American Mineralogist, Volume 88, pages 1919–1932, 2003 Efflorescent iron sulfate minerals: Paragenesis, relative stability, and environmental impact JEANETTE K. JERZ* AND J. DONALD RIMSTIDT Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061, U.S.A. ABSTRACT This study of a pyrrhotite-dominated massive sulfide deposit in the Blue Ridge province in south- western Virginia shows that sulfate minerals formed by the oxidation of the pyrrhotite transform from one to another by a combination of oxidation, dehydration, and neutralization reactions. Significant quantities of sulfate minerals occur in the underground adits (Area I) and under overhangs along the high sidewall of the adjoining open pit (Area II). Samples from this site were analyzed to determine mineralogy, equilibrium relative humidity, chemical composition, and acid generation potential. In Area I, pyrrhotite oxidizes to marcasite + melanterite, which eventually oxidizes to melanterite + sul- furic acid. Melanterite is extruded from the rocks as a result of the volume change associated with this reaction. It accumulates in piles where halotrichite, copiapite, and fibroferrite form. In Area II, FeSO4 solutions produced by pyrrhotite oxidation migrate to the exposed pit face, where they evaporate to form melanterite. The melanterite rapidly dehydrates to form rozenite, which falls into a pile at the base of the wall, where melanterite, copiapite, and halotrichite are present. The observed paragenesis a a can be understood using a log O2 – log H2O diagram that we developed from published thermody- namic data and observations of coexisting phases. Dissolution experiments showed that fibroferrite-rich samples had the highest acid producing po- tential, followed by copiapite-rich samples and then halotrichite-rich samples. The most abundant metals in solutions produced by dissolving impure bulk samples of the salts were Mg, Al, Zn, Cu, Ca, and Pb. The molar concentrations of the metals varied with mineralogy. However, all of the sulfate minerals release metals and acid when they dissolve and therefore represent a potentially significant environmental risk. INTRODUCTION Fe S93- x O5- x H O()1 x Fe(OH) + H SO (2) 1- x ++4 2 2 2324 =- Acid mine drainage (AMD) environments provide of a rich tapestry of mineralogy and geochemistry. Although the het- Iron sulfide and iron oxyhydroxide geochemistries have erogeneity and complexity of AMD systems can make them been well studied (see Nordstrom and Alpers 1999 and refer- difficult to study, we expect them to show a systematic pattern ences therein), but the equally important iron sulfate phases of mineral paragenesis. Knowledge of this paragenesis is im- are less well understood. The objective of this study is to es- portant for quantifying the environmental impact of the miner- tablish the geochemical relationships of some important iron als, predicting the evolution of AMD sites, and identifying sulfate minerals that occur in AMD environments. effective AMD remediation methods. A first step toward ad- Numerous hydrated iron sulfate minerals are known (Table dressing the paragenesis problem is to consider only reactions 1), and their mineralogy and geochemistry are discussed in a involving the most important AMD elements: iron, sulfur, oxy- recent review by Jambor et al. (2000). Efflorescent iron sulfate gen, and hydrogen. The important acid mine drainage minerals minerals are common in base metal deposits, coal deposits, and can be divided into three types: iron sulfides, iron sulfates, and tailings and waste rock piles where they are frequently associ- iron oxyhydroxides. The overall AMD-forming process in- ated with oxidizing iron sulfide minerals. Because of their high volves a complex chain of reactions that link the oxidation of solubility, most iron sulfate minerals only persist under rock iron sulfides to the precipitation of iron oxyhydroxides and the overhangs and similar sheltered sites where they are protected release of sulfuric acid to receiving waters. For pyrite and mar- from dissolution during rain events. The minerals can also form casite, the overall reaction is: in the open during drier times as sulfate-rich solutions migrate to the surface and evaporate. 15 7 These iron sulfate minerals form when solutions rich in iron FeS2 +4 O2 +2 H2324 O = Fe(OH) + 2H SO (1) sulfate and in sulfuric acid evaporate in surficial environments. and for pyrrhotite the overall reaction is: The evolution from ferrous sulfate minerals to iron oxyhydroxide minerals occurs by a series of oxidation, dehy- dration, and neutralization reactions. The mineralogy that de- * Present address: Department of Geology and Geography, velops at any particular site will be controlled by the relative DePauw University, 602 South College Avenue, Greencastle, rates of each of these types of reactions. We can predict the Indiana 46135, U.S.A. E-mail: [email protected] order in which the minerals in Table 1 are likely to form by 0003-004X/03/1112–1919$05.00 1919 1920 JERZ AND RIMSTIDT: EFFLORESCENT IRON SULFATE MINERALS examining their stoichiometries in terms of these three reac- are related by oxidation, neutralization, and dehydration reac- tion types. The axes of Figure 1, based on the atomic ratios of tions. The oxidation axis expresses the amount of cationic chemical constituents, are designed to show how the minerals charge provided by ferric iron divided by the total cationic charge. This number increases when ferrous iron oxidizes to TABLE 1. Minerals that occur in the system Fe-S-O-H ferric iron. The neutralization axis expresses the ratio of an- Name and abbreviation Formula ionic charge provided by hydroxide to the total charge pro- 3+ hematite hem Fe2 O3 vided by all anions. This number increases as minerals goethite goe Fe3+OOH 3+ incorporate hydroxide from solution or as sulfate is lost to so- hydrous ferric oxide* hfo Fe (OH)3·nH2O 3+ maghemite† mhem Fe2 O3 lution as H2SO4. The dehydration axis expresses the number of 3+ quenstedtite quen Fe2 (SO4)3·10H2O moles of water molecules per total moles of cationic charge. 3+ coquimbite coq Fe2 (SO4)3·9H2O 3+ This ratio decreases as water is lost by dehydration. The bold paracoquimbite‡ pcoq Fe2 (SO4)3·9H2O 3+ kornelite korn Fe2 (SO4)3·7H2O arrow shows the general trend of compositions as ferrous sul- 3+ lausenite laus Fe2 (SO4)3·6H2O fates evolve to iron oxyhydroxides. The diagram explains why 3+ hohmannite hoh Fe2 (SO4)2(OH)2·7H2O 3+ minerals such as lausenite (lau) are rare in AMD environments; metahohmannite mhoh Fe2 (SO4)2(OH)2·3H2O 3+ fibroferrite fib Fe (SO4)(OH)·5H2O they can form only under strongly acidic conditions. More com- 3+ amarantite amar Fe (SO4)(OH)·3H2O 3+ mon minerals, such as copiapite (cop), fall near the most direct butlerite but Fe (SO4)(OH)·2H2O 3+ parabutlerite pbut Fe (SO4)(OH)·2H2O path leading from melanterite (mel) to goethite (goe). By ex- 2+ 3+ bilinite bil Fe Fe2 (SO4)4·22H2O amining the geochemistry, mineralogy, and occurrence of these 2+ 3+ römerite röm Fe Fe2 (SO4)4·14H2O 2+ 3+ phases, we can begin to map out the important reactions that copiapite cop Fe Fe4 (SO4)6(OH)2·20H2O 3+ ferricopiapite fcop Fe5 O(SO4)6OH·20H2O result in the overall transformation processes. 2+ melanterite mel Fe SO4·7H2O Knowledge of the sulfate mineralogy and paragenesis at an 2+ ferrohexahydrite fhex Fe SO4·6H2O 2+ AMD site is important because different sulfate minerals carry siderotil sid Fe SO4·5H2O 2+ rozenite roz Fe SO4·4H2O different amounts of trace elements and produce different 2+ szomolnokite szo Fe SO4·H2O 3+ amounts of acid upon dissolution. The dissolution of iron sul- rhomboclase rhom H3OFe (SO4)2·3H2O 3+ hydronium jarosite h-jar H3OFe3 (SO4)2(OH)6 fate minerals during rain events can dramatically affect aquatic 3+ schwertmannite sch Fe 16O16(OH)12(SO4)2 ecosystems. Dagenhart (1980) showed a clear relationship be- Note: Not all of these minerals are found in the AMD environment. The tween stream discharge and stream chemistry during rain events abbreviations are used in subsequent figures and tables in this paper. * A complex group of phases that includes ferrihydrite and lepidocrocite. (Fig. 2). His data show that during the initial phase of a rain † Polymorph of hematite. ‡ Polymorph of coquimbite. 4.00 3.75 mel 3.50 fer 3.25 4 sid 3.00 roz 2000 bil 3 1500 röm 2 1000 szo que coq cop 500 kor fib 1 lau fcop amarhoh ff 600 but 0 mhoh 0 0 0.2 400 0.2 0.4 0.4 goe 0.6 0.6 200 0.8 hem0.8 1.0 0 0 25 50 75 100 Time (hrs) FIGURE 1. The stoichiometries of iron sulfate and iron oxide FIGURE 2. Hydrograph of Contrary Creek, Virginia, showing the minerals are directly related to the reactions that transform one phase relationship between stream discharge and stream chemistry for rain into another. Mineral stoichiometries and abbreviations are given in events in June, 1978 (Dagenhart 1980). The bold vertical lines represent Table 1. Changes in the ratios along the axes correspond to chemical rain events. Runoff from the first storm event dissolved sulfate minerals reactions that are responsible for iron sulfate mineral transformations and carried the resulting acidic, sulfate-rich solution to the stream, in acid mine drainage environments. These reactions are dehydration causing an increase in acidity and conductivity. After a short time, (z-axis), neutralization (y-axis), and oxidation (x-axis). The bold line relatively fresher water from upstream diluted the stream. Later storm shows a generalized paragenesis path involving these three reactions events produce smaller excursions in conductivity and pH because the that converts ferrous sulfate to iron oxyhydroxides. sulfate minerals had been previously washed away. JERZ AND RIMSTIDT: EFFLORESCENT IRON SULFATE MINERALS 1921 event, there was a marked decrease in pH and an increase in This paper builds on the results of these previous studies of dissolved solids as iron sulfate minerals dissolved and the acid the paragenesis of sulfate minerals and their environmental im- sulfate solutions were carried by overland flow into the stream.