Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 8 January 2021 doi:10.20944/preprints202101.0154.v1 Article Thermal Annealing and Phase Transformation of Serpentine- like Garnierite Arun Kumar1,2, Michele Cassetta1, Marco Giarola3, Marco Zanatta4, Monique Le Guen5, Gian Domenico Soraru6, Gino Mariotto1,* 1Department of Computer Science, University of Verona, 37134, Verona, Italy 2CNR-Institute for Microelectronics and Microsystems, Agrate Brianza, 20864, Agrate, Italy 3Centro Piattaforme Tecnologiche (CPT), University of Verona, 37134, Verona, Italy 4Department of Physics, University of Trento, 38123, Povo, Trento, Italy 5Mining Division, ERAMET, 78190, Trappes, France 6Department of Industrial Engineering, University of Trento, 38123, Povo, Trento, Italy *Correspondence: [email protected] Abstract: This study deals with vibrational and crystallographic aspects of the thermally induced transformation of serpentine-like garnierite into quartz, forsterite, and enstatite occurring at about 620 °C. Powder specimens of garnierite have been annealed in static air between room temperature and 1000 °C. The resulting products from the transformations detected based on thermogravimetric and differential thermal analysis, have been extensively characterized via microRaman spectros- copy, 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. Keywords: Garnierite, Phase transformation, TGA/DSC, XRD, micro Raman spectroscopy 1. Introduction In 1863, Jules Garnier discovered a nickel ore in New Caledonia, a magnesium-based nickel silicate later named as Garnierite [1]. Garnierite is mined from laterite deposits, and it is formed by weathering and secondary mineralization of igneous ultra-mafic rocks. Garnierite with the serpentine group of minerals is generally a mix of various Ni and Ni- Mg layer silicates [2-4]. However, there has always been difficulty in the identification of the garnierite minerals [1,5,6], as these minerals are poorly crystalline, naturally fine- grained nature as well as their frequent occurrence as mixtures of serpentine, talc, chlorite, sepiolite, and smectite minerals. Further, it has been shown that Ni2+ can substitute for Mg2+ in the serpentinite mineral lizardite since the two cations have similar ionic radii [2,7]. Because of their fine-grained, inhomogeneous distribution and their generally poor crystalline order, garnierite would seem to be very appropriate materials for characteri- zation. However, if the initial minerals are mixtures of hydrous silicates, the phase devel- opment will depend on the distribution of nickel inside the mineral, they transform inde- pendently. Thus, the thermal transformations of garnierite are of interest for studying the primary nature of the minerals [8,9]. Furthermore, the thermal decomposition of these hydrous silicates provides insight about the thermal stability of garnierite. This is an im- portant tool for the assessment of these types of materials in terms of mineral chemistry and shed a light on the amount of water on their structure [5], that has been confirmed very insightful to differentiate among the serpentine minerals [5,8,9]. In this study, thermal analysis has been applied to the study of the structural trans- formation of garnierite powder, which was unraveled by means of X-ray diffraction (XRD) and micro-Raman spectroscopy. In particular, microRaman spectroscopy constitutes a very powerful and in the meanwhile not invasive tool for mineral phase identification in geological samples, as well as in the characterization of the heterogeneous materials in © 2021 by the author(s). Distributed under a Creative Commons CC BY license. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 8 January 2021 doi:10.20944/preprints202101.0154.v1 terms of component phases [10-13]. It has been employed here to investigate both the serpen- tine-like garnierite and its thermal phase transformation, which is not reported in previous stud- ies.Therefore, the results of the present study can be compared only with the published literature using XRD analysis to study phase transformation of garnierite [14]. 2. Materials and Methods A garnierite sample (labeled as GN) coming from a mine of New Caledonia, was provided by ERAMET RESEARCH GROUP within the EU-H2020 SOLSA project grant no. 689868 (http://www.solsa-mining.eu), funded within the EU-H2020 Raw Material pro- gram. The sample was quite hard and compact rock, grey/brown colored, of irregular form, and some cm in size. For this study, some pieces of the solid rock were grinded manually using the mortar pestle to a finely grounded powder. The optical image of the sample taken with a high-quality camera is shown in Fig. 1. Figure 1. Optical image of the garnierite sample. The thermal stability of the garnierite powder sample was investigated by differen- tial scanning calorimetry (DSC) and thermogravimetric analysis (TGA) using an STA 449C Jupiter® analyzer (Netzsch-Gerätebau GmbH, Selb, Germany) from room tempera- ture 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 pow- der. The mineral characterization of the pristine and treated powders was done by means of XRD and micro-Raman spectroscopy. XRD measurements were acquired with a Thermo ARL X’TRA powder diffractome- ter, 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. Before the XRD measure- ments, the diffractometer was calibrated with a reference silicon powder (d111 = 3.1353 Å). Micro-Raman spectra were acquired in backscattering geometry with Horiba Jobin Yvon, LabRAM (HR800) spectrometer equipped with a 632.8 nm He-Ne laser and an op- tical microscope (Olympus, BX41). The laser beam was narrowed and focused through a 75-µm entrance pinhole and an 80x long working distance objective having 0.75 NA yield- ing a spatial resolution of about 1 µm and an irradiation power of 1.5 mW at the sample surface. The scattered light were dispersed using a grating with 600 lines/mm and col- lected with 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 cm- 1 and 4000 cm-1 was obtained by using the LabSpec (version 5) software package that merges several spectra with different wavenumber ranges by averaging the spectral in- tensities in the overlapping regions. Before performing measurements, the spectrometer was calibrated by using the 520.4 cm-1 peak of crystal Si. 3. Results and Discussion The thermogravimetric analysis of the serpentine type garnierite, weight loss curve, and the phases formed at the various temperatures as presented in Fig. 2. Optimum loss of hydration water is observed at 40-90 °C (first endothermic peak on DTA curves). Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 8 January 2021 doi:10.20944/preprints202101.0154.v1 Optimum temperatures of dihydroxylation of the layered silicates occur at 560-645 °C for the 7 Å- type (second endothermic peak) and 815-825 °C for the 10 Å - type (third endo- thermic peak), as already reported in the literature [7,15] for Ni-rich ‘garnierite’ veins, serpentine-like as well as serpentine + talc-like, respectively. This garnierite sample shows three distinct weight loss percentages; the first one is a continuous weight loss of about 3% from room temperature to up to 500 °C. The second one is a well-observed weight loss of about 5.5% at 620 °C, well known as an endothermic peak and the third one is of about 1.5% at 813 °C, which is well defined as an exothermic peak. The endothermic peak at 620 °C is assigned to the de-hydroxylation of the Ni incapacitated serpentine. After the de- hydroxylation at 620 °C, the anhydrous garnierite like serpentine breakdown and for- mation of anhydrous silicates crystals (olivine, enstatite) at 813 °C occurs. Further, the well-established mechanism of thermal transformation for the equilibrium of serpentine and talc [8,15,16] are shown below: Mg3Si205(OH)4 = Mg2SiO4 + MgSiO3 + 2 H2O (1) serpentine olivine enstatite Mg3Si2O5(OH)4 = (3/2) Mg2SiO4 + (1/2) SiO2 + 2 H2O (2) serpentine olivine quartz Mg3Si4O10(OH)2 = 3 MgSiO3 + SiO2 + H2O (3) talc enstatite It was shown that up to 900 °C the serpentine mainly forms olivine but if it serpen- tine-like garnierite with low Ni content, up to this temperature, it will transform mainly into olivine but possibly also enstatite as shown in equation (1). But, if the content of Ni is high, it gives two main phases, olivine (forsterite) and quartz as shown in equation (2). Further, 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. From the above well-established liter- ature in this type of minerals, we were able to demonstrate that our material is basically, serpentine-like garnierite with low Ni contents, which upon thermalization up to 900 °C transforms into olivine (forsterite), and small segregation of enstatite was also observed. However, the presence of quartz was already there in the initial mixture, and it was re- ported that the phase transformation depends on the initial minerals mixtures, and the distribution of nickel between the components, gives rise to transform them individually [8], which completely fit our results with XRD and microRaman spectroscopy. Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 8 January 2021 doi:10.20944/preprints202101.0154.v1 Figure 2.