The use of mineral interfaces in sand-sized volcanic rock fragments to infer durability

The use of mineral interfaces, in sand-sized rock fragments, to infer the influence exerted by durability on the generation of sediments, has been determined for plutoniclastic sand (e.g., Heins, 1995; Caracciolo et al., 2012; Weltjie et al., 2018). Conversely, for volcaniclastic sand, it has received much less attention, and, to our knowledge, this is the first attempt to make use of the volcaniclastic interfacial modal mineralogy of epiclastic fragments, to infer durability control at modern beach environment. Volcaniclastic sand was collected along five beaches developed on five islands (Alicudi, Filicudi, Salina, Panarea and Stromboli) of the Aeolian Archipelago, whereas one was sampled near the crateric centre of Stromboli volcano. Each sample was sieved and thin sectioned for petrographic analysis. The modal mineralogy of the very coarse, coarse and medium sand fractions was determined by point-counting of the interfacial boundaries discriminating 38 types of interfaces categories, both not-isomineralic and/or not iso-structural (e.g., phenocryst/glassy groundmass) and iso-mineralic, inside volcanic lithics with lathwork and porphyric textures. A total of 47386 interfacial boundaries have been counted and, the most representative series of interfaces, from the highest to the lowest durability, can be grouped as follows: a. ultrastable interfaces Plagioclase/Glassy groundmass»Pyroxene/Glassy groundmass»Olivine/Glassy groundmass»Plagioclase/Microlitic groundmass»Plagioclase/Pyroxene»Pyroxene/Opaque»Opaque/Glassy groundmass»Plagioclase/Opaque b. stable interfaces Pyroxene/Olivine»Plagioclase/Olivine»Hornblende/Glassy groundmass»Pyroxene/Microlitic groundmass»Opaque/Microlitic groundmass»Hornblende/Opaque»Olivine/Iddingsite»Hornblende/Plagioclase c. moderately stable interfaces Hornblende/Pyroxene»Iddingsite/Glassy groundmass»Olivine/Microlitic groundmass»Biotite/Glassy groundmass»Iddingsite/Pyroxene»Iddingsite/Plagioclase»Biotite/Opaque»Olivine/Hornblende and d. unstable interfaces Opaque/Olivine»Hornblende/Microlitic groundmass»Biotite/Plagioclase»Biotite/Pyroxene»Olivine/Biotite»Biotite/Hornblende»Rutile/Glassy groundmass»Rutile/Microlitic groundmass. Grains, eroded from the volcanic bedrock, if affected solely by abrasion, developed a rounded and smoothed form, with prevailing not-isostructural interfaces such as Plagioclase/Glassy groundmass, Pyroxene/Glassy groundmass and Olivine/Glassy groundmass interfaces. Grains that during transport suffered fracturing and percussion have a sharp and angular form: this combined transport mechanisms produce mainly iso-structural interfaces such as Plagioclase/Plagioclase, Pyroxene/Pyroxene, Hornblende/Hornblende, and, to a lesser extent, Biotite/Opaque and Biotite/Glassy groundmass interfaces.