Genesis of Quartz in Mediterranean Soils

Abstract

The sand fraction of soil (50-2000 μm) is assumed to be inherited from the parent rock and has historically been considered useful for the provision of information about the type of parent material and its degree of alteration. However, there is a gap regarding the potential of this fraction in pedogenetic studies, since it has rarely been used as an object of study. Quartz is an important component of soils and is concentrated in the sand fraction mainly due to inheritance, both as a clastic constituent from the parent rock by physical disintegration (fragmentation) or by dissolution (as occurs in carbonate rocks), as by aeolian inputs to the ground surface. Quartz is one of the chemically purest minerals in nature, after diamond and graphite. It contains, however, lattice bound trace elements in low concentrations due to atomic substitution of Si and interstitial elemental incorporation within the crystal structure. In addition, its high hardness and low solubility in water makes quartz a very stable mineral facilitating its enrichment in many sediments and clastic rocks. This enables it to collect and maintain features acquired by alteration (physical or chemical) and/or precipitation on its surface, which are used with great efficiency in environmental interpretations of sediments, soils and materials. This PhD Thesis evaluates the sand fraction, and the quartz contained in that fraction, in materials from Mediterranean area. Our work is based on soils belonging to a chronosequence of the fluvial terraces of the Guadalquivir River, and on aeolian materials deposited in the south of the Iberian Peninsula. The main hypothesis is that the sand fraction and its main constituent, mineral quartz, are indicators of the genesis and origin of soil materials and atmospheric dust from the Mediterranean environment. Mineralogical (X-ray diffraction -XRD-), elemental composition (major and minor including rare earth elements -REE-) (inductively coupled plasma atomic emission spectroscopy -ICP-AES-, inductively coupled mass spectrometry -ICPMS-, laser ablation inductively coupled plasma mass spectrometry -LA-ICP-MS, scanning electron microscope backscattered electron -SEM-BSE-, scanning electron microscopy–energy dispersive X-ray spectroscopy -SEM-EDX-, variable pressure scanning electron microscope equipped with an EDX detector, and with a structural and chemical analyzer by Raman spectroscopy -VP-SEM-EDX Raman-, high resolution transmission electron microscopy –HRTEM-) and morphological (SEM-EDX, SEM-BSE, scanning electron microscope cathodoluminescence -SEM-CL-, image anaysis –IA-, on mineral species, mainly quartz) studies are carried out. Mineralogy and elemental composition (major and minor including REE, and also geochemical indices of alteration/evolution) studies have demonstrated the suitability of the soil sand fraction as a provider of pedogenetic information in Mediterranean environments, allowing: a discussion of the origin of the different minerals it contains and, therefore of the soil material; and knowledge of the pedogenetic alteration suffered by this granulometric fraction and the rate at which its properties and alteration evolve through the establishment of chronofunctions. In addition, the study of the quartz trace element content as well as its crystalline structure, in the soil sand fraction of a chronosequence of the Guadalquivir River, have made it possible to establish both its lithological origin (six types of quartz have been distinguished: metamorphic quartz, undeformed granitic quartz, strongly altered granitic quartz, recrystallised –deformed- granitic quartz, sandstone quartz and hydrothermal quartz) as its provenance (Iberian Massif, Betic Cordilleras or Cenozoic Guadalquivir Basin). The granulometric and compositional characterization (mineralogy, major and minor chemical elements) of the atmospheric dust (and the iberulites contained in it) deposited in a Mediterranean environment provides information on its origin. Atmospheric dust samples were from both local and African sources. According to their mineralogy, the samples came from both the north-northwest of African and regional soil sources. Moreover, the morphology of the aeolian quartz grains is indicative of the origin and the alteration suffered in atmospheric transport. The quartz grains of African origin, with respect to those of local origin, have the following properties: a) greater presence of bulbous edges and upturned plates microtextures on the surface of quartz grains; and b) as regards the quartz shape parameters, minor values for area, perimeter, maximum feret, minimum feret, major axis, minor axis, and shape factor, and higher values for aspect ratio. All the above demonstrate the potential of the sand granulometric fraction as a pedogenesis informant in Mediterranean soils. In addition, they demonstrate the suitability of mineral quartz as an indicator of origin and alteration both in soils and in aeolian materials, being able to provide (by itself) information of great interest for Earth Sciences.