# Identification and composition of carbonate minerals of the calcite structure by Raman and infrared spectroscopies using portable devices

Yonghwi Kim, Marie-Camille Caumon, Odile Barres, Amadou Sall, Jean Cauzid

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Yonghwi Kim, Marie-Camille Caumon, Odile Barres, Amadou Sall, Jean Cauzid. Identification and composition of carbonate minerals of the calcite structure by Raman and infrared spectroscopies using portable devices. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy [1994-..], 2021, 261, pp.119980. 10.1016/j.saa.2021.119980. hal-05224353

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Spectrochimica Acta Part A: Molecular and
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# Identification and composition of carbonate minerals of the calcite structure by Raman and infrared spectroscopies using portable devices

<figure><img src="image_4.png" /></figure>

Yonghwi Kim *, Marie-Camille Caumon, Odile Barres, Amadou Sall, Jean Cauzid

GeoResources Laboratory – Université de Lorraine, Faculté des Sciences et Technologies, BP 70239, F-54506 Vandœuvre-lès-Nancy, France

## HIGHLIGHTS

* Characterization of carbonate minerals by using portable Raman and IR instruments.

* Identification and composition determination of Ca-Mg-Fe-Mn carbonate minerals.

* Elemental variation in carbonates using band shifts in Raman and infrared spectra.

## GRAPHICAL ABSTRACT

<figure><img src="image_5.png" /></figure>

## ARTICLE INFO

**Article history:**

Received 9 February 2021
Received in revised form 20 April 2021
Accepted 17 May 2021
Available online 26 May 2021

**Keywords:**

Qualitative analysis
Carbonate minerals
Raman spectroscopy
Infrared spectroscopy
Portable spectrometers

## ABSTRACT

A portable Raman device with a 532 nm excitation laser and a portable infrared spectrometer with ATR (Attenuated Total Reflection) mode were used to analyse the spectral features associated with the identification and compositional variation of Ca-Mg-Fe-Mn natural carbonate minerals with a calcite structure (calcite, ankerite, dolomite, siderite, rhodochrosite, and magnesite). A systematic study of the variations of the peak positions with various compositional ratios was carried out. Most of the band positions were shifted to lower wavenumbers with increasing ionic radius or atomic mass of the divalent cations but the band of the translational lattice (T) mode in Raman and the symmetric bending (ν₄) band in the mid-infrared were the most sensitive. Therefore, the elemental variation of the Ca-Mg-Fe-Mn ratio in this carbonate series can be estimated from Raman and infrared band positions from spectra acquired with portable spectrometers.

© 2021 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

## 1. Introduction

Carbonate rocks act as reservoirs in sedimentary rocks and can also be host rocks related to numerous metalliferous ore deposits in magmatic, hydrothermal, or sedimentary contexts [32]. As carbonates are one of the minerals which can be prone to be altered according to geological activities, understanding the compositional changes in carbonate minerals is important as is the spatial distribution

of alteration zones in detail surveys [29]. For example, a geological model of the Elvira Volcanogenic Massive Sulfide (VMS) deposit in the south-eastern Iberian Pyrite Belt (IPB) is currently being developed in the framework of the NEXT (New Exploration Technologies) project (G.A. No. 776804) funded by the European Union's Horizon 2020 research and innovation programme. The hydrothermal carbonates associated with the ores in Sotiel-Migollas, which is the eastward continuation of the Elvira deposit, show a systematic variability according to depth with a predominance of dolomite in the deepest part under the ore, which is gradually replaced by ferroan dolomite in the intermediate zone

<footnote>* Corresponding author.
E-mail address: yonghwi.kim@univ-lorraine.fr (Y. Kim).</footnote>
<figure><img src="image_6.png" /><figcaption>**Fig. 1.** Chemical composition of monomineralic carbonate samples plotted in the Ca-Mg-Fe + Mn ternary diagram.</figcaption></figure>

to ankerite and even siderite in the shallowest part [13,36]. Therefore, quick identification of carbonates in the field can be effectively used as a discriminant parameter as an inexpensive and fast guide to determine the implication for ore mineralization in hidden ore explorations.

XRD (X-ray Diffraction), VNIR-SWIR (Visible, Near-InfraRed and ShortWave-InfraRed), Raman and FTIR (Fourier-Transform Infra-Red) spectroscopies have commonly been used for the mineralogical determination of carbonate minerals [6,12,19,34]. However, some of these techniques present several limitations. XRD analysis is often time-consuming and requires extensive sample preparation. Therefore, analysis needs lengthy experimental and evaluation time despite being one of the most common and reliable techniques for mineralogical characterization. In VNIR-SWIR spectroscopy, the carbonate minerals have a main band located

between 2300 and 2400 nm. However, qualitative analysis is difficult due to overlapping with the absorption bands of the hydroxyl-bearing minerals in this region [30]. Furthermore, the carbonate bands can be disturbed by water, which has a strong signal in the SWIR region near 1900 nm. Raman and FTIR spectroscopies, however, are useful techniques for carbonate identification. These are non-destructive methods that require little or no sample preparation. Moreover, all the bands related to carbonates in the mid-infrared region have characteristic positions which make it easy to differentiate from other minerals [3,38]. Raman and FTIR analyses of carbonates have long been used in mineralogical and geochemical research but newly developed portable devices are reliable, fast, and economic tools that allow new applications [18,33]. Furthermore, Raman and IR spectroscopies for archaeological and mineralogical applications associated with cultural heritage materials provide more important practical information on both the proper conservation but reconstruction of damaged ancient objects and paintings that have lost their original appearance [2,20]. Remote-sensing Raman and IR techniques that can be measured at a long distance have been installed in the SuperCam instrument on the Mars 2020 Rover, Perseverance, to attempt to recognize carbonates, silicates, sulfates, and, phosphates [39]. This shows that carbonate discrimination using Raman and IR spectroscopies has continued to expand the field of its research applications in diverse ways.

Carbonate minerals are constituted by the combination of one $(CO_3)^{2-}$ ion and various monovalent or divalent cations. Rhombohedral carbonates such as calcite, magnesite, siderite, and rhodochrosite crystallize in the R-3c group [14]). Dolomite and ankerite minerals are structurally similar to the rhombohedral carbonates (R-3 group) but with different site symmetry due to substitutions by divalent atoms such as Ca, Mg, Fe, and Mn in the crystal structure. Despite this site symmetry difference, the molecular symmetry as observed by Raman and infrared spectroscopy is not affected [1]. The minerals in these carbonate groups have six active modes; four internal vibrations within the $(CO_3)^{2-}$ unit and two external vibrations of the crystal lattice which involve translational and rotational oscillations of a carbonate group [16,23]. Previous Raman studies have demonstrated this tech-

<figure><img src="image_7.png" /><figcaption>**Fig. 2.** A) Raman spectrum and B) infrared spectrum (ATR mode) of calcite.</figcaption></figure>
nique's capacity to characterize carbonates but only for a few groups of solid solutions using laboratory devices [3,5,9,28,33,34]. First carbonate identification using calcite under field conditions by portable Raman instruments was reviewed by Jehlička et al. [22]. Previous portable Raman studies showed latent possibilities by determining Raman peaks in various carbonate groups, but these handheld spectrometers used in the studies were equipped with red to near-infrared diode lasers excitation only, not considering a green laser excitation [21,37]. For this reason, decreased sensitivity and limited spectral range can result from near-infrared excitation despite fluorescence suppression. Furthermore, no comprehensive comparison among all rhombohedral carbonates has been carried out by a portable Raman instrument.

Some carbonate minerals have also been investigated by infrared spectroscopy [1,4,8,19,27,38]. However, the studies were limited to a few minerals, and only took measurements with a transmission technique using pellets prepared with potassium bromide (KBr). This is hardly feasible in field conditions due to the hygroscopy of KBr. Spectral comparisons using both Raman and infrared spectroscopies were carried out by Gunasekaran et al. [15], but calcite and dolomite were the only minerals studied and these were analysed with a laboratory device. To date, no

study has reported using a combination of a portable Raman and a portable IR instrument for the identification and characterisation of the carbonate minerals in the calcite structure.

This study aimed to obtain an optimized methodology to discriminate between different carbonates during field analyses by using Raman and IR spectroscopies. For this study, a set of monomineralic carbonate minerals in the Ca-Mg-Fe-Mn system were measured by portable Raman and IR spectrometers. The band shifts were associated with mineralogical and compositional changes to determine the most reliable parameters and methodology.

## 2. Materials and methods

### 2.1. Materials

#### 2.1.1. Samples

Two sets of samples were used in this study. The first set consisting of 20 carbonate samples was analysed using atomic absorption spectroscopy (A800; Perkin Elmer, Norwalk, CT) to determine their elemental composition in Si, Mg, Ca, Mn and Fe. The elemental data were then used to calibrate the pXRF values. Among these samples, 7 were taken out of the subsequent study for at least one

<table><caption>**Table 1**
Position of the bands (cm⁻¹) in the Raman and infrared spectra and Mg# calculated from the corrected pXRF data.</caption><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Group</th><th colspan="4">Raman</th><th colspan="3">Infrared</th><th rowspan="2">Mg#<sup>**</sup></th></tr><tr><th>E<sub>g</sub> (T)</th><th>E<sub>g</sub> (L)</th><th>E<sub>g</sub> (v<sub>4</sub>)</th><th>A<sub>1g</sub> (v<sub>1</sub>)</th><th>v<sub>4</sub></th><th>v<sub>2</sub></th><th>v<sub>3</sub></th></tr></thead><tbody><tr><td>Cal_1</td><td>calcite</td><td>154</td><td>280</td><td>711</td><td>1084</td><td>712</td><td>872</td><td>1395</td><td>0.00</td></tr><tr><td>Cal_2</td><td>calcite</td><td>154</td><td>280</td><td>710</td><td>1085</td><td>712</td><td>871</td><td>1396</td><td>0.00</td></tr><tr><td>R040070<sup>*</sup></td><td>calcite</td><td>154</td><td>281</td><td>712</td><td>1086</td><td>712</td><td>872</td><td>1394</td><td>0.01</td></tr><tr><td>Cal_3</td><td>calcite</td><td>154</td><td>280</td><td>711</td><td>1084</td><td>713</td><td>872</td><td>1395</td><td>0.00</td></tr><tr><td>Cal_4</td><td>calcite</td><td>154</td><td>280</td><td>710</td><td>1085</td><td>712</td><td>871</td><td>1394</td><td>0.00</td></tr><tr><td>R050009<sup>*</sup></td><td>calcite</td><td>155</td><td>280</td><td>712</td><td>1086</td><td>712</td><td>872</td><td>1396</td><td>0.00</td></tr><tr><td>R050048<sup>*</sup></td><td>calcite</td><td>155</td><td>280</td><td>712</td><td>1086</td><td>712</td><td>872</td><td>1394</td><td>0.00</td></tr><tr><td>R040170<sup>*</sup></td><td>calcite</td><td>155</td><td>279</td><td>711</td><td>1086</td><td>712</td><td>872</td><td>1398</td><td>0.01</td></tr><tr><td>R050307<sup>*</sup></td><td>calcite</td><td>155</td><td>281</td><td>712</td><td>1086</td><td>712</td><td>872</td><td>1394</td><td>0.00</td></tr><tr><td>Cal_5</td><td>calcite</td><td>155</td><td>281</td><td>711</td><td>1086</td><td>713</td><td>872</td><td>1396</td><td>0.00</td></tr><tr><td>R050127<sup>*</sup></td><td>calcite</td><td>156</td><td>282</td><td>713</td><td>1087</td><td>712</td><td>872</td><td>1394</td><td>0.00</td></tr><tr><td>R050130<sup>*</sup></td><td>calcite</td><td>156</td><td>282</td><td>713</td><td>1087</td><td>712</td><td>872</td><td>1394</td><td>0.00</td></tr><tr><td>Cal_6</td><td>calcite</td><td>156</td><td>281</td><td>710</td><td>1085</td><td>713</td><td>872</td><td>1396</td><td>0.00</td></tr><tr><td>Ank_1</td><td>ankerite</td><td>167</td><td>285</td><td>718</td><td>1091</td><td>726</td><td>873</td><td>1416</td><td>0.18</td></tr><tr><td>R050181<sup>*</sup></td><td>ankerite</td><td>167</td><td>281</td><td>721</td><td>1092</td><td>725</td><td>870</td><td>1408</td><td>0.18</td></tr><tr><td>R050197<sup>*</sup></td><td>ankerite</td><td>168</td><td>283</td><td>721</td><td>1092</td><td>-</td><td>-</td><td>-</td><td>0.18</td></tr><tr><td>Ank_2</td><td>ankerite</td><td>168</td><td>289</td><td>719</td><td>1090</td><td>726</td><td>873</td><td>1405</td><td>0.23</td></tr><tr><td>Ank_3</td><td>ankerite</td><td>169</td><td>287</td><td>721</td><td>1091</td><td>724</td><td>871</td><td>1412</td><td>0.16</td></tr><tr><td>Ank_4</td><td>ankerite</td><td>171</td><td>291</td><td>721</td><td>1092</td><td>727</td><td>874</td><td>1419</td><td>0.30</td></tr><tr><td>R050129<sup>*</sup></td><td>dolomite</td><td>171</td><td>289</td><td>721</td><td>1094</td><td>727</td><td>876</td><td>1410</td><td>0.31</td></tr><tr><td>R050272<sup>*</sup></td><td>dolomite</td><td>173</td><td>294</td><td>722</td><td>1095</td><td>727</td><td>874</td><td>1412</td><td>0.32</td></tr><tr><td>R050370<sup>*</sup></td><td>dolomite</td><td>174</td><td>296</td><td>723</td><td>1096</td><td>727</td><td>874</td><td>1408</td><td>0.49</td></tr><tr><td>Dol_1</td><td>dolomite</td><td>175</td><td>300</td><td>722</td><td>1096</td><td>728</td><td>877</td><td>1418</td><td>0.33</td></tr><tr><td>Dol_2</td><td>dolomite</td><td>176</td><td>298</td><td>721</td><td>1095</td><td>728</td><td>877</td><td>1418</td><td>0.38</td></tr><tr><td>Dol_3</td><td>dolomite</td><td>176</td><td>298</td><td>724</td><td>1096</td><td>729</td><td>877</td><td>1419</td><td>0.40</td></tr><tr><td>R040030<sup>*</sup></td><td>dolomite</td><td>176</td><td>300</td><td>724</td><td>1099</td><td>729</td><td>877</td><td>1425</td><td>0.51</td></tr><tr><td>R050357<sup>*</sup></td><td>dolomite</td><td>177</td><td>300</td><td>724</td><td>1099</td><td>729</td><td>877</td><td>1419</td><td>0.49</td></tr><tr><td>R050241<sup>*</sup></td><td>dolomite</td><td>178</td><td>301</td><td>725</td><td>1099</td><td>729</td><td>881</td><td>1435</td><td>0.51</td></tr><tr><td>Sd_1</td><td>siderite</td><td>181</td><td>280</td><td>726</td><td>1082</td><td>737</td><td>862</td><td>1403</td><td>0.00</td></tr><tr><td>Sd_2</td><td>siderite</td><td>181</td><td>283</td><td>729</td><td>1083</td><td>736</td><td>862</td><td>1403</td><td>0.00</td></tr><tr><td>R050349<sup>*</sup></td><td>siderite</td><td>185</td><td>286</td><td>729</td><td>1086</td><td>737</td><td>862</td><td>1400</td><td>0.09</td></tr><tr><td>Sd_3</td><td>siderite</td><td>189</td><td>293</td><td>730</td><td>1085</td><td>738</td><td>865</td><td>1405</td><td>0.21</td></tr><tr><td>R040034<sup>*</sup></td><td>siderite</td><td>190</td><td>294</td><td>730</td><td>1088</td><td>739</td><td>866</td><td>1406</td><td>0.24</td></tr><tr><td>R050262<sup>*</sup></td><td>siderite</td><td>192</td><td>296</td><td>731</td><td>1089</td><td>741</td><td>866</td><td>1410</td><td>0.42</td></tr><tr><td>Rds_1</td><td>rhodochrosite</td><td>183</td><td>287</td><td>717</td><td>1083</td><td>724</td><td>861</td><td>1394</td><td>0.00</td></tr><tr><td>Rds_2</td><td>rhodochrosite</td><td>183</td><td>290</td><td>716</td><td>1083</td><td>726</td><td>860</td><td>1397</td><td>0.00</td></tr><tr><td>R050019<sup>*</sup></td><td>rhodochrosite</td><td>184</td><td>288</td><td>718</td><td>1085</td><td>725</td><td>860</td><td>1392</td><td>0.00</td></tr><tr><td>R040133<sup>*</sup></td><td>rhodochrosite</td><td>184</td><td>289</td><td>718</td><td>1085</td><td>726</td><td>860</td><td>1398</td><td>0.02</td></tr><tr><td>R050676<sup>*</sup></td><td>magnesite</td><td>209</td><td>324</td><td>737</td><td>1093</td><td>746</td><td>877</td><td>1427</td><td>0.87</td></tr><tr><td>Mgs_1</td><td>magnesite</td><td>210</td><td>325</td><td>734</td><td>1092</td><td>747</td><td>879</td><td>1426</td><td>0.88</td></tr><tr><td>Mgs_2</td><td>magnesite</td><td>211</td><td>325</td><td>735</td><td>1093</td><td>747</td><td>880</td><td>1430</td><td>0.92</td></tr><tr><td>R040114<sup>*</sup></td><td>magnesite</td><td>213</td><td>330</td><td>738</td><td>1094</td><td>746</td><td>881</td><td>1433</td><td>0.99</td></tr><tr><td>R050443<sup>*</sup></td><td>magnesite</td><td>213</td><td>331</td><td>738</td><td>1094</td><td>748</td><td>879</td><td>1431</td><td>1.00</td></tr></tbody></table>

<footnote>* reference data from RRUFF [25].</footnote>

<footnote>** Mg# (Mg/[Ca + Mg + Fe + Mn]) calculated from corrected pXRF data.</footnote>
<figure><img src="image_8.png" /><figcaption>**Fig. 3.** Correlation between the different peak regions in the Raman spectrum and Mg#. Calibration curves among calcite, ankerite, dolomite, and magnesite (R² = 0.97), and between siderite and magnesite (R² = 0.99).</figcaption></figure>

of the following reasons: a mixture of several carbonate minerals; out of the expected Raman region for the main carbonate peak; fluorescence during Raman acquisition, and presence of aragonite. The remaining 13 samples were added to 7 extra samples of unchecked elemental composition providing an interpretable Raman signal. In the end, 20 crystals were used, distributed amongst 6 phases covering the most common natural carbonate minerals: calcite (CaCO₃), magnesite (MgCO₃), siderite (FeCO₃), rhodochrosite (MnCO₃), dolomite (CaMg[CO₃]₂), and ankerite (Ca[Fe,Mg,Mn][CO₃]₂).

### 2.1.2. Sample preparation

The samples were prepared as powders and pellets. ATR spectra were measured on the powders while Raman and portable X-Ray Fluorescence (pXRF) spectra were measured on 13 mm diameter and 2 mm thick pellets prepared by using a 10 tons hydraulic press. Furthermore, raw data files of Raman spectra and infrared spectra in ATR mode which are chemically determined and come from the RRUFF database were added to extend the dataset in this study and compare them with the results from our samples [25].

## 2.2. Methods

### 2.2.1. Portable X-ray fluorescence spectroscopy

XRF data were measured with a Thermo Fisher Scientific Niton XL3t GOLDD + handheld spectrometer. The instrument used an Ag-anode X-ray tube with a voltage of 50 kV and was equipped with a

silicon drift detector (SDD). The X-ray beam covered a circular area of 8 mm in diameter on the sample. It operated the Main, Low, High, and Light filters with a duration of 30 s for each filter. Each sample was measured 5 times at the same point to obtain statistically reliable data. The spectral data was automatically interpreted using the Mining Cu/Zn mode, which is particularly suitable for measuring lighter elements such as Mg, Al, Si, P, and S. The samples from the RRUFF database were analysed by UA's modified CAMECA SX50 electron microprobe [25]. The elemental concentrations measured by the pXRF device were corrected by drawing calibration curves built by plotting pXRF values against the corresponding values determined by atomic absorption spectroscopy (A800; Perkin Elmer, Norwalk, CT). The cation composition of carbonate standards was plotted in the Mg-Ca-Fe-Mn ternary diagram (Fig. 1). To compare the band positions from Raman and ATR-FTIR spectroscopy with chemical composition, the modified Mg ratio [Mg# = Mg/(Ca + Fe + Mg + Mn)] was used to represent the cation composition in our target minerals.

### 2.2.2. Portable Raman spectroscopy

Raman spectra were collected using a RaPort portable spectrometer (Enhanced Spectrometry, Inc., San José, CA, United States). This handheld device (weight c. 2 kg) was equipped with a green Nd:YAG laser (532 nm) for excitation with a maximum output power of 30 mW. The minimum spot size was around 15 µm. The spectral range was 130–4000 cm⁻¹ at a spectral resolution of 6 cm⁻¹. A baseline was automatically subtracted by EnSpectr soft-
<figure><img src="image_9.png" /><figcaption>Fig. 4. Correlation between the peak positions in the T mode and the different compositional ratios.</figcaption></figure>

While the total acquisition time was set to 30 s, the number of accumulations and the duration of each individual acquisition were automatically defined. The selection of the peak position was determined by the wavenumber at maximum intensity after two correction steps: i) a baseline correction of the raw spectra by EnSpectr software followed by ii) an automated background removal which was installed in the Crystal Sleuth software [24].

### 2.2.3. Portable IR spectroscopy

IR analyses in ATR (Attenuated Total Reflection) mode were performed using a portable infrared spectrometer ALPHA (Bruker, Optik GmbH, Ettlingen, Germany). The instrument consisted of a base module with an exchangeable sampling module. The module used for this study was the Platinum ATR module. The IR spectra were obtained on powders placed on a diamond crystal and pressed with an anvil. In this technique, the penetration depth of the infrared radiation is greater at low wavenumbers. The spectra were acquired from 4000 to 500 cm⁻¹, and 24 scans were averaged with a spectral resolution of 2 cm⁻¹.

## 3. Result

### 3.1. Raman characterization of carbonate minerals

Calcite structure referred to the D₃d group has six Raman active modes in a Raman spectrum [34]. The four prominent peaks in the

range 135–1200 cm⁻¹ were selected for this study because they are narrow, sharp, and intense. In the Raman spectrum of calcite, for example, the positions of the peak related to translational (T) and librational (L) lattice mode from the external vibration of the CO₃ group are at 154 and 279 cm⁻¹ (Fig. 2A). The Raman peaks at 710 and 1083 cm⁻¹ are assigned respectively to the out-of-plane bending (v₄) and stretching (v₁) internal vibration modes. The v₁ mode is the most intense, while the v₄ mode is weaker. The shifts of the peak positions for the different carbonate minerals with diverse mineral chemistries are reported in Table 1. The two peaks of the external modes (T and L) arise from 154 to 214 cm⁻¹, and 279 to 331 cm⁻¹, respectively. The two peaks of the internal modes vary from 710 to 738 cm⁻¹ (v₄) and 1082 to 1100 cm⁻¹ (v₁).

Fig. 3 shows the Raman peak positions plotted against Mg# in order to determine the spectral region which is the most sensitive to compositional variations. Two factors should be considered: the range and the distribution of the peak positions within each group, and the unique characteristic of the relation between peak characteristics and mineral chemistry. The Mg# plotted against the Raman peak positions of the T mode displayed two distinct trend lines from calcite to magnesite on one hand and from siderite to magnesite on the other hand. This mode also shows the largest range of the peak positions (60 cm⁻¹) (Fig. 3A). Furthermore, the distribution of the peak positions for each mineral phase has a small dispersion in the T mode which led to the phases being separated from each other.
<figure><img src="image_10.png" /><figcaption>**Fig. 5.** Correlation between the different bands of the infrared spectrum and Mg#. Calibration curves among calcite, ankerite, dolomite, and magnesite (R² = 0.95), and between siderite and magnesite (R² = 0.98).</figcaption></figure>

The L mode has a narrower peak interval (51 cm⁻¹) with a wider range of peak dispersion than the T mode, whereas the peak shift was well-reflected in Mg# regardless of mineral phases, showing a single linear trend (Fig. 3B). The v₄ mode (Fig. 3C) exhibits two trends of peak positions like the T mode, but over a much narrower interval (28 cm⁻¹). Although two trend lines (i.e., calcite-ankeritedolomite and rhodochrosite-siderite-magnesite) are identified in the v₁ mode, these trends show a large level of dispersion (Fig. 3D). Thus, the T mode is the best region for carbonate discrimination. Two trend lines can be highlighted with a regression coefficient (R²) greater than 0.97 (Fig. 3A). The L mode can be used for determining Mg# in carbonate minerals within the regions.

The other compositional combinations were plotted against the Raman peak position of the T mode: another Mg# (Mg#2 = Mg/[Ca + Mg]), Fe# (Fe# = Fe/[Ca + Mg + Fe + Mn]) and two Ca# (Ca#1 = Ca/[Ca + Fe + Mg + Mn], and Ca#2 = Ca/[Ca + Fe + Mn]) were plotted against the Raman wavenumbers of the T mode (Fig. 4). Mg#2 shows a positive trend with the peak positions except for rhodochrosite, and Fe# has two correlations between ankerite and dolomite, and between siderite and magnesite (Fig. 4). No clear trends are observed with Ca#. These results confirm that Mg# against the Raman peak position of the T mode is the best parameter to discriminate and characterize the carbonate minerals with this Raman portable spectrometer.

## 3.2. IR characterization of carbonate minerals

Four internal vibration modes of CO₃²⁻ at 1396 cm⁻¹ (asymmetric stretching, v₃), 1089 cm⁻¹ (symmetric stretching, v₁), 872 cm⁻¹ (asymmetric bending, v₂) and 713 cm⁻¹ (symmetric bending, v₄) are found to be active in the infrared spectrum measured from the calcite reference (Fig. 2B). The two bending bands v₂ and v₄ are narrow and sharp. The v₃ band gives rise to a strong and very broad feature in the infrared spectrum, whereas the Raman spectrum displays a very weak peak in that region [38]. In contrast, the intensity of the v₁ band is very weak, while the peak is very strong in the Raman spectrum. Therefore, the v₁ band was not considered for the IR part of this study.

The v₄ and v₂ bands are observed respectively from 711 to 749 cm⁻¹ and from 860 to 882 cm⁻¹, and the v₃ band arises from 1392 to 1435 cm⁻¹ (Table 1). The position of the bands in the v₄, v₂, and the v₃ modes moves generally towards high wavenumbers as Mg# increases, except for rhodochrosite. Fig. 5 shows the infrared band positions for the three modes plotted against Mg#. In a similar fashion to the T mode in the Raman spectra, two trend lines in four regions are observed within the v₄ mode. The Ca-Mg and Fe-Mg trend lines have a strong correlation between the band position and the chemical composition of carbonates, showing high R² values of 0.95 and 0.98, respectively. In the v₄ mode, the
distribution of the band position can be well distinguished for each phase with a low level of dispersion except for the overlap of ankerite-dolomite series and rhodochrosite (Fig. 5A). The v₂ mode exhibits a narrower range (21 cm⁻¹) than the v₄ mode and also a with a broader dispersion. The band shift displays two correlations with Mg# from calcite to magnesite and from rhodochrosite to magnesite. Rhodochrosite can be easily identified by the lowest wavenumbers in this spectral range (Fig. 5B). The v₃ mode displays the broader peak range (42 cm⁻¹) and the largest dispersion for the positions of the bands for each group, without a clear correlation between the position of the band and Mg# (Fig. 5C). The v₄ spectral range is therefore the best region for carbonate discrimination allied with using the v₂ mode for rhodochrosite.

Fig. 6 displays the position of the v₄ bands plotted against Ca#1, Ca#2, Mg#2, and Fe#. Mg#2 shows a positive correlation with the band positions except for rhodochrosite and siderite. Fe# displays a linear trend between siderite and magnesite, but dolomite, ankerite, and rhodochrosite are close to each other between 730 and 724 cm⁻¹. The band shift in the ankerite-dolomite samples has an unclear trend with Ca#1 in comparison with Mg#. Ca#2 displays a linear correlation between ankerite and dolomite, whereas magnesite does not correlate with the peak position. Thus, Mg# is the best-correlated ratio with band shift in the v₄ mode.

### 3.3. Combination of chemical composition and spectral characteristics

Fig. 7 shows the ATR band positions of the v₄ mode plotted against the Raman shift of the T mode. The combination of these

<figure><img src="image_11.png" /><figcaption>Fig. 7. Correlation between the positions of Raman peak and the positions of infrared band.</figcaption></figure>

two spectral data shows that there are well-defined clusters for the calcite and magnesite phases separately from other phases. Rhodochrosite plots together with siderite in Raman data but with ankerite and dolomite in ATR, making both siderite and rhodochrosite recognizable. Ankerite and dolomite are the most difficult to distinguish, with a non-sharp transition from ankerite below

<figure><img src="image_12.png" /><figcaption>Fig. 6. Correlation between the positions of the band in the v₄ mode and the different compositional ratios.</figcaption></figure>
171 and 727 cm⁻¹ to dolomite above 170 and 727 cm⁻¹ from the Raman and infrared spectra, respectively. We thus confirm that the combination of Raman and infrared spectroscopies enables discrimination between all the carbonates, with some remaining overlap between Mn-Fe rich dolomites and Ca-Mg rich ankerites.

Fig. 8 shows the band positions from the Raman and infrared spectra and chemical composition (pXRF) in a 3D plot, making the identification of carbonates even easier by adding pXRF. The calcite samples (0.00–0.01 Mg#) are very well distinguished due to a very low Mg# and very low band positions in both Raman and infrared spectra (153–159 cm⁻¹ and 712–713 cm⁻¹, respectively). The magnesite samples, the most Mg-rich carbonate (0.79–1.00 Mg#), exhibit the highest wavenumbers from 209 to 213 cm⁻¹ in the Raman spectra and from 746 to 748 cm⁻¹ in the infrared spectra. The Mg# of ankerite samples (0.11–0.18) corresponds to a spectral range of 165–171 cm⁻¹ and 725–726 cm⁻¹ in the Raman and infrared spectra, respectively. The dolomite samples have a strong band from 170 to 178 cm⁻¹ in the Raman spectra and 727–729 cm⁻¹ in the infrared spectra with 0.18–0.51 Mg#. The band of siderite samples is between 181 and 192 cm⁻¹ in the Raman spectra and between 736 and 741 cm⁻¹ within the infrared spectra with 0.00–0.42 Mg# and the rhodochrosite samples (0.00–0.04 Mg#) are identified with a band position at 183–184 cm⁻¹ in the Raman spectra and 725–726 cm⁻¹ in the infrared spectra.

# 4. Discussion

## 4.1. Divalent cation in the carbonate minerals

For the same carbonate structure, changes in the atomic constitution induce variations of chemical bond strengths and atomic masses, which have consequences on the vibration frequencies and thus on spectral band positions [5]. As Mg# increased, the band position in the Raman spectrum shifted from 150 to 215 cm⁻¹ and from 710 to 750 cm⁻¹ in the infrared spectrum

(Fig. 8). We noticed that the band positions are shifted to lower wavenumbers with respect to cation radius and atomic mass in both Raman and infrared spectra. Fisler et al. [10] revealed a positive correlation between the ionic radius of the divalent cation, i.e., Ca, Mg, Mn, Fe, Cd, Zn, Co, and Ni, and the calculated lattice energies in carbonate minerals. We, therefore, added an otavite (CdCO₃), smithsonite (ZnCO₃), and spherocobaltite (CoCO₃) sample from the RRUFF database to our data set to highlight the trend between peak position and ionic radius (Fig. 9A). Ionic radius and atomic mass were represented as mean values weighted by the cation composition ratio (i.e., Ca, Mg, Fe, and Mn). The peak positions of siderite and rhodochrosite in this study overlap in the Raman spectrum between 180 cm⁻¹ and 185 cm⁻¹. This overlapping region associated with the lattice mode can be explained by the similar effective atomic mass of iron (55.85 AMU) and manganese (54.94 AMU), respectively (Fig. 9B). Infrared spectra also show a correlation between the band position in the v₄ region and ionic radius (Fig. 10A). This is consistent with the finding showing that a decrease in the ionic radius induces an increase of the wavenumber [17]. Furthermore, the positions of the bands are slightly shifted as the masses of the divalent cations decrease from calcium to magnesium (Fig. 10B). Siderite and rhodochrosite have different band regions in the infrared spectrum but overlap in the Raman spectrum. This can be explained by the use of different vibration modes in the two techniques.

## 4.2. Mg:Fe ratio in ankerite-dolomite relationship

Mineral identification in a solid ankerite-dolomite solution is always a difficult and complicated challenge due to a continuous variation in terms of chemical composition with Mg, Fe, and Mn. The term dolomite is restricted to the ratio Mg:Fe > 4, while ankerite is defined by the ratio Mg:Fe ≤ 4:1 [7]. Our ankerite samples, including the RRUFF database, have Mg:Fe lower than 4.5 and the dolomite samples have Mg:Fe ratio between 4.3 and 117.3 (Fig. 11). The ankerite and dolomite samples show that an increase in Mg:Fe ratio appears to be associated with a shift of the Raman

<figure><img src="image_13.png" /><figcaption>Fig. 8. 3D plot among the band positions in Raman and infrared spectra with Mg#.</figcaption></figure>
<figure><img src="image_14.png" /><figcaption>Fig. 9. Comparison between the ionic radius and atomic mass in the carbonate standards against Raman peak positions.</figcaption></figure>

<figure><img src="image_15.png" /><figcaption>Fig. 10. Comparison between the ionic radius and atomic mass in the carbonate standards against infrared band positions.</figcaption></figure>

<figure><img src="image_16.png" /><figcaption>Fig. 11. Variation diagram showing Mg/Fe ratio against the Raman peak position in the ankerite and dolomite samples.</figcaption></figure>

peak position to higher wavenumbers. The vertical demi error bars of two ankerite samples correspond to the uncertainty on the mean value of the atomic % for the magnesium and iron (Fig. 11). The propagation of uncertainty for the Mg:Fe ratio is defined in Eqs. (1),

$$ \frac{\delta \frac{Mg}{Fe}}{\frac{Mg}{Fe}} = \sqrt{\left(\frac{\delta Mg}{Mg}\right)^2 + \left(\frac{\delta Fe}{Fe}\right)^2} \quad (1) $$

$$ \delta Mg = \frac{\sigma Mg}{\sqrt{N}} \quad \text{and} \quad \delta Fe = \frac{\sigma Fe}{\sqrt{N}} $$

where Mg and Fe are the values of the atomic percentage calculated from the weighted percentage provided by pXRF, and δMg and δFe the standard deviations of magnesium and iron atomic values divided by the square root of the number of measurements (5 times at the same point on each sample). The horizontal demi error bars indicate the standard deviation of peak position divided by the square root of the number of measurements (5 random locations on each sample). Most of the ankerite samples have a position at 166–169 cm⁻¹ with Mg:Fe < 4 except for one sample at 171.3 cm⁻¹ with Mg:Fe = 4.5 (Fig. 11). The dolomite sample with the lowest Mg:Fe shows a similar position to that
of the highest Mg:Fe ankerite at Mg:Fe = 4.3 and 170.5 cm⁻¹.
Therefore, 170 – 172 cm⁻¹ in the Raman spectrum can be inter-
preted as the value corresponding to the border or transition
region between ankerite and dolomite.

4.3. Applicability of portable devices in the field

This study was carried out on the flat surface of pelletized sam-
ples composed of a single (i.e., monomineralic) mineral in order to
unify sample conditions and represent the homogeneity of the

samples. However, mixed samples, including various particle sizes
and diverse mineral groups, are more common in the field. Raman
analysis can be performed without sample preparation. Solid sam-
ples have generally lower absolute intensity than pressed pellets
and the signal-to-noise ratio can be affected by the surface condi-
tion. However, these factors are not critical for position
determination (Fig. 12). Furthermore, Raman analysis also suffers
from fluorescence effects, precluding the identification of some
carbonates. That effect was often encountered in this study and
lead to the exclusion of 2 samples from the study, siderite, and
magnesite, respectively. ATR requires the extraction of a small
piece of the sample. It is however much simpler to apply than
XRD as no crushing and sieving of the <100 µm are required. Both
techniques can be applied to the same grains as the Raman beam is
smaller than the grain to be analysed by ATR. Adding pXRF on the
field would facilitate mineral recognition but the large beam size
(8 mm) makes it difficult to apply to heterogeneous samples.
Assuming that only two techniques are to be applied, the combina-
tion of i) Raman and pXRF or ii) Raman and IR analysis is the most
appropriate to identify carbonate minerals. i) The Raman and pXRF
combination can be applied directly to a rock or core sample and is
capable of providing elemental and mineralogical information
simultaneously. ii) Raman and IR analyses are effective in identify-
ing minerals in mixed samples made up of the same carbonate
group and also various mineral groups such as silicates and sul-
fates. Furthermore, Raman analysis would be the most effective
to distinguish carbonate minerals if only one method in the three
techniques was required on a limited basis. Unlike pXRF with the
large beam size and IR analysis that cannot distinguish ankerite
and dolomite relationship, Raman spectroscopy is able to discrim-
inate between the other carbonate phases except for siderite and
rhodochrosite, which are readily identified with the naked eye.

<figure><img src="image_17.png" /><figcaption>Fig. 12. Changes of Raman spectra from the surface condition on carbonate minerals.</figcaption></figure>

4.4. Principal Component analysis (PCA)

To investigate the chemometric capability of the discrimination
of carbonate minerals, a Principal Component Analysis (PCA) which
reduces the number of variables of the input spectra was applied
(Fig. 13). PCA analysis was carried out by FactoMinR (Factor Anal-
ysis and Data Mining with R) package in R platform [26,31,35]. The
spectral data for PCA consisted of the position of each band for
Raman and infrared spectra, and the concentration of the four
cations measured by pXRF. The two principal components (i.e.,

<figure><img src="image_18.png" /><figcaption>Fig. 13. PCA scores bi-plot using each band position from Raman and infrared spectra, and corrected concentration data of the four cations (Ca, Mg, Fe, and Mn) from pXRF.</figcaption></figure>
<table><caption>**Table 2**
Coefficients of determination between primary variables and principal components.</caption><thead><tr><th rowspan="2"></th><th>Raman</th><th>IR</th><th>Ca</th><th>Mg</th><th>Fe</th><th>Mn</th></tr><tr><th colspan="2">Position (cm<sup>-1</sup>)</th><th colspan="4">Concentration (wt.%)</th></tr></thead><tbody><tr><td>PC1</td><td>0.96</td><td>0.98</td><td>0.86</td><td>0.57</td><td>0.17</td><td>0.01</td></tr><tr><td>PC2</td><td>0.00</td><td>0.01</td><td>0.13</td><td>0.34</td><td>0.10</td><td>0.72</td></tr><tr><td>PC3</td><td>0.03</td><td>0.00</td><td>0.00</td><td>0.08</td><td>0.73</td><td>0.26</td></tr><tr><td>PC4</td><td>0.01</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.00</td></tr></tbody></table>

PC1 and PC2) account for 81% of the variance, with 59.3% and 21.7%, respectively (Fig. 13). The PC 1/PC 2 score plot indicates a clear separation of the pure carbonate clusters from each other, except for the ankerite and dolomite that are close to each other and close to the origin in the PC 1/PC 2 field, meaning that neither of these variables is critical in distinguishing them. However, they remain separated by positive PC1 values for dolomite and negative PC1 values for ankerite. These analogous plots can be interpreted as a result of a similar composition that results in similar band positions between the ankerite-dolomite series. The siderite samples show a linear distribution on the score plot due to the variation in composition within the siderite group as described above. Table 2 shows that Raman, IR, and Ca are strongly correlated to PC1 (coefficient close to 1). Mn and Fe are highly correlated in PC2 and PC3 with both showing coefficients above 0.70. Thus, the PCA plot provides a well-classified graphical representation for the carbonate endmembers based on the combination of the three different spectral datasets.

## 5. Conclusions

This study aimed to compare chemical compositions with spectral properties using 20 samples of six natural carbonate minerals (calcite, ankerite, dolomite, siderite, rhodochrosite, and magnesite) in order to effectively identify the minerals from spectral differences. We clearly demonstrated a strong correlation between Mg# acquired from a portable XRF device and the band positions from both a portable Raman and a portable infrared spectrometer with an ATR module. The compositional variation is highly correlated to the spectral regions with respect to translational (T) lattice mode in the Raman spectra and symmetric bending vibration (ν₄) in the infrared spectra. Two different trends for the calcite-ankerite-dolomite-magnesite group and the siderite-magnesite group were identified by combining the chemical composition and the wavenumbers from Raman and IR spectra. Our detailed mineralogical and spectral study suggests that spectral analysis using portable tools is effectively applicable to the determination of carbonate minerals in the field. Most processing and plots in this study, including the PCA, can be automatized to accurately identify unknown samples against references using handheld devices. Another alternative to pXRF would be handheld Laser-induced breakdown spectroscopy (HH LIBS). While pXRF has limitations in measuring low-Mg carbonates, HH LIBS is able to analyse the elements that are lighter than Mg such as lithium, beryllium, and fluorine with a small laser spot size (100 µm) [11]. Finally, the continual shifts in Raman and ATR linked to the variation of the Ca-Mg-Fe-Mn ratio in the carbonate minerals suggest that a semi-quantitative analysis of carbonates by Raman or ATR is most probably feasible.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgment

This research is part of the NEXT project, supported by funding from the European Union's Horizon 2020 research and innovation programme under Grant Agreement No. 776804 – H2020-SC5-2017. We thank all the anonymous reviewers for their critical reviews and comments.

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