NIR-MID Reflectance and Emissivity Study at Different Temperatures of Sodium Carbonate Minerals: Spectra Characterization and Implication for Remote Sensing Identification

Abstract

Recent satellite observations and the deconvolution of remote sensing data have shown the existence of various carbonate minerals in different solar system bodies. Emissivity, from 403 to 803 K, and reflectance spectra at 300 and 193 K of selected carbonates minerals with different ratios of Na:Ca and water amounts were respectively collected at 3–20 μm and at 1–16 μm. All reflectance spectra show absorption features in the 1.9–2.5 and 3.4–4.0 μm areas due to overtone and a combination of CO₃²⁻ and fundamental vibrational models at ≈9.09, 11.35, 7.06, and 14.7 μm. The increase of the Na:Ca ratio in anhydrous samples produces a shift of the absorption features in the 3.4–4.0 μm area toward shorter wavelengths, and the peak at 3.9 μm doubles in the presence of a CO₃²⁻ oxygen group shared with two cations in minerals having more complex structures. The comparison of the bands at ≈669 and ≈794 cm⁻¹ in the emissivity spectra collected at high temperatures indicates that around 600 K, phase transitions occurred in natrite and thermonatrite. The reflectance spectra measured at 193 K reveal a fine structure compared to spectra collected at room temperature. The comparison of laboratory results with the spectrum of Ceres’s brightest crater Occator from the Dawn mission, taken as a case study, showed how the anhydrous samples, shortite and nyerereite, studied in this work can also be hypothesized for Ceres’s surface beyond that already suggested (trona, natrite, thermonatrite).

1. Introduction

Near-infrared spectroscopy (NIR) and mid-infrared spectroscopy (MID) permit the identification of minerals according to their compositional/structural properties. The assignment of experimental data to remote sensing observations of planetary body surfaces (e.g., [1,2,3]) is of paramount importance to detect the mineralogical composition of the surfaces of planetary bodies in the solar system.

Carbonates are relevant minerals to understand the surface composition of several planetary bodies and are crucial environmental markers. Some lines of evidence indicate that different types of carbonate minerals that occur on the surface of Mars, Ceres, Enceladus, and Europa [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19].

Carbonates were detected on the Martian surface through remote sensing techniques and in situ observations by rovers [4,5]. The presence of carbonates on the surface is attested by many spectral evidences; e.g., Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) detected carbonates in Noachian terrains using near-infrared reflectance [5]. These minerals have been also discovered at the Phoenix and Gusev landing sites [6,7]. Carbonate is an expected weathering product of water and basalt in an atmosphere with CO₂ [20]. They are important records of fluid activity in the Martian surface and water–rock system interaction in relation to potentially living systems in the ancient past of Mars [21].

Ceres’s spectrum shows a diagnostic carbonate signature in the area at 3.3–4 μm confirmed by the Dawn mission [8]. The average surface of Ceres is composed of phyllosilicate, ammonium-bearing minerals, Mg-Ca carbonates, water ice, and salts [10]. A large amount of Na-carbonate is present in the brightest area in Ceres’s Occator crater [9]. The spectral fit of the Occator bright spot presents natrite (Na₂CO₃), mixed with other elements, as the best carbonate mineral candidate [9]. Carbonates was also found on Ahuna Mons [11] and on additional bright faculae distributed on Ceres’s surface [8]. Carrozzo et al. [8] report the additional presence of hydrate forms of sodium carbonate such as thermonatrite (Na₂CO₃·H₂O) and trona [Na₃(CO₃)(HCO₃)·2(H₂O)]. The main hypothesis for the formation of bright spots is the minerals’ genesis from the liquid brine reservoir, i.e., the material produced by impacts that induce heating of the surface brine [12]. By the above-mentioned results, Ceres’s dwarf planet has been included in the category of celestial bodies with possible oceans underneath the crust (e.g., Europe, Ganymede, Callisto) [12,13].

The surface composition of Enceladus has some analogies with that of Ceres; in fact, plumes erupted by Enceladus are formed of ammonium-bearing minerals, water, and carbonate salts such as NaHCO₃ and NaCO₃ [14]. The genesis of these salts is attributed to the presence of oceans underneath the surface rich in Na elements [15].

Moreover, hydrated sulfate and carbonate minerals are the principal candidates for composing the wide non-icy part of the Europa surface, which is the second Galilean satellite outward from Jupiter [16,17]. Recently, De Angelis et al. [18] showed how the spectra of some hydrated Na-carbonate minerals strongly match with reflectance spectra remotely coming from Europa. Magnesium and sodium sulfates, sodium carbonate, and a mixture of these salts provided a good fit for the non-ice material [19]. Zolotov et al. [22] demonstrate how hydrated salts are thermodynamically stable in ice-bearing deposits on the surface of Europa.

The origin of the hydrated sulfates and sodium carbonate minerals on the surface of Jovian moons was attributed to the base of the density analogy between the bulk icy satellites and the CI carbonaceous chondrites [23,24,25]: the icy satellites were supposed to be accreted from CI chondrite during primordial stages of heating that caused the decomposition of carbonates (formed by oxidation of organics) and CO₂ degassing. The icy shell and possibly the ocean tend to trap the CO₂, which would be available to drive the formation of carbonates and bicarbonates [25].

On Earth, alkaline carbonate minerals are often attributed to a magmatic origin even if they were also found in carbonatite deposits as melt inclusions [26]. Na-Ca carbonates were discovered in natrocarbonatite magmas erupted by the Oldoinyo Lengai volcano (East African Rift System, northern Tanzania), which is the only active carbonatite volcano on Earth [27,28].

Nyerereite and shortite (attributed to the subsolidus breakdown of nyerereite) were identified in mantle-derived melt inclusions in kimberlites ([29,30,31,32] and references therein) as well as in minerals from multiphase solid inclusions in diamonds [33].

The occurrence of alkaline carbonates both in natrocarbonatite magmas and in deep-mantle derived products is of paramount importance as it offers unique insight into the interior of the Earth, i.e., the composition and origin of kimberlites and mantle-derived melts and deep Earth composition.

Recently, natrocarbontes have attracted the attention of the scientific community as they have been found to play a key role in carbon capture and storage acting as CO₂ trapping by mineral carbonation. [34].

An interesting topic is the chemical composition and structure of the icy planets in relation to the presence of oceans underneath the crust, as the strong interaction between oceanic crust and the surface is crucial for their habitability. The occurrence of the mineral phases listed above gives important indications about the internal dynamic of the icy planetary bodies [35], as their presence is closely related to variation in pressure (P) and temperature (T) during the evolution of these bodies. Indeed, fluid and volatile storage are functions of P/T conditions inside minerals’ structure, according to the topological changes in crystalline structures [36]. Consequently, to elaborate hypotheses about the depth of one or more oceans, as well as the thickness of the ice crust, it is necessary to focus on the behavior of non-ice components and their evolution under the crust of an icy planet. Moreover, phase transitions occurring under different pressure and temperature conditions can drastically change the mineral density and can affect the buoyancy and internal dynamics of planetary bodies [37,38,39].

Several studies report carbonate measurements in the laboratory e.g., [40], using VIS-NIR reflectance spectroscopy, although there are very few investigated carbonates at non-ambient temperature. In some cases, researchers have also not collected data in the spectral range larger than 5 μm. Gaffey [41] reports reflectance spectra of anhydrous carbonate minerals in the 0.35–2.55 μm range. Some literature data e.g., [42] focused on the study of natrite (NaCO₃) reflectance spectra at various temperature and in the spectral range up to 25 μm, taking into account the phase transitions at high temperature of this phase [43]. Recently, De Angelis et al. [18] analyzed the reflectance spectra of Na-carbonates (1–4.8 μm) at cryogenic temperature (down to 90 K). Notwithstanding the abundant literature on natrocarbonates spectroscopic studies, the whole set of alkaline and earth-alkaline carbonate minerals reflectance spectra has not been covered. For nyerereite, only Raman spectra [44] is reported, even though this mineral is very important in the geological scenario, as previously documented, and likely having an important role also in the planetary context.

A set of hydrated and anhydrous Na-Ca carbonate minerals, which should be relevant to the planetological context, has been selected for this work. The aims of this project are as follows: (1) to improve the spectral libraries of carbonate minerals with reflectance and emissivity spectra at different temperatures and wide spectral range; (2) to understand how crystal–chemical changes affect the spectral features in each sample; (3) to evaluate temperature changes such as dehydration and/or phase transitions; (4) to compare new anhydrous carbonates (shortite and nyerereite) to Ceres’s surface; and (5) to present a new algorithm to compare remote sensing data collected by spacecraft satellites with laboratory measured spectra of selected mineral mixtures.

X-ray diffraction analyses were performed on the selected samples before and after thermal treatments to study the evolution of their crystal structure. The spectral evolution was also studied in a wide temperature range, from 193 K to around 803 K, for all samples (except for nyerereite for which we only have reflectance data).

Once we analyzed the minerals spectra and their evolutions with T, we applied our data to a case study, the Ceres Cerealia bright spot taken from [45]. A quantitative fitting of the spectra coming from this area and collected from the Dawn mission was performed by using an algorithm, mixture fit, based on the Hapke model [46,47] and working by applying an areal (linear) mixing. The detailed procedure is reported in Appendix A.

Spectroscopy Properties of CO₃²⁻ and H₂O/OH Groups

The free CO₃²⁻- ion (D_3h symmetry) has four fundamental IR-active vibrational modes: symmetric stretching (ν₁), out-of-plane bend (ν₂), degenerate antisymmetric stretch (ν₃), and degenerated bending mode (ν₄) [48]. Generally, strong IR modes appear near 1100 cm⁻¹ due to the symmetric stretching vibration (ν₁) and at 7.14 µm (1400 cm⁻¹) generated to antisymmetric stretch (ν₃). Absorption bands near 12.5 µm (800 cm⁻¹) are due to out-of-plane bend (ν₂), while features around 14.28 µm (700 cm⁻¹) are due to in-plane bending modes (ν₄) [49]. Overtone and combinations of CO₃²⁻ are expected in the areas at 1.85–1.87 µm (ν₁ + 3ν₃), 1.97–2.00 µm (2ν₁ + 2ν₃), 2.12–2.16 µm (3ν₁ + 2ν₄), 2.30–2.35 µm (3ν₃), and 2.50–2.55 µm (ν₁ + 2ν₃) [50]. Absorption features located in the range from 3.40 to 4.00 µm (29412–500 cm⁻¹) can be assigned to combination and vibrational modes due to ν₂ + ν₄/2ν₂ + ν₄/ν₃ [51]. H₂O free molecule has three fundamental vibrational modes: at 2.73 µm (3657 cm⁻¹) (ν₁) symmetric stretching, 6.26 µm (1595 cm⁻¹) (ν₂) symmetric bending, and at 2.66 µm (3756 cm⁻¹) (ν₃) asymmetric stretching [52]. The free HCO₃⁻ ion has also four modes: ν₁ symmetric stretching at 9.04 µm (1063 cm⁻¹), ν₂ symmetric bending at 11.37 µm (879 cm⁻¹), ν₃ asymmetric stretching at 7.06 µm (1415 cm⁻¹), and ν₄ asymmetric bending at 14.7 µm (680 cm⁻¹). The H₂O free molecule presents a symmetric H-O-H stretch at 2.73 μm (3663 cm⁻¹), an asymmetric H-O-H stretch at 2.66 μm (3759 cm⁻¹), H-O-H bending at 6.27 μm (1594 cm⁻¹) [53], and a rotational fundamental band at 20 μm (500 cm⁻¹) [54]. OH has a single stretching vibration, near 2.8 μm (3571 cm⁻¹). The overtones of these vibrational modes are expected near 1.4 μm and 0.45 μm (7142 and 22,222 cm⁻¹) [55]. OH also has rotational vibration bands in the 9.2–5 μm (1086–400 cm⁻¹) range and translational vibration bands in the 1.5–3.3 μm (666–303 cm⁻¹) range [56]. Peaks located around 1.9 μm (5263 cm⁻¹) are defined as water combination bands, whereas the strongest OH overtones are in the region 1.38–1.41 μm (7246–7092 cm⁻¹) [57]. The 3 μm (3333 cm⁻¹) region of all the spectra shows well-defined H₂O stretching fundamental bands. The minerals examined in this manuscript have H₂O in structurally different sites, as explained carefully below, and they are involved in hydrogen bonds (mention as H₂O⁺), which may have different lengths and resistances sensitive to both temperature and crystalline structure.

2. Materials and Methods

2.1. Samples

We analyzed the samples reported in

Table 1. Chemical formula, minerals name, and source for selected samples used in this work.

Minerals	Chemical Formula	Source
Natrite	Na2CO3	Koashva, Kola, Russia
Nyerereite	Na2Ca(CO3)2	Synthetic
Shortite	Na2Ca2(CO3)3	Green River, Wyoming, USA
Thermonatrite	Na2CO3·H2O	MT. Koashva, Kola, Russia
Trona	Na2(CO3)(HCO3)·2(H2O)	Wadi El-Natrun, Westen Desert, Egypt

.

Reflectance and emissivity measurements on the proposed samples were performed at the PSL (Planetary Spectroscopy Laboratory, DLR, Berlin, Germany).

2.2. Emissivity and Reflectance Measurements

At PSL, two identical Bruker Vertex 80 V FTIR spectrometers are equipped, each with an external emissivity chamber. The chamber used for medium-low temperature (from 303 to 343 K) emissivity measurements is continuously purged with dry air (atmospheric water and CO₂ removed) at room pressure. A second emissivity chamber, which can be evacuated at 0.7 mbars, to obtain measurements free of atmospheric spectral feature [58], and is directly attached to the second spectrometer, allows measuring the emissivity of samples under vacuum at high temperature, up to approximately 803 K. Under this vacuum emissivity setup, the entire stainless-steel cup is heated up using an induction system, allowing heating the samples from below as well as from the side of the cup. The temperature was measured by a thermocouple in contact with the emitting source. The emissivity spectra were scaled by a comparison with the blackbody emission spectrum measured at the same temperature and under the same setup configuration. Details on the emissivity measurements can be found in [38].

Details on the reflectance collection process and PSL build-up are described in [38,59]. For reflectance, approximately 1 mm thick uniform layer of samples is placed metal cups for measurements. Bi-conical reflectance measurements were performed by means of a Bruker A513 accessory. The reflectance standard is a gold-coated sandpaper for measurements along the entire 1 to 100 μm. The standard spectral resolution is 4 cm⁻¹, and the spot size is 48 mm for emissivity measurements. The spectral range of our analysis is from 0.5 to 16 μm, namely the infrared spectrum (IR) from Visible-Near (VNIR) to Thermal (TIR). The reflectance spectra of all samples are measured at room temperature (RT) and 193 K, and these are frozen by means of a FRIKA cold box B 35–85.

2.3. Emissivity and Reflectance Data Reduction

The collected emissivity and reflectance spectra were reduced to clean them from instrumental features. The background was removed by a linear interpolation between the estimated points on the two sides of a maximum, and a straight line has been drawn between the edges for each band. The band depth is determined using the equation D = (R_c − R_b)/R_c. The area was computed by a simple integration. The width has been computed by intersecting the continuum-removed band with a horizontal line passing through half depth. More information on the reduction of emissivity and reflectance data can be found in the work of Comodi et al. [38].

2.4. X-ray Diffraction Measurements and Rietveld Analysis

X-ray powder diffraction measurements were made with a Philips PW 1830 diffractometer (Koninklijke Philips N.V., Amsterdam, The Netherlands), with a graphite monochromator and using the CuK_α radiation (λ = 1.54184 Å). Data were taken with a step scan of 0.02 °/step and a step time of 100 s/step.

Quantitative measurements of the collected data were carried out by means of the Rietveld refinement method [60], as implemented in the EXP-GUI GSAS software (Windows version 2001, University of California, Berkeley, CA, USA) [61,62]. The starting crystal structure data were selected from the American Mineralogist Crystal Structure Database [63]. Crystal structures were plotted using CrystalMaker software (Software version 10.6, CrystalMaker Software Limited, Oxford, UK) [64]. We used scattering factors for neutral atoms. The refined parameters were background, fitted with a 17 to 28-term Chebyshev polynomial function, profile functions of pseudo-Voigt type, scale factor, instrument zero point, and the crystal lattice constants. When needed, we applied the March–Dollase formulation for preferred orientation [65] correction.

3. Results

The emissivity and reflectance spectra collected at different temperatures are reported subsequently for each sample.

Table 2. CO₃²⁻, H₂O, and OH band positions in the reflectance spectra collected at 298K for all the investigated samples. Absorption band assignments were made following De Angelis et al. [18] (a,c), Frost and Palmer [49], Clark [50], Zhu et al. [51], Adler and Kerr [66] (b), and Harner and Gilmore [40] (d). The symbols + and – indicate bands assigned to structural water (H₂O⁺) and hydroscopic water (H₂O⁻); see text for details.

Mineral Name	CO32− Fundamentals (µm)				CO32− Overtones and Combinations (µm)					OH Bands (µm)	H2O Bands (µm)
	ν1	ν2	ν3	ν4	2ν3	2ν1 + 2ν3	ν1 + ν3	3ν3	ν1 + 2v3
Natrite a Na2CO3	9.36 9.68 9.90	11.29 11.75 12.78	6.24 7.07 7.16	14.25 14.41	3.35 3.51	1.94/H2O 2.05	3.86 4.01	2.34	2.54	1.52− 1.74− 1.94−	2.88− 5.62− 5.91−
Nyerereite Na2Ca(CO3)2	9.33	11.24 11.55 11.84	6.61 6.70 6.85 7.04 7.20	13.83 14.78	3.35 3.43	1.95/H2O 2.06	3.85 3.95 4.04	2.31	2.46 2.51	1.57− 1.80− 1.95−	2.90− 5.55− 5.70− 6.19−
Shortite b Na2Ca2(CO3)3	9.18 9.33	11.43 11.71 12.10 12.47	6.15 6.93 7.01 7.13	13.68 13.90 14.10 14.36	3.41 3.50	1.91/H2O	3.80 3.95 4.06	2.31 2.35	2.51	1.72− 1.76− 1.91−	3.02− 3.12− 5.51−
Thermonatrite c Na2CO3·H2O	9.57	11.31	6.40 7.04	14.44 14.97		2.04	4.05 4.42	2.22	2.44	1.52+ 1.74+ 1.94+	2.83− 3− 5.68−
Trona d Na3(CO3)(HCO3)·2(H2O)	9.53		6.44 7.05	14.33 14.97		2.04		2.21 2.39		1.52+ 1.74+ 1.94+	~3− 5.61+

reports the position and assignation of the observed bands of the reflectance spectra, which was collected at 298 K. The data collected at 603 K together with the temperature dependence of the bands parameters, full width at half maximum (FWHM) and depth, considering that the evolution with temperature is generally linear [18], for emissivity data are in Supplementary Table S1.

For hydrous samples, in agreement with literature data e.g., [18,66], the H₂O and OH absorption fundamental and combination bands were considered. In the spectra collected at low temperature, additional peaks related to the ice formation in the sample chamber are evident at approximately 1.4 µm and 1.9 µm. For all the samples, the common features related to the CO₃²⁻ group were also observed. Common features of the studied anhydrous carbonates (natrite, shortite, and nyerereite) are the presence of absorption bands in the 11.24–6.66 µm (889–1500 cm⁻¹) range corresponding to fundamental vibrational modes of CO₃²⁻. In addition, a prominent band around 5.68 µm (1760 cm⁻¹) is present in almost all the studied samples due to either the structural water in hydrated samples or the hygroscopic water in anhydrous samples. Some anhydrous carbonates show H₂O bands, even if H₂O is not part of the mineral structure (indicated as H₂O⁺), which is probably due to a small amount of hygroscopic H₂O trapped at grain boundaries. The comparison of emissivity spectra collected at different temperatures for each sample are reported, using the same scale to highlight linear and nonlinear changes. The temperature usually increases the deepening of most absorption bands, whereas it does not affect the band position. The low temperature reflectance spectra display more detailed and define absorption bands. The temperature range is not low enough to estimate the presence of additional bands and evaluate some bands parameters trends such as the decreasing temperature. For some modes, nonlinear changes were observed with temperature, indicating that either dehydration or phase transitions occurred.

3.1. Natrite Na₂CO₃

Natrite at room temperature is monoclinic with a space group C2/m and density of 2.54 g/cm³ [67]. The mean structure of natrite can be described globally by layers of 9-coordinated Na cationic sites (Na3) and CO₃²⁻ ions (Figure 1a). Additional Na octahedral sites (Na1,2) are found in pseudo-hexagonal channels [68]. The face that shares the Na1,2 octahedra forms a column, which is connected by triangles of CO₃²⁻ [69]. Sodium carbonate has four phase transitions (α, β, γ, δ) in the T range from 170 up to 800K [68]. The γ-Na₂CO₃ is stable below 605 K and refined as an incommensurately modulated structure [70]. Two temperature-dependence phase transitions are detected at HT: β-Na₂CO₃ at 628K and α-Na₂CO₃ at 757 K [68,69,71] (Figure 2d). The cell parameters and symmetry of natrite polymorphs are reported in

Table 3. Sodium carbonate (Na₂CO₃) phase transitions in the temperature range investigated. Crystallographic data are taken from Dusek et al. [68], Arakcheeva et al. [69], Arakcheeva et al. [70], Swainson et al. [71]. T (K) are phases stability temperatures ranges.

Phase	T (K)	Symmetry	Cell Parameters (Å)
α	757–1000	Hexagonal P63/mmm	a = 5.209(2), c = 6.454–6.676
β	628–757	Monoclinic C2/m	a = 9.00(2), b = 5.23(2), c = 6.21–6.44, β = 101.87 (8)
γ	170–628	Monoclinic C2/m	a = 8.920(7), b = 5.245(5), c = 6.050(5), β = 101.35 (8)
δ	≤170	Monoclinic P21/n	a = 8.898 (7), b = 5.237 (5), c = 5.996 (5), β = 101.87 (8)

. Details on the spectral behavior after phase transitions are reported in Section 4.

The reflectance spectra (1 to 5 µm) (Figure 2a) show very weak CO₃²⁻ absorption features due to overtones and combinations at 2.05. 2.34, 2.54, 3.35, 3.51, 3.86, and 4.01 µm. In the spectral range from 5 to 16 µm (Figure 2b), the bands are less deep and defined, and they are generated by fundamental vibrational modes of CO₃²⁻. The absorption features at 6.24, 7.07, and 7.16 µm are assigned to ν₃(CO₃²⁻) vibrational modes. The ν₂(CO₃²⁻) fundamental modes generate the peaks at 11.29. 11.75, and 12.78 µm. The last part of the natrite reflectance spectra is characterized by the presence of one evident absorption band at 14.25 µm generated by ν₄ vibrational modes. The emissivity spectra (Figure 2c) exhibit CO₃²⁻ ν₄ and ν₂ modes at 681 and 796 cm⁻¹. The overlapping of several overtones/combinations and fundamental vibrational modes of CO₃²⁻ generates the features centered at 1178 and 1487 cm⁻¹. The well-defined bands at 2068, 2243, and 2657 cm⁻¹ are due to overtone and the combination of carbonates group relative to ν₁ + ν₃ and 2ν₃. The absorbed water molecules cause peaks at 1764, 2974, and 3215 cm⁻¹. The shifts in the centers of the absorption bands and the variation in their shape in the spectra collected at T = 603 K are caused by phase transitions (γ-Na₂CO₃ → β-Na₂CO₃; β-Na₂CO₃ → α-Na₂CO₃), as reported by Harris and Salje [43] (Figure 2d).

3.2. Nyerereite Na₂Ca(CO₃)₂

Synthetic nyerereite has a three-component orthorhombic twinned structure with either Pbca or P2₁ca space groups [72,73] and density 2.54 g/cm³ [72]. The unit cell of nyerereite is a = 10.0713(5) Å, b = 8.7220(2) Å, and c = 12.2460(4) Å [72]. The framework of Na₂Ca(CO₃)₂ can be described as a stacking of close-packed Na- and Ca-layers in a six-layered close-packed structure, the layers orientation is (001) [72] (Figure 3a).

The reflectance spectra of nyerereite show evident peaks in the first part of the spectra (Figure 4a). We identify overtone and combination CO₃²⁻ bands located at 2.31 (3ν₃), 2.46, and 2.51 µm (ν₁ + 2ν₃). Diagnostic overtones and combinations are present at 3.35 and 3.43 µm due to 2ν₃(CO₃²⁻), whereas absorption features at 3.85, 3.95, and 4.04 µm are due to ν₁ + ν₃(CO₃²⁻) vibrational modes. In the last part of the spectra (Figure 4b), we detect fundamental absorption features situated near 6, 9, 12, and 14 µm due to ν₃, ν₁, ν₂, and ν₄ CO₃²⁻ vibrational modes, respectively.

3.3. Shortite Na₂Ca₂(CO₃)₃

Shortite is orthorhombic with space group Amm2, a = 4.947(1) Å, b = 11.032(2) Å, and c = 7.108(1) Å and a density of 2.62 g/cm³ [74]. The structure (Figure 5a) is made by distinct layers of Na-CO₃ and Ca₂Na(CO₃)₂. Within the unit cell, two sodium sites (Na1 and Na2) and one Ca site are located, lying in a disordered trigonal prismatic site, seven-fold site, and nine-fold site, respectively [27]. Two distinct carbonate ions are present: C1, which goes out of plane with respect to the oxygens of approximately 0.03 Å, and C2, where carbonate is symmetry constrained to be flat [27]. As both C1 and C2 are approximately aligned with their edges along the Ca site at a regular interval [27], the Ca site achieves a nine-fold coordination.

The reflectance spectra of shortite are characterized by well-defined absorption bands in the range from 1 to 5 µm (Figure 6a) with features at 2.31 (3ν₃), 2.35 (3ν₃), and 2.51 (ν₂ + ν₃) µm due to the overtone and combination bands of the CO₃²⁻ group. These modes generate diagnostic and interesting features at 3.41 and 3.50 µm caused by 2ν₃ (CO₃²⁻), and at 3.80, 3.95, and 4.06 µm assigned to ν₁ + ν₂ (CO₃²⁻). Fundamental vibrational modes are present in the spectral range over 6 µm (Figure 6b). From 6 to 7.5 µm, the bands are due to ν₃(CO₃²⁻); whereas in the rage from 11 to 12.5 µm, the absorption features are ascribed to ν₂(CO₃²⁻) vibrational modes. The two bands at 13.68 and 14.36 µm are generated by ν₄(CO₃²⁻) mode. The emissivity spectra (Figure 6c) show defined peaks at 1226 and 1486 cm⁻¹ due to the ν₃ fundamental vibrational modes of the CO₃²⁻. The overtones and combinations of this group are detected at 2067, 2283, 2604, and 2699 cm⁻¹. In this sample, the absorption features ν₄ modes are detected at 831 cm⁻¹ and there are no emissivity spectra above 403K due to its dissolution after this temperature.

3.4. Thermonatrite Na₂CO₃·H₂O

Thermonatrite, the sodium carbonate monohydrate, is orthorhombic with space group P2₁ab, unit cell parameters a = 6.472(2) Å, b =10.724(3) Å, c = 5.259(2) Å [75], and density of 2.25 g/cm³ [76]. The CO₃²⁻ groups within the thermonatrite crystal structure have planes almost perpendicular to the a-axis forming sheets that are staked in columns along the same axes (Figure 7a). Two adjacent CO₃²⁻ groups within the same column are both ionically bonded to four Na ions and hydrogen bonded by a water molecule; whereas adjacent CO₃²⁻ molecules in the same sheet are linked by the Na ions that lie above and below the sheet [75] (Figure 8a). The H₂O molecule is lost in the temperature range between 360 and 363K; over these temperatures, only sodium carbonate occurs [77].

The reflectance spectra of thermonatrite (Figure 8a,b) are characterized by the presence of poorly marked absorption features due to the presence of water that overlaps the bands of the CO₃²⁻ group. In the first part of the spectra (1–5 µm), the absorption features of CO₃²⁻, caused by overtone and a combination at 2.04 (2ν₁ + 2ν₃), 2.22 (3ν₃), 4.05 (ν₁ + ν₃), and 4.42 (ν₁ + ν₃) µm are detected. The bands related to CO₃²⁻ fundamental vibrational modes are present at 6.40 (ν₃), 7.04 (ν₃), 9.57 (ν₁), 11.31 (ν₂), 14.44 (ν₄), and 14.97 (ν₄) µm. The emissivity spectrum (Figure 8c) is closely related to the natrite mineral. In fact, ν₄ and ν₂ bands are present at 651 and 790 cm⁻¹. The band centered at 1117 cm⁻¹ is made by the overlap of several CO₃²⁻ overtone and combination features. Other absorption bands related to overtone and combination are located at 2013, 2245, and 2677 cm⁻¹ due to ν₁ + ν₃/2ν₃ (CO₃²⁻). The presence of water molecule generates absorption features at 1768 and 3241 cm⁻¹. As reported in the natrite sample, the bands shifting and changing shape in the emissivity spectra collected at 703 and 803 K are related to phase transition from γ- to β-Na₂CO₃ and β- to α-Na₂CO₃ (Figure 8d). In the thermonatrite sample, the variations of the fundamental vibrational modes during heating are much more evident than in the natrite sample.

3.5. Trona Na₃(CO₃)(HCO₃)·2(H₂O)

Trona is an interesting mineral that contains the unusual combination of both a carbonate and bicarbonate anion [78] (Figure 9a). It crystallizes in the monoclinic space group C2/c with cell parameters a = 20.41(3) Å, b = 3.49(1) Å, c = 10.31(1) Å, β = 106.2(1)° [79], and density 2.13 g/cm³ [80]. The crystal structure of the minerals is characterized by units of three edge-sharing sodium polyhedra cross-linked by carbonate groups and hydrogen bonds [78]. Thus, each Na ion is surrounded by six O atoms, some belonging to the carbonate ion and some belonging to the water molecule. Two different Na sites are present, being the one octahedrally coordinated and the other one at the center of a slightly distorted trigonal prism [78]. The CO₃²⁻ group has one oxygen bonded by a hydrogen bridge to an oxygen of another carbonate ion, forming what has been usually called a complex [HC₂O₆]³⁻ anion. The disordering of H in the [HC₂O₆]³⁻ complex gives rise to the sharing of one H between the carbonate units, thus producing the mixed carbonate/bicarbonate character of these anion groups [79].

The reflectance spectra of trona (Figure 10a,b) have poorly marked or absent bands due to the presence of water molecules (15.94 wt %). We observed evident and typical H₂O absorption features at 1.52, 1.94, and 5.61 µm. CO₃²⁻ overtone and combination vibrational modes are detected at 2.04 (2ν₁ + ν₃), 2.21 (3ν₃), and 2.39 (3ν₃) µm. Absorption bands at 6.44(ν₃), 7.05(ν₃), and 9.53(ν₁) µm are due to fundamental vibrational modes of CO₃²⁻. The emissivity spectra (Figure 10c) are strictly related to natrite and thermonatrite samples, as they are subject to the same phase transitions at the investigated temperature. In this case, we do not have a split peak before phase transitions. In fact, the absorption features present at 660 and 791 cm⁻¹ are due to the fundamental CO₃²⁻ vibrational modes ν₁ and ν₄, respectively, and the bands position at high temperature probably shift due to phase transitions (Figure 10d). The overtone and combination of this group (ν₁ + ν₃ and 2ν₃) generate the bands at 2001, 2244, and 2670 cm⁻¹. Water in trona shows two bands at 1768 and 3238 cm⁻¹. Trona decomposes in two steps, in relation of water loss between 373 and 407K [81], generating sodium carbonate as confirmed by XRPD data collected after the eating process (Figure 11).

4. Discussion

In the reflectance spectra, the absorption bands near 3.4 and 3.9 µm are considered diagnostic for the detection and selection of carbonate minerals [18,82]. The anhydrous samples examined in this paper show distinct and well-defined bands in the above-mentioned ranges. Comparing the spectra of the shortite, nyerereite, and natrite samples (Figure 12a), a variation of the band positions in relation to the change of the Ca:Na ratio is shown, all considering that: in Na₂Ca₂(CO₃)₃-shortite, the Ca:Na ratio is 2:2; in Na₂Ca(CO₃)_2-nyerereite, Ca:Na = 1:2, and in Na₂CO₃-natrite, Ca:Na = 1:0. The plot (Figure 12b,c) shows that decreasing the Ca:Na ratio, the 3.4 and 3.9 µm bands shift toward shorter wavelengths. Thus, the variation in Na:Ca concentration influences the position of the bands area that decreases as the ratio increases.

Moreover, the bands located at 3.9 µm show an additional interesting behavior depending on the carbonate mineral structure and chemistry. In fact, the simple salts such as Na₂CO₃-natrite are characterized by an uncomplete split broad band. On the other hand, double carbonates such as Na₂Ca₂(CO₃)₂-shortite and Na₂Ca(CO₃)₂-nyerereite show well-defined doublet peaks at 3.9 µm (Figure 13). Features at 3.4 and 3.9 µm are due to C-O stretching frequencies [83]. In double carbonates, some C-O bonds are shared between two cations (in the shortite Na⁺ and Ca²⁺), and these can generate a different arrangement of the cations in the crystal structure. In this configuration, the CO₃²⁻ group in double carbonate shares an oxygen atom with two different cations, than it is possible to conclude that when carbonate groups are distorted by regular planar symmetry, this mode splits into two components [84].

Among the investigated minerals, natrite, thermonatrite, and trona undergo complete de-hydration between 373 and 600 K, which determine a new phase with higher density with respect to the low-temperature ones. The increase of density can be relevant in trona, where it approaches 19.2%. The density variation at high temperature is an important parameter that strongly influences geodynamics and volcanism [85]. The density change connected to phase transition could have a strong impact on the formation of free water due to the structural rearrangements, which may affect the planetological models [38].

The occurrence of possible phase transitions in sodium carbonate samples by emissivity data is well evaluated by looking at the ≈669 and ≈794 cm⁻¹ bands that are due to ν₄ and ν₂ CO₃²⁻ fundamental vibrational modes, respectively, and these are the most sensitive to temperature variations. Thermonatrite and trona at 400K become natrite, Na₂CO₃ as reported in the results section. In the temperature range between 600 and 800 K, both are subject to two phase transitions (γ-Na₂CO₃ → β-Na₂CO₃; β-Na₂CO₃ → α-Na₂CO₃) [68,69]. In our measurements, the ν₄ frequencies change at high temperature (above 600K) as shown in Figure 2d, Figure 8d, and Figure 10d, where the ν₄ in the α-Na₂CO₃ shows a single band in agreement with Harris and Salje [43]. The different shapes of the absorption bands are due to small impurities within the samples because they are natural minerals.

Carbonates on the Ceres Occattor Area

As reported in the introduction section, in recent years, the composition of the surface of Ceres has attracted the interest of the scientific community, and a mixture of Mg-phyllosilicates, ammoniated clays, carbonates, and salts has been proposed [86,87]. The suggest minerals on the surface also could imply the presence of water in the form of an ancient ocean or localized brine underneath the crust [12]. The mineralogy of Ceres is largely coherent with the bulk composition of carbonaceous chondrite [88], and Na-carbonates gave good fits [9], even if the addition of Ca-carbonates (e.g., calcite) also produces equally fine results [8]. The presence of carbonate minerals on the surface of the dwarf planet Ceres was suggested taking in consideration the absorption features in the 4 µm region of telescopic spectra. Moreover, the 3.4 and 3.9 µm absorption features in the VIR spectra of Ceres confirm the presence of carbonates, and spectral models identify anhydrous sodium carbonate (e.g., Na₂CO₃) as the best candidate for Occator crater’s bright material [45].

Occator is the most well-preserved large crater where all the ancient features and textures of the formation are quite intact. [89]. The bright spots on Ceres are presumed to be the results of cryovolcanism and/or post-impact hydrothermal activity [90,91]. These phenomena imply the rising of material and the interaction between the surface and the subsurface [92]. Moreover, some bright faculae were modelized using hydrated sodium carbonates minerals (thermonatrite and trona) [9]. The presence of this hydrated form, unstable at Ceres’s surface condition, is strictly related to the continuous genesis of new material [12,93].

In this paper, we compare the VIR measured spectra by the Dawn mission of the Ceres Occator crater (taken from [45]) and our anhydrous samples collected at room temperature (Figure 14a). The band’s positions in the 3–4 µm region of the three anhydrous samples are in good agreement with those detected on the surface spectra, without considering the band’s shapes. As reported above, the band’s position shifts between the samples are related to the different Na:Ca ratios. As a conclusion, on the basis of a qualitative comparison, the presence of double carbonates (Na-Ca) cannot be excluded in the Occator area. The possible occurrence of these two compounds does not imply a change in the assumptions made about the genesis of carbonates on Ceres [8], as they are minerals produced by the rock–fluid interaction, too. However, as observed in the Ca-Mg carbonate system, some important physical properties, such as solubility, are influenced by the Ca:Mg ratio in the mineral crystal structure, being quite different in calcite and in dolomite also as a function of cation disorder [94,95].

In the Na-Ca system, natrite is very soluble in water (solubility ≈30 g/100mL at room T), whereas calcite has a lower solubility in CO₂ free water (solubility ≈0.0013g/100mL at room T). That is, double Na-Ca salts can have solubility and dissolution rates different from the end members, strongly affecting physical properties such as permeability. Discerning about the likely occurrence of Na-Ca double carbonates could be relevant to model the genetic environments of carbonates in the Occator area and in planetary bodies.

Once the qualitative comparison has been made, using an algorithm mixture-fit, we carried out a quantitative comparison, and we obtained the mixtures in Figure 15 to represent the surface composition of the Ceres Occator area. Details and descriptions of how the tool works can be found in Appendix A. We aimed to test if the Na carbonate minerals studied within this work, shortite and nyerereite, provide a good fit and if they can be considered as possible alternatives with respect to literature data [8,9] to represent the surface. Focusing on the phases proposed by Carrozzo et al. [8] and De Sanctis et al. [9], we modeled the spectrum of Ceres Cerealia as a mix of magnetite (dark material), illite (low-albedo material), sal-ammoniac, calcite, and Na-carbonates (natrite, termonatrite, and trona). We replace the latter with the spectra of the samples studied within this work. The spectral fit using hydrated carbonates and natrite (Figure 15b,d,e) are reported to evaluate the goodness of spectral fit made by our algorithm as a check before inserting the two proposed anhydrous phases.

For each combination, we consider albedo constants, end-member percentages, temperatures, and effective reflectance as free parameters. On the other side, the grain size parameter is fixed in relation to the single granulometry of the data collected inside this work. In the future, is necessary to implement within the algorithm the particle size parameters, and optical constants to obtain more realistic phase percentages and accurate fits. For these reasons, some discrepancies are evident in our fit of overtones, especially in the area near doublet bands.

The resulting mixtures show how shortite and neyerereite provide the same fine spectral fit of Na-carbonates, those made by our algorithm, as those already tested and evaluated by previous work, so we cannot exclude their presence and they could be additional good candidates to represent the surface of the Occator bright area. Given the differences in the solubility behavior of Na-Ca carbonates in water as a function of their chemical composition, the existence of the two Na-Ca carbonates may be relevant to speculate on the composition and field stability of the brines from which the surface salts are derived.

5. Conclusions

The collected data and the detailed analysis of the spectral features of CO₃²⁻, in the different mineral species as a function of their crystal structure and cation types, give more information for the remote sensing identification of the selected phases. We focused on how the bands located at ≈3.4 and 3.9 µm, which are essential for carbonates mineral identification, are affected by chemical composition and crystal structure, and we found a strict relation between the band’s position and the Na:Ca ratio. Moreover, bands located at ≈3.4 and 3.9 µm could be used for giving information about the structural configuration of carbonates salts: a complete doublet of the ≈3.9 µm feature is present only in spectra of minerals with the metal–oxygen bond length shared between two cations.

A comparison of the 3–4 μm region spectra of the Ceres Occator area to our laboratory spectra of anhydrous Na:Ca phases suggests that the laboratory spectra have absorption features that are consistent with those seen in the Ceres Occator area. High-temperature emissivity spectra are useful to determine the dehydration conditions of hydrated Na:Ca carbonates, especially when they are either in unstable surface condition or periodically replaced by fresh material. In addition, the behavior of these salts at the temperature that can be found inside icy bodies [96,97] can constrain both the composition of the subsurface brain and/or oceans and the depth at which they release water.

The here-proposed laboratory spectra should help the future remote sensing interpretation (e.g., Juice mission with MAJIS spectro-imager) emphasizing the spectral range as more sensible to the variation due to different mineral species.