For commercial reasons, elements, minerals or ores are preferentially mined in large scale at places with big and localized deposits. However, worldwide several deposits/mines with a huge variety of crystal chemically different minerals and ores are known. The important ones appear with more than 100 different species.
One of the scientifically important places in France is Cap Garonne in the Département du Var (région Provence-Alpes-Côte d’Azur) with its three separate mines: mine Nord (north mine), mine Sud (south mine) and La Gavaresse (an annex of the north mine) . Meanwhile 127 different minerals and ores have been discovered, including inter alia arsenates, sulfates and halides of Al, Fe, Co, Ni, Cu, Zn, Ag, Hg, Pb and U . The most recent example is the twinned structure of zippeite  a complex uranyl mineral of composition K2.8[(UO2)4(SO4)2O2.8(OH)1.2](H2O)3. So far not all of the Cap Garonne minerals and ores have been characterized with respect to their correct composition and crystal structure.
Herein we report on elemental orthorhombic sulfur which grew on galena matrices in the north mine (mine Nord) of Cap Garonne. The structure of the low-temperature modification, α-S8 was first determined in 1935 by Warren and Burwell  using crystals grown from CS2 solution via rotation and oscillation film data. Subsequent studies focused on precise determinations of the lattice parameters, higher accuracy structure refinements, lattice thermal expansion experiments, analyses of thermal vibrations, charge density studies as well as high-pressure experiments up to the amorphization limit , , , , , , , , , , , , . The early work has competently been summarized by Donohue , later in reviews on all sulfur modifications by Meyer  and Steudel and Eckert .
Based on our literature survey and the data compiled in the Pearson data base , the present data set seems to be the first one from a natural source.
2.1 Sample selection
The orthorhombic sulfur crystals originate from Cap Garonne, Mine du Pradet. Elemental sulfur at Cap Garonne was first signaled in the thesis of Guillemin . He discovered yellow transparent fragments with up to 0.5 mm crystal size. His analyses stated the absence of selenium. The crystals used for the present investigation were collected in the north mine (mine Nord), in the entrance to the Salle du Grand Bassin (Fig. 1), the lead-containing part of the mine. The transparent yellow crystals grew on a galena matrix (Fig. 2), most likely through sulfide oxidation , , . The existence of anglesit crystals (PbSO4) close to galena and sulfur supports this hypothesis.
2.2 X-ray diffraction
Irregularly shaped crystal fragments were carefully, mechanically broken from the galena matrix. The crystals were fixed on glass fibers using varnish and their quality was checked using a Buerger precession camera with white Mo radiation and an image plate technique. Intensity data was collected using a Stoe StadiVari diffractometer which was equipped with a Mo micro focus source and a Pilatus detection system. Due to a Gaussian-shaped profile of the StadiVari X-ray source scaling was applied along with the numerical absorption correction. The first crystals (up to 200 μm edge length) shattered during data collection, most likely due to internal stress. We have then cut smaller specimen and fixed them to glass fibers with a thin film of Paratone®-N. Data was finally collected in a nitrogen stream at T=150 K. Details of the data collection and the crystallographic parameters are listed in Table 1.
2.3 Structure refinement
The data set of the sulfur crystal showed a face-centered orthorhombic lattice, and the systematic extinctions were in agreement with space group Fddd, readily indicating that the natural sample consisted of α-S8. The standardized atomic parameters of α-S8 listed in the Pearson data base  were taken as starting values and the structure was refined on F2 values with the Jana2006 software package  with anisotropic atomic displacement parameters for all atoms. Separate refinements of the occupancy parameters revealed full occupancy for all sites. The final difference Fourier synthesis revealed no significant residual electron density. The refined atomic positions, displacement parameters, and interatomic distances are given in Tables 2 and 3.
CCDC 1871601 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
2.4 EDX data
Several crystal fragments that were mechanically separated from the sulfur agglomerate grown on the galena matrix were further studied by energy dispersive analyses of X-rays (EDX) using a Zeiss EVO® MA10 scanning electron microscope in variable pressure mode (60 Pa) with FeS2 as standard. All fragments exclusively showed sulfur. Within the accuracy of the instrument we got no hint for impurity elements in the yellow transparent crystals.
3 Crystal chemistry
The single crystal data of the naturally grown crystals of orthorhombic α-S8 (orthorhombic sulfur; o-S8) fully confirm the literature data. The monomeric unit, the S8 crown, is shown in Fig. 3. The S–S distances show a very narrow range from 204.9 to 205.4 pm and the S–S–S angels range from 107.9 to 109.0°. These small distortions result from packing effects. The intermolecular distances (van-der-Waals contacts) are much longer. Each sulfur atom has between two and four sulfur neighbors at 330.4–345.2 pm (Table 3). Although all sulfur sites have site symmetry 1, the S8 crown exhibits a two-fold axis. The packing of the S8 crowns is shown in Fig. 4. The S8 crowns show the typical crankshaft arrangement. The crystal chemistry of orthorhombic S8 and the various metastable sulfur modifications is summarized in  and .
The thermal expansion of orthorhombic sulfur has been studied through temperature-dependent X-ray powder and single-crystal diffraction data , . Our 150 K data show reasonable agreement with these studies. One should keep in mind that these studies used sulfur from different sources, i.e. crystals grown by evaporation of a CS2 solution  and microcrystalline α-S8 that resulted from desulfurization reactions in a biorefinery .
We thank M.Sc. Fabian Eustermann for the EDX analyses and Jérôme Gibert, Agent du Patrimoine du Musée de la Mine de Cap Garonne for support.
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