The nature and tropospheric formation of iberulites: Pinkish mineral microspherulites

Abstract

The circum-Mediterranean area has one of the highest dust accretion rates in the world. We have found pinkish mineral microspherulites (here referred to as iberulites), a new type of aerosol particle formed under special atmospheric conditions in periods corresponding to the highest levels of solid additions (summer). Because these particles are labile, they have gone unnoticed until now. Image analysis shows that these particles are spherical in shape, most commonly 60–90 μm in diameter, and show a typical depression (vortex). Iberulites are considered complex mineral assemblages with different hygroscopic characteristics and also contain biological remains (plants, silica shells, plankton and probably viruses). Their bulk mineralogy includes silicates, carbonates, sulfates, halides, oxides and phosphate-vanadates. This mineralogical composition indicates that likely source areas are the Sahara and Sahel for primary minerals (inherited), while other minerals are the result of atmospheric neoformation (gypsum and alunite-jarosite). We thus define an iberulite as a coassociation with axial geometry, constituted by well-defined mineral grains together with non-crystalline compounds, structured on a coarse-grained core and a smectite rind, with only one vortex and pinkish color, formed in the troposphere by complex aerosol–water–gas interactions. We suggest the aqueous interphase hypothesis as the mechanism for tropospheric formation of iberulites, mainly in summer: interactions between water droplets and aerosols create complex hydrodynamic conditions, causing possible collisions (wake and front capture) that produce the “precursor water droplets” of iberulites. Atmospheric processing is required from this stage to iberulite maturation. To date, we do not know to what extent these iberulites are present in time and space or whether they can serve as markers for environmental or paleoclimatic analyses or even lead to uncertainty in radiative transfer models.