Distribution of secondary minerals in crusts developed on sandstone exposures

Abstract

Previous research on rock weathering crusts has revealed their large variability depending on the type of host rocks and development of weathering processes. The composition of crusts developed on natural sandstone exposures is less documented in the literature in comparison to those developed on architectonic stones. In both cases, previous research has focused mainly on the progress of salt weathering. This study considers the surfaces of sandstone tors in the Polish Outer Carpathians. The exposed parts of the rocks in this area are often covered by crust, which is up to several centimetres thick, and differs from the internal part in colour and composition. The crusts were characterized using light and electron microscopy, X‐ray diffractometry, thermal analyses, Mössbauer spectroscopy, bulk chemical analyses and sequential chemical extractions. Porosity was estimated by digital image processing. The following two hardened zones were observed: (1) thin (up to 30 µm), black, external layer, rich in carbon and composed of opal‐type silica, covered in places by sulphate incrustations and numerous spherical particles of anthropogenic origin; (2) thicker (up to several millimetres), internal part composed of a set of laminae of variable colouration, enriched in iron (oxyhydr)oxides (goethite and hematite) in comparison to the rock interior. Development of the crust results from silicon and iron redistribution during the sandstone alteration. The chief source of silica is hydrolysis of aluminosilicates, whilst that of iron is decomposition of aluminosilicates, carbonates and sulphides. Hematite is probably a result of goethite transformation. However, air pollutants may play an important role in the formation of sulphates. Silica and iron compounds affect the properties of the rock, hardening the surface and lowering porosity by formation of secondary cement. Crystallization of sulphate salts, in turn, may contribute to mechanical disintegration of the rock. Copyright © 2013 John Wiley & Sons, Ltd.

Pickeringite from the Stone Town Nature Reserve in Ciężkowice (the Outer Carpathians, Poland)

“…Granular aggregates of glauconite occur as a minor admixture. Spherical microaggregates of pyrite framboids and/or their alteration products were sometimes observed [38,52]. The average porosity of the Ciężkowice sandstones from the Stone Town tors is 6.78% [52], reaching 12.0% in the weathered varieties [38].…”

“…The weathered surface of the sandstone tors is covered in places by hard crusts composed mainly of iron minerals (among others hematite Fe 2 O 3 and goethite FeOOH) and various salts that may also occur in the form of very thin layers as subflorescences beneath the surface [52,53] [30,52,54,55]. The efflorescences sometimes cover wider surfaces of the tors, as their crystallization and preservation depend on the atmospheric conditions and rock exposition.…”

“…In case of the Ratusz tor sandstone, such minerals include the mica group, mainly biotite, and feldspars (plagioclases). Kaolinite, illite, glauconite, as well a small amount of the interstratified illite-smectite group minerals identified in the matrix of the Ciężkowice sandstones, can also provide these cations [38,51,52].…”

The impact of rock climbing on the micro-relief of sandstone surfaces: The case study of the Carpathian tors

“…The natural protective layer, irregularly covering the surfaces of Carpathian tors, consists of a hard weathering crust, usually 1-2 cm thick, and -more rarely -several centimetres thick (Alexandrowicz and Pawlikowski, 1982;Rzepa et al, 2011;Alexandrowicz et al, 2012Alexandrowicz et al, , 2014Marszałek et al, 2012). It is composed of very thin laminae, macroscopically differing in colour, running either as a continual or broken layer, in parallel to the rock surface.…”

“…In natural conditions, it enables the successive regeneration of damaged surfaces by developing a new protective crust. The weathering crust and co-existing weathering micro-forms are at a different stage of development, thus testifying to the continuity and progress of the process of transformation of the rock surface (e.g., Robinson and Williams, 1994;Cílek, 1997;Turkington and Paradise, 2005;Mikuláš, 2007b;Robinson, 2007;Alexandrowicz et al, 2014). The measurements demonstrated that the particular height-zones of the tors showed differences in the thermo-humidity regime of the exposed surfaces (Alexandrowicz and Brzeźniak, 1989).…”