Orange Pickeringite from the Algares 30-Level Adit, Aljustrel Mine, Iberian Pyrite Belt, Portugal

: The sheltered environment of the Algares +30 level adit (underground mine gallery) contributes to the preservation of secondary water-soluble minerals formed on the tunnel walls. The massive sulphide and related stockwork zone are hosted by the Mine Tuff volcanic unit and are exposed in the walls of the gallery, showing intense oxidation and hydrothermal alteration. Minerals from the halotrichite group were identiﬁed on the efﬂorescent salts, typically white ﬁne-acicular crystals but also on aggregates with dark orange/brownish colour. Mineral characterization was performed using several methods and analytical techniques (XRD, XRF-WDS, SEM-EDS, DTA-TG), and the chemical formulas were calculated maintaining the ratio A:B ∼ = 1:2 in accordance with the general formula of the halotrichite group, AB 2 (SO 4 ) 4 · 22H 2 O. This methodology allowed the assignment of the orange colour to the presence of trivalent iron on iron-rich pickeringite in partial substitution of aluminium.

In the 19th century, modern exploration began mainly through wells or adits (galleries), reaching a depth of about 360 m [12][13][14][15]. Presently, the deposit is considered exhausted and the area is being rehabilitated for mining and geological tourism. To promote new educational visits, the Company Empresa de Desenvolvimento Mineiro (EDM) and the Aljustrel Municipality carried out rehabilitation works inside the Algares +30 level adit (approximately 500 m long). During the mine rehabilitation project, the Algares mine gallery was cleaned and enlarged for public visits. Protection structures were constructed including wood pillars, concrete walls and metallic networks. Thematic underground visits will be promoted by the Aljustrel Municipality, including in the future Aljustrel Mining Park.
The Algares +30 level adit intersects two distinct volcanic units of the IPB Volcano-Sedimentary Complex (Tournaisian-Late Visean age at the Aljustrel Antiform) [16]. The massive sulphide and related stockwork zone are hosted by the Mine Tuff volcanic unit and are exposed in the walls of the gallery, showing intense hydrothermal alteration. The primary mineralization is strongly affected by supergene oxidation alteration due to the near surface location of the studied mine gallery. The two gossans Algares Oeste and Algares Este are present in the gallery. The oxides and sulphides in the moist air resulted in the formation of simple and complex multi-coloured sulphates (secondary minerals), making the adit a natural "mineralogical museum". These particular conditions include direct access to the Vipasca mine shaft (and connection with deeper flooded mine levels) and old Roman mine galleries (with 2000 years of mineral exposure to the open air). In summary, the minerals present in the mine gallery are: primary mineralization (sulphides + sulphosalts + hydrothermal assemblage, e.g., chlorite and sericite), supergene-oxidationstage mineralization (e.g., hematite, goethite, limonite) and neoformation minerals/crystals (e.g., oxides, hydroxides, sulphates). The sheltered environment of this underground mine gallery contributes to the preservation of water-soluble minerals. The most common minerals found were melanterite (FeSO 4 ·7H 2 O) and chalcanthite (CuSO 4 ·5H 2 O), resulting from the weathering brought about by the percolation of acidic fluids and forming essentially massive or crystalline aggregates, ranging from greenish to bluish colours [16]. Other minerals identified in the walls of the adit, including those prevailing in stalactites/stalagmites, were: alunite [(K,Na)Al 3 (SO 4 ) 2 (OH) 6  . Indeed, these minerals are frequently found in acid mine drainage (AMD) environments (e.g., [17][18][19][20][21][22]), but their presence as a coating of the gallery walls has preserved them from dissolution.
Minerals from the halotrichite group are usually colourless or white [23,24]. Colours like pale pink or yellow appear when Mn 2+ or Fe 3+ are present, respectively [25]. The possibility of a Fe 3+ substitution for Al is also foreseen [19,[25][26][27]. Furthermore, Parafiniuk [25] points out that Fe 3+ rich pickeringite may be distinguished macroscopically by its colour (creamy, yellow, light brown of various tints), while pure pickeringite is white. He also mentions that Fe 3+ -rich pickeringite is much darker than in its dry state, and could even have a chocolate colour, which gradually changes to pale-brown and yellow upon drying.
In fact, it was the dark orange/brownish colour of some aggregates, in contrast to the typically white fine-acicular crystals, that caught our attention. It was specifically this aspect that gave rise to this thorough mineralogical (XRD) study on minerals from the halotrichite group, particularly concerning white halotrichite, as well as white and orange pickeringite, from the Algares adit. Chemical analysis (XRF-WDS), scanning electron microscopy (SEM-EDS) and thermo-analytical techniques (DTA-TG) were also used to characterize the efflorescent samples, aiming to contribute to the knowledge of the unusual orange pickeringite.  [26,28]. There is complete miscibility between Mg 2+ and Fe 2+ in the pickeringite (A = Mg 2+ )halotrichite (A = Fe 2+ ) series, the substitution of aluminium (B = Al 3+ ) by Fe 3+ being restricted [24,29] [26,30]. The other species of the halotrichite group are uncommon [28]. Due to the occurrence of solid solutions and to the similar powder diffraction patterns, the identification of these minerals is difficult [25,27,31,32]. Moreover, the accurate crystal-chemical characterisation has been problematic because of the impossibility of obtaining pure samples due to the small dimension of the crystals and by the occurrence in nature of very complex mixtures with other sulphates [30,33].
The structure of these minerals (an example of pickeringite in Figure 1) is based on finite clusters of (SO 4 ) tetrahedra (four per asymmetric unit), one AØ 6 octahedra (Ø = five water oxygens plus one sulphate oxygen) and two independent BØ 6 octahedra (Ø = six water oxygens). There are seventeen ligand water molecules and five isolated water molecules [29,33,34].
The structure of these minerals (an example of pickeringite in Figure 1) is based on finite clusters of (SO4) tetrahedra (four per asymmetric unit), one AØ6 octahedra (Ø = five water oxygens plus one sulphate oxygen) and two independent BØ6 octahedra (Ø = six water oxygens). There are seventeen ligand water molecules and five isolated water molecules [29,33,34].  [35]. The water molecules are not represented for simplification: left side-unit cell, right side-asymmetric unit.

Materials and Methods
More than 100 efflorescent samples were collected in September 2018 from the walls of the Algares +30 level adit along its entire length, considering both colour and crystal morphology [16]. From those, 19 are the focus of the present study. Several fragments for X-ray diffraction (XRD) were selected (25) using the stereomicroscope (Stemi SV-11, Zeiss, Jena, Germany), with images collected using a digital Zeiss camera (Axio-Cam Mrc, Jena, Germany). Several images were mustered from the same fragment with the focus set at different levels, all of which were compiled together in Adobe Photoshop TM using the focus stacking method.
For X-ray diffraction (XRD), a Philips PW 1500 powder diffractometer (Philips, Aachen, Germany) with Bragg-Brentano geometry was used, equipped with a largeanode copper tube operating at 50 kV-40 mA and a curved graphite crystal monochromator. A semi-quantitative chemical analysis was also performed through X-  [35]. The water molecules are not represented for simplification: left side-unit cell, right side-asymmetric unit.

Materials and Methods
More than 100 efflorescent samples were collected in September 2018 from the walls of the Algares +30 level adit along its entire length, considering both colour and crystal morphology [16]. From those, 19 are the focus of the present study. Several fragments for X-ray diffraction (XRD) were selected (25) using the stereomicroscope (Stemi SV-11, Zeiss, Jena, Germany), with images collected using a digital Zeiss camera (Axio-Cam Mrc, Jena, Germany). Several images were mustered from the same fragment with the focus set at different levels, all of which were compiled together in Adobe Photoshop TM using the focus stacking method.
For X-ray diffraction (XRD), a Philips PW 1500 powder diffractometer (Philips, Aachen, Germany) with Bragg-Brentano geometry was used, equipped with a large-anode copper tube operating at 50 kV-40 mA and a curved graphite crystal monochromator. A semiquantitative chemical analysis was also performed through X-ray fluorescence spectrometry with a wavelength dispersive system (XRF-WDS), using a PANalytical 4.0 AXIOS sequential spectrometer (Rh X-ray tube) (Malvern PANalytical, Malvern, UK) under He flow. SEM-EDS was performed using a tabletop scanning electron microscope (Hitachi TM3030Plus) (Hitachi High-Technologies Corporation, Tokyo, Japan), coupled with a Quantax70 Energy Dispersive X-Ray Spectrometer, operated at an acceleration voltage of 15 kV at ambient temperature and under low vacuum.
Samples were mounted with no preparation (non-destructive characterization) using a double-sided carbon tape.
Thermo-analytical techniques-simultaneous differential thermal analysis (DTA) and thermogravimetry (TG)-were also used, using a SETARAM 92-16.18 apparatus (KEP Technologies Group, Sophia Antipolis, France), incorporating a microbalance with a controlled argon gas flow (inert atmosphere). About 60 mg of milled sample were deposited in an alumina (α-Al 2 O 3 ) crucible. The reference material was alumina powder. Therefore, the DTA-TG assays were performed in a heating temperature range from ambient to 1000 • C, at a heating rate of 10 • C min −1 .

Results and Discussion
Minerals from the halotrichite group were identified in 25 fragments, the majority as whitish pickeringite, two iron-rich pickeringite (orange) and one white halotrichite. They were mainly in mixtures with melanterite, but also with alunogen or chalcanthite, over quartz + pyrite, quartz + hematite, jarosite + quartz or quartz + chlorite/clinochlore + mica. The stability of halotrichite group minerals with alunogen or melanterite in saturated solutions has already been mentioned [26]. These sulphates occurred mainly as white fine-acicular crystals (Figures 2 and 3) but also as orange/brownish aggregates of crystals in two samples ( Figure 4). Indeed, the orange samples collected in September 2018 [16] and kept in the laboratory in sealed containers are now brown/yellowish with a dry appearance in some zones. The halotrichite group minerals were formed over felsic volcanic rocks with stockwork and disseminated mineralization. Generally, these rocks are well-cleaved and present hydrothermal alteration (chlorite ± sericite).
Minerals 2021, 11, x FOR PEER REVIEW 4 of 15 ray fluorescence spectrometry with a wavelength dispersive system (XRF-WDS), using a PANalytical 4.0 AXIOS sequential spectrometer (Rh X-ray tube) (Malvern PANalytical, Malvern, UK) under He flow. SEM-EDS was performed using a tabletop scanning electron microscope (Hitachi TM3030Plus) (Hitachi High-Technologies Corporation, Tokyo, Japan), coupled with a Quantax70 Energy Dispersive X-Ray Spectrometer, operated at an acceleration voltage of 15 kV at ambient temperature and under low vacuum.
Samples were mounted with no preparation (non-destructive characterization) using a double-sided carbon tape.
Thermo-analytical techniques-simultaneous differential thermal analysis (DTA) and thermogravimetry (TG)-were also used, using a SETARAM 92-16.18 apparatus (KEP Technologies Group, Sophia Antipolis, France), incorporating a microbalance with a controlled argon gas flow (inert atmosphere). About 60 mg of milled sample were deposited in an alumina (-Al2O3) crucible. The reference material was alumina powder. Therefore, the DTA-TG assays were performed in a heating temperature range from ambient to 1000 °C, at a heating rate of 10° C min −1 .

Results and Discussion
Minerals from the halotrichite group were identified in 25 fragments, the majority as whitish pickeringite, two iron-rich pickeringite (orange) and one white halotrichite. They were mainly in mixtures with melanterite, but also with alunogen or chalcanthite, over quartz + pyrite, quartz + hematite, jarosite + quartz or quartz + chlorite/clinochlore + mica. The stability of halotrichite group minerals with alunogen or melanterite in saturated solutions has already been mentioned [26]. These sulphates occurred mainly as white fineacicular crystals (Figures 2 and 3) but also as orange/brownish aggregates of crystals in two samples ( Figure 4). Indeed, the orange samples collected in September 2018 [16] and kept in the laboratory in sealed containers are now brown/yellowish with a dry appearance in some zones. The halotrichite group minerals were formed over felsic volcanic rocks with stockwork and disseminated mineralization. Generally, these rocks are well-cleaved and present hydrothermal alteration (chlorite ± sericite).      Table 1).      Table 1).   Table 1).  As already mentioned, the mineralogical identification was very difficult due to the possibility of solid solution between the halotrichite group members. The free program PowderCell [35] was used to simulate the XRD pattern of pickeringite [MgAl2(SO4)4·22H2O] based on data published for the crystal structure, where the proportion of Mg:Mn occupancy is 0.93:0.07 and the positions Al1 plus Al2 are completely occupied only by aluminium [31]. The substitution of part of the Mg by Zn or Fe 2+ changed the intensity of some XRD lines: for example, the lines corresponding to hkl (011), (002) and (023) increased in opposition to the lines (012) and (041) that decreased ( Figure 6). By other means, the substitution of part of the Al by Fe 3+ revealed differences in the intensity of those lines depending on if it occurred on Al1, Al2 or both sites (Figure 7). Nevertheless, the observation of these differences in the experimental patterns was not clear; the intensity of lines (011) and (023) was very low in all XRD patterns, and lines (002), (012) plus (041) were less intense in all samples compared to sample 22-white.   As already mentioned, the mineralogical identification was very difficult due to the possibility of solid solution between the halotrichite group members. The free program Pow-derCell [35] was used to simulate the XRD pattern of pickeringite [MgAl 2 (SO 4 ) 4 ·22H 2 O] based on data published for the crystal structure, where the proportion of Mg:Mn occupancy is 0.93:0.07 and the positions Al1 plus Al2 are completely occupied only by aluminium [31]. The substitution of part of the Mg by Zn or Fe 2+ changed the intensity of some XRD lines: for example, the lines corresponding to hkl (011), (002) and (023) increased in opposition to the lines (012) and (041) that decreased ( Figure 6). By other means, the substitution of part of the Al by Fe 3+ revealed differences in the intensity of those lines depending on if it occurred on Al1, Al2 or both sites (Figure 7). Nevertheless, the observation of these differences in the experimental patterns was not clear; the intensity of lines (011) and (023) was very low in all XRD patterns, and lines (002), (012) plus (041) were less intense in all samples compared to sample 22-white.  Halotrichite, pickeringite and even apjohnite have similar XRD patterns, the distinction between them shown only through chemical analysis [25]. Thus, chemical analyses were obtained for the two white and two orange samples (Table 2), and the number of atoms on the basis of four sulphur atoms was then calculated. The possibility of the presence of phases in vestigial content not detected by XRD ( Figure 5), due to the low content or overlap with other lines, cannot be ruled out. In this way, the approximate chemical formula, AB2(SO4)4·22H2O, can be calculated (Table 3). Samples 20 and 22 are both white, and chemically they are halotrichite and pickeringite, respectively (Table 3, Figure 8). Since samples 19 and 25 were both orange, to maintain the theoretical ratio A:B = 1:2, all the iron was assumed to be Fe 3+ , in substitution of aluminium in the case of fragment 19, and ¼ Fe 2+ plus ¾ Fe 3+ in fragment 25. The ternary diagram plotted in Figure  8 shows that they are both pickeringite. Halotrichite, pickeringite and even apjohnite have similar XRD patterns, the distinction between them shown only through chemical analysis [25]. Thus, chemical analyses were obtained for the two white and two orange samples (Table 2), and the number of atoms on the basis of four sulphur atoms was then calculated. The possibility of the presence of phases in vestigial content not detected by XRD (Figure 5), due to the low content or overlap with other lines, cannot be ruled out. In this way, the approximate chemical formula, AB 2 (SO 4 ) 4 ·22H 2 O, can be calculated (Table 3). Samples 20 and 22 are both white, and chemically they are halotrichite and pickeringite, respectively (Table 3, Figure 8). Since samples 19 and 25 were both orange, to maintain the theoretical ratio A:B = 1:2, all the iron was assumed to be Fe 3+ , in substitution of aluminium in the case of fragment 19, and 1 4 Fe 2+ plus 3 4 Fe 3+ in fragment 25. The ternary diagram plotted in Figure 8 shows that they are both pickeringite.     Collected SEM images illustrate the aggregates of fine-acicular crystals (Figure 9), showing the typical morphology of these minerals in accordance with published papers (e.g., [22,26,[36][37][38]). The approximate chemical formula was also calculated through EDSquantification (Tables 4 and 5), following the same methodology used before, based on Collected SEM images illustrate the aggregates of fine-acicular crystals (Figure 9), showing the typical morphology of these minerals in accordance with published papers (e.g., [22,26,[36][37][38]). The approximate chemical formula was also calculated through EDSquantification (Tables 4 and 5     DTA-TG assays were also performed ( Figure 11). The intense endothermic peaks at approximately 150 °C and those at 310-340 °C correspond to dehydration [27]. The peak at 405-420 °C is attributed to the liberation of the most strongly bound water molecules or to the decomposition of intermediate products containing hydroxyl groups. The large endothermic peak at about 800 °C corresponds to the dissociation of aluminium and iron sulphates. The increase of total iron in the pickeringite structure caused a decrease in the temperature of this peak [25]; indeed, for pickeringite samples, the observed temperature was 808 °C for fragment 22-white, plus 778 and 772 °C for 19-orange and 25-orange, DTA-TG assays were also performed ( Figure 11). The intense endothermic peaks at approximately 150 • C and those at 310-340 • C correspond to dehydration [27]. The peak at 405-420 • C is attributed to the liberation of the most strongly bound water molecules or to the decomposition of intermediate products containing hydroxyl groups. The large endothermic peak at about 800 • C corresponds to the dissociation of aluminium and iron sulphates. The increase of total iron in the pickeringite structure caused a decrease in the temperature of this peak [25]; indeed, for pickeringite samples, the observed temperature was 808 • C for fragment 22-white, plus 778 and 772 • C for 19-orange and 25-orange, respectively; the iron content was 0.03%, 1.40% and 1.56%, respectively ( Table 4). The temperature observed for sample 20-white (halotrichite) was 762 • C, with an iron content of 2.42%.

Conclusions
Several methods and analytical techniques (XRD, XRF-WDS, SEM-EDS, DTA-TG) were used in this study to characterise crystal aggregate samples belonging to the halotrichite group, collected from the walls of the Algares 30-level adit in the Aljustrel mine (Algares deposit, IPB massive sulphides) during a rehabilitation intervention for its valorisation as a mining and geological heritage site. Indeed, the existence of solid solution between members of this group makes their identification only through XRD difficult. Of the 25 fragments selected from the 19 samples, the majority were identified as whitish pickeringite, along with two iron-rich pickeringite (the orange ones) and one white halotrichite. In fact, pickeringite was the only magnesium mineral found coating the adit walls [16], resulting from the oxidation of sulphides and from the percolation of aqueous solution of Mg 2+ /Fe 2+ /Fe 3+ combined with Al 3+ from silicates weathering. Hydrothermal Mg-rich chlorite (present in the host felsic volcanic rocks) can also be considered as a source of the magnesium.
If it is not the high iron content that is responsible for the orange colour, as is the case of the white halotrichite, surely the trivalent state of iron in partial substitution of aluminium is responsible for the samples' colour, even when the mixed divalent-trivalent state is present (iron-rich pickeringite). Future spectroscopic studies concerning the presence of Fe 2+ and/or Fe 3+ are foreseen using synchrotron radiation. The adsorbed water of the samples, due to the constant humidity of the adit walls, intensifies their colour, giving them the peculiar orange/brownish hue.

Conclusions
Several methods and analytical techniques (XRD, XRF-WDS, SEM-EDS, DTA-TG) were used in this study to characterise crystal aggregate samples belonging to the halotrichite group, collected from the walls of the Algares 30-level adit in the Aljustrel mine (Algares deposit, IPB massive sulphides) during a rehabilitation intervention for its valorisation as a mining and geological heritage site. Indeed, the existence of solid solution between members of this group makes their identification only through XRD difficult. Of the 25 fragments selected from the 19 samples, the majority were identified as whitish pickeringite, along with two iron-rich pickeringite (the orange ones) and one white halotrichite. In fact, pickeringite was the only magnesium mineral found coating the adit walls [16], resulting from the oxidation of sulphides and from the percolation of aqueous solution of Mg 2+ /Fe 2+ /Fe 3+ combined with Al 3+ from silicates weathering. Hydrothermal Mg-rich chlorite (present in the host felsic volcanic rocks) can also be considered as a source of the magnesium.
Quantitative analyses obtained through SEM-EDS were used to determine the approximate chemical formula of the halotrichite s.s. fragment, corresponding to the formula ( If it is not the high iron content that is responsible for the orange colour, as is the case of the white halotrichite, surely the trivalent state of iron in partial substitution of aluminium is responsible for the samples' colour, even when the mixed divalent-trivalent state is present (iron-rich pickeringite). Future spectroscopic studies concerning the presence of Fe 2+ and/or Fe 3+ are foreseen using synchrotron radiation. The adsorbed water of the samples, due to the constant humidity of the adit walls, intensifies their colour, giving them the peculiar orange/brownish hue.