From the type description of coquimbite to the first X-ray diffraction studies

Coquimbite was described as a new mineral species from Coquimbo, Chile by Rose (Reference Rose1833), who called it “neutrales schwefelsaures Eisenoxyd mit Krystallisationswasser”. According to the type description, this mineral was found “in der Provinz Coquimbo […] und zwar im district Copiapo” (Rose, Reference Rose1833). Chemical data reported in the type description indicate only minor Al₂O₃ (0.92 wt.%). The name ‘coquimbite’, after the type locality, was first introduced by Breithaupt (Reference Breithaupt1841).

Arzruni (Reference Arzruni1879) gave additional information about the occurrence of coquimbite and discussed its morphology and chemistry. In that paper, the results of the chemical analyses performed by Bamberger were presented, indicating that the formula of coquimbite should be written as ‘(¼ Al₂ + ¾ Fe₂)(SO₄)₃ + 9 H₂O’. By recalculating this formula on the basis of six (SO₄) groups per formula unit, it becomes AlFe₃(SO₄)₆⋅18H₂O.

New chemical data were given by Linck (Reference Linck1889). No Al₂O₃ was reported, in contrast to the previous data given by Arzruni (Reference Arzruni1879). Subsequent data given by Collins (Reference Collins1923), who described this mineral from the Concepcion mine, Spain, found 2.25 wt.% Al₂O₃, also confirmed by Lausen (Reference Lausen1928), who pointed out the high Al content (6.93 wt.% Al₂O₃) occurring in coquimbite from the United Verde mine, Arizona, USA. On the contrary, Scharizer (Reference Scharizer1927) and Bandy (Reference Bandy1938) gave Al-free chemical data for coquimbite, in agreement with Linck (Reference Linck1889).

It is worth noting that Bandy (Reference Bandy1938) gave similar data for both coquimbite and paracoquimbite.

The latter phase was described by Ungemach (Reference Ungemach1933, Reference Ungemach1935) during the re-examination of the specimens studied by Linck (Reference Linck1889), stored in the mineralogical collection of the University of Strasbourg, France, as a dimorph of coquimbite. The distinction between these two minerals was based upon some morphological and physical features as well as upon X-ray data collected through the Laue method. X-ray diffraction proved that coquimbite is trigonal with a primitive lattice, whereas paracoquimbite has a rhombohedral lattice. According to the data given by Ungemach (Reference Ungemach1935), the chemistry of these two minerals is very similar and both display only trace amounts of Al₂O₃.

Owing to the uncertain Al content, the ideal formula Fe₂(SO₄)₃⋅9H₂O was used throughout for both coquimbite and paracoquimbite (e.g. Palache et al., Reference Palache, Berman and Frondel1951). This is the same chemical formula that was reported in the official IMA List of Minerals (Pasero, Reference Pasero2019), ignoring the results of the crystallographic studies carried out in the 1970s.

Structural studies and the role of Al in coquimbite

Cesbron (Reference Cesbron1964) determined the space group of coquimbite as P $\bar{3}$1c, using a sample from Tierra Amarilla, Chile (Musèum National d'Histoire Naturelle, Paris sample MHNN 13373).

The crystal structure of coquimbite was first solved through single-crystal X-ray diffraction data by Fang and Robinson (Reference Fang and Robinson1970) and Giacovazzo et al. (Reference Giacovazzo, Menchetti and Scordari1970). The former, using a sample from Tierra Amarilla stored in the National Museum of Natural History (Smithsonian Institution) in Washington DC, USA (catalogue number 12548), found the presence of an Al-centred octahedron with the cation at (0, 0, 0) (site 2b), with site occupancy (Al_0.90Fe_0.10). The structural data were confirmed by wet chemical analysis. Giacovazzo et al. (Reference Giacovazzo, Menchetti and Scordari1970) achieved similar results studying a sample from the Dexter mine, Utah, USA, obtaining an occupancy for the octahedral 2b site of (Al_0.63Fe_0.37). Aluminium and iron were determined chemically through colorimetric methods.

The crystal structure of the related mineral paracoquimbite was solved by Robinson and Fang (Reference Robinson and Fang1971) using a sample from Chuquicamata, Chile stored in the National Museum of Natural History (Smithsonian Institution, USA; catalogue number 115161). As pointed out by the authors, no Al₂O₃ was detected, contrasting with the extensive Al-to-Fe substitution observed in coquimbite. The relationship between paracoquimbite and coquimbite was later discussed by Fang and Robinson (Reference Fang and Robinson1974).

Majzlan et al. (Reference Majzlan, Navrotsky, McCleskey and Alpers2006) refined the crystal structure of coquimbite from the Richmond mine, Iron Mountain, California, USA, using synchrotron powder X-ray diffraction data, further confirming the occurrence of an Al-dominant octahedron in the crystal structure of coquimbite, with site occupancy (Al_0.91Fe_0.09). Minor Al was found at the Fe(3) site at (⅔, ⅓, z), with site occupancy (Fe_0.93Al_0.07). Inductively-coupled plasma optical emission spectroscopy confirmed the occurrence of Al in coquimbite.

Demartin et al. (Reference Demartin, Castellano, Gramaccioli and Campostrini2010a) studied three samples of coquimbite from Vulcano, Italy; Alcaparrosa, Chile; and from the Dexter No. 7 mine, Utah, USA. In all cases, the 2b site was occupied by Al, with only a partial replacement by Fe³⁺ (i.e. Al_0.76Fe_0.24) in the Chilean specimen. Structural data were supported through energy-dispersive spectroscopy (EDS) chemical analyses. A fourth sample, from Vulcano, was particularly enriched in Al and it was described as the new mineral species aluminocoquimbite (Demartin et al., Reference Demartin, Castellano, Gramaccioli and Campostrini2010a,Reference Demartin, Castellano, Gramaccioli and Campostrinib). Demartin et al. (Reference Demartin, Castellano, Gramaccioli and Campostrini2010a) were the first to locate the hydrogen atoms in the crystal structure of coquimbite. Their data were confirmed through neutron diffraction by Majzlan et al. (Reference Majzlan, Dordević, Kolitsch and Schefer2010) using a sample from the Dexter No. 7 mine, Utah, USA. This study confirmed, through neutron and X-ray diffraction, that the 2b site is occupied dominantly by Al. Meanwhile, a new refinement performed by Ackermann et al. (Reference Ackermann, Armbruster, Lazic, Doyle, Grevel and Majzlan2009) on the synthetic analogue of paracoquimbite confirmed the previous results of Robinson and Fang (Reference Robinson and Fang1971), indicating that all octahedral sites in this phase are occupied by Fe³⁺ only.

Frost et al. (Reference Frost, Žigovečki Gobac, López, Xi, Scholz, Lana and Fernandes Lima2014) discussed the infrared and Raman spectra of coquimbite, using a specimen from the Javier Ortega mine, Peru. Chemical analysis, performed using EDS, gave the formula (Fe_1.37Al_0.63)(SO₄)₃⋅9H₂O, corresponding to Al_1.26Fe_2.74(SO₄)₆⋅18H₂O (Z = 2).

Finally, Yang and Giester (Reference Yang and Giester2018) re-examined the crystal structures of both coquimbite and paracoquimbite, giving the location of hydrogen atoms in the latter species. For the first time, they proposed the correct formula AlFe₃(SO₄)₆⋅18H₂O (Z = 2) for coquimbite, suggesting that a revision of its chemical formula is necessary. Unfortunately, their structural data, indicating the site occupancy (Al_0.66Fe_0.34) for the 2b site, were not supported by chemical data, which indicate the occurrence of only negligible Al. Paracoquimbite, on the contrary, was confirmed to be a pure Fe³⁺-sulfate.

Chemistry

Quantitative chemical data were collected using a Superprobe JEOL JXA 8200 electron microprobe (wavelength-dispersive spectrosopy mode, 15 kV, 10 nA and 10 μm beam diameter) at the Eugen F. Stumpfl laboratory, Leoben University, Austria. Standards (element, emission line) were: hematite (FeKα); kaersutite (AlKα); and baryte (SKα). The average of 20 spot analyses gave (in wt.%): SO₃ 42.88(45), Al₂O₃ 4.38(7), Fe₂O₃ 22.28(38), H₂O_(calc) 29.10, total 98.64. The H₂O content was calculated on the basis of structural data, which confirmed the occurrence of 18 H₂O groups. The empirical formula, based on 42 oxygen atoms, is Al_0.96Fe_3.11S_5.97O₂₄⋅18H₂O. X-ray maps, collected using the same analytical conditions reported for electron microprobe data, show the homogeneous distribution of Al and Fe within the grain studied (Fig. 2), which was associated closely with euhedral krausite, ideally KFe(SO₄)₂⋅H₂O.

Fig. 2.

Back-scattered electron image (a) and X-ray maps for Al (b) and Fe (c) of coquimbite (Coq) associated with krausite (Kra) from Monte Arsiccio.

Micro-Raman and infrared spectroscopies

Micro-Raman spectrum of coquimbite (Fig. 3) was collected on an unpolished sample using a Horiba Jobin-Yvon XploRA Plus apparatus, with a 10× objective lens and the 532 nm line of a solid-state laser attenuated to 10% (i.e. 2.5 mW) in order to minimise sample damage. The spectrum was collected through multiple acquisitions with single counting times of 60 s. Back-scattered radiation was analysed with a 1200 grooves per cm grating monochromator. The spectral features fully agree with the results of Frost et al. (Reference Frost, Žigovečki Gobac, López, Xi, Scholz, Lana and Fernandes Lima2014). The strongest band occurs at 1024 cm^–1 and is due to the ν ₁ mode of the SO₄ groups. Three low intensity bands at 1199, 1163, and 1096 cm^–1 can be assigned to the antisymmetrical stretching mode ν ₃ of SO₄ groups. The bending mode of SO₄ groups are represented by bands at 507, 456 (ν ₂ mode) and 597 cm^–1 (ν ₄ mode). Lattice, Fe-O and Al-O vibration modes are observed at 311, 283, 255, 209, 181 and 150 cm^–1. Broad and intense Raman bands are observed in the region between 3600–2900 cm^–1. These Raman bands at 3577, 3423, 3179 and 2997 cm^–1, can be attributed to the stretching vibration mode of O–H bonds.

Fig. 3.

Raman spectrum of coquimbite from Monte Arsiccio and band interpretation.

Unpolarised Fourier transfer infrared absorption spectra of coquimbite (Fig. 4) were measured on powdered sample material mixed with KCl and pressed to a pellet. A Bruker Vertex spectrometer equipped with a Globar source, a KBr beam-splitter, an MCT detector, and a Hyperion 2000 microscope was used to acquire spectra in the wavenumber range 4000–600 cm^–1 with a resolution of 4 cm^–1. The spectrum shows strong similarities with that of Frost et al. (Reference Frost, Žigovečki Gobac, López, Xi, Scholz, Lana and Fernandes Lima2014), with a strong absorption band with some shoulder features in the 3700–2900 cm^–1 range caused by O–H stretching motions, a distinctive band at 1655 cm^–1 due to the H₂O bending mode, and a number of more narrow bands in the range 1200–1000 cm^–1 that can be attributed to antisymmetric and symmetric stretching modes of SO₄ groups.

Fig. 4.

Fourier-transfer infrared (FTIR) spectrum of coquimbite from Monte Arsiccio.

Single-crystal X-ray diffraction and crystal-structure refinement

X-ray diffraction intensity data were collected using a Bruker Smart Breeze diffractometer (50 kV and 30 mA) equipped with a Photon II CCD detector. Graphite monochromatised MoKα radiation was used. The detector-to-crystal working distance was 50 mm. A total of 654 frames were collected using φ and ω scan modes with an exposure time of 10 s per frame. The data were integrated and corrected for Lorentz-polarisation, background effects and absorption, using the Apex3 software package (Bruker AXS Inc., 2016), resulting in a set of 16,979 reflections. The refined unit-cell parameters are a = 10.9318(12), c = 17.069(2) Å and V = 1766.6(4) ų. The statistical tests on |E| values and systematic absences indicated the space-group symmetry P $\bar{3}$1c. The crystal structure of coquimbite was refined using Shelxl-2018 (Sheldrick, Reference Sheldrick2015) starting from the atomic coordinates given by Demartin et al. (Reference Demartin, Castellano, Gramaccioli and Campostrini2010a). Taking into account the results of the electron microprobe analysis, the site scattering at the three metal sites was modelled using the following scattering curves, taken from the International Tables for Crystallography (Wilson, Reference Wilson1992): Al vs. Fe at the Al site; Fe at the Fe(1) and Fe(2) sites; and S at the S site. The scattering curve of O and H were used at all the O and H positions, respectively. Although the hydrogen coordinates have been given in previous studies (e.g. Demartin et al., Reference Demartin, Castellano, Gramaccioli and Campostrini2010a), their positions were sought in the difference-Fourier maps. Soft restraints were used in order to avoid O–H distances that were too short. An anisotropic model for all the atom positions (except for H atoms) converged to 0.0219 for 1307 reflections with F _o>4σ(F _o) and 110 refined parameters. Details of data collection and refinement are given in Table 2. Atomic coordinates and isotropic or equivalent isotropic displacement parameters for coquimbite are given in Table 3, and selected bond distances are given in Table 4. In agreement with the previous studies, the crystal structure of coquimbite is characterised by the occurrence of clusters of composition [FeFe₂(SO₄)₆(H₂O)₆]^3- and by isolated [Al(H₂O)₆]³⁺ octahedra. Six symmetry-related H₂O groups, held in the structure through H-bonding only, are arranged in a cyclohexane-like chair conformation (e.g. Demartin et al., Reference Demartin, Castellano, Gramaccioli and Campostrini2010a). Table 5 reports the geometrical features of the H-bonds, whereas Table 6 gives the bond-valence balance. The crystallographic information files have been deposited with the Principal Editor of Mineralogical Magazine and are available as Supplementary material (see below).

Table 2.

Summary of crystal data and parameters describing data collection and refinement for coquimbite.

Weighting scheme is defined as w = 1/[σ²(F _o²)+ (aP)² + bP], where P = [2F _c² + Max(F _o²,0)]/3. The a and b values are 0.0293 and 2.3592, respectively.

Table 3.

Sites, Wyckoff positions, site occupancy factors (s.o.f.), fractional atom coordinates and isotropic (*) or equivalent isotropic displacement parameters in coquimbite.

Table 4.

Selected bond distances (Å) in coquimbite.

Table 5.

Hydrogen-bond lengths (d in Å) and angles (in °) for coquimbite.

D – donor; A – acceptor.

Table 6.

Bond-valence sums (in valence units) for coquimbite.*

* The number of equivalent bonds involving anions are indicated by ×↓. Bond parameters after Brese and O'Keeffe (Reference Brese and O'Keeffe1991). The bond-valence sums at the anion sites were corrected taking into account O⋅⋅⋅O distances (see Table 5) and the relationship of Ferraris and Ivaldi (Reference Ferraris and Ivaldi1988).