and enstatite occurring at about 620 °C. Powder specimens of garnierite were annealed in static air between room temperature and 1000 °C. The kinetic of the transformation was investigated by means of thermogravimetric and differential thermal analysis, and the final product was extensively characterized via micro Raman spectroscopy and X-ray diffraction. Our study shows that serpentine-like garnierite consists of a mixture of different mineral species. Furthermore, these garnierites and their composition can provide details based on the mineralogy and the crystalline phases resulting from the thermal treatment.


Introduction
In 1863, Jules Garnier discovered a nickel ore in New Caledonia, a magnesium-based nickel silicate later named garnierite [1]. This rock is mined from laterite deposits, and it is formed by weathering and secondary mineralization of igneous ultra-mafic rocks. Garnierite is classified as a hydrous Ni-Mg-phyllosilicate [2][3][4]. However, the identification of the garnierite minerals composition is a well-known open problem [1,5,6], as these minerals are poorly crystalline, naturally fine-grained, inhomogeneously distributed, and frequently consist of mixtures of serpentines, talc, chlorite, sepiolite, and smectite. Nickel is prevalently hosted in serpentine-like, talc-like, or clay-like phases [7,8]. For instance, in Ni-bearing serpentines, it has been shown that Ni 2+ can substitute for Mg 2+ in the serpentine mineral lizardite, since the two cations have similar ionic radii [2,7]. The global request of nickel for industrial and technological application makes the study of possible sources as garnierite ores extremely interesting [1]. Since the garnierite mineral are a mixture of hydrous silicates, the phase occurrence will depend on the distribution of nickel inside the mineral, and each phase transforms independently. Thus, the thermal transformations of garnierite are of interest for studying the primary nature of the minerals [9,10]. Furthermore, the thermal decomposition of these hydrous silicates provides insight about the thermal stability of garnierite. This is an important tool for the assessment of these types of materials in terms of mineral chemistry and sheds light on the amount of water on their structure [5], which has been confirmed as very insightful for differentiating among the serpentine minerals [5,9,10].
In this study, thermal analysis has been applied to study the structural transformation of garnierite powder, whose mineralogy was extensively characterized by means of X-ray diffraction (XRD) and micro -Raman spectroscopy. In particular, micro -Raman spectroscopy constitutes a very powerful and non-invasive tool for the identification of mineral phases in geological samples, as well as for the characterization of the heterogeneous materials [11][12][13][14]. Here, micro -Raman spectroscopy has been employed to investigate both the serpentine-like garnierite and its phase transformation under thermal treatment, which is not reported in previous studies as per the best of our knowledge.

Materials and Methods
A garnierite sample coming from a mine of New Caledonia was provided by ERAMET IDEAS within the EU-H2020 SOLSA project grant no. 689868 (http://www.solsamining.eu), which was funded within the EU-H2020 Raw Material program. The sample was a quite hard and compact dark grayish rock with white veins, of irregular form, and some cm in size. An optical image is shown in Figure 1 (leftmost image). For the thermal treatment, some pieces of the garnierite sample were manually ground with a pestle in a mortar, thus obtaining a fine brown powder (rightmost image in Figure 1). The elemental composition of this pristine powder was determined by an environmental scanning electron microscope (ESEM) coupled with an energy-dispersive X-ray (EDX) analyzer. Both elemental spectrum and elemental composition are shown in Figure S1 and in Table  S1, respectively (Supplementary Material). The thermal stability of the garnierite powder sample was investigated by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) using an STA 449C Jupiter® analyzer (Netzsch-Gerätebau GmbH, Selb, Germany) from room temperature to 1000 °C at a heating rate of 5 °C min -1 and a static air flux of 40 ml min -1 . For TGA, 50 mg garnierite powder was measured along with a reference sample of aluminum powder.
Micro -Raman spectra were acquired in backscattering geometry with a Horiba Jobin Yvon LabRAM HR800 spectrometer equipped with a 632.8 nm He-Ne laser and an optical microscope (Olympus, BX41). The laser beam was spatially filtered by a 75µm pinhole and focused onto the sample surface by a long working distance (LWD) 80x objective having 0.75 NA. This yielded to a spatial resolution of about 1 µm with an irradiation power at the sample surface of 1.5 mW. The scattered light was analyzed using a 600 lines/mm grating and collected by a Si-based CCD (charge-coupled device) detector of 1024 × 256 pixels cooled with liquid nitrogen. A composite spectrum covering the spectral range between 150 and 4000 cm -1 was obtained by using the LabSpec (version 5) package that merges several partial spectra with different wavenumber ranges. Before the measurements, the spectrometer was calibrated using the 520.4 cm -1 peak of crystal Si. XRD measurements were acquired with a Thermo ARL X'TRA powder diffractometer, operating in the Bragg-Brentano geometry and equipped with a Cu-anode X-ray source (Kα, λ = 1.5405 Å), using a Peltier Si(Li) cooled solid-state detector. The patterns were collected with a scan step of 0.03° in the 10-70 2θ range. The diffractometer was calibrated with a reference silicon powder (d111 = 3.1353 Å).

Results and Discussion
The starting garnierite sample was extensively characterized by means of micro Raman spectroscopy. In particular, measurements were acquired along the gray veins (see Fig. 1) correlating the sample surface texture shown by the microscopic image with the mineral phases revealed by the Raman spectra. Three typical results for such an analysis are reported in Figure 2 for the three major mineral phases, i.e., the quartz and two hydrous Mg-Ni silicates. In the Raman spectra, we highlighted the low wavenumber (150-1100 cm -1 ) and the high-wavenumber (3550-3950 cm -1 ) regions: the first contains the vibrational fingerprints of the mineral species, while the second displays the bands of the structural water, which is pivotal to identify the serpentine forms.
As already mentioned, quartz is one of the major phases detected. Its Raman spectrum is displayed in Figure 2a: the fingerprints of the quartz are a strong narrow peak at 464 cm -1 together with a band centered at about 207 cm -1 and a small peak occurring at about 356 cm -1 [15][16][17]. Figure 2b shows the typical Raman spectrum of serpentine-like garnierites (Ni-Serpentine), which is characterized by strong Raman bands at about 227, 382, and 691 cm -1 . The peaks at about 227 and 382 cm -1 can be assigned either to minerals of the serpentine group or to talc. These peaks are less intense (or even absent) when the garnierite contains high amounts of kerolite-pimelite [18]. We can say that a low amount talc-like phase is also present in the pristine garnierite ore. In the literature, the band at 227 cm -1 is attributed to either metal-oxygen [17] or O-H-O vibrations [19,20]; whereas the peaks at about 345 cm -1 and 382 cm -1 are due to metal-oxygen [18], SiO4 bending/symmetric [19,20], and the one at about 689 cm -1 to Si-O vibrations [19][20][21], in all serpentine minerals. Finally, Figure 3b shows the spectrum of talc with a strong band peaked at about 680 cm -1 , but the other intense bands shown by Ni-serpentine are absent. In contrast, it shows intense bands at around 187 cm -1 and a strong peak at 376 cm -1 .
The spectra of both the garnierite types show a feature due to Si-O vibrations. However, while in serpentine-like garnierite, it is a strong peak at about 691 cm -1 , in the talk-like one, it appears to have a band at about 680 cm -1 . Therefore, this difference can be used to discriminate between the two species [19][20][21][22][23]. Raman spectra are also sensitive to the effect of the Ni content in Ni-bearing phyllosilicates. As a matter of fact, the effect of the cations in the octahedral site and the Raman band positions below 600 cm -1 in phyllosilicates are well discussed in the literature [24]. However, this effect is small, and upon comparing the spectra of garnierite and Ni-free serpentine or talc ones, no significant differences were observed at low frequency [5]. Conversely, the spectral region above 3500 cm -1 provides very interesting structural insights about the serpentine minerals. In fact, serpentine-like garnierite shows a broad spectral feature consisting of a strong band centered at about 3685 cm -1 and of a pronounced shoulder peaked at 3697 cm -1 ; see Figure 2b. In talc-like garnierite, a broader feature, consisting of three spectral components, peaked at about 3647, 3669, and 3685 cm -1 respectively, is observed; see   Figure 3 shows the thermal analysis on garnierite powder with the results of the TGA (blue line) and DTA (red line). Considering the TGA results, this garnierite sample shows that we detect three distinct weight loss steps. The first one is a continuous weight loss of about 2%, going from room temperature up to 500 °C. Conversely, the second step is a well-defined weight loss of about 5.5% at 620 °C and corresponds to an endothermic peak in the DTA curve. Finally, the third weight loss is of about 1.5% and occurs at 813 °C, and it is accompanied by a strong exothermic peak in the DTA. Several studies performed on Ni-rich garnierite veins [7,28] have displayed that optimum temperatures of dihydroxylation of the layered silicates occur at 560-645 °C for the 7 Å-type, which match with serpentine-like mineral. The same authors propose that dihydroxylation taking place in the range of 815-825 °C can be assigned to the 10 Å-type, which matches with serpentine + talc-like minerals. Consequently, the endothermic peak at 620 °C can be assigned to the de-hydroxylation of the Ni incapacitated serpentine. Increasing the temperature, the anhydrous serpentine-like garnierite breaks down, and the exothermic at 813 °C can be ascribed to the formation of anhydrous silicates crystals such as olivine and enstatite. Furthermore, the well-established mechanisms of thermal transformation for the equilibrium of serpentine and talc [9,28,29,30]  It was shown that up to 900 °C, serpentine mainly transforms into olivine, while serpentine-like garnierite with low Ni content turns into olivine and possibly also into enstatite, as shown in Equation (1). However, if the content of Ni is high, it transforms into two main phases, olivine (forsterite) and quartz, as shown in Equation (2). Furthermore, for talc-like garnierite, with low Ni content, the phase transformation gives rise to enstatite, as shown in Equation (3), whereas with high Ni content, it transforms into olivine phase up to the same range of temperature. Consequently, the identification of the mineral composition of the treated powder can shed some light on the nature of our garnierite.
The above-mentioned phase transformation scenario was confirmed by XRD and micro-Raman on treated powders. Figure 4 shows the XRD pattern of the garnierite powder before and after the thermal treatment. As expected, the pristine garnierite powder is heterogeneous, and it mainly consists of quartz. The XRD pattern of quartz shows the major diffraction peaks at about 20.77°, 26 [34,35]. These broad peaks could be due to the intermixing of talc and quartz phases. XRD results reveal that the diffraction peaks of serpentine-like garnierite dominate over the quartz peaks with respect to the Ni content. After the thermal treatment, the XRD pattern still shows the presence of quartz peaks that progressively become sharper and better defined. Another major change observed is the degradation of the Bragg peaks of serpentine minerals at about 36°, 42°, and 52.2° and their replacement by small reflections or shoulders. Specifically, the sharp overlapping reflections originating from the broad band at about 2θ = 36.2° for the untreated mixed serpentines (i.e., 2.45 Å for chrysotile-2M, 2.51 Å for lizardite-1T, and 2.55 Å for antigorite) tend to wane and be replaced by a broad two-dimensional diffraction band of olivine (forsterite) and enstatite (see Figure 4), indicating a high degree of stacking disorder in silicate layers [28,30,32,33]. These results indicate that the phase transformation of the garnierite to the olivine (forsterite) and enstatite has occurred and they are in good agreement with the previously reported literature [9,30].
The phase transformation from garnierite to olivine (forsterite) and enstatite is also evidenced by micro Raman spectroscopy; see Figure 5. In order to identify the mineral phases, the recorded Raman spectra were compared to reference spectra from both the RRUFF database [31, 36,37]    In some cases, besides these typical phases, the occurrence of enstatite was also detected. This is the case of Figure 5b, where the contribution of forsterite is dominant but enstatite is clearly present. The characteristic bands of enstatite are peaked at about 343, 662, and 684 cm -1 [45,46]. In general, Raman spectra recorded from the treated powders show the co-existence of two or more phases, which turn out in fair agreement with those detected by XRD.

Conclusions
In conclusion, if we combine the results of the thermal analysis and the phase characterization of the treated powder, we can assess the nature of the starting garnierite sample. Indeed, combining our results, we demonstrate that our material is basically serpentine-like garnierite with low Ni contents, which upon thermalization up to 900 °C transforms into olivine (forsterite) with a small segregation of enstatite. Furthermore, we can also infer that the low amount talc-like phase is present in the pristine garnierite ore. Thermogravimetric analysis proves there are at least three decomposition steps attributed to dehydration, dihydroxylation, and decomposition of the formed phases. The mechanism for the thermal decomposition of garnierite is highlighted. Serpentine-like garnierite transforms between 800 and 1000 °C, depending on its NiO content. Serpentinelike garnierite, with low NiO content, transforms mainly to an olivine phase. Consequently, the formation of olivine and enstatite as a result of the thermal annealing of garnierite is supported both by XRD and micro Raman spectroscopy results. These results may help identify mixed crystalline phases in the initial garnierite ores. Such an approach can also help in geometallurgy, to study the transformations of mineral phases in a furnace during a pyrometallurgical process.