Mineralogy of jarosites from the Bányabérc waste dump (Mátra Mts, Hungary) and their environmental significance

Abstract

The amount of leached metals in sulphidic mine waste dumps is controlled by the mineralogical composition, the
degree of oxidation of sulphides and the secondary minerals formed at the site. This study focuses on one of the most
common secondary iron-sulphate families — the jarosite-group — which is occurs at the Bányabérc sulphidic waste dump,
in the Mátra Mts, Hungary. The jarosites were identified on the basis of their mineralogical characteristics. Their presence
indicate a low pH, high sulfate content and highly oxidative media. The samples of values above 1.7 Fe/S atomic ratio were
regarded as mixtures of jarosite and iron oxy-hydroxides (goethite), but with jarosite as the dominant mineral phase
(“jarositic”). The mixed nature of these samples was proved also by Raman spectroscopy. In some samples P and As were
also present, indicating (AsO4)
3– and (PO4)
3– as substitution in the (SO4)
2– position. No replacement of Fe for Al in the
jarosite structure was directly proved, though Al occurred in some of the samples.
The chemical data show, that most of the the Bányabérc jarosites form a solid solution along the jarosite -
hydroniumjarosite join (continuous K+ – H3O+ substitution, without Na in the structure). That observation was confirmed
also by unit cell data: the c0 unit cell dimensions in our samples are of lower values [17.082(2), 17.087(2), 17.104(4),
17.124(5), 17.136(2), 17.125(5), 17.091(2)] than for the pure jarosite (17.2043(2) Å; BASCIANO & PETERSON 2007). These
lower values of c unit-cell parameter justify the H3O+ substitution. The lattice constant a0, which is sensitive to substitution
(e.g. Al3+) in the Fe site, is very close to that reported for pure end-member K-jarosite (compare 7.3346(4), 7.3325(6),
7.3238(5), 7.3117(5), 7.3241(4), 7.3143(7), 7.3317(5) with 7.30293(8) Å, BASCIANO & PETERSON 2007). A minor amount of
Al was measured in some samples and this may reflect other included minerals.
Well-crystalline plumbojarosite forms an outer ring around the anglesite and the latter encloses an unaltered core of
galena. These textural relationships suggest that galena was the Pb source for the two secondary minerals. The secondary
anglesite coatings on galena may increase the stability of the latter; this is due to the low solubility of anglesite and its
protection of galena from direct contact with oxidizing agents. Thus, the galena in the Bányabérc dump may persist for
longer periods of time than other sulphides. The Raman spectra of jarosite-group minerals slightly differ from each other.
The plumbojarosite spectrum has some special features. Around 472 cm–1 and 153 cm–1 there are two extra peaks which
cannot be identified in the other jarosite spectra. There are three broad bands at 990–1030 cm–1 [P1(SO4)2–], around
1080–1120 cm–1 [P3(SO4)2–] and around 1160–1180 cm–1 [P3(SO4)2–] due to overlapping of multiple peaks.
The presence of jarosites in large quantities at the Bányabérc waste dump leads to the conclusion that the oxidative
processes are still in progress. As an oxidation product of pyrite, the jarosite is ubiquitous in the waste dump, indicating
an acidic environment. In the waste material from Bányabérc, in addition to pyrite oxidation, the transformation of
jarosite into goethite is also an acid producer.