Arsenic-rich stalactites from abandoned mines: Mineralogy and biogeochemistry

Highlights: • Stalactites consist of HFA, schwertmannite, HFO and minor HMO and kaňkite. • As-rich HFO and schwertmannite contain coprecipitated HFA. • As in the drip water is linked to Fe/As ratio in the solid phase. • Aqueous As is controlled by HFA solubility and/or sorption to HFO/schwertmannite. Stalactites containing 0.13–294 g kg⁻¹ As were collected from abandoned adits of the former Mikulov and Plavno mines (NW Czech Republic), and were then characterized by: X-ray diffraction, bulk chemical analysis, electron microprobe and Raman microspectrometry, chemical composition of the drip water, and analyses of the microbial communities. Several assemblages of mineral phases were identified: (i) straws of X-ray amorphous hydrous ferric arsenate (HFA) with younger kaňkite, (ii) pure HFA straws, (iii) straws composed of schwertmannite and HFA co-precipitates, and (iv) massive stalactites composed of X-ray amorphous hydrous ferric oxide (HFO) and HFA co-precipitates and minor hydrous manganese oxide (HMO). The chemistry of the drip water was closely linked to the solid phase composition of the stalactites. HFA- and schwertmannite-rich straws formed at pH < 4.4, while HFO-rich stalactites precipitated at a higher pH (>6.6). The concentrations of As and other trace elements (namely Pb and Zn) in the drip water is controlled by the solubility of HFA and/or sorption affinity of these elements to the schwertmannite, HFO, and HMO phases. Drying out of the HFA straws may lead to recrystallization and rearrangements of HFA, which result in formation of kaňkite and chemically anomalous HFA domains enriched in Ca, K, Mn, Pb, S, and Zn. Analyses of the stalactite's microbial communities revealed autotrophic oxidation of Fe, As, and S as the main factors driving formation of secondary minerals. Contrasting communities were found in similar mineral assemblages of stalactites, suggesting the high variability of microhabitats within each stalactite.