Plasma Spectroscopy of Various Types of Gypsum : 2 An ideal Terrestrial Analogue

The first detection of gypsum (CaSO4.2H2O) by the Mars Science Laboratory (MSL) rover 14 Curiosity in the Gale Crater, Mars created a profound impact on planetary science and exploration. 15 The unique capability of plasma spectroscopy involving in situ elemental analysis in extraterrestrial 16 environments, suggesting the presence of water in the red planet based on phase characterization 17 and providing a clue to Martian paleoclimate. The key to gypsum as an ideal paleoclimate proxy lies 18 in its textural variants, and in this study terrestrial gypsum samples from varied locations and 19 textural types have been analyzed with laser-induced breakdown spectroscopy (LIBS). Petrographic, 20 sub-microscopic and powder X-ray diffraction characterizations confirm the presence of gypsum 21 (hydrated calcium sulphate; CaSO4.2H2O), bassanite (semi-hydrated calcium sulphate; 22 CaSO4.1/2H2O) and anhydrite (anhydrous calcium sulphate; CaSO4) along with accessory phases 23 (quartz and jarosite). The principal component analysis of LIBS spectra from texturally varied 24 gypsums can be differentiated from one another because of the chemical variability in their 25 elemental concentrations. The concentration of gypsum is determined from the partial least-square 26 regressions model. Rapid characterization of gypsum samples with LIBS is expected to work well in 27 extraterrestrial environments. 28


Introduction
Laser Induced Breakdown Spectroscopy (LIBS) is an atomic emission spectroscopic technique in which high power laser pulse is focused on the surface of the material and plasma is created.Light from the plasma is fed to the spectrometer to get spectral lines of the corresponding elements present in the material.The LIBS technique is applied in mineral and rock analyses in field and laboratory environments [1][2] for rapid measurements without involving sample preparation.It has a unique in-situ point detection capability.LIBS technique was introduced in planetary exploration for the first time in 2012 when the Mars Science Laboratory (MSL) rover Curiosity studied the Gale Crater [3][4].The ChemCam identified gypsum, a water-bearing non-metallic calcium sulphate mineral (CaSO4.2H2O)as veins on the rim of Endeavour Crater, Mars as veins [5][6].Consequently, it was suggested that gypsum on Mars formed due to evaporation of a large lake [7][8][9]  physico-chemical environment.Sulphate salts are useful markers to decode paleoenvironments as they precipitate from aqueous fluids suggesting the presence of liquid water.Further the aqueous geochemistry promotes the proliferation of microbial life and its consequent preservation in planetary environments [10].
Gypsum provides ideal habitat for endolithic microbial communities, protection from harmful ultra-violet radiations and supports their proliferation even under High Arctic conditions [11] to hot springs [12] environments with temperatures larger than 50°C.Microbes thrive within gypsum crystals in terrestrial impact craters, such as Haughton structure [13][14][15] and the extraterrestrial landscapes are endowed with such structures [16].Based on terrestrial evidences, the possible presence of microbial life is predicted in gypsum veins and in other sulfate minerals on Mars [17][18], other planets of the solar system, their moons, asteroids and comets.
The MSL Curiosity rover characterized many calcium sulfate veins at Gale crater, Mars including gypsum at Martian surface with nominal change in elemental composition [19].Dunes of gypsum are also detected at the Martian North pole [20][21].Presence of sulphates associated with clays at the base of the Mount Sharp by Compact Reconnaissance Imaging Spectrometer (CRISM) is also pointed out by Milliken et al. 2010 [22] and also by Opportunity rover at Meridiani Planum [23].
Various hypotheses explaining the origin of the massive Martian sulphate deposits exist advocating hydrothermal processes, oxidation of sulphides, evaporative deposition, and bedrock leaching by volcanic vapors [23][24][25][26][27][28][29].The report of gypsum veins from Mars, its formation in various terrestrial environments and suitability for the proliferation of life have made this hydrated sulfate mineral an ideal terrestrial analogue [30].In addition, the texturally different types of gypsum, which in turn control the physical and chemical conditions of their formation, have attracted significant attention [31].
In the present work, gypsum samples from varied terrestrial environments and textural types have been studied for their inter-sample comparison using LIBS data combined with multivariate statistical analysis methods.The data generated can be compared with the available ChemCam data on gypsum from Mars and upcoming Chandrayaan-2 experiments from Moon [32].

Mineralogy and geochemistry of gypsum
Gypsum crystallizes in monoclinic system and its structure contains parallel layers of (SO4) -2 group bonded with (Ca) +2 .The CaSO4 molecules are separeated by sheets of H2O-molecules having very weak bonds and breakdown on heating [33][34].Calcium atoms are enclosed by two water molecules and six sulphate oxygen atoms, though the sulphur atoms are at the middle of sulphate oxygen tetrahedra.The water molecules have two oxygen atoms and calcium as their first neighbours.The arrangement consists of SO4 tetrahedra, CaO8 polyhedra and weak hydrogen bonds made among the O atoms of nearest-neighbour SO4 tetrahedra and intercalated H2O molecules [35].The diaphaneity of gypsum varies from transparent to opaque and is usually light in color.The habit of gypsum is platy, columnar, fibrous, needle-like, lenticular, forms massive aggregates and twins-swallowtail.It is a relatively soft mineral (1.5-2: Mohs' Scale of Hardness) and gets easily weathered.Gypsum constitutes 79.1% of calcium sulphate and 20.9 % water by weight.Since gypsum is known to form by several processes (digenesis [36]; chemical weathering [37]; evaporation [38]; hydrothermal activities etc. [39]) and under varied physical and chemical environments and this has led to useful reconstruction of the paleoenvironment [40][41].Sometimes  [42][43].Formation of gypsum takes place in through a complex multi-step processes [43].It can form in evaporite beds during evaporation of water near lake and sea water.Gypsum can form diagenetically by oxidation.It can form in association with sedimentary rocks as oxidation of existing sediments leading to gypsum formation and replaces the other minerals in the sediments [44].It can also occur in hot spring from volcanic vapors as well as in sulfate solutions in veins [19].Hydrothermal anhydrite in veins is commonly hydrated to gypsum by groundwater in near-surface exposures.Gypsum can also occur in a flower-like form with embedded sand grains called desert rose in arid areas [45].
The SEM images of all five samples are shown in Figure 2.
They form an array of circular nearly flat plates, where plates are having sharper edges and relatively thick center.The diameter of biggest plate is about 5 cm and smallest is around 1.5 cm.The color is pinkish brown similar to sand (5YR 4/6; Munsell Color Chart).The sand encrustation on the top surface is about a few mm thick [45].Acicular gypsum (MAG; Morocco) constitutes column-like aggregate of needle-shaped gypsum crystals.The length of the gypsum is around 24 cm and width around 3 cm.The color of the gypsum is white (5GY 9/2; Munsell Color Chart).Visibly the sample does not contain any impurity [46].Kachchh gypsum (KG; Gujarat, India) is light brown color (7.5 YR 7/8; Munsell Color Chart), contains randomly arranges sub-millimeter-thin flakes/plates.The effect of surficial weathering in the sample is evident.The length of the sample is about 10 cm, and its width is 2 cm [48][49][50][51].
Rajpura-Dariba gypsum 2 (RDG2; Dariba Mines, Rajasthan) is off-white in color (5 Y 9/2; Munsell Color Chart).The sample is having blade like petal having no fixed orientation and the blades are very thin (around 0.5 cm).The sample length and width are 12.3 and 2.5 cm, respectively [52].

Powder X-ray diffraction (XRD)
Powdered gypsum samples were characterized by X-ray Diffractometer (Philips PanAnalytical) using CuKα radiation and Ni filter.The scans were made between 10 to 75° 2θ at a speed of 2° per minute, divergence slit = 1°, anti-scatter slit =1°, receiving slit = 0.30 mm, generator voltage of 40 kV and tube current of 30 mA.

Experimental Set-up
The experimental setup consists of a frequency-doubled (532 nm) Nd:YAG laser (Continuum Surelite III-10).The plasma is created using a 15 mJ laser pulse having a pulse width of 4ns with 10

Multivariate analysis
Multivariate analysis is a tool that can be used to maximize the information extracted from a huge number of datasets, here in the present case spectral data of LIBS spectra of the samples.Single spectra of LIBS can contain more than 8000 data points thus it is very difficult to distinguish the spectral variables (wavelength, intensity) of the samples with chemical heterogeneity [54][55].
Multivariate analyses techniques such as PCA and PSLR are used in the present study.PCA is an unsupervised method that reduces the dimensionality (number of features) within a dataset while still retaining as much information as possible.It is an orthogonal transformation that reduces the spectral data by projecting them into lower dimensions called principal components [56].For the computation of the first principal component it is the linear combination of variables on which the most data variation can be projected.And for succeeding component it is perpendicular to the For making the regression model the concentration of Calcium and sulfur is taken from the analysis of Electron Microprobe Analyzer [57].

X-ray diffraction analysis
X-ray diffraction studies on powered samples reveal the presence of gypsum and anhydrite in all samples, illustrated in Figure 3       If the concentration of any element in the sample is very high, its molecular bands must be present in the laser induced plasma of the sample [60].The presence of sulfur lines (542.8 nm, 543.2 nm, 545.3 nm) in the LIBS spectra of gypsum also confirms its presence, as indicated in Figure 8.

LIBS analysis
Similarly, the presence of carbon line at 247.8 nm and oxygen at 777.1 nm indicates the presence of carbonates in the present study.Strong spectral lines of magnesium at wavelengths 279.5 nm, 285.2 nm, 383.8 nm etc. are present in four gypsum samples except in acicular sample (MAG).Magnesium occurs in the defect sites of the gypsum lattice since they form large soluble complexes [61].The intensity of spectral line of magnesium varies depending upon its concentration in the sample.
Presence of prominent line of Si in MDRG is due to the presence of quartz grains from the sand which bonds with moisture on the top of gypsum plates.Ca lines are prominent in all gypsum samples except MDRG as the analyzed spots are dominantly masked by quartz grains.Presence of spectral lines of Fe in KG and MDRG are due to jarosite and quartz, respectively.Gypsum comprises abundant fluid phase, which is a mixture of various elements K, Ti, Mn and Sr in minor to trace amounts.Their concentration in gypsum is used in stratigraphic correlation, paleosalinity estimation and to decipher the origin of brine and diagenesis [62,63].The presence of spectral lines of Mn and Fe in KG indicates the terrigenic deposit mainly siliciclastic sediments or clay minerals into the sedimentary basin.Sr lines are found in RDG2 as Sr can replace Ca in the gypsum lattice [64].
Presence of strontium in gypsum can take place in many ways like digenetic process, bacterial sulphate reduction or by dissolution or recrystallization.Abundance and scarcity of Sr content in gypsum indicate the episodic salinity fluctuation of the basin and this also reveals the paleohydrology of the basin [65].Similarly, Presence of spectral line of Ba (455.4 nm and 614.1 nm) may be correlated as a possible tracer of fluid circulation in hydrothermal water [66].The wavelength of the spectral lines observed in the LIBS spectra of all five gypsums are summarized in Tables 1 and 2. The same calcium lines are identified in all samples and the wavelengths are listed below Table 2.
Hydration level in gypsum depends upon the hydrogen and oxygen content, calcium sulfate either occurs as non-hydrated (anhydrite CaSO4) or hydrated phase (bassinite: CaSO4.1/2H2Oandgypsum: CaSO4.2H2O).H alpha emission line at 656.5nm is sensitive to hydrogen content which can differentiate hydration levels of calcium sulfates in the samples.Similarly, the emission line of oxygen at 777.1 nm is sensitive to correlate the hydration level in the sample [67].
Spectral lines of both elements are detected but respective intensities of the spectral lines of hydrogen and oxygen in samples RDG-1 are very low.Thus, their concentration in RDG1 is also low which establishes the calcium sulfates observed may be bassanite.deviate very little from each other.Thus, the model adopted in the present study is strongly correlated.Concentration of all calcium and sulfur is determined using above PSLR model and the values are tabulated in Table 3.
pulses per second of repetition rate.A convex lens of focal length 15 cm is used to focus the laser on the sample to create hot plasma.The light emitted by the plasma at the sample surface was collected by a set of collection optics (CC52 collimator, Andor Technology, USA) and focused into the optical fibre bundle.The optical fiber bundle delivered the light to the Mechelle spectrograph (ME5000, Andor Technology, USA) equipped with an ICCD camera (iStar 334, Andor Technology, USA).The spectral signatures of atoms and ions form of the plasma are collected after a short interval of time to avoid the background continuum.Therefore, the gate delay and gate width are 0.7 µ s and 4 µ s, respectively.A detailed analysis of spectral lines uses Andor SOLIS software and NIST atomic spectroscopic database[53].

Figure 4 .
Figure 4. Typical LIBS spectra (250-600 nm) of different gypsum samples analyzed during the present study stacked to show the relative intensity variations amongst the samples.

Figure 5 .Figure 6 .
Figure 5. Spectral regions of recorded Al, Mg, Ca, and Fe data for the gypsum samples.

Figure 7 .
Figure 7. Spectral regions of recorded H, K, and O atomic data for the gypsum samples.

Figure 8 .
Figure 8. Spectral regions of recorded H, K, and O atomic data for the gypsum samples.

Figures 5 to 8
Figures 5 to 8 indicate that the intensities of spectral lines of the elements are different in the LIBS spectra of gypsums formed in different conditions/environment.Thus, with the information of elemental analysis or composition of gypsum one can predict the environment and the process of formation which cannot be achieved by their physical appearance.Persistent spectral lines along with the other weak spectral lines of calcium are observed in all five samples.Similarly, in the observation of Forni et al.[58,59], a molecular band near 606 nm probably due to presence of CaOH is also observed in the LIBS spectra of all five gypsum samples.

Five replicate spectra of
each gypsum sample are classified in five different groups using PCA.These groups indicate variation in elemental concentrations in gypsums.Replicates of each gypsum sample are very close to each other forming a clustering in the score plot.Clearly, the samples of each class tend to cluster and in almost all cases they are well-separated from the other classes.The PC1 classifies gypsum samples into two groups (see Fig. 9).The first group contains MAG, RDG1 and RDG2 having positive correlation.The second groups KG and MRD show negative correlation.MRD and KG are different from MAG, RDG1 and RDG2 as they contain Si, Fe, Al, Mg and Ti.The above observation is also consistent with the loading plot, as illustrated in Figure 10.A positive correlation observed in the loading plot corresponds to the emission lines of Ca which is greater in RDG1, RDG2 and MAG while negative correlation is noted in Si, Al, Ti, Mg and Fe.In the second group (MRD and KG), Fe and Mn are present in only KG.The Si content is very high in MRD in comparison to KG.This is the main reason leading to the classification of these samples into two different clusters based on PC2.Further the cluster of KG is comparatively spread out suggesting sample heterogeneity.Similarly, in the first group PC2 is classified into two different categories (RDG1and MAG) due to the less hydration in RGD1.The clustering of RGD2 and MAG are very close to each other as they contain meager impurities.But RGD2 and MAG also form two groups because of Sr and Ba present in RDG2.

Figure 10 .
Figure 10.Loading plot for the principal component analysis of the LIBS spectral data.

Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 2 July 2019 doi:10.20944/preprints201907.0033.v1
Peer-reviewed version available at Atoms 2019, 7, 72; doi:10.3390/atoms7030072previous principal component so that it contains maximum variation.Partial Least Squares Regression (PLSR) is one of the multivariate methods used in the interpretation of spectra (independent variables) to predict concentrations (dependent variables) of element in the sample.

Table 1 .
Spectral wavelengths of atoms observed in laser induced breakdown spectroscopic spectra of different gypsum samples: Hydrogen, nitrogen, oxygen, sodium, magnesium, aluminium, and silicon.

Table 2 :
Spectral wavelengths of atoms observed in laser induced breakdown spectroscopic spectra of different gypsum samples: Sulfur, potassium, titanium, manganese, iron, strontium, barium.In the present study PCA is used to analyze the LIBS spectra of gypsum samples.The spectra obtained from different gypsum samples are arranged in the form of matrix which is made up with variables having 8848 features.The Unscrambler-X software (CAMO Software India Pvt.Ltd.) is used to solve the data matrix to generate PCA plot.Figure9illustrates that the combination of PC1 and PC2 is 92% which is very close to 100% that explain most of the variance present in the dataset.

Table 3 .
Reference and predicted values of calcium and sulfur in analyzed gypsum samples.