These are primarily salts that form black crust and efflorescences on the rock surfaces and accumulate in subsurface layers Thomachot and Jeannette ; Charola et al. The mechanism and path of these processes have been well known and explained e.
In the case of sandstones, the salt weathering usually leads to their disintegration due to an increase in intergranular pressure caused by growing salt crystals, hydration of salts Goudie and Viles ; Winkler and dissolution of silicates by saline fluids. Only few studies, however, focused on comparing the structures and compositions of weathering crusts developing in various environments but on comparable rock substrates Siedel and Klemm ; Bai et al. The present authors investigated secondary substances of crusts formed on sandstones influenced to various degrees by air pollutants. The aims of this paper are to study the damage mechanisms affecting lithologically and genetically similar sandstones in the natural outcrops and in the urban environment and to correlate the patterns and composition of the weathering crust with different components of atmospheric aerosol.
The analyses were conducted on the outermost parts of weathering crusts, black or dark grey in colour, developed on the surfaces of natural sandstone tors occurring within the Carpathian Foothill of the Polish Outer Flysch Carpathians Fig. Fenger ; Mira-Salama et al. Among dominating thin- and medium-layered strata are widely developed lithofacies of thick-bedded sandstones and conglomerates.
Many outcrops of these rocks were shaped during Quaternary into tors due to various denudation processes. Their cement is scarce, composed of clay minerals, sometimes of silica, occasionally of calcite. The sandstone complexes mentioned consist of very thick beds often above several meters and are characterised by high water absorbability, low or medium compression strength and very high frost resistance Bromowicz et al.
This lithological type of sandstones has been quarried within the Carpathian Foothill since the early Middle Ages. The Carpathian sandstones have been used in constructing works, as the rocks perfectly suitable for building purposes because of their good physical and chemical parameters.
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Thick bedding of the massive sandstone facilitated the exploitation. The exposed surfaces of the rock are covered with black crusts. Photos by Z. Current quarrying of these sandstones is limited. On the other hand, some of the quarries and most of natural exposures of sandstone tors are subject of protection as examples of various types of sandstones, elements of the natural landscape, and also due to the presence of their specific sedimentary structures and weathering processes Alexandrowicz , They have been protected separately as nine reserves and 38 monuments of the inanimate nature Fig.
In terms of lithology, the sandstones considered are similar. The lamination is chiefly associated with variable proportions and mineralogy of iron compounds and is a separate issue discussed in detail by Alexandrowicz et al.
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The outermost part of the weathering crust is often developed as a thin hard, black or dark grey layer. Because of its properties, chiefly the hardness associated with the presence of, e. It is also termed patina , whereas its analogues formed as a result of pollution on stony architectonic elements in urban areas are called false patinas Manecki et al.
A total of 30 samples were collected for the study, and the results of investigating 14 of them have been presented here. The samples selected reveal a distinctly developed black or dark grey outer layer affected by atmospheric factors and are usually clearly laminated underneath. General view and examples of black crust development of: a , d the Ethnographic Museum, b , e the Czartoryski Museum and c , f the Czapski Palace.
Photos: d by T. Sokal, e by A. The collection was also housed in neighbouring buildings: part of an old Piarist monastery and three tenement houses that were merged in the nineteenth century into a single Czartoryski Palace building. The Arsenal, or so-called the Small Monastery, and the Palace are linked by corridor-like, overhead passages. The single most valuable piece of art in the museum is the Lady with an Ermine by Leonardo da Vinci. The Czapski Palace also a branch of the National Museum was erected in At present, the building houses the European Centre of Polish Numismatics.
The samples from the Czartoryski Museum seven specimens were collected from plinths and window sills at a similar height, whereas those from the Czapski Palace four specimens from columns of a balcony. The samples were collected during the dry season, in June and July of , utilizing a restoration campaign.
Although the study was to be centred on the outermost sandstone layers, i. The black crust and the weathering zone are collectively termed the weathering crust. Depending on the local conditions, samples were collected with a hammer or a knife; their volume ranged from single to several tens cubic centimetres.
Microscopic observations including petrographic descriptions of natural unaltered rocks as well were carried out using optical transmission microscopy with an Olympus BX instrument and an Olympus DP digital camera. The samples were ground in an agate mortar.
Al 2 O 3 powder was used as the thermally inert substance. The chemical compositions were determined by applying different analytical methods at the Activation Laboratories Ltd. After a 7-day period, the samples were counted on a high purity Ge detector with resolution of better than 1. Using the flux wires, the decay-corrected activities were compared to a calibration developed from multiple certified international reference materials. Optical and scanning electron observations were made on universal polished thin sections cut perpendicular to stone surfaces.
Additionally, broken surfaces of outer parts were studied using the SEM method. The phase and chemical composition were established for samples of outermost parts of the sandstones layer reaching to ca. Due to a limited size of samples, it was impossible to conduct all determinations for each of them. All the rocks examined are medium-grained sandstones with very similar lithological characteristics.
Their grain framework comprises quartz, feldspars, rock fragments, micas biotite and muscovite and accessory minerals e. Feldspars are significantly altered by kaolinitisation and sericitisation. Also altered are the remaining, albeit less resistant aluminosilicates. The contact-porous cement is composed chiefly of clay minerals and iron oxyhydr oxides. It is locally enriched in carbonates, while glauconite aggregates were also found in some places.
Arid and Semi-Arid Geomorphology
The XRD analysis has revealed among clay components kaolinite, illite and probably illite-smectite mixed-layer minerals Fig. The outer, altered rock crusts a few centimetres thick are covered by a thin, hard, often discontinuous dark grey to black layer of variable morphology. As the identification of the components of this layer under an optical microscope was not possible because of their cryptocrystalline nature, the electron scanning microscope observations provided more information Fig.
The framework minerals of the outer zone are covered by a thin several to several dozen micrometers thick , amorphous film of a variable chemical composition, which includes C, Si, Al, Fe, P, Cl and K. Amorphous silica predominates among its components, accompanied by organic matter in various morphological forms, occasionally found grains of iron oxides and spherical particles of aluminosilicate glass from industrial emissions. Scanning electron microscope image of the weathering crust developed on the sandstone tor surface sample KB BC black crust, Fe iron oxyhydr oxides incrustation between quartz grains, Qz quartz.
Scanning electron microscope images of the sandstone surface: a , b from natural outcrops sample SMC and c , d from architectonic details sample ME Gp gypsum, Alum- K potassium alum, Hl halite, partial dissolution of mineral grains is also visible d. Immediately beneath the above described hard and black crust, there is a zone of sandstone altered by weathering Fig. Alexandrowicz et al. The layers closer to the surface of the weathering crust contain often major hematite and are richer in iron Fig.
Such stratification is a result of redistribution of iron during weathering. Iron released during the decomposition of primary aluminosilicates, and—to a lesser extent—of carbonates and sulphides, is transported towards the rock surface, where it precipitates and can undergo further transformations Alexandrowicz et al. Additionally, physical changes of rock-forming minerals have been noted in the weathering crust in the form of cracks in quartz grains and fissures running along the cleavage surfaces of feldspars.
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These cracks and fissures are filled with iron compounds, principally goethite and hematite, whose particles were also found within other minerals Fig. Microphotographs: sandstone from natural outcrops— a outer layer covered by ferruginous compounds sample SMC, 1P , b iron oxides between mineral grains sample SMC, 1P , and c iron compounds occurring in cleavage fissures of feldspar sample SMC, XP ; sandstone from architectonic detail— d black crust composed of gypsum covering the outer layer of the detail sample MC, 1P , e gypsum crystals filling pores and cracks, replacing stone cement sample MC, 1P , and f gypsum filling cleavage on feldspar sample MC, XP.
Optical microscope, thin sections cut perpendicular to the stone surface. The XRD analyses confirmed the presence of quartz, feldspars, clay and carbonate minerals Fig. It is a proof of minor amounts of the salt-group phases in the overall composition of the crust. The thermal curves obtained for the surface-closest crust samples point to a higher content of iron oxyhydroxides and a lower content of clay minerals than those of the inner parts of the rock Alexandrowicz et al.
The thermal effects of salt minerals have not been recorded. The precipitation of amorphous silica silicification leads to a hardening and sealing of the rock surfaces. The hydrolytic decomposition of less resistant aluminosilicate minerals, such as feldspars and biotite, which to a variable degree have been replaced by clay minerals and iron compounds, is the main source of the silica.
Dissolution of aluminosilicates is favoured by an acidic reaction within the rock, low pH being a result of weathering of sulphides and precipitating iron oxyhydr oxides, as well as intensive microbial activity or infiltration of acid waters from atmospheric precipitation Mcarthur et al.
The first two processes seem to be more significant in the sandstones studied Alexandrowicz et al. The dark grey to black colour of the outer layer can be associated with the presence of atmospheric dusts of industrial origin and also an activity of microorganisms, such as cyanobacteria, bacteria oxidising Fe and Mn, and melanin-producing fungi and actinomycetes Gaylarde et al. The presence of salts e.
The majority of anions necessary to form these minerals are probably associated with the effect of air pollutants e. The last process must be particularly important in the case of jarosite, whose formation requires a highly acidic environment Bingham and Nordstrom The alteration of aluminosilicate e. The supply of ions via capillary transport of groundwater is less likely.
The sandstones of the outer zones of architectonic details are generally loose and friable. Their surface is covered by a discontinuous, grey or black crust up to 0. Gypsum also fills the cracks in quartz grains and the fissures along the cleavage surfaces of feldspars, as well as spaces between flakes of micas.