Mineralogical transformations in carbonate soils affected by a spill of pyrite tailings

INÉS GARCÍA,; FRANCISCO MARTÍN,; CARLOS DORRONSORO,; MARIANO SIMÓN,; DIEZ, M.; BOUZA, P.; JOSÉ AGUILAR,

Budapest

Congreso; 4th International Congress of the European Society for Soil Conservation; 2004

The aim of this work is to analyze the effect of the pyrite tailing oxidation, over time, on soil mineralogy The soils in which the reddish-yellow layers (RYL) develop are carbonated (8%) and sandy (46%) soils, with low organic matter content (1.7%), alkaline pH (8.0) and slightly developed profile (Xerorthent and Xerofluvent (Soil Survey Staff, 1999)). Over time, the pH decrease with the progress of the contamination, reaching the value of 2.5 at five years from the staying of the tailings in soil, causing the decrease and even the dissolution of the carbonates; in this sense, two month after the spill carbonate content is reduced up to 58%, the samples from 15 months and 5 years are completely decarbonated, having a strong fall in the cation exchange capacity values. The iron within this layer strongly increased, indicating a high degree of pollution. Also, from this Fe distribution, we deduce that this metal was responsible for the reddish-yellow coloration. Mineralogy of uncontaminated soil was dominated by quartz, feldspars, calcite and dolomite, and the infiltration of the acidic solution produced and intense weathering of the calcite, totally disappearing in soil at 15 months from the pollution. The other constituents have less substantial variations. In this sense, feldspars and phyllosilicates are reduced approximately to half at five years from the spill. On the other hand, the peaks in the X-ray diffractograms of the resistant-to-hydrolysis minerals, like quartz, increase its intensity. Very important are the neoformations produced by the high quantity of iron and sulphates dissolved in the contaminant solution. In this way, the calcium released in the weathering of the calcite reacts with the sulphates to form gypsum which increases gradually until reach the 15% of the total soil; meanwhile the sulphate, iron and potassium form jarosite which, as the contamination progressing, trend to change to plumbojarosite. So the plumbojarosite is relatively abundant in the decoloured zone of the layer formed at 5 years. The electron microscopy (SEM_EDS) reveals, both crystals of gypsum and jarosite, as other neoformations like the needled crystals of S and Fe (melanterite ?) or the plated-habit crystals of S and Al (alunite?). According to the clay mineralogy, the oriented aggregates of the clay fraction in the samples from the soil underlying the reddish-yellow layer gave diffractograms showing illite, kaolinite, smectite, interstratified complexes of chlorite/smectite, calcite and feldspars. In the samples of the reddish-yellow layer, the peaks become broader and lower, indicating a generalized breakdown of minerals, both phyllosilicates as well as feldspars. The presence and alteration of the components in the soil (mainly, feldspars, micas and clays) boosted the capacity of acid neutralization and retention of released metals. Identification of such mineral alteration is not easy, the phyllosilicates and particularly smectites apparently being the most active in neutralizing the acidity.