Evaluation of the test-mineral method for studying minesoil geochemistry

The test-mineral method was evaluated as a means of assessing the chemical environment and soil processes in mine soils. This study was carried out on lignite mine tailings at As Pontes (northwestern Spain). The spoils are heterogeneous and mainly consist of carbonaceous and noncarbonaceous clays and slates, often containing pyrite. Vermiculite was used as a reference mineral and placed in ion-permeable polyamide bags that were then buried in various reclaimed minesoils. One bag was recovered from each soil after 13, 26, and 52 wk and analyzed for pH, elemental composition, exchangeable cations, cation-exchange capacity (CEC), extractable Al and Fe, and mineralogy (x-ray diffractometry [XRD], differential thermal analysis [DTA], and thermogravimetry [TGA]). Vermiculite rapidly underwent geochemical and mineralogical changes. The greatest changes took place in soils developed from minesoils rich in sulfides; exchangeable Na on the vermiculite was replaced with Ca, Mg, Al, or H and there was an important decrease in CEC due to interlayer Al-polymerization. This behavior is typical of an acid environment. By contrast, the CEC of vermiculite in the low-sulfide soils did not change and exchangeable Na was displaced by Ca, with virtually no involvement of other cations. This test vermiculite proved to be an excellent indicator of the chemical environment of minesoils