Efflorescent Sulfate Crystallization on Fractured and Polished Colloform Pyrite Surfaces: A Migration Pathway of Trace Elements

: The colloform pyrite variety incorporates many trace elements that are released in the environment during rapid oxidation. Colloform pyrite from the Chiprovtsi silver–lead deposit in Bulgaria and its oxidation efflorescent products were studied using X-ray diffractometry, scanning electron microscopy, electron microprobe analysis, and laser ablation inductively coupled plasma mass spectrometry. Pyrite is enriched with (in ppm): Co (0.1–964), Ni (1.8–3858), Cu (2.9–3188), Zn (3.1–77), Ag (1.2–1771), As (8179–52,787), Se (2.7–21.7), Sb (48–17792), Hg (4–2854), Tl (1.7–2336), Pb (13–7072), and Au (0.07–2.77). Gypsum, anhydrite, szomolnokite, halotrichite, römerite, copiapite, and ammoniomagnesiovoltaite were identified in the efflorescent sulfate assemblage. Sulfate minerals contain not only inherited elements from pyrite (Cr, Fe, Co, Ni, Cu, Zn, Ag, In, As, Sb, Hg, Tl, and Pb), but also newly introduced elements (Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Mn, Ga, Rb, Sr, Y, Zr, Sn, Cs, Ba, REE, U, and Th). Voltaite group minerals, copiapite, magnesiocopiapite, and römerite incorporate most of the trace elements, especially the most hazardous As, Sb, Hg, and Tl. Colloform pyrite occurrence in the Chiprovtsi deposit is limited. Its association with marbles would further restrict the oxidation and release of hazardous elements into the environment.


Introduction
Pyrite is the most worldwide abundant sulfide mineral, occurring as a major constituent in ore mineralizations of various types and origin (from magmatic and hydrothermal to sedimentary geologic environments). Pyrite (FeS2) incorporates metals, metalloids, and non-metals in the structure through isovalent (Mn 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , Hg 2+ , Pb 2+ for Fe 2+ ; Se and Te for S) and heterovalent (Cu + , Ag + , Au + , Bi + , Tl + , As 3+ , for Fe 2+ ; As − and Sb − for S) substitutions to an extent of percents [1][2][3][4][5][6][7][8]. Recent studies of its trace element geochemistry show trends of element associations in different deposit types [2][3][4][5][6][7][8]. This feature determined pyrite as a mineral marker for the evolution of a hydrothermal fluid both in time and space. Pyrite has been proved to be a major carrier of gold where it occurs either as "invisible gold" (nanoparticulate and structurally bound) [9][10][11][12][13][14][15][16][17][18] or precipitates within the cracks of earlier As-rich pyrite as microscopically visible gold. It is considered that As content electrochemically facilitates Au deposition in the cracks [1,19]. It is used as a source for the extraction of gold and other trace metals, such as thallium, and the production of sulfuric acid. However, its abundance makes pyrite also the major constituent of mine waste and the main contributor to the formation of acid mine drainage in some mining areas. The latter being a result of the high reactivity to oxygen and water. Pyrite oxidation [20][21][22][23][24] leads to the formation of a number of water-soluble sulfates [25][26][27][28][29][30]. Their formation in situ on pyrite-containing wastes in tailing impoundments or other waste storage facilities is more abundant but can be observed during certain environmental conditions, usually employing low humidity. However, the "colloform"/spherulitic pyrite tends to accommodate more trace elements due to the conditions of its formation (supersaturated fluids and rapid crystal growth [31][32][33][34][35][36][37]) and also oxidizes more rapidly to produce efflorescent sulfates due to the larger reactivity area.
The phenomenon of sulfate crystallization on samples containing "colloform"/spherulitic pyrite during their laboratory storage gives a unique opportunity of direct observation of this process, and information to what extent the trace elements incorporated in pyrite can be mobilized into the environment and behave as pollutants.
In this study, we report data on the trace element composition of colloform pyrite from the Ag-Pb Chiprovtsi deposit (NW Bulgaria) and the mineralogy and trace element composition of the efflorescent sulfates formed during its oxidation in laboratory conditions. The results are interpreted with respect to the partitioning of inherited trace elements among newly formed sulfates and their environmental significance.

Geological Background
The Chiprovtsi silver-lead deposit represents the central and eastern part of an ore zone that extends from the outcropping in NW-W direction Sveti Nikola granite to the E of the Zhelezna village in the Western Balkan Mountains (Bulgaria) (Figure 1). The western part of the ore zone is known as the Martinovo iron skarn deposit. The formation of the ore zone is considered by some researchers [38,39] to be associated with the syntectonic emplacement of the Sveti Nikola granite pluton (311.9 ± 4.1 Ma, [40]) into the low-metamorphic rocks (marbles and schists) of the so-called diabase-phyllitoid complex (DFC) in Bulgaria during the last Variscan collisional events. Others [41,42] suggest the influence of another undiscovered magmatic source of hydrothermal fluids in depth in addition to the Sveti Nikola pluton. The low-metamorphic rocks of DFC, which form the Paleozoic core of the Alpine Berkovitsa anticline, are a set of ophiolite (Cherni Vrah complex), island-arc (Berkovitsa complex), and continental remnants (Dalgi Del complex) related to the evolution of the Proto-Tethys Ocean (Cambrian-Ordovician age, [43][44][45]), as the ophiolite part was dated back to be Early Devonian and related to the Southern Variscan suture [46]. The low-metamorphic rocks of Dalgi Del complex occur to the south of the region shown on Figure 1.
The Ag-Pb metasomatic replacement mineralization spatially associates with siderite bodies, which are hosted by thick marble layers in the metamorphic series (Berkovitsa complex) outlined on an area extending 0.5 km north-northwest of the village of Martinovo to the north of the village of Zhelezna. The siderite bodies (>30) occur as mostly concordant layers and lenses, with various thicknesses (10-15 m) and lengths (up to 200 m), with banded, columnar, or tubular shapes. Some of them are bent, boudinaged, and affected by brittle deformations. Transverse right reverse-slip faults with N-NW and N-NE direction break and transpose parts of the marble layers, forming zones of intensive fracturing and brecciation. These zones together with the foliation and brittle deformations in siderite are a favorable environment for the deposition of the hydrothermal ore mineralization [47,48]. The morphology of the ore bodies mostly concurs with the morphology of the siderite bodies. Ore mineralization occurs as veins, veinlets, metasomatic nests, and impregnations within the siderite bodies. The mineral association comprises galena (~90%), pyrite, sphalerite, chalcopyrite, tetrahedrite-tennantite, and other Ag, Pb-Ag sulfosalts. A low-temperature (<250 °C) quartz-barite-fluorite mineralization is deposited in zones of intensive faulting and displacement of the marble layers in the eastern part of the deposit (Velin Dol and Lukina Padina sections of the Chiprovtsi mine; Figure 1), as barite and fluorite pervasively replace the calcite marbles. This mineralization spatially associates with late polymetallic galena-chalcopyrite-sulfosalt assemblage, where colloform/spherulitic pyrite and cinnabar locally occur as thin veinlets, metasomatic nests, and breccia fillings, with quartz, calcite, ankerite, and fluorite.
The Chiprovtsi mine extracting the Pb-Ag ores from the Chiprovtsi deposit started underground operation in 1951. It was closed in 1999 after producing 4.79 Mt of Pb at 1.84% [49]. The Chiprovtsi mine was also the largest Ag mine in Bulgaria, although public data on the extracted metal and Ag content of the ore is not available. The extraction of fluorite was done for a short period (2010-2016) in the Velin Dol (VD) and Lukina Padina (LP) mine sections, which is located in the easternmost part of the deposit.

Samples
Two types of samples have been studied: (1) aggregates containing abundant colloform pyrite from the low-temperature hydrothermal mineralization occurring in the eastern part of the Chiprovtsi deposit; underground mine workings at level 550 (Velin Dol section) and at levels 495, 606, and 630 (Lukina Padina section) of the mine (Figures 1 and 2); and (2) efflorescent sulfates that have been formed in laboratory conditions on the polished and fractured surfaces of the colloform pyrite containing aggregates from the same deposit. The mineral relationships were studied by reflected light microscopy in polished sections. The chemical composition (major, minor, and trace elements) of colloform pyrite was determined in four polished sections. The sulfate aggregates were observed on five large (~6-7 cm × 10-12 cm × 20 cm) and four small (2-4 cm × 3-5 cm × 4-5 cm) samples. They were hand-picked under binocular microscope from the surface of the large samples, i.e., not from the part that was cut for polished section preparation. Monomineral sulfate aggregates for LA-ICP-MS analysis were carefully hand-picked at highest magnification (60×) under binocular microscope.

Analytical Procedures
3.2.1. X-Ray Diffractometry (XRD), Scanning Electron Microscopy (SEM), and Electron Microprobe Analysis (EMPA) A bulk sample of powdered efflorescent aggregates was used for mineral identification by XRD using a PANalytical Empyrean (Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria), equipped with a multichannel detector (Pixel 3D) using (Cu Kα 45 kV-40 mA) radiation in the 20-115° 2θ range, with a scan step of 0.01° for 20 s. The morphology of the newly formed crystal aggregates was examined on gold-coated specimens using a Jeol 5510 SEM (Sofia University, Faculty of Chemistry) in secondary electron operating mode. The chemical composition of sulfates was determined on carbon-coated specimens by a Jeol JXA-8500F microprobe instrument (Museum für Naturkunde, Berlin, Germany). Microprobe analyses of colloform pyrite were performed on a JEOL Superprobe 733, equipped with an ORTEC energy-dispersive system (Geological Institute, Bulgarian Academy of Sciences) at accelerating voltage of the probe 25 keV and using the following standards: pure metals Ag, Ni, Co, Sb2S3 (for Sb), FeS2 and CuFeS2 (for Fe), Cu3AsS4 (for Cu, As, and S), ZnS (for Zn), CdS (for Cd), and HgS (for Hg).

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)
Trace element concentrations in pyrite and efflorescent sulfates were measured using a PerkinElmer ELAN DRC-e ICP mass spectrometer combined with a New Wave UP193-FX excimer laser ablation system (Geological Institute, Bulgarian Academy of Sciences, Sofia, Bulgaria). The ablation was done in He medium. The ICP-MS was optimized daily to maximize sensitivity and tune the oxide production rate of ThO/Th to <0.5%. The laser system was operated at: (1) 10 Hz repetition rate; 50 µ m spot size, and energy density on pyrite sample and standards 2.5-3.0 J/cm 2 ; (2) 4 Hz repetition rate; 35 µ m spot size, and energy density on sulfate samples and standards 2.5-2.8 J/cm 2 . The analysis time was 100 s, comprising a 40-s measurement of background and a 60-s analysis with a laser-on the sample. The acquisition dwell time for all masses was set to 0.01 s. The following isotopes were monitored in pyrite: 49 Ti, 51 V, 53 Cr, 55 Mn, 57 Fe, 59 Co, 60 Ni, 65 Cu, 66 Zn, 71 Ga, 73 Ge, 75 As, 77 232 Th, and 238 U, monitored in sulfates. Analyses were performed on predefined areas of the polished pyrite-containing samples to avoid mineral inclusions. External standardization was done using NIST SRM 610 glass and MASS1 [51] sulfide standards, which were analyzed recurrently throughout the experiment. Data reduction was done using Fe as an internal standard and the SILLS software [52]. The unexpected peak-shaped fluctuations of the intensity signal of some isotopes were avoided during data reduction to exclude the influence of other minerals on the chemical composition of both pyrite and sulfates.
An SEM study of colloform pyrite spherulites associating with cinnabar reveals that the outer surface of the spherulite is not smooth, but consists of numerous pyritohedron crystals with sizes up to 50-60 µ m (Figure 3b). Transverse sections of spherulites (Figures 3b and 4) show that they are composed of radial-divergent elongated needle-like crystals along the [001] crystallographic axis of pyrite, which start growing from single or multiple crystal nuclei located on the mineral surface (quartz, calcite), as well as metasomatically replace carbonate medium (Figure 3c). The growth zone width varies from micrometers to 200-300 µ m (outer zone). The semi-concentric to concentric zonal banding is determined mostly by differences in chemical composition, the presence of discrete mineral inclusions enveloped by pyrite during crystal growth (tetrahedrite-tenantite, quartz; Figure  4), or interstitial pore space infillings of other minerals (cinnabar, barite, and native silver) ( Figures  3b and 4).
These interstitial pore spaces significantly increase the surface of pyrite aggregates, which is exposed to air. Immediate oxidation reactions occur on the pyrite surface when exposed to water or air.   Table 1). LA-ICP-MS spot analyses reveal the presence of other trace elements, such as Cr, Mn, Co, Zn, Se, Cd, In, Te, Au, Hg, Tl, Pb, and Bi (Table 2).
A number of trace elements were detected in colloform pyrite by LA-ICP-MS. All intensities of trace element isotopes in time-resolved down-hole ablation profiles were thoroughly examined during data reduction and sudden peak-shaped fluctuations, denoting mineral inclusions and gangue mineral crosscuttings, were avoided/excluded. Flat profiles, featuring a gradual decrease or increase of isotope intensities, were presumed to be a result of stable content of the element in one zone or smooth transition into another adjacent growth zone. However, the ablation of a volume containing a dense cluster of nanoparticulate mineral phases within a host mineral would produce also a subtle flat intensity profile. Thus, the interpretation of the LA-ICP-MS results requires caution. It is seen that Co, Ni, Cu, Ag, As, Sb, Tl, Hg, and Pb are frequent constituents of the colloform pyrite chemistry. Their contents vary in wide range, from tens to thousands of parts per million (ppm), not only on the micronscale (neighboring growth zones) within one spherulite aggregate, but also in ore bodies from Velin Dol and Lukina Padina sections ( Figure 5). In some pyrites, the variation range of concentrations in neighboring growth zones is more subtle, in others, it is more distinct, with change even in the element profile. Pyrite from the Lukina Padina section is more enriched in Ni, Co, Hg, and Pb compared to pyrite from the Velin Dol section. The latter one is more enriched in Sb, Tl, Se, and Zn. Arsenic is characteristic for pyrite, with contents usually above 1 wt.%, reaching up to 5.2 wt.%. Although on a different scale, As distribution patterns concur with those of Hg, Tl, and Sb ( Figure 5). Arsenic and Sb were also detected everywhere using EMPA, thus suggesting their incorporation in the lattice.
Cobalt, Ni, and Pb have common patterns of distribution. Copper, Ag, Zn, and Cd form another group of elements that show similar distribution patterns in growth zones. Manganese, Zn, and Se are detected in all analyzed spots in constant, but low concentrations: Mn (5-9 ppm), Zn (3-25 ppm), and Se (4-21 ppm). Single significantly higher concentrations of Zn coincide with elevated contents of Cu, Ag, and Cd, attesting probably an inclusion of tetrahedrite-tennantite. Such discrete inclusions are visible on Figure 4. Single high concentrations of Cr and Mn can also be attributed to mineral influence. Cadmium, In, Te, and Bi were detected sporadically with concentrations below 1 ppm.
Gold shows strong attachment to colloform pyrite. Although microscopically visible gold was not observed, it was detected in almost all analyzed spots. Its concentration varies from 0.07 to 2.77 ppm, and it was usually around 1 ppm.

Mineralogy
As a result of colloform pyrite rapid oxidation in air, iron sulfate phases start to crystallize on its surface and become macroscopically visible in days or months, depending mostly on the air humidity. In years, colloform pyrite aggregates stored in laboratory conditions (approximately 20-25 °C and 20-40% air humidity), usually disintegrate entirely. Sulfates examined in this study gradually formed on the colloform fractured and polished surfaces over several months and continue to crystallize at the moment. The minerals were identified by XRD, SEM-EDS, and LA-ICP-MS techniques. Table 3 summarizes the identified minerals, their chemical formula, the mineral group and the crystal system affiliation (class and space group), the abundance within the assemblage, and the method of identification. This study acknowledged the presence of gypsum, anhydrite, römerite, szomolnokite, and halotrichite as major minerals. Copiapite, coquimbite, and voltaite group minerals occur as minor and accessory phases.  Halotrichite occurs on the pyrite surface as radial, divergent, matted aggregates of transparent to white acicular (fiber needle-like) crystals, elongated along (001) (Figure 6a-d). Halotrichite closely associates with voltaite and szomolnokite, which crystallize within its aggregates, on the prismatic faces of the crystals. (Figure 6b-d).
Szomolnokite is the most abundant iron sulfate in the mineral assemblage (33%). It forms creamy, ivory-white fine-grained, "massive" aggregates ( Figure 6c). SEM revealed that szomolnokite occurs as dense clusters of 5-50-µm-sized distorted pseudo-dipyramidal crystals, which actually represent the complex twinning of prismatic crystals along the elongation axis (Figure 6d,g). The extensive oxidation of pyrite provides a constant source for crystallization, and the twinned crystals are overgrown by new crystals, as the whole scepter-like twin strives to preserve the overall dipyramidal shape (Figure 6d). It also forms ball-shaped clusters of complex twinned crystals (Figure 6h), which is probably a result of bacteria activity.

Chemical Composition of the Identified Efflorescent Minerals
The chemical composition of efflorescent minerals was determined in situ on carbon-coated aggregates by EMPA. Representative microprobe analyses of szomolnokite, aluminocopiapite, copiapite, magnesiocopiapite, coquimbite, voltaite, and halotrichite are included in Table 4. Phosphorous and Si were determined in some analyses. Arsenic is present almost everywhere.
Trace element composition was determined by LA-ICP-MS on gently pressed fiber aggregates of halotrichite and monomineral crystal aggregates of szomolnokite, römerite, copiapite, and voltaite varieties. The estimated concentrations are given in Table 5. This method identified the presence of ammoniomagnesiovoltaite. Although the ammonium content obviously cannot be measured by LA-ICP-MS, the content of other elements coincides with those reported in [53]. All analyses show that sulfate minerals contain not only inherited elements from pyrite, but also new introduced elements. The inherited trace elements are Cr, Fe, Co, Ni, Cu, Zn, Ag, In, As, Sb, Hg, Tl, and Pb. The newly introduced elements are Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Mn, Ga, Rb, Sr, Y, Zr, Sn, Cs, Ba, REE (La, Ce, Nd, Sm, Eu), U, and Th. Gold was not detected in sulfates. The most hazardous elements (As, Sb, Tl, and Hg, excluding Pb) were included in the new minerals. Among these, Hg is least incorporated in sulfates. The others occur in reduced concentrations. Silver was detected only in halotrichite and szomolnokite. However, these concentrations could have been higher depending on the colloform pyrite source. Therefore, it can be assumed that As, Sb, and Tl are entirely inherited in the efflorescent sulfates. The same can be suggested for Co, Ni, Cu, Zn, and Cd. They occur as concentrations commensurable to those detected in pyrite.
Sodium, Mg, and Mn were determined in all sulfates. Aluminum is absent in szomolnokite and magnesiocopiapite, whereas Cs was determined in copiapite and magnesiocopiapite. Voltaite group minerals incorporate most of the trace elements, including the rarest P, Si, V, Rb, Y, Zr, REE, U, and Th. The concentrations of trace elements in sulfate minerals vary spatially based on the colloform pyrite-bearing assemblage that is involved in the oxidation.

Colloform Pyrite Chemistry: Mode of Occurrence of Trace Elements and Genetic Implications
Based on the flat down-hole ablation profiles of As and Sb, and their negative correlations to S (As, r = −0.66; Sb, r = −0.34; As + Sb, r = 0.59), a typical As − , Sb − substitution for S exists. However, their negative correlations with Fe (As, r = −0.58; Sb, r = −0.42) could suggest also the presence of heterovalent substitutions of As and Sb for Fe [16,54], although they are not well defined. Arsenic and Sb show common distribution patterns with Tl and Hg in growth zones ( Figure 5), as well as strong positive correlations (r > 0.75), whereas Tl and Hg correlate negatively toward Fe (r = −0.34 and −0.64, respectively). This could denote a coupled substitution 2 2+ ↔ 3+ + + , where M 3+ are As 3+ and Sb 3+ . Such substitution mechanisms were suggested by [55,56] and proved by [57]. The stronger positive correlation between Tl and Sb (r = 0.98), than Tl with As (r = 0.75) supports a predominant Sb 3+ + Tl + coupled substitution.  Ba  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  La  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  Ce  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  Nd  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  Sm  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  Eu  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  bdl  Hg  bdl  Mercury contents in pyrite were reported mostly for colloform and framboidal varieties that occur in sedimentary environments ( [58,59] and references therein) and Carlin-type [2,15,60,61] deposits. Its occurrence in pyrite is usually related to As-rich zones, because As-rich pyrites imply non-stoichiometric substitutions, which distort the crystal structure and require charge balance (semi-conducting behavior [1,62]). This could be achieved in colloform pyrite by incorporating other metal cations or even nanoparticulate minerals, as it was reported by [55,61]. Here, the positive correlation of Hg and As, and the negative correlation with Fe (r = −0.64), as well as the flat down-hole ablation profiles suggest that Hg could occur as an isovalent substitution for Fe within As-rich growth zones. Since cinnabar is also observed in the assemblage, a nanoparticulate form within colloform pyrite is also very possible.
Cobalt and Ni isovalent substitutions for Fe in pyrite lattice are examined as solid solution [63] and discussed in [1,64]. In this study, Co and Ni show similar distribution patterns and strong positive correlation with Pb (r = 0.73 and 0.90, respectively). Negative correlations of Co, Ni, and Pb with Fe are insignificant to missing. Such correlations suggest the presence as nanoparticulate form rather than substitution. Galena occurs within the colloform pyrite-bearing assemblage; thus, it is more likely that it was attached to the growing surfaces of colloform pyrite and overgrown. Higher Co and Ni contents for this colloform pyrite, as well as cobaltian polydymite, were reported by [38].
Gold content does not show particular dependence on the As content, or strong association with other trace elements, excluding Se (r = 0.50). Weak (r = 0.35) positive correlations are established with Co, Ni, and Pb. Although the contents of Au and As plot well below the solubility limit defined by [15], suggesting its presence as structurally bound species, the absence of any correlation with As, and the association with Co, Ni, Pb, and Se argues for nanoparticulate presence. Gold apparently comes with fluid portions enriched with Co, Ni, Pb, and Se, and deposits predominantly as Au 0 [61], rather than Au + in coupled substitution with As 3+ [16,54]. It is possible to form nanoparticulate Au-Se phases.
Well-defined positive correlations exist among Cu, Ag, Zn, and Cd (r = 0.78-0.97), which show similar distribution patterns in growth zones ( Figure 5). Copper and Zn have weak to insignificant positive correlations with Pb. Correlation with Fe is missing. This fact refers their relation to nano-to micron-sized inclusions of tetrahedrite-tennantite or chalcopyrite together with galena. Tellurium, Bi, and In were detected in very low concentrations. They are probably related to galena or tetrahedrite-tennantite.
The trace element signature of colloform pyrite (enrichment with As, Sb, Hg, and Tl) is characteristic for the low-temperature final stages of hydrothermal activity in the ore zone (<200 °C) [65,66]. The colloform pyrite precipitated from supersaturated sulfide-rich fluids with almost constant As activity and pulses enriched with Sb, Hg, Tl, Au, Ag, Ni, Co that were buffered by the calcite and siderite marble host rocks. The supersaturation of the fluid allowed crystallization from multiple nucleation centers. The nucleation of other mineral phases occurred on the rough surface of the rapidly growing acicular crystals with unbalanced chemical composition. The charge deficiency caused by As and other cations substituting in the crystal lattice could have attracted the deposition of nanoparticulate Au and other minerals, such as galena, tetrahedrite-tennantite, cinnabar, and possibly polydymite. The mineralization precipitated from evolved hydrothermal fluid (intrusion derived mixed with heated meteoric water), which interacted additionally with the country rocks along faults before depositing in the calcite-siderite marble bodies.

Transformation Mechanisms
Pyrite oxidation mechanism has been studied extensively in several experimental studies simulating natural environmental conditions-in air, water solutions with different pH and the presence of carbonates [20][21][22][23][24]. [20,23] inferred that the rate of oxidation is a linear function of the pyrite surface area. Pyrites composed by micron-to submicron-sized crystals with porous texture (framboidal and colloform) naturally possess high surface area that would react during oxidation, causing more rapid transformation (physical and chemical). [23] summarized the oxidation in water solution in two reactions: (1) FeS2 + 3.5O2 + H2O = Fe 2+ + 2H + + 2SO 4 2− ; (2) FeS2 + 14Fe 3+ + 8H2O = 15Fe 2+ + 16H + + 2SO 4 2− . These reactions are in part applicable for oxidation in air. [24,67] suggested that the fracturing of the pyrite in air ruptures the S-S bonds and enhances the production of Fe 3+ and S 2− surface species. Thus, it is possible that ferrous and ferric ions exist concurrently and are incorporated in the formation of szomolnokite and coquimbite. The oxidation of the rough porous surface of the fractured colloform pyrite in air (30-40% humidity) will produce micron-sized supersaturated acidic salt vapor droplets (with H2SO4), from which precipitates rapidly szomolnokite (?) as shapeless masses, coquimbite as micron-sized hexagonal plates, and pseudorombohedral copiapite (Figure 8a,b). The crystal growth promotes the further weathering and fracturing of pyrite (Figure 8b). The oxidation of pyrite adjacent to carbonate matter (vein calcite or marble particles) will be neutralized by the formation of gypsum and anhydrite and release some Mg. Sericite and chlorite that occur in small quantity in marbles will be affected by the acidic droplets (sulfuric acid) and release K, Al, Si, Mg, and Fe. The trace elements in pyrite are released in the same way. For example, the substitution As − for S 2− naturally occurring in pyrite can be represented in the following simple reaction: FeAs − S 2− + 4O2 + H2O = Fe 2+ + 2H + + SO 4 2− + AsO 4 3− , where Asis oxidized to As 5+ and is incorporated as tetrahedral oxyanion. The growth zones that accommodate As 3+ Tl + or Sb 3+ Tl + substitutions for Fe will require the transfer of less electrons to oxidize the cations to As 5+ and Sb 5+ . Therefore, these growth zones will oxidize more rapidly. Thallium will probably preserve its monovalent oxidation state, which is typical for most environments [68]. The metallic cations that are substituted for Fe in pyrite, as well as newly introduced and oxidized, are incorporated in MO6 octahedra, where M is divalent or trivalent cations (Fe 2+ , Mn 2+ , Mg 2+ , Zn 2+ , Al 3+ , Fe 3+ , etc.) [69]. Sulfates started to precipitate and grow on the pyrite surface. Halotrichite hardly touched the pyrite surface and crystallized in the fracture open space or over the pyrite surface. This suggests that the immediate air layer above the pyrite surface contains a dense aerosol of liquid salty droplets from which halotrichite crystallizes. Its crystals serve as a nucleation surface for voltaite and szomolnokite ( Figure 6). The oxidation of pyrite is probably facilitated by acidophilic bacteria, such as T. ferrooxidans and L. ferrooxidans ([70] and references therein). T. ferrooxidans was reclassified to Acidithiobacillus ferrooxidans (A. ferrooxidance) in 2000 [71]. The formation of sphere-shaped szomolnokite aggregates (Figure 6h) could be evidence of their activity. The existence of efflorescences is controlled by water vapor in air. If air humidity increases, the supersaturation of the salty liquid droplets decreases and dissolution of the precipitated phases occurs. Dissolution affects the edges of the crystals (copiapite and römerite), less developed crystal faces (szomolnokite), down a trigonal prism-shaped pits on dipyramidal faces in coquimbite (Figure 8c-f). Our observations show that the order of sulfate crystallization is similar to that described in [25,26]: ferrous (szomolnokite and halotrichite) to ferrous-ferric (copiapite, römerite, and voltaite) and ferric sulfates (coquimbite).

Partitioning of Inherited Trace Elements in Sulfates
The element budget inherited from pyrite is distributed among sulfate phases. Figure 9 shows the trace element concentrations of sulfates normalized to the mean content of trace elements in colloform pyrite. The plot discriminates the element affinity toward particular sulfate phases. The sulfate/mean pyrite <1 of Fe is explained with the redistribution of the initial Fe content among the various sulfates. All newly formed sulfates have lower Fe content than pyrite. It can be seen that V, Cr, and Mn came from external sources. Manganese mostly partitions among voltaite, ammoniomagnesiovoltaite, and magnesiocopiapite, where it enters in the M 2+ position of Fe 2+ as reported by [53,69,72,73]. Nickel and Co show affinity to copiapite, halotrichite, and voltaite, also occuring in M 2+ . Copper partitions among halotrichite, voltaite, szomolnokite, copiapite, and römerite (highest content). Zinc is incorporated in copiapite and voltaite. As it was suggested by [74,75], Cu and Zn occur in the M 2+ position. Most of the As is included in voltaite and magnesiocopiapite. Antimony and Tl are related to voltaite and copiapite group minerals. Mercury and Pb enter in voltaite and magnesiocopiapite. Tin also come from external sources and concentrates in römerite. Cadmium and In also enter in voltaite and copiapite. Generally, all elements from Se to Pb are partly resistant to incorporation in sulfates. Notably, Gold and Se stay out of the new phases. Once released from pyrite, Au was not incorporated in the new phases. This supports the presence of Au in nanoparticulate form (Au 0 ). Silver is more reactive in low-temperature environments. It could be included in acanthite, as such, it was also observed as recent exsolution from Ag-rich tetrahedrites in samples containing the main Ag-Pb ore mineralization in Chiprovtsi deposit. The most hazardous elements (As, Sb, and Tl) are incorporated in the efflorescent minerals. Thallium has a large ionic radius [76]; thus, it probably occurs in the position of K in voltaite minerals, whereas in copiapite it probably occurs in the position of M 2+ .

Sources of Introduced Elements in Sulfates and Incorporation
A great number of new elements are introduced in sulfates: Na, Mg, Al, Si, P, K, Ca, Sc, Ti, V, Cr, Mn, Ga, Rb, Sr, Y, Zr, Sn, Cs, Ba, REE (La, Ce, Nd, Sm, Eu), U, and Th. Sources for these elements are other gangue minerals occurring in the mineral assemblage, such as carbonates (calcite and siderite marble particles, and late vein ankerite, calcite), quartz, barite, and fluorite. The host rocks (marbles and chlorite-sericite schists) contain quartz, sericite, and chlorite. The interaction of these minerals with the salty acidic droplets produced by pyrite oxidation releases the following elements: (1) carbonates-Ca, Mg, Mn, Fe, Sr, Ba, and REE; (2) fluorite-Ca, Na, P, Rb, Y, REE, and Cs; (3) barite-Ba, Sr, Rb, and U; (4) sericite-K, Al, Rb, Si, Cs, V, Cr, Ga, Sc, REE, possibly Zr and Sn; (5) chlorite-Al, Mg, Fe, V, Cr, Cs, REE, U, and Th. They are deposited as gypsum, anhydrite, halotrichite, and voltaite. Aluminum and Mg are included as minor constituents in copiapite and coquimbite. The alkaline elements Na, Rb, and Cs, having large ionic radii [76], probably take the position of K + in the voltaite group. Scandium probably occurs in the Al 3+ position in voltaite. Vanadium and Cr are incorporated in the Al 3+ position in halotrichite. Tin, Zr, REE, Y, U, and Th are rarely found mostly in voltaite. Their position is hard to elaborate. REE and Y also have large ionic radii, so they can occur in the K position, or in crystal defects. Tin, Th, and Zr may substitute for Al 3+ , Fe 3+ , or Mg 2+ , and Fe 2+ creating vacancies and electron charges. Uranium in oxidizing conditions forms a uranyl (UO 2 2+ ) cation that may occur in the position of Fe 2+ . Phosphorus occurs as a PO4 3− tetrahedral oxyanion. As it is seen, voltaite group minerals incorporate most trace elements, as inferred in [53,72,73].

Environmental Significance
Most of the reported efflorescent mineral formations so far occur in areas associated with (1) ore mining deposition from highly acidic solutions/streams caused by acid mine drainage (AMD) during dry seasons, waste heaps, and dumps [26][27][28][29][30]70,[77][78][79][80]; (2) coal mining within coal seams and on coal fuel heaps in thermoelectric plants as a result of framboidal pyrite oxidation [81]; (3) natural precipitation from streams during dry seasons on the surface and underground; (4) volcanic and other hydrothermal activity [82], etc. In natural conditions, such formations are subject to periodical dissolution by rainfalls.
Since the newly formed minerals inherit the trace element signature of colloform pyrite, they act as temporary water-soluble carriers before releasing them into the environment when dissolved. Therefore, abundant colloform pyrite in some deposits can become extremely harmful for the areas close to the mines. In this case, colloform pyrite occurrence is very limited. Its association with carbonate host rocks would further restrict its influence on a large scale. However, concentrations of As and Sb were detected in water discharges from abandoned underground mine workings in the Velin Dol and Lukina Padina mine sections, but they do not pose a threat for the Chiprovska Ogosta river flowing through the area [83].

Conclusions
The oxidation of colloform pyrite aggregates is carried out more rapidly compared to euhedral and anhedral pyrite. The growth zones/bands containing more trace elements react first. The richness of efflorescent mineral assemblage is controlled directly by the mineral composition of the colloform pyrite-bearing assemblage. Most of these minerals take part in the oxidation mechanism and transformation reactions. They act as a source of elements, which do not occur in pyrite, but are essential for the formation of gypsum, anhydrite, halotrichite, voltaite, the Al-and Mg-varieties of copiapite, and coquimbite. The overall order of crystallization starts from ferrous (szomolnokite, halotrichite) to ferrous-ferric (copiapite, römerite, voltaite) and ferric sulfates (coquimbite). Gypsum and anhydrite precipitate as a result of neutralization by the carbonate minerals. The preservation of the efflorescent sulfates is controlled mainly by air humidity; thus, in the laboratory, they continue to growth in relatively steady conditions, with limited dissolution and transformation from one sulfate mineral to another. They form until the entire deterioration of pyrite samples. Most of the trace elements in colloform pyrite are inherited and partitioned among the newly formed phases. Thus, colloform pyrite could be a major cause of environmental pollution. Funding: This study was financially supported by the RNF01/0006 and DNTS China01/7 grants of the Bulgarian Science Fund and an exchange mobility grant within the CEEPUS program.