Coupled SEM-microRaman system: A powerful tool to characterize a micrometric aluminum-phosphate–sulfate (APS)

man m- ahfid 5060 Or � A coupled SEM-microRaman spectrometer was used to differentiate APS phases. � Based on literature, band assignment of the Raman spectra was performed. � Sulfate substitution by two distinct phosphate groups could be metric aluminum-phosphate–sulfate (APS) occurring in a complex geological Keywords: cm�1 decrease as a ects well th as also sho werful tool tify micro-minerals of the alunite supergroup occurring in a complex geological sample. � 2013 Elsevier B.V. All rights re 1. Introduction Interest in the minerals of the alunite supergroup has surged in recent decades particularly because of the occurrence of these minerals in a wide range of geological environments, including metamorphic, sedimentary, magmatic rocks and soils, and their large domain of stability (pH, temperature, redox) [1]. In these environments, these minerals are potential scavengers of toxic ele- ments such as heavy metals, radionuclides or rare earths and play an important role in the transport of these elements in the environ- ment [2–4]. Some of these minerals have also been found on Mars ⇑ Corresponding author. Tel.: +33 2 38 64 35 92; fax: +33 2 38 64 39 25. Journal of Molecular Structure 1048 (2013) 33–40 Contents lists available at ec E-mail address: [email protected] (N. Maubec). Aluminum-phosphate–sulfate Raman spectroscopy Micrometric size Substitutions established that the intensity of the sulfate bands located at around 653 and 1026 function of the mole fraction of phosphate in the APS minerals. This correlation refl of the substitution of sulfate anions by phosphate anions. In addition, this study h the use of a coupling system between a SEM and a microRaman spectrometer is a po 0022-2860/$ - see front matter � 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.05.019 e effect wn that to iden- served. Article history: Received 16 January 2013 Received in revised form 13 May 2013 Accepted 13 May 2013 Available online 23 May 2013 A micrometric aluminum-phosphate–sulfate (APS) with an intermediate composition between the min- erals woodhouseite (CaAl3(PO4)(SO4)(OH)6) and crandallite (Ca(PO4)2(OH)5�H2O) has been identified in a natural sample and characterized using a coupled SEM-microRaman system. The Raman spectrum obtained was characterized by various bands assigned to vibrational modes of sulfate and phosphate units. A comparison with other APS minerals, with different PO4/SO4 ratios, enables some differences to be highlighted, which concern particularly the intensities of some sulfate and phosphate bands. It is evidenced. � Raman spectroscopy is proposed as a new tool to follow PO4/SO4 substitution. a r t i c l e i n f o a b s t r a c t system. The capability to get BSE contrast, WDS elemental mapping and Raman data on tiny grains, allowed new structural insight for an APS with a high PO4/SO4 ratio and capability to distinguish com- pounds with complex variable compositions between the minerals alunite (KAl3(SO4)2(OH)6) and cran- drallite (Ca(PO4)2(OH)5�H2O). � APSs (aluminum-phosphate–sulfate) occur as complex micrometric grains. Here we used a coupling d trometer to analyze micro Coupled SEM-microRa a micrometric aluminu N. Maubec ⇑, C. Lerouge, A. L BRGM, 3 Avenue Claude Guillemin, BP 36009, 4 h i g h l i g h t s system: A powerful tool to characterize phosphate–sulfate (APS) , G. Wille, K. Michel, X. Bourrat léans Cédex 2, France g r a p h i c a l a b s t r a c t evice between a scanning electron microscope (SEM) and a microRaman spec- journal homepage: www.elsevier .com/ locate /molst ruc Journal of Mol SciVerse ScienceDirect ular Structure 2.3. Instrumentation cula [5]. Such a discovery is important for Mars exploration since it en- ables insights into Martian surficial processes; the presence of this kind of minerals suggests aqueous components in processes under oxidizing and acid-sulfate conditions [6,7]. Minerals of the alunite supergroup have a general formula given by AB3(XO4)2(OH)6, where A may be occupied by Na, K, Ca, Pb, Ba, U, light REE (such as La, Ce, Pr, Nd, Sm), etc., B is the trivalent cat- ion, Al3+ or Fe3+, but also Cu2+ or Zn2+ and X is the tetrahedral anion occupied by S, P or As [8,9]. Members of this supergroup crystallize in a rhombohedral system, with a space group R�3m. A trigonal symmetry C3v, with a � 7 Å and c � 17 Å, characterize most of the alunite supergroup minerals. The general structure can be de- scribed as a combination of BO2(OH)4 – octahedra and XO4 – tetra- hedra. The A-cations reside between the BO2(OH)4 – octahedral sheets, in an icosahedral site. Knowledge of the structure of alunite is important, since it helps to define and elucidate the interaction with toxic elements and determine the retention mechanisms with minerals, and en- ables fingerprints of minerals to be acquired in order to facilitate their identification. Vibrational spectroscopy has been applied to characterize vari- ous minerals, and Raman spectra of alunite supergroup compounds and synthetic analogs have been reported in numerous studies [10–24]. Raman spectroscopy has been used to acquire structural information and distinguish compounds, but most of the results concern the characterization of sulfate minerals, such as alunite (AAl3(SO4)2(OH)6) or jarosites (AFe3(SO4)2(OH)6) with different cat- ions in A-sites. In contrast, limited Raman spectroscopic data is available on minerals with various ions in the X-site, such as the aluminum-phosphate–sulfates (APSs) of the alunite supergroup [24–26]. Woodhouseite (CaAl3(SO4)(PO4)(OH)6) and svanbergite (SrAl3(SO4)(PO4)(OH)6) are the most widespread among the APS minerals and extensive solid solutions exist between huangite (Ca0.5Al3(SO4)2(OH)6) – woodhouseite – crandallite (CaAl3(PO4)2(- OH)5�H2O) and svanbergite – goyazite (SrAl3(PO4)2(OH)5�H2O) ser- ies. Such minerals therefore have complex variable compositions and it has been reported that some samples can contain rare earth elements in measurable amounts [27]. Therefore, APS are minerals of interest not just in mineral exploration and resources, but also in the environment since they have a capacity to incorporate heavy metals into their structure. The only Raman spectra of APS that have been described are those for which the PO4/SO4 ratios are close to 1 or below, but no Raman spectroscopic study of APS, with a PO4/SO4 ratio higher than 1, has been published. However, natural APS minerals often occur as clusters of micro- nic crystals ( oupl N. Maubec et al. / Journal of Molecular Structure 1048 (2013) 33–40 35 system is presented in Fig. 1. The coupling interface results in a parabolic mirror inserted beneath the SEM pole piece for laser beam injection, optical image and Raman signal collection. This system is configured to work with a focal length of 6 mm and en- ables incident laser beam to be focused onto a micrometer order Fig. 1. Schematic diagram of the SEMSCA c sized spot on a sample in the SEM [29]. Electron beam injection is allowed by a hole in the mirror, aligned with SEM electron op- tics. The incident laser beam and the Raman scattered photons are both transmitted between the microRaman spectrometer and the SEMSCA coupling interface via optical fibers. SEM observations were made under low vacuum conditions (P = 20 Pa) at 25 kV. The sample was not carbon coated to avoid artifacts associated with carbon layers on Raman spectra. SEM images were collected by a backscattered electron detector (BSE). An EDAX TEAM Energy Dispersive X-ray Spectroscopy system (EDS) (EDAX Mahwah – USA) equipped with a Silicon Drift Detec- tor (SDD) was used to determine the chemical element composi- tion of the APS mineral. Spectral resolution is 126 eV at Mn Ka X-ray peak. Fig. 2. Overview of the sample studied observed by SEM (BSE image mode) with lo During the acquisition of the Raman spectra, the electron beam was blanked to minimize electron bombardment of the sample, which can result in sample degradation and/or excessive carbon contamination. For the investigation, the laser power at the sample was set to 0.5 mW. The spectra were acquired with two accumula- ing interface, according to Jarvis et al. [29]. tions and an acquisition time of 60 s, over a 100–1400 cm�1 spec- tral range. The spectral resolution is 1 cm�1. Calibration was performed using the 520.4 cm�1 line of silicon. Instrument control and Raman measurements were performed with RenishawWIRE™ software. 3. Results and discussion 3.1. Chemical characterization Fig. 2 presents a SEM image, obtained in backscattered electron mode (BSE), which gives an overview of the examined material. The BSE image shows significant phase contrasts due to chemical w (a) and high (b) magnification showing alunite particles with different sizes. variations. The darkest areas are phases with low mean atomic number, whereas the lightest areas are composed of elements with heavier atomic numbers. It provides evidence of the texture of the quartz–alunite–iron hydroxide assemblage and clearly reveals that the 20–40 lm sized grains of alunite exhibit a pseudocubic form and exhibit very fine chemical zoning. In Fig. 3, SEM images show APS minerals that occur as 5–10 lm pseudocubic grains intimately associated within alunite, which un- der natural light illumination of an optical microscope are not dis- tinguished (Fig. 3c). The use of a conventional micro-Raman spectrometer is, therefore, inappropriate to characterize APS min- erals, because of the difficulty in locating them and the impossibil- ity of focusing the laser beam on a precise micro-zone under the optical microscope. Consequently, in the present study, these particles were tar- geted and analyzed by a coupled SEM-microRaman system. EDS analysis of this particle reveals the presence of phosphorus and divalent ions such as Ca2+, Sr2+, Ba2+ (Fig. 4). The quantitative chemical analysis, performed by EPMA, enables a chemical formula to be defined, which can be expressed as (Ca0.7Sr0.1Ba0.1K0.1)Al2.8 Fig. 3. Backscattered electron (BSE) images of the area studied observed with different magnifications (a and b). The cross indicates the point where EDS and Raman spectroscopy were performed. Image (c) is an optical image of the same area. 36 N. Maubec et al. / Journal of Molecular Structure 1048 (2013) 33–40 Fig. 4. Chemical analyses (EDS) of the a luminum-phosphate–sulfate (APS). ctru ecula (PO4)1.5(SO4)0.6(OH)4.6. The particle is a Ca-rich APS whose compo- sition is a solid solution between woodhouseite and crandallite end-members of the alunite supergroup. 3.2. Raman spectroscopy The recorded Raman spectrum of the AP1.5S0.5 is displayed in Fig. 5. This figure shows the frequencies and relative intensities of the bands in the Raman spectrum. The Raman spectrum is characterized by an intense sharp band at 1026 cm�1, assigned to the m1ðSO2�4 Þ symmetric stretching band [26]. The presence of sulfate is also marked by the occurrence of bands centered at 483, 610–640, 653, 1101 and 1180 cm�1. The first band is attributed to the m2ðSO2�4 Þ bending mode. This band was also reported at 485 cm�1 by Frost et al. [26] in the spectrum of woodhouseite. However, these authors indicated that there was a possible second band attributable to this vibration centered at 475 cm�1. In our study, this band is not observed because of both lower sulfate content in the AP1.5S0.5 structure, compared to the �1 Fig. 5. Raman spe N. Maubec et al. / Journal of Mol woodhouseite, and a presence of a band at 465 cm , which can hide other bands. The band centered at 653 cm�1 is due to m4ðSO2�4 Þ bending modes. This band has a shoulder, with at least two bands, also attributable to this vibration [24]. The positions of these bands are 618 and 638 cm�1 in an APS with a PO4/SO4 ratio close to 1. The positions of the two broad bands centered at 1101 and 1180 cm�1 are in good agreement with a previous study of the authors [24] and with the Raman spectrum of a woodhouseite downloaded from the RRUFF database (http://rruff.info), where vibrational bands are observed at 1095 and 1179 cm�1. These bands are attributed to the anti-symmetric stretching vibrational mode of the sulfate ion ðm1ðSO2�4 ÞÞ [24]. However, Frost et al. [35] assigned the band located at 1101 cm�1 to the m3ðPO3�4 Þ anti-symmetric stretching mode. Indeed, in their study of the crandallite mineral (CaAl3(PO4)2(OH)5�H2O) without a sulfate group in the structure, a band with a similar position was re- ported even though there is no sulfate group in the structure. Therefore, it can be believed that both anti-symmetric stretching modes of sulfate and phosphate occur in similar positions for APS minerals. The presence of phosphate is also marked by the occurrence of bands centered at 403, 530, 985 and 1006 cm�1. The two first bands are attributed to m2 and m4ðPO3�4 Þ bending vibrations, respectively. These are reported at 408 and 534 cm�1 by Frost et al. [26] for a woodhouseite mineral. The shift observed for the two bands is due to the nature of the cations in the A-site. Indeed, it was shown by Maubec et al. [24] that these bands were cation dependent and their positions decrease in wavenumbers with the ionic radius of the ions in the A-site. The shift is due to the polarizing power of cations, which affects the vibration wavenumbers [36]. The wave- number decreases as the polarizing power decreases. The polariz- ing power is expressed as Z/r2, where Z is the ion charge and r the radius [37]. Therefore, since the mineral studied here is com- posed not just of Ca2+ but also ions such Sr2+, Ba2+ and K+, which have higher ionic radii than Ca2+, a shift to lower wavenumbers is observed compared to the mineral characterized by Frost et al. [26], where only Ca2+ is present in the A-site. The presence of various cations may also contribute to the pres- ence of broad bands that occur between 350 and 403 cm�1 and be- tween 500 and 530 cm�1, and which are due to multiple phosphate bending modes. However, the presence of these multiple bands may also be due to the presence of different phosphate units in m of the AP1.5S0.5. r Structure 1048 (2013) 33–40 37 the structure. Indeed, according to Stoffregen and Alpers [38], two phosphate groups, described as PO3�4 and HPO 2� 4 , characterize APS minerals when phosphate is in excess of one mole per formula unit. This hypothesis is based on the negative charge created by this excess, which is balanced by the addition of H+. Therefore, for these authors, this additional proton implies that there are two different phosphate groups, PO3�4 and HPO 2� 4 . The presence of these two groups may contribute to giving multiple bands with close wavenumbers, such as those observed for the two regions 350–403 cm�1 and 500–530 cm�1. The observation of two bands at 985 and 1006 cm�1 is also due to vibrations of the PO3�4 and HPO 2� 4 groups. These are assigned to m1ðHPO2�4 Þ and m1ðPO3�4 Þ symmetric stretching modes, respectively. The presence of bands with similar wavenumbers is also reported in woodhouseite [24,26] and in crandallite [16,35]. The observation of two phosphate symmetric stretching modes confirms there are two non-equivalent phosphate units in the APS structure. In their study of the crandallite mineral (CaAl3(PO4)2(OH)5�H2O), Breitinger et al. [16] observed an additional band at 465 cm�1, which they also attributed to phosphate groups, in particular to the m2(PO4) bending mode vibration. In our study, a band at this po- sition is also observed, but its assignment is questionable. Indeed, since the APS particle is located in a complex matrix, it is possible that this band may also come from quartz present below or near the particle analyzed. Therefore, the present authors suggest that the band at 465 cm�1 is the result of vibrations related to the phos- phate groups from the APS and Si–O–Si bonds from quartz. Finally, in the low frequency region ( 1 (a ecula Fig. 8. Raman spectra of aluminum-phosphate–sulfates (APSs) in the 550–750 cm� phosphate band at 985 cm�1. N. Maubec et al. / Journal of Mol difference in the band shape may be due to higher disorder of the AP1.5S0.5 structure, compared to the other APS. This hypothesis is based on the description of the APS structure by Stoffregen and Al- pers [38] who indicate that there are two different phosphate groups, described as PO3�4 and HPO 2� 4 . According to Breitinger et al. [16], these units are randomly distributed in the crandallite type mineral structure and induce a disorder. For the second wavenumber region, it appears that the intensity of the band at 653 cm�1 remains quite similar between the three APS minerals. In contrast, the bands with lower wavenumbers (be- tween 550 and 650 cm�1) are almost absent in the AP1.5S0.5 Raman spectrum. This result means that the sulfate environment is af- fected by the structural change of the mineral with increasing phosphate content. A structural change was also noted in a previ- ous work which studied the influence of sulfate anion substitution by phosphate anions. Maubec et al. [24] compared an alunite (K0.8- Na0.2)Al3(SO4)2(OH)6 to AP0.5S1.5 [24] and observed that the substi- tution of sulfate by phosphate induces a reduction in symmetry of the sulfate units. These are in C3v site symmetry in the alunite com- pounds and the m4ðSO2�4 Þ bending modes are characterized by only one band at 653 cm�1, in contrast to the APS mineral where multi- ple bands are observed from 600 to 653 cm�1. As shown in Fig. 7, this effect is true and more pronounced with an increase in phos- phate until one mole per formula unit occurs. On the other hand, with an increase in phosphate above 1 mole per formula unit, the effect on the sulfate groups is reversed: the magnitude of the mul- tiple bands decreases. In the present case, this analysis suggests Fig. 9. Variation of I(PO4)/I(SO4) ratios as a function of PO4/(PO4 + SO4) mole fraction of th bands at 653 cm�1 (A) and 1026 cm�1 (B). ) and 950–1060 cm�1 (b) regions. For both regions, spectra are normalized to the r Structure 1048 (2013) 33–40 39 that most of the sulfate groups are in C3v site symmetry and there are minor effects of the local environment on the symmetry of the sulfate anions. In addition, as mentioned by Adler and Kerr [42], the low intensity of the split can also be due to the low sulfate group content in the structure. A comparison of different APS can also be made by watching the relative intensities of some characteristic bands of phosphate and sulfate groups. Indeed, by normalizing the spectra to the phosphate band at 985 cm�1, it appears from the Fig. 8 that the intensities of the sulfate bands located at 653 and 1026 cm�1 change. The evolu- tion of the relative intensities I(PO4)/I(SO4) is summarized on the Fig. 9 and shows, that when phosphate replaces sulfate, there ex- ists a tendency to the increase in the PO4/SO4 intensity ratio. This trend is consistent with the substitution of sulfate by phosphate in the APS structure. This figure reveals that Raman spectroscopy can, firstly, be useful to compare qualitatively APS minerals with different phosphate and sulfate contents and secondly be a power- ful tool to identify minerals with complex compositions. 4. Conclusions The present investigation focuses on the characterization of a micrometric aluminum-phosphate–sulfate (APS) mineral by a cou- pled SEM-microRaman spectroscopy system. SEM images show the presence of APS crystals is intimately associated with alunite and quartz particles. Chemical analyses of the APS crystals indicated e APS minerals. I(PO4) is determined from the band at 985 cm�1 and I(SO4) from the the presence of a compound with a composition between wood- houseite and crandallite end members of the alunite supergroup. A spectroscopic study, based on previously published data of APS minerals, allowed us to assign the different bands observed in the 100–1400 cm�1 region of the Raman spectrum and compare different APS minerals with various PO4/SO4 ratios. Some differ- ences between the Raman spectra were observed and attributed to structural changes due to sulfate anion substitution by phos- phate anions and also by the presence of two distinct phosphate groups, PO3�4 and HPO 2� 4 , which occur when phosphate is in excess of one mole per formula unit. These structural changes induce both a modification of the band shapes around 400 cm�1 and 650 cm�1, which are assigned to vibrational modes of phosphate and sulfate, respectively, and variations in intensities of some bands. A correlation between the I(PO4)/I(SO4) intensity ratios and PO4/(PO4 + SO4) mole fractions of the APS minerals showed that an increase in the phosphate content led to an increase in I(PO4)/ I(SO4) intensity ratios. This correlation indicated direct evidence Economou, J. Farmer, W. Farrand, W. Folkner, M. Golombek, S. Gorevan, J.A. Grant, R. Greeley, J. Grotzinger, L. Haskin, K.E. Herkenhoff, S. Hviid, J. Johnson, G. Klingelhofer, A.H. Knoll, G. Landis, M. Lemmon, R. Li, M.B. Madsen, M.C. Malin, S.M. McLennan, H.Y. McSween, D.W. Ming, J. Moersch, R.V. Morris, T. Parker, J.W. Rice, L. Richter, R. Rieder, M. Sims, M. Smith, P. Smith, L.A. Soderblom, R. Sutlivan, H. Wanke, T. Wdowiak, M. Wolff, A. Yen, Science 306 (2004) 1698. [7] S.W. Squyres, J.P. Grotzinger, R.E. Arvidson, J.F. Bell, W. Calvin, P.R. Christensen, B.C. Clark, J.A. Crisp, W.H. Farrand, K.E. Herkenhoff, J.R. Johnson, G. Klingelhofer, A.H. Knoll, S.M. McLennan, H.Y. McSween, R.V. Morris, J.W. Rice, R. Rieder, L.A. 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