Szomolnokite

Hydronium Jarosite Natrojarosite Jarosite Kornelile Melanleritc Rhomboclase Rome rite Rozenile Schwertmannite Szomolnokite... 

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

The presence of iron sulfates such as szomolnokite FeS04 H20 hindered the identification of the clays at room temperature. The ferric sulfates can be detected either by treating the sample with dilute HC1 or by carrying out Moessbauer measurements at low temperatures. This is of particular importance for the identification of jarosites. Other minerals like siderite can be identified with relative ease. An extensive review of this area can be found in ref [11]. 

Figure 7. Mossbauer spectrum of a Waynesburg coal (RT). Szomolnokite, illite, and pyrite are identifiable.

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). 

In Figure 10 a Mossbauer spectrum for a mixture of szomolnokite (A) and rozenite (B) is shown. The sample was characterized by X-ray diffraction as well as Mossbauer spectroscopy. After the LTA was obtained, the Mossbauer spectrum shows the presence of szomolnokite (no rozenite) and ferric sulfate (Figure 11). This conversion was observed for all the coal samples studied that contained sulfates. In Table III a list of the iron sulfates and their respective Mossbauer parameters is given. 

Figure 11. Mossbauer spectrum of szomolnokite and rozenite after LTA obtained (90°C). All the rozenite has disappeared (RT).

The results of the fitting procedure (table i) reveal chat the divalent iron phase is Szomolnokite (FeS04 H20). The trivalent iron phase is probably a hydroxylated basic irline width of its spectral contribution. The presence of die sextuplet in the spectrum of the used catalyst B shows that still some iron(III) oxide is present as also found by XRD. 

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... 

Freshly mined coal rarely exhibits oxidation, but if coal is exposed to oxidative weathering some forms of pyrite or marcasite, especially the nodular forms, rapidly oxidize to iron sulfates. For completeness, these alteration products are included in the epigenetic occurrences as proposed by Mackowsky (28). Some forms of pyrite are known to oxidize in situ to Szomolnokite (FeSO/i HoO) prior to exposure to the atmosphere (39). 

Lewis acid-base theory (Jensen, 1980) is an outgrowth of the Lewis model of chemical bonds. A Lewis acid is a chemical species that can accept an electron pair. Lewis acids can be cations like Fe or Cu or they can be species with empty or partially empty valence orbitals such as CO2 or SO2. Lewis bases can donate an electron pair. Lewis bases are anions like OH" or S or they can be species with lone pairs such as HjO or NHj. The transfer of cations from a solid, such as szomolnokite (FeS04 H20), to form a hydrated ferrous ion in solution is a typical Lewis acid-base reaction. 

Fig. 3.20 RT spectra of some sulfates szomolnokite (adapted from Van Alboom et al. 2009...

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