Heavy minerals distribution and provenance in modern beach sands of Campania, Italy

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

“…Those grains relating to weathering have been termed epiclastic, and, according to Garrels and MacKenzie (1971), volcanogenic rocks form 26% of the total sedimentary mass and, of this, a significant part is volcanic sand. In the Aeolian islands, chemical weathering and leaching potential operate at low rate (Garzanti andAndò 2007a, 2007b) but, conversely, steep gradients in rocks and soils, sparse vegetation cover and strong winds, are favorable to a physical weathering of the source rocks producing epiclastic sand mainly in the range of very coarse, coarse and medium grain-sizes (e.g., Critelli et al 1993;Morrone et al 2017Morrone et al , 2018) with a detrital mineral composition strongly sizedependent for both light (Morrone et al 2017) and heavy minerals (Garzanti and Andò 2007a) within the sandy fraction.…”

“…Moreover, these authors state that impact shattering may cause effective mineralogic disaggregation of smaller fragments like sand and silt as the length of intercrystalline boundaries approaches the diameter of these smaller grains. In a previous study on volcaniclastic sand on the Aeolian archipelago (Morrone et al 2018), the quantitative roundness analysis for the Lipari Island beach sand, indicates that mechanical preservation potential for this sedimentary environment is Lvv > Lvf > Lvl > Lvmi [(volcanic lithic grains with vitric (Lvv) > felsitic (Lvf) > lathwork (Lvl) > microlitic (Lvmi) textures], for volcanic lithic grains of the medium sand fraction. The comminution of more brittle vitric (Lvv) lithic grains may result in grain fracture, splintering them into fragment, whereas felsitic (Lvf), lathwork (Lvl) and microlitic (Lvmi) volcanic particles, are primarily influenced by rounding through abrasion, with roundness categories ranging from 3 (sub-rounded grains) to 6 (well rounded grains).…”

“…Volcanic lithic grains of the Aeolian islands sand are characterized by glass-rich interfaces and the dominant nature of the source rocks, including phenocrysts-rich basaltic, basaltic andesitic and andesite lava flows and pyroclastic rocks, and minor dacites, latites and rhyolites , have certainly an influence on the compositions, textures and abundance of these interface types. However, in addition to the source lithotypes controls on sand composition (e.g., Heins and Kairo 2007), abrasion and breakdown of the grains, during the transit from ephemeral stream channels to beach environment (e.g., Mack 1978;Le Pera and Critelli 1997;Morrone et al 2017Morrone et al , 2018, should be considered to understand the mechanical durability of these no-isostructural interfaces, and which of them are preferentially retained or removed at beach environment.…”

“…Deduction of interface properties from analysis of natural sediments is very often difficult (e.g., Weltje 2012;Weltje et al 2018) and, even more for volcaniclastic lithic grains which are highly sensitive to chemical alteration (e.g., Marsaglia 1992;Johnsson et al 1993; Le Pera and Morrone 2018) and to destruction by transport (Cameron and Blatt 1971;Davies et al 1978;Cather and Folk 1991;Morrone et al 2018) before the particles are buried by later pyroclastic events or reworked into the marine record (e.g., Morrone et al 2017Morrone et al , 2020. The use of mineral interfaces in sand-sized rock fragments derived from effusive and pyroclastic source rocks has received much less attention, probably because they, especially their glassy component, are strongly dependent by chemical weathering (e.g., Johnsson et al 1993), such as the very active and high rate of glass hydrolysis (e.g., White et al 1986), glass breakdown and transformation to clay minerals (e.g., Lowe 1986;King 1986) or to both analcime and palagonite (e.g., Saetre et al 2018Saetre et al , 2019.…”

“…Modal mineralogy analysis on the types of mineral interfaces (e.g., Heins 1995), preserved in rock fragments of modern volcaniclastic Aeolian islands beach sand, has been carried out as a complementary research integrating, and further refining, existing petrographic analyses of both light framework grains (Critelli et al 1993;Morrone et al 2017Morrone et al , 2018 and heavy detrital minerals (Garzanti andAndò 2007a, 2007b) from an undissected volcanic arc setting sand (e.g., Dickinson 1985) such as the Aeolian magmatic arc.…”