Ferrantia • 44 / 2005 177Ferrantia • 44 / 2005 177Ferrantia • 44 / 2005

J. Schweigstillová et al. Salt weathering of Cretaceous sandstones in Northern Bohemia,Czech Republic

New investigations on the salt weathering of Cretaceous sandstones, Czech Republic

Jana SCHWEIGSTILLOVÁInstitute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic

V Holešovičkách 41, CZ-182 09 Prague 8Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic

V Holešovičkách 41, CZ-182 09 Prague 8Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic

fi ny@seznam.czV Holešovičkách 41, CZ-182 09 Prague 8

fi ny@seznam.czV Holešovičkách 41, CZ-182 09 Prague 8

Veronika ŠÍMOVÁ & David HRADILInstitute of Inorganic Chemistry, Academy of Sciences of the Czech Republic

CZ-250 68 ŘežInstitute of Inorganic Chemistry, Academy of Sciences of the Czech Republic

CZ-250 68 ŘežInstitute of Inorganic Chemistry, Academy of Sciences of the Czech Republic

hradil@iic.cas.cz

Keywords:Keywords: Aluminium; rainwater; sandstone; salt crust

Introduction

The results discussed previously show that the presence of calcium-rich salts in crusts from sandstone of Bohemian Cretaceous Basin does not necessarily indicate the reaction of water with calcite or lime; extra inputs of K+ and NH4

+ are not critically needful to form e.g. syngenite, boussin-

4critically needful to form e.g. syngenite, boussin-

4

gaultite or alums, found also in weathering crusts. The only element, concentration of that is principally insuffi cient in the pure rainwater, is aluminium (Soukupová et al. 2002).Because the matrix of North Bohemian sandstones is fairly kaolinitic, we discuss clay minerals as a probable source of aluminium. Within this work, we focus on the distribution of elements in the salt crust to demonstrate the reaction of the penetrating solution with the low pH values in the sandstone matrix, especially with clay minerals (kaolinite).

Crust stratigraphy and morphology

Selected samples of salt crusts were cast in a polyester resin and polished cross-sections were created. They were observed in a normal optical microscope and in a scanning electron microscope (SEM) with Energy Dispersive X-Ray Microa-nalysis (EDX).

SEM/EDX analyses confi rmed the presence of gypsum and alums in the salt crusts. They showed

that gypsum crystals appears homogenously in free pores as both effl orescence and subfl orescence but alums crystallize only locally in pores as subfl ores-cence (Fig. 1). It could indicate the local reaction of acid-sulphate solutions with Al-rich phases in the sandstone. Similarly to previous XRD results some amounts of Mg-sulphate (e.g. boussingaultite) were observed. Contrarily to XRD analyses, SEM/EDX analyses indicated the high content of P-rich phases in some cases together with aluminium abundance. Amorphous clay mineral P-alophane is also expected by Cílek et al. (1998), aluminium phosphates of the crandallite group are described from North Bohemian sandstones of the Cenom-

Fig. 1: Crystals of potassium alum KAl(SO4)2.12H2O in the pore space of the sandstone from Bohemi-an Switzerland, cross-section, normal visible light. Photo: J. Schweigstillová.

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J. Schweigstillová et al. Salt weathering of Cretaceous sandstones in Northern Bohemia,Czech Republic

anian age (e.g. Hradil & Hostomský 1999). Because of their reactivity, amorphous or poorly crystalline Al-rich phases could be locally very important source of aluminium to form alums, instead of clay minerals. Several distinct layers in the sample of opaline crust from Bohemian Paradise are visible in normal and Several distinct layers in the sample of opaline crust from Bohemian Paradise are visible in normal and Several distinct layers in the sample of opaline crust

UV light in the fi gures 2a and 2b, respectively. Each layer is fi nished by a thin cover of atmospheric UV light in the fi gures 2a and 2b, respectively. Each layer is fi nished by a thin cover of atmospheric UV light in the fi gures 2a and 2b, respectively. Each

impurities (dust). "Young" solutions migrate from layer is fi nished by a thin cover of atmospheric impurities (dust). "Young" solutions migrate from layer is fi nished by a thin cover of atmospheric

the pores of the stone through the "old" compact impurities (dust). "Young" solutions migrate from the pores of the stone through the "old" compact impurities (dust). "Young" solutions migrate from

layers causing their ruptures. More diffi cult the pores of the stone through the "old" compact layers causing their ruptures. More diffi cult the pores of the stone through the "old" compact

migration caused by the previous impregnation of layers causing their ruptures. More diffi cult migration caused by the previous impregnation of layers causing their ruptures. More diffi cult

stone leads to the eff ect of diff erent thickness of migration caused by the previous impregnation of stone leads to the eff ect of diff erent thickness of migration caused by the previous impregnation of

layers – "younger" (upper) layers are thinner. The places of infi ltration and evaporation are diff erent. layers – "younger" (upper) layers are thinner. The places of infi ltration and evaporation are diff erent. layers – "younger" (upper) layers are thinner. The

The migration of solutions through the sandstone places of infi ltration and evaporation are diff erent. The migration of solutions through the sandstone places of infi ltration and evaporation are diff erent.

before their evaporation is evident. The migration of solutions through the sandstone before their evaporation is evident. The migration of solutions through the sandstone

Layered profi le of the salt crust on the sandstone is clearly visible in fi gure 3 where the layer of Layered profi le of the salt crust on the sandstone is clearly visible in fi gure 3 where the layer of Layered profi le of the salt crust on the sandstone

gypsum (Fig. 3: 2, 3) is overlapped by younger is clearly visible in fi gure 3 where the layer of gypsum (Fig. 3: 2, 3) is overlapped by younger is clearly visible in fi gure 3 where the layer of

layer of syngenite (Fig. 3: 1, 4).gypsum (Fig. 3: 2, 3) is overlapped by younger layer of syngenite (Fig. 3: 1, 4).gypsum (Fig. 3: 2, 3) is overlapped by younger

Conclusions

Scanning electron microscopy with EDX helped to answer the question of aluminium origin. The Scanning electron microscopy with EDX helped to answer the question of aluminium origin. The Scanning electron microscopy with EDX helped

reaction of penetrating low pH acid-sulphate to answer the question of aluminium origin. The reaction of penetrating low pH acid-sulphate to answer the question of aluminium origin. The

solutions with Al-phosphates or even clay reaction of penetrating low pH acid-sulphate solutions with Al-phosphates or even clay reaction of penetrating low pH acid-sulphate

minerals in the sandstone matrix is leading to solutions with Al-phosphates or even clay minerals in the sandstone matrix is leading to solutions with Al-phosphates or even clay

a local formation of alums seems to be evident minerals in the sandstone matrix is leading to a local formation of alums seems to be evident minerals in the sandstone matrix is leading to

from the elemental composition in the salt cross-section. It confi rms our previous assumption from the elemental composition in the salt cross-section. It confi rms our previous assumption from the elemental composition in the salt cross-

that aluminium derives from local source, based section. It confi rms our previous assumption that aluminium derives from local source, based section. It confi rms our previous assumption

on theoretical reaction model (Schweigstillová and Hradil, in press). The stratigraphy of neo-on theoretical reaction model (Schweigstillová and Hradil, in press). The stratigraphy of neo-on theoretical reaction model (Schweigstillová

formed layers of salt crusts clearly indicates the and Hradil, in press). The stratigraphy of neo-formed layers of salt crusts clearly indicates the and Hradil, in press). The stratigraphy of neo-

penetration of "younger" solutions through the formed layers of salt crusts clearly indicates the penetration of "younger" solutions through the formed layers of salt crusts clearly indicates the

older generation of salts.penetration of "younger" solutions through the older generation of salts.penetration of "younger" solutions through the

Acknowledgments

This work was supported by the research plan of the Institute of Rocks Structures and Mechanics This work was supported by the research plan of the Institute of Rocks Structures and Mechanics This work was supported by the research plan of

AS CR (A VOZ3046908). Janka Hradilová from the Academy of Fine Arts in Prague is acknowledged for her help with photos.Academy of Fine Arts in Prague is acknowledged for her help with photos.Academy of Fine Arts in Prague is acknowledged

a

b

Fig. 2: Opaline (SiO2.nH2O) crust from on the sandstone from Bohemian Paradise with many layers covered by atmospheric dust. a. Under normal visible light (magni-fi cation 250×). b. Under UV light (magnifi cation 250×). Photo: J. Hradilová.

Fig. 3: Sulphate crust on the sandstone from Bohemian Paradise with two different sulphate layers, cross-sec-tion, SEM image, back-scattered electrons. 1,4. Synge-nite K2Ca(SO4)2.H2O. 2,3. Gypsum CaSO4.2H2O.Photo: V. Šímová.

2Photo: V. Šímová.

2Ca(SOPhoto: V. Šímová.

Ca(SO4Photo: V. Šímová.

4)Photo: V. Šímová.

)2Photo: V. Šímová.

2 2Photo: V. Šímová.

2

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J. Schweigstillová et al. Salt weathering of Cretaceous sandstones in Northern Bohemia,Czech Republic

References

Hradil D. & Hostomský J. 1999. - Dissolution kinetics of natural kaolinites at low pH sulfuric acid solutions – an example from Stráž pod kinetics of natural kaolinites at low pH sulfuric acid solutions – an example from Stráž pod kinetics of natural kaolinites at low pH sulfuric

Ralskem mineral deposit. Acta Universitatis Carolinae, Geologica 43: 537–543.

Soukupová J., Hradil D. & Přikryl R. 2002. - Chemical weathering of clay – rich sandstone

matrix: controls and case studies, in Přikryl R. & Viles H. A. (eds.), Understanding and Managing Stone Decay. Proceeding of the Inter-national Conference Stone Weathering and Atmospheric Pollution Network (SWAPNET 2001), pp. 263–271. Karolinum Press.

Schweigstillová J., Hradil D. (in press). - Salt formation on the Cretaceous sandstones in the North and Northwest Bohemia.

Les résultats établis précédemment montrent que la présence de sels riches en calcium en croûtes dans le grès du bassin crétacé de Bohème n’indique pas néces-sairement la réaction de l’eau avec la calcite ou la chaux. Des additions supplémentaires de K+ et de NH4+ ne sont pas strictement nécessaires pour former par exemple la syngenite, la boussingaultite ou les aluns - l’eau de pluie est suffi sante. Le seul élément, dont la concentration est en principe insuffi sante pour former des aluns très abondants, est l’aluminium. Nous avons évoqué des minéraux des argiles dans la matrice du grès comme source locale potentiel de cet élément.

Un exemple, nous nous sommes concentrés sur la distri-bution des éléments dans la croûte de sel pour démontrer la réaction de la solution pénétrante dans la matrice du grès à valeur de pH plus basique, particulièrement avec de la kaolinite. Des échantillons choisis de croûtes de sel ont été moulées dans une résine de polyester et des sections polies transversales ont été créées. On les a observés au microscope optique et au microscope d’élec-

trons à balayage (SEM) avec microanalyse au rayon X (EDX).

L’analyse EDX a confi rmé la présence de gypse et d’aluns dans les croûtes de sel, mais également a indiqué une teneur élevée de phosphore dans certains cas ainsi que l’abondance d’aluminium. Des phosphates d’aluminium n’ont jamais été identifi és auparavant par diff raction de rayon X sur poudre. Basé sur la distribution des éléments dans les pores observé par l’intermédiaire de SEM/EDX, une substitution directe de minéraux argileux par des aluns semble possible. Ceci confi rme des hypothèses postulées antérieurement par des modèles théoriques de ce� e réaction. Le gypse cristallise à la surface ou dans les pores vides. Plusieurs couches distinctes ont été trouvées sur l’échantillon de la croûte d’opale, chacune d’elles se termine par un fi lm mince d’impuretés atmosphériques (poussières). La stratigraphie des couches d’opale néo-formées indique clairement une pénétration d’une solution «plus jeune» à travers les sels de génération antérieur.

Nouvelles études sur la désagrégation de sel du Crétacé de Bohême, République Tchèque

Résumé de la présentation

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