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Over the past 15 years several expeditions by French, American and especially Italian cavers have unveiled over 50 caves in the Cordillera de la Sal (Atacama Desert, Northern Chile). Many of these caves contain a variety of speleothems and minerals, some of which have rarely been observed within karst systems. Most of the secondary deposits in these caves are composed of halite, but also other halide, carbonate, sulphate, nitrate, phosphate, and silicate minerals have been found. Among the sixteen cave mineral species recognized, atacamite, darapskite, blödite, leonite, anhydrite, and especially antarcticite are worth mentioning. In one of the samples an unknown Ca-Sr-bearing chloride mineral has also been discovered, but it has not been possible to carry out detailed mineralogical analyses. These often-rare minerals have formed in this region due to the very extreme hyperarid and salt-rich environment. This research reports the mineralogical results and proposes the genetical mechanisms leading to the formation of antarcticite, powdery anhydrite, and the paragenesis of the halite-darapskite-blödite. This study also shows that Atacama caves may be excellent analogues to study weathering processes and subsurface secondary minerals in hyperarid and hypersaline environments on Mars.

International audience; Guano is a typical deposit found in caves derived from the excretions of bats and in minor cases of birds. These organic deposits decompose and form a series of acid fluids and gases that can interact with the minerals, sediments, and rocks present in the cave. Over sixty phosphates are known and described from caves, but guano decay also often leads to the formation of nitrates and sulfates. In this study twenty-two European caves were investigated for their guano-related secondary minerals. Using various analytical techniques, seventeen phosphates, along with one sulfate (gypsum), were recognized as secondary products of guano decay. Among those minerals, some are very rare and result from the interaction of guano leachates with clays, fluvial deposits, or pyrite. Some of these minerals are even found only in the studied caves (spheniscidite, robertsite). The most common minerals belong to the apatite group. The common mineral association present in fresh decaying guano is brushite-ardealite-gypsum, minerals that usually are not present in older deposits because of their higher solubility. Most minerals are in hydrated form because of the wet cave environment; however, some specific dry conditions may favor the presence of dehydrated minerals, such as berlinite, formed during guano combustion. Investigation on the acidity of guano piles shows pH values as low as 3.5 with an increase of acidity with age and depth. Finally, cave guano deposits should be better studied in the future because of their role in paleoenvironmental and paleoclimatic reconstructions and because it is important to better understand the origin of guano-related minerals, especially the phosphates and sulfates. Among all of the caves studied, Corona ’e sa Craba (Italy) and Domica-Baradla Cave (Slovakia-Hungary) are considered to be outstanding sites with respect to their phosphate mineralogy.

Durante una visita alla grotta Eraldo, un’interessante crovassa a sviluppo verticale intercettata da una galleria mineraria della miniera di Barega, è stato ritrovato un minerale bianco depositato sulle pareti e su dei terrazzini del pozzo principale. Una sua successiva analisi al diffrattometro per polveri a raggi X ha appurato inequivocabilmente che si tratta del minerale argilloso Halloysite-10Å (Al2Si2O5(OH)4 • 2H2O). L’Halloysite appartiene al gruppo della Caolinite ed è conosciuto sia nella forma deidrata Al2Si2O5(OH)4 chiamata Halloysite-7Å e sia nella forma idrata, nota anche come Endellite, trovata nella grotta Eraldo. È abbastanza raro trovare tale minerale in un ambiente di grotta. Finora è stato segnalato nelle seguenti località: Mbobomkulu Cave (Sud Africa), Mishima-Fuketsu Cave (Giappone), grotta di Faggeto Tondo (Umbria, Italia), Carlsbad Cavern e Lechuguilla Cave (Nuovo Messico, USA), due grotte nel granito della Sierra Nevada (USA), in ambiente di grotta di miniera nel Kirghizistan ed in alcuni paleokarst nel Belgio, nel Nord della Francia e nella Repubblica Ceca. In tutti questi ambienti l’Halloysite sembra essersi formato da alterazione a bassa temperatura di rocce e/o sedimenti silicatici con il possibile locale intervento di soluzioni acide arricchite in H2SO4. Se questo meccanismo è responsabile anche della presenza di Halloysite nella grotta Eraldo, ciò avvallerebbe l’ipotesi di una produzione di acido solforico a causa dell’ossidazione di solfuri in ambiente supergenico.