Secondary minerals from halite caves in the Atacama Desert (Chile)

In the past 15 years several expeditions by French, American and especially Italian cavers have surveyed over 15 km of salt cave passages in the Cordillera de la Sal, close to San Pedro de Atacama village (Atacama Desert, Northern Chile). Over 50 caves have been explored up to now at an elevation around 2,500 m asl. These karst systems are characterized by in-cave temperature of around 17 °C and a relative humidity always very low, with a maximum of 15%. This extreme aridity is due to the severe conditions of the area with only a couple millimeters annual rainfall and several years without rain. Currently the rare precipitation events are enough to allow the dissolution of the salt rock and crusts, and the deepening of underground meandering river passages. Moreover, after the sporadic rain events, the water penetrating the cave’s host rock along fractures and bedding plains leads to the dissolution of primary minerals and allows the formation of seeping brines with dissolved salts. Both these processes selectively add solutes to the incoming undersaturated rainwater. The evaporation of these resulting salt-rich fluids at the cave atmosphere interface causes secondary minerals to precipitate. Mineral samples have been collected in eight caves, and include stalactites, flowstones, precipitates that form crusts in the streambeds and at the groundwater seeps, parietal coatings, earthy masses from the cave floors and efflorescence salts on ceiling rock outcrops. Most secondary deposits are composed of halite, but also other halides, carbonates, sulphates, nitrates, phosphates, and silicates have been discovered. Among the sixteen observed minerals, antarcticite, leonite, darapskite, blödite, atacamite and anhydrite are worth mentioning. The peculiar climate (extremely arid) and the very special environment dominated by NaCl and CaSO4, allow the crystallization primarily of halite. Atacamite was found where local enrichment in Cu (of hydrothermal origin) occurs, and antarcticite precipitates by the final evaporation of SO4-depleted brine (after early precipitation of anhydrite). Among sulphates, the metals necessary for the formation of these mineral species (magnesium, potassium, sulphate) derive from the cave sediments while nitrates are supplied by bird guano. Salt mineral precipitation is controlled by the temperature dependence solubility of the species in saline water, so that different secondary minerals were observed.