An Overview of Authigenic Magnesian Clays

Abstract

Clay authigenesis mostly concerns: (a) the formation of clays by direct precipitation from solution, called "neoformation" and (b) development of clays by transformation of precursor minerals. Precipitation from solution implies that a new mineral structure crystallizes, so that a prior mineral structure is not inherited. Transformation of precursor detrital minerals, a process also termed "neoformation by addition", can be conducted whether throughout precipitation on pre-existing natural surfaces or transformation and reaction on pre-existing surfaces. Both processes have been recognized as effective mechanisms in the formation of Mg-clays, which mostly include 2:1 clay minerals, such as talc-kerolite and Mg-smectites, as well as fibrous clays (sepiolite, palygorskite). Authigenic Mg-clay minerals occur in both modern and ancient marine and non-marine depositional environments, although formation of these clays in hydrothermal continental and seafloor settings must be also outlined. Most favourable conditions for the formation of Mg-clays on earth surface are found in evaporitic depositional environments, especially where parent rocks are enriched in ferromagnesian minerals. In these settings, Mg-clays are important constituent of weathering profiles and soils and can form thick deposits of significant economic interest. Based on this review of authigenic clay deposits, we propose three geochemical pathways, mainly related to continental environments, for the origin of authigenic Mg-clays: formation of Al-bearing Mg-clays (pathway 1), formation of Al-free Mg clays (pathway 2) and formation of sepiolite from other Mg-clay minerals (pathway 3).

Spanish Bentonites: A Review and New Data on Their Geology, Mineralogy, and Crystal Chemistry

“…The Green Clays are sedimentary deposits that correspond to mud flat facies of the alluvial fans: the detrital dioctahedral minerals became unstable in such Mg-rich environments and underwent a weathering process that resulted in the formation of trioctahedral smectites (saponites). Small amounts of detrital micas still persist in the Green Clays together with minor amounts of other detrital minerals, such as quartz and feldspars ( [31,65,68,75,76], among others). The smectites that appear in the proximal areas of the alluvial fans in arkosic sediments, however, are dioctahedral [42].…”

Natural and Resource Potential of the Region as Part of Investment Resources: Evidence from the Ganja-Gazakh Economic Region of Azerbaijan

“…Clay and gilded deposits are mostly discovered in flat areas [17]. Local people have been using the raw material since ancient times to produce burnt bricks, water-pipes, ceramics [18].…”

Controls on Associations of Clay Minerals in Phanerozoic Evaporite Formations: An Overview

“…It is known that the tendency of slightly elevated magnesium content is inherent in almost all (except illite) clay minerals of evaporite deposits (e.g. [5,100]).…”

“…Allogenic aluminosilicates, brought into a hypersaline environment of an evaporite basin as terrigenous and pyroclastic material, were destroyed or transformed, giving rise to authigenic, neoformed and transformed clay minerals [1,2]. It is assumed that the vast majority of clay minerals of evaporites are authigenic [3][4][5]. The previous study of clays indicated that from Archean to recent times their potassium content was decreasing, and the sodium content was increasing from the Archean to Late Mesozoic, and then was decreasing [6,7].…”