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Advances and Gaps in the Knowledge of Thermodynamics and Crystallography of Acid Mine Drainage Sulfate Minerals

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Advances and Gaps in the Knowledge of Thermodynamics and Crystallography of Acid Mine Drainage Sulfate Minerals MINERALOGY CHIMIA 2010, 64, No. 10 699 doi:10.2533/chimia.2010.699 Chimia 64 (2010) 699–704 © Schweizerische Chemische Gesellschaft Advances and Gaps in the Knowledge of Thermodynamics and Crystallography of Acid Mine Drainage Sulfate Minerals Juraj Majzlan* Abstract: Acidic and metal-rich waters produced by sulfide decomposition at mining sites are termed acid mine drainage (AMD). They precipitate a number of minerals, very often sulfates. The recent advances in thermodyna- mic properties and crystallography of these sulfates are reviewed here. There is a reasonable amount of data for the divalent (Mg, Ni, Co, Fe2+, Cu, Zn) sulfates and these data may be combined with and optimized by tempe- rature-relative humidity brackets available in the literature. For the sulfates with Fe3+, most data exist for jarosite; for other minerals and phases in this system, a few calorimetric studies were reported. No data whatsoever are available for the Fe2+-Fe3+ sulfates. A significant advance is the development of the Pitzer model for Fe3+-sulfate solutions and its confrontation with the available thermodynamic and solubility data. In summary, our knowledge about the thermodynamic properties of the AMD sulfates is unsatisfactory and fragmented. Keywords: Acid mine drainage Introduction population matches the quantities of the relatively recently.[12,13] A sizeable body of overall natural mass flow.[4] The mining of data accumulated during the last ten years, The ever-increasing demand for metals ores and salts does not dominate the an- partially because of interest in AMD, par- dictates the steady increase in the exploi- thropogenic mass flow but cannot be ne- tially because sulfate minerals have been tation of ore deposits, and, to a smaller glected. identified in Martian meteorites[14] or pre- extent, attempts to recycle the useful ele- The extraction of ores from the litho- sumed to be present on the surface of Mars ments. For example, 3,547 tonnes of gold sphere, fine grinding and disposal of un- (e.g. ref. [15]) and to have implications for were extracted in 2007.[1] This is a large wanted waste exposes many ore minerals, the understanding of the ancient evolution amount for a metal handled so cautiously in a fine particulate form, to the atmo- of this planet.[16,17] and carefully even in the smallest quanti- sphere and hydrosphere, thereby acceler- ties. Correspondingly larger amounts of ating their decomposition rates by many other metals are needed, some of them orders of magnitude.[5] The released acid- Mineral Parageneses reaching unimaginable magnitudes. The ity, metal cations, and anions can reach for- global population moves into cities which midable levels.[6] This phenomenon is well Acidic, metal-rich waters can be pro- [2] are becoming vast new ore deposits with known under the name acid mine drainage duced by weathering of pyrite (FeS2) or their buildings, traffic systems, and other (AMD) and is certainly not new, as can be pyrrhotite (Fe1-xS), rarely other sulfides, daily components of life. The human- documented by detailed descriptions in ref. in contact with atmosphere and surface induced fluxes of some elements have [7] among others. The mineralogy of AMD or underground water.[5] Such weathering already exceeded the natural fluxes (e.g. systems is rich, although many minerals most commonly occurs in waste products Cd[3]). Apart from creation and decompo- are only ephemeral, being washed away by of mining activities and the process and sition of the biomass, the today’s human rain or surface streams after a dry period. its products are called acid mine drainage Much of the mineralogical work on AMD (AMD). The composition of the two pri- has been reviewed.[5,8–10] Here I summarize mary sulfides (pyrite and pyrrhotite) dic- the recent advances in the thermodynamics tates the bulk ion load of the AMD waters, and crystallography of the sulfates found that is, their usually high concentration of in AMD systems. The thermodynamic iron, either ferrous or ferric, and sulfate. properties of iron oxides, halogenides, se- The acid waters attack and dissolve other lected sulfates, and many aqueous species minerals present and release their con- with iron have been critically summarized stituents into the aqueous fluids. These in an excellent review of Lemire et al.[11] constituents comprise major rock-forming and need not to be discussed here. Because elements such K, Na, Mg, Ca, Al but also the thermodynamic properties must be re- toxic and heavy metals and metalloids, for ferred to phases with known composition example Cu, Ni, Zn, Cd, As, and others. All and structure, this review provides also an these cations may combine with sulfate to *Correspondence: Prof. Dr. J. Majzlan update for each group of considered com- form AMD minerals. Beside sulfates, iron Friedrich-Schiller University Institute of Geosciences pounds in terms of their crystal structures. oxides, be they poorly or well crystalline, Burgweg 11 This update documents how variable is are abundant. The iron oxides are power- D-07749 Jena this ‘well known’ group of structures. No ful sorbents of many ions and thus in some Tel.: +49 3641 948700 Fax: +49 3641 948602 comprehensive overview of the sulfate cases prevent the formation of independent E-mail: [email protected] structures is necessary; this has been done iron-free or iron-poor phases. 700 CHIMIA 2010, 64, No. 10 MINERALOGY temperature [°C] temperature Fig. 1. Eh-pH diagram that shows two possible pathways for relative humidity [%] the evolution of an acid mine drainage system. The minerals not mentioned in the text but shown here are: goethite – α-FeOOH, green Fig. 2. Stability of sulfate minerals as a function of relative air rust – Fe2+-Fe3+ hydroxide with additional anions, schwertmannite humidity and temperature. The curves calculated from optimized [26] 2+ – ~FeO(OH)3/4(SO4)1/8, and ferrihydrite – ~Fe(OH)3. Schwertmannite thermodynamic data are: Mg sulfates, Cu, Ni, Zn, Fe , Co sulfates and ferrihydrite are poorly crystalline and metastable with respect to n (Grevel and Majzlan, in preparation). The curves for the Fe2(SO4)3· H2O goethite. were calculated from measured and estimated thermodynamic [30] data. The curve for rhomboclase and (H3O)Fe2(SO4)2 is the best fit to the experimental data in ref. [31]. The curves in the range of very high relative humidity are dashed because the sulfate minerals In general, an AMD system proceeds deliquesce in this region. Minerals shown in the figure: bianchite ZnSO ·6H O, bieberite CoSO ·7H O, bonattite CuSO ·3H O, through several stages of development 4 2 4 2 4 2 chalcanthite CuSO ·5H O, epsomite MgSO ·7H O, goslarite before the final, stable or metastable but 4 2 4 2 ZnSO ·7H O, hexahydrite MgSO ·6H O, kieserite MgSO ·H O, kornelite kinetically inert state is achieved.[5] Fig. 1 4 2 4 2 4 2 Fe (SO ) ·~7H O, melanterite FeSO ·7H O, moorhouseite CoSO ·6H O, shows possible evolution pathways of an 2 4 3 2 4 2 4 2 morenosite NiSO4·7H2O, paracoquimbite Fe2(SO4)3·9H2O, retgersite AMD system. Crystallization of the solu- NiSO ·6H O, rhomboclase (H O )Fe(SO ) ·2H O, rozenite FeSO ·4H O. ble divalent metal sulfates depends on the 4 2 5 2 4 2 2 4 2 availability of the divalent cations and the degree of evaporation. As the system ma- tures, iron is being oxidized and is stored in iron sulfates or oxides, depending on the to polymerize (as opposed to octahedra ious MgSO4·nH2O phases by varying rela- pH value of the fluids. Note that the system that house Fe3+), the chemical composition tive humidity and temperature. Because must achieve relatively low pH values (<2) of these minerals is simply MSO4·nH2O. they were able to reverse the reactions, that to form the crystalline Fe3+ (or Fe2+-Fe3+) Even with this simple composition, the is, to observe hydration of the lower hy- sulfates described below. It may seem, variability in this group is remarkable. The drate and dehydration of the correspond- therefore, that these minerals are rare and structures of most of these phases have ing higher hydrate, these experiments are restricted to the large AMD systems which been solved earlier, reviewed in ref. [12] brackets in the sense of the brackets com- achieve such low pH values routinely. To- and further discussed in ref. [13]. A recent monly determined for high-temperature sca et al.[18] have shown that evaporation thorough study of the Mg sulfates identi- and high-pressure silicate equilibria. The of pools of waters with relatively high pH fied several new phases.[19] The structure univariant curves calculated from avail- [24,25] (~3) can lead to a final pH of almost ~0 and of the MgSO4 dihydrate (mineral sand- able thermodynamic values signifi- precipitation of many minerals mentioned erite[20]), 2.5 hydrate,[21] and the dodeca- cantly deviate from the experimentally in this paper. A common example from hydrate (mineral meridianiite[22]) were observed reversals.[27,28] To alleviate this many AMD sites are the powdery aggre- solved. The systems with the other diva- problem, Grevel and Majzlan[26] applied gates of copiapite which probably crystal- lent cations await similar attention and it is the mathematic programming analysis (cf. lize from small isolated evaporating pools very likely that new phases exist in those ref. [29]), a method that varies the avail- of acidic waters and disappear with the first systems as well. able thermodynamic values within their rain. As the acid-generating capacity of an The thermodynamic data for these uncertainties. The result of the calculations AMD system vanishes, the pH rises and phases have been summarized in an ex- is a phase diagram constructed from opti- the final product is goethite (Fig.
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