Melanterite

White and yellow evaporite minerals form thin crusts on the surface of and within fissures in the tailings. XRD analyses identified the hydrated iron sulphates, melanterite (FeS04-7H20), Zn-melanterite ((Zn,Fe)S04-7H20),... 

In this case study, the selected phases are pyrite, amorphous FeS, calcite (present in limestones in the roof strata Fig. 5), dolomite (possibly also present in the limestones), siderite (which occurs as nodules in roof-strata mudstones), ankerite (present on coal cleats in the Shilbottle Seam), melanterite and potassium-jarosite (representing the hydroxysulphate minerals see Table 3), amorphous ferric hydroxide (i.e., the ochre commonly observed in these workings, forming by precipitation from ferruginous mine waters), and gypsum (a mineral known to precipitate subaqueously from mine waters with SO4 contents in excess of about 2500 mg/L at ambient groundwater temperatures in this region, and with which most of the mine waters in the district are known to be in equilibrium). In addition, sorption reactions were included in some of the simulations, to contribute to the mole transfer balances for Ca, Na, and Fe. 

An indication of the degree of exothermicity of sulphide oxidation reactions can be gained by comparing the enthalpy of formation (A//f), that is, a measure of the energy locked up in each chemical species, relative to native elements. The difference in enthalpies of formation of all reactants and all products defines the enthalpy (heat released or absorbed) of the reaction. Thermodynamic data on sulphide minerals, such as pyrite, are notoriously varied and disputed, and the values in Table 4 must be treated with caution. Nevertheless, depending on whether one defines the reaction as ending in an aqueous solution (equation 5), an intermediate secondary sulphate (e.g., melanterite - equation 6) or in complete oxidation to an oxyhydroxide (equation 7), the calculated reaction enthalpy (AH°) released is of the order of at least 1000 kJ/mol. 

FeS04 -2 to 25 1.9 FeS04-H20 (szomolnokite), FeS04-7H20 (melanterite) Marion et al. 2003a Reardon and Beckie 1987... 

Chalcanthite is copper sulfate, often used as a poison, and sometimes as a pigment. Melanterite is a highly poisonous product of the decomposition of pyrite and marcasite. This white powdery material can often be found on deteriorating jewelry or other items made of iron sulfides. It is mentioned here because of its extreme toxicity. 

Most mineral collections contain potentially harmful species or forms. Toxic species may be the primary components in a specimen, but they may also occur as deterioration products. An example of this is melanterite, which is a common by-product of the decomposition of pyrite and marcasite. This highly poisonous sulfate of iron occurs as a white powder or crust on decomposed specimens. 

When oxidised by weathering, the final product of pyrites is hydrated ferric oxide or rust, as stated above, but the method of oxidation depends upon circumstances. In the absence of carbon dioxide or carbonates, it is usual for oxidation to ferrous sulphate to first take place whence the mineral copperas or melanterite (see below). This, in course of time, oxidises to limonite. In the presence of carbon dioxide, however, it appears that the sulphate is not first formed, but the readily oxidisable carbonate, which is then converted into limonite.3... 

In Figure 5 we have plotted S.I. values for melanterite indicating a trend towards saturation for the Hornet effluent (labeled B). All of the other waters (collected at downstream sites) have been diluted and oxidized and therefore appear undersaturated. The results of these calculations compare quite favorably with field observations. Unfortunately, there is a large uncertainty associated with the thermodynamic solubility constant for melanterite. Although its solubility is well-known, the thermodynamic equilibrium constant is difficult to obtain because the compound is highly soluble and therefore becomes saturated only at high ionic strenths. 

More importantly, it should be noted that melanterite commonly forms on rock surfaces, especially along fractures where water can be pulled up from a water-saturated zone to the surface by capillary forces. Thus, melanterite saturation is occurring in a microenvironment in many instances which makes sampling and interpretation difficult. 

Copiapite. Theoretically, melanterite can oxidize to form copiapite by the reaction ... 

As mentioned earlier, large amounts of copiapite have been found accumulating on ore surfaces in areas protected for direct rainfall. The mechanism of its formation is not at all clear but from field observations it appears that acid mine water is being drawn by capillary forces to an exposed surface where it quickly evaporates to melanterite and/or copiapite. Coquimbite is intimately associated with copiapite and these two minerals appear to be very stable as long as they are protected from rainfall or running water. No thermodynamic data... 

Figure 5. Saturation indices for melanterite plotted as a function of the log conductivity showing an approach to saturation for the most concentrated ferrous-rich waters of the Hornet effluent (B)...

Mineral saturation indices for melanterite and amorphous iron hydroxide agree quite well with field occurrences of the same minerals. Jarosite, however, appears to be supersaturated for nearly all of the samples regardless of the presence or absence of the mineral in these streams. Field observations indicate that jarosite precipitation occurs in the microenvironment of bacterial colonies where the chemical conditions may be quite different from the bulk solution. These considerations lead us to suggest that there is a kinetic barrier which hinders jarosite precipitation but does not hinder ferric hydroxide precipitation and that this barrier is overcome by the surfaces of bacterial colonies (both iron-oxidizers and unidentified nonoxidizers ). ... 

We have observed melanterite, FeSOi 7H2O, to be one of the coimnon sulfate minerals produced by the oxidation of pyrite during weathering. Unfortunately, its solubility and related thermodynamic properties are not well established. The log K for melanterite dissolution has been derived from the free energies of... 

The standard technique used for detection of sulfates is X-ray diffraction of the LTA. Nevertheless, we have observed that in some cases sulfates are present in the coal, but the X-ray does not show any line attributable to them (44). The most abundant divalent iron sulfate observed in the coals studied is FeS04 H20 (szomolnokite), a monoclinic crystal with a tetramolecular unit cell (45). This compound orders antiferromagnetically around 10 K with an effective internal field of 359 kOe (22, 23). Other sulfate minerals found less frequently are FeS04 -4H20 (rozenite) and FeS04 7H20 (melanterite) anhydrous ferrous sulfate was detected when the coal was stored under vacuum. The ferric sulfates commonly observed in several coals are coquimbite and jarosites (16). 

The aluminum sulfates (e.g., alunite, alunogen, basaluminite) do not significantly buffer acid pH values (see Chap. 5). However, the ferric and ferrous sulfates (e.g., coquimbite, the jarosites, melanterite, and szomolnokite) are strong acid buffers that can keep pH values at or below 3 until they are dissolved. The upper pH expected when jarosite is present probably reflects its equilibrium with practically ubiquitous HFO in such systems and the reaction... 

---