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In-Situ X-Ray Photoelectron Spectroscopy and Raman Microscopy of Roselite Crystals, Ca2(Co2+,Mg)

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In-Situ X-Ray Photoelectron Spectroscopy and Raman Microscopy of Roselite Crystals, Ca2(Co2+,Mg) crystals Article In-Situ X-ray Photoelectron Spectroscopy and Raman 2+ Microscopy of Roselite Crystals, Ca2(Co ,Mg)(AsO4)2 2H2O, from the Aghbar Mine, Morocco Jacob Teunis Kloprogge 1,2,* , Barry James Wood 3,† and Danilo Octaviano Ortillo 2 1 School of Earth and Environmental Sciences, The University of Queensland, Brisbane, QLD 4072, Australia 2 Department of Chemistry, College of Arts and Sciences, University of the Philippines Visayas, Miagao, Iloilo 5023, Philippines; [email protected] 3 Centre for Microscopy & Microanalysis, The University of Queensland, Brisbane, QLD 4072, Australia; [email protected] * Correspondence: [email protected] † Deceased. 2+ Abstract: Roselite from the Aghbar Mine, Morocco, [Ca2(Co ,Mg)(AsO4)2 2H2O], was investigated by X-ray Photoelectron and Raman spectroscopy. X-ray Photoelectron Spectroscopy revealed a cobalt to magnesium ratio of 3:1. Magnesium, cobalt and calcium showed single bands associated with unique crystallographic positions. The oxygen 1s spectrum displayed two bands associated with the arsenate group and crystal water. Arsenic 3d exhibited bands with a ratio close to that of the cobalt to magnesium ratio, indicative of the local arsenic environment being sensitive to the substitution of magnesium for cobalt. The Raman arsenate symmetric and antisymmetric modes were all split with the antisymmetric modes observed around 865 and 818 cm−1, while the symmetric modes were Citation: Kloprogge, J.T.; Wood, B.J.; found around 980 and 709 cm−1. An overlapping water-libration mode was observed at 709 cm−1. Ortillo, D.O. In-Situ X-ray The region at 400–500 cm−1 showed splitting of the arsenate antisymmetric mode with bands at 499, Photoelectron Spectroscopy and 475, 450 and 425 cm−1. The 300–400 cm−1 region showed the corresponding symmetric bending Raman Microscopy of Roselite modes at 377, 353, 336 and 304 cm−1. The bands below 300 cm−1 were assigned to lattice modes. 2+ Crystals, Ca2(Co ,Mg)(AsO4)2 2H2O, from the Aghbar Mine, Morocco. Keywords: arsenate; Raman microscopy; roselite; X-ray Photoelectron Spectroscopy Crystals 2021, 11, 670. https://doi. org/10.3390/cryst11060670 Academic Editor: Sergio Brutti 1. Introduction Received: 17 May 2021 Roselite is a rare secondary arsenate mineral in cobalt-bearing hydrothermal min- Accepted: 8 June 2021 eral deposits forming a solid solution with wendwilsonite with Mg substituting for Co2+. Published: 10 June 2021 Since both minerals are very similar in appearance, it is almost impossible to distinguish between the two minerals without proper chemical analysis. The crystals of roselite Publisher’s Note: MDPI stays neutral are transparent to translucent with a pale pink to dark rose-pink colour. Occasionally with regard to jurisdictional claims in colour-zoning can be observed and is a function of the cobalt content [1]. Roselite is published maps and institutional affil- often associated with related arsenates such as wendwilsonite, talmessite and erythrite, iations. as can be observed at Bou Azzer, Morocco [2–5]. Minerals of the roselite group can by characterized by the general formula X2M(TO4)2.2H2O with X = Ca, Na; M = Mg, Mn2+, Co2+, Cu2+, Zn and T = As5+,S6+. The roselite group consists of the end-member minerals: roselite Ca2Co(AsO4)2.2H2O, wendwilsonite Ca2Mg(AsO4)2.2H2O, brandtite Copyright: © 2021 by the authors. Ca2Mn(AsO4)2.2H2O, zincroselite Ca2Zn(AsO4)2.2H2O and kröhnkite Na2Cu(SO4).2H2O. Licensee MDPI, Basel, Switzerland. Recently a new member was added to the group with the chemical composition This article is an open access article Ca2Cu(AsO4)2.2H2O named rruffite [6]. distributed under the terms and Roselite was named in 1824 by French mathematician and mineralogist Armand conditions of the Creative Commons Lévy (14 November 1795–29 July 1841) in honour of the German mineralogist Gustav Attribution (CC BY) license (https:// Rose (18 March 1798–15 July 1873), who was a Professor of Mineralogy at the University creativecommons.org/licenses/by/ of Berlin, as an orthorhombic arsenate of lime and cobalt [7]. About fifty years later Schrauf 4.0/). Crystals 2021, 11, 670. https://doi.org/10.3390/cryst11060670 https://www.mdpi.com/journal/crystals Crystals 2021, 11, 670 2 of 10 Crystals 2021, 11, x FOR PEER REVIEW 2 of 10 Berlin, as an orthorhombic arsenate of lime and cobalt [7]. About fifty years later Schrauf concludedconcluded that that roselite roselite did did not possessnot possess orthorhombic orthorhombic symmetry, symmetry, although although it showed it showed pseudo-orthorhombicpseudo-orthorhombic triclinic triclinic elements elements due to due the factto the the fact crystals the exhibitedcrystals exhibited lamellar twin-lamellar ningtwinning on as many on as as many five of as the five six of pseudo-symmetry the six pseudo-symmetry elements elemen of thets chosen of the latticechosen[ 8lattice–10]. [8– Peacock10]. Peacock however however definitively definitively proved thatproved roselite that belongsroselite tobelongs the monoclinic to the monoclinic symmetry sym- metry class [11] with point group 2/m (space group P21/c), a = 5.801(1) Å, b = 12.898(3) Å, class [11] with point group 2/m (space group P21/c), a = 5.801(1) Å, b = 12.898(3) Å, c = 5.617(1)c = 5.617(1) Å, b Å,= 107β = ◦107°42(2)42(2)0, Z =′, Z 2. = Roselite 2. Roselite crystals crystals have have a tendency a tendency to be to elongated be elongated along along [100][100] and and are frequentlyare frequently twinned twinned on {100}on {100} with with {100} {100} as the as compositionthe composition plane plane [2,12 [2,12,13].,13]. FigureFigure1 shows 1 shows the crystal the crystal structure structure and formand form of roselite. of roselite. (a) (b) FigureFigure 1. (a )1. Crystal (a) Crystal structure structure (top) (top) of roselite of roselite based based on crystal on crystal data data from from Hawthorne Hawthorne and Fergusonand Ferguson [13]; ([13];b) crystal (b) crystal form. form. WhileWhile there there is available is available data ondata the on crystal the crysta structurel structure and form and of form roselite, of roselite, infrared andinfrared Ramanand spectroscopic Raman spectroscopic data are limiteddata are while limited there while are there no reported are no reported X-ray Photoelectron X-ray Photoelectron data anddata more and work more is needed work is on needed this rare on arsenate this rare mineral arsenate [6 ,mineral14,15]. Both[6,14,15]. techniques Both techniques allow for theallow non-destructive for the non-destructive analysis of analysis small crystals of small in crystals situ. As in partsitu. ofAs an part ongoing of an ongoing study on study arsenateon arsenate minerals minerals [16–25 ][16–25] this study this study aims toaims better to better understand understand the X-ray the X-ray photoelectron photoelectron spectroscopyspectroscopy (XPS) (XPS) and and Raman Raman spectra spectra of well of well crystallised crystallised roselite roselite from from the Aghbarthe Aghbar Mine, Mine, MoroccoMorocco [3–5 [3–5].]. A betterA better understanding understanding of of the the XPS XPS andand Raman spectra of of roselite roselite and and other otherarsenate arsenate minerals minerals will will support support the the identificati identificationon of of these these secondary secondary minerals minerals in in e.g., e.g., con- contaminatedtaminated soilssoils duedue toto leachingleaching fromfrom minemine tailings. tailings. 2. Materials and Methods 2. Materials and Methods The roselite sample used in this study is part of the author’s private collection under The roselite sample used in this study is part of the author’s private collection under catalogue number 05802 (Figure2) and originates from the Aghbar Mine, Aghbar, Bou catalogue number 05802 (Figure 2) and originates from the Aghbar Mine, Aghbar, Bou Azer District (Bou Azzer District), Tazenakht, Ouarzazate Province, Souss-Massa-Draâ Azer District (Bou Azzer District), Tazenakht, Ouarzazate Province, Souss-Massa-Draâ Region, Morocco [3–5]. Region, Morocco [3–5]. The XPS analyses were performed on a Kratos AXIS Ultra (Kratos Analytical, Manch- ester, UK) with a monochromatic Al X-ray source at 225 W under ultrahigh vacuum conditions. Each analysis started with a survey scan from 0 to 1200 eV with a dwell time of 100 milliseconds, pass energy of 160 eV at steps of 1 eV with 1 sweep. For the high resolution analyses the number of sweeps was increased, the pass energy was lowered to 20 eV at steps of 100 meV and the dwell time was changed to 250 milliseconds. The spectra were charge corrected using the adventitious C 1s signal at 284.8 eV. The sample with the roselite crystal in situ was placed in the XPS after a wash with alcohol. Prior to the analyses the surface of the crystal was cleaned by Argon ion etching for 20 min. Despite the fact that XPS is generally considered a surface analysis technique, in reality more than 90% of the signal is received from the bulk crystal structure beneath the surface. The roselite sample was orientated on a polished metal surface on the stage of an Olympus BHSM microscope equipped with 10× and 50× objectives. The microscope is part of a Renishaw 1000 Raman microscope system (Renishaw, New Mills, Wotton-under- Edge, Gloucestershire, UK), which also includes a monochromator, a filter system and Crystals 2021, 11, 670 3 of 10 a Charge Coupled Device (CCD). The Raman spectra were excited by a Spectra-Physics model-127 H-Ne laser producing highly polarised light at 633 nm. Spectra were obtained at a nominal resolution of 2 cm−1 and a precision of around 1 cm−1 in the range between −1 Crystals 2021, 11, x FOR PEER REVIEW1500 and 100 cm . 3 of 10 FigureFigure 2.2. MicroscopeMicroscope photophoto ofof roselite,roselite, cataloguecatalogue no.no.
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