*3.2. Mineralogical Transformations Along with Activation Temperatures*

The evolution in the mineral composition at different temperatures and residence times has been studied using XRPD. Serpentine shows high crystallinity in samples U, A, and B, respectively untreated, treated at 550 ◦C for 15 min, and treated for 60 min. It then decreases in samples C, D, E, and F, respectively treated at 650 ◦C for 15 min, 30 min, and 60 min, and at 750 ◦C for 15 min. Crystalline features disappear in samples F and G, respectively, at 750 ◦C for 15 and 30 min. Amorphous contents can be identified in all of the treated samples as the crystallinity decreases. Forsterite is observed in samples E, G, and H, shown by highly crystalline peaks.

The remaining magnetite (the small proportion not removed during gravimetric separation) shows peaks in all samples, whereas the hematite (Fe2O3) which appears during the duration and temperature of the test increases. Due to the tests being performed in atmospheric conditions, the iron in the ferrous form (Fe2<sup>+</sup>) contained in the serpentine structure is oxidized into ferric iron (Fe3+) [50]. As iron rich olivine (fayalite—Fe2SiO4) can essentially incorporate Fe2<sup>+</sup> in its structure [51], hematite (Fe2O3) is preferentially formed.

Table 5 presents phases quantification as measured using the Rietveld refinement. Three issues are faced: (i) these values do not consider the mass loss occurring during thermal treatment (ii) amorphous components are identified in the untreated sample, due to the stacking disorder of serpentine, making the identification of thermally induced amorphous components difficult, and finally (iii) a small peak is observed in the low angle that can be attributed to illite thus undermining the observation of the formation of meta-serpentine as described by [21]. Wilson et al. [38] determine that absolute quantification errors (wt %) for serpentine (chrysotile) and non-serpentine phases, regardless of their abundance in a sample, to be under 5.0 wt %. Consequently, illite is not considered in the Rietveld refinement as their peaks are too low and would fall under the estimation limit.


**Table 5.** Mineral composition using Rietveld refinements on XRD patterns, given in wt %.

In an attempt to overcome these issues, a mass factor (MF in Equation (5)) is computed based on the mass loss of each sample (Table 4). Using this factor, the abundance of each phase can be expressed as grams per 100 g of starting material as given in Equation (5).

Equation (5): Proportion of phases expressed in mass

$$m\_{\text{phase}} = \sqrt[\text{\text{\textquotedblleft}phase}]{\text{\textquotedblleft}phase} \times \left(\frac{100}{100 + \text{\textquotedblleft}\text{\textquotedblright}\text{\textquotedblright}}\right) = \sqrt[\text{\textquotedblleft}phase} \times \text{MF.} \tag{5}$$

As dehydroxylation is considered to be the loss of H2O from the structure, the mass of H2O lost per gram of serpentine is computed in order to obtain the proportion of dehydroxylated serpentine (Equation (6)). The value used as maximum mass loss "%mass loss max" was obtained experimentally and found to be 14.2% for this material.

Equation (6): Proportion of dehydroxylated serpentine

$$\% \text{ dehydration} \times \text{perpe} = \frac{\% \text{mass loss} / \sum (m\_{\text{amphaus}} + m\_{\text{surpino}})}{\% \text{ mass loss}^{\text{max}}} = \frac{m\_{\text{MEO lost}}}{\% \text{mass loss}^{\text{max}}}.\tag{6}$$

The initial remaining material is decomposed into a non-reacted serpentine (serpentine(i)) associated with a non-reacted amorphous phase (amorphous(i)) induced by the layered structure of the serpentine. Their masses are calculated according to Equation (7), assuming that amorphous phase and crystalline initial serpentine both dehydroxylated in the same proportion.

Equation (7): Mass of initial phases

$$m\_{\text{phase}\_{(i)}} = (m\_{\text{phase}}) - (m\_{\text{phase}} \times \% \text{ debyday} \text{}).\tag{7}$$

The amount of dehydroxylated serpentine and amorphous phase corresponding to the first amorphous observed, (respectively named serpentine(d) and amorphous(d)) are given by Equation (8). Equation (8): Mass of intermediate amorphous phases

*m*phase(d) = *m*phase − *m*phase(i). (8)

Further dehydroxylation leads to the formation of meta-serpentine, whose mass is obtained by Equation (9) This formation is marked by the total loss of the hydroxyls groups at close to 10 wt % of the starting material mass.

Equation (9): Mass of meta-serpentine

$$m\_{\text{meta-sepertion}} = \left(m\_{\text{amorphous}} \times m\_{\text{H2O lost}}\right) - m\_{\text{amorphous}\_{(i)}} \,. \tag{9}$$

As a result, three phases emerge from this calculation: first an initial serpentine, resulting from the sum of amorphous(i) and serpentine(i), then an intermediate amorphous components which is the sum of amorphous(d) and serpentine(d) corresponding to the first stage of amorphization, and finally meta-serpentine. Forsterite and iron oxides (magnetite and hematite) remain unaltered by the calculation.

As shown in Table 6, Serpentine is gradually replaced by intermediate amorphous phases in samples treated at temperatures lower than 650 ◦C and peaks for 60 min treatment at 70.3 g/100 g of starting material. Meta-serpentine is first found in samples treated at 650 ◦C for 15 min. Its proportion increases with the temperature and peaks at 27.2 g/100 g of starting material in the sample treated at 750 ◦C for 15 min. The increase of meta-serpentine is combined with a decrease of intermediate amorphous components contents. As seen previously (Table 5), forsterite is observed in samples E, G and H, respectively treated at 650 ◦C for 60 min and at 750 ◦C for 30 and 60 min. A treatment at 750 ◦C for 15 min produced a sample with no initial serpentine and no forsterite but only amorphous phases, associated with iron oxides. These observations are in agreement with previous studies which observed the formation of an intermediate amorphous component, α meta-serpentine, progressively replacing serpentine below 580 ◦C. It is then followed by the appearance of an amorphous meta-serpentine material by 650 ◦C prevailing by 750 ◦C [21].


**Table 6.** Mineralogical compositions based on Rietveld refinements, expressed in grams per 100 g of starting material) at given temperature and residence times. In. Serp: Initial serpentine, Inter. Am.: Intermediate amorphous components, Meta-serp.: Meta-serpentine, For.: forsterite, Mag: magnetite, Hem: hematite and ML: Mass loss.
