MINERALOGICAL STUDY of STONE DECAY in CHARLES BRIDGE, PRAGUE SUMMARY Charles Bridge Over the Vltava River in Prague

MINERALOGICAL STUDY OF STONE DECAY IN CHARLES BRIDGE, PRAGUE

SULOVSKY, PETR; GREGEROVA, MIROSLAVA Dept. of Mineralogy, Petrology and Geochemistry, Masaryk University, Kotlarska 2, 611 37 Brno, Czech Republic; POSPISIL, PAVEL Technical University of Brno, Dept. of Civil Engineering, Veveri , 600 00 Brno, Czech Republic

SUMMARY Charles Bridge over the Vltava river in Prague (Czech Republic) belongs to the most valuable gothic monuments in central Europe. The still increasing degree of deterioration since the general bridge reconstruction in the 1970's made it imperative that a complex re-assessment of its condition was completed prior to the presumed reconstruction of the bridge. An integral part of this project has been petrological, mineralogical and geochemical study of the weathered building stone, reported in this paper. With respect to rock types used for building and repairs as well as to local variability of atmospheric conditions (temperature, wind, shading from sun or rain), the bridge is not a homogeneous structure. As a consequence, an assemblage of neo-formed minerals more variegated than any other earlier described in building object has developed there. The depth distribution of these minerals displays two main types. The more common one consists of a gypsum crust covering the stone, beneath which occurs a zone of highly porous and friable rock. In some places, the zoning is as follows (from the surface inwards): thin cover of iron oxidohydroxides - zone with jarosite I gelous silica cement - zone with gypsum cement - zone with iron oxidohydroxide occurrences. Similar zonation has until now only been reported from concretes. In places shaded from precipitation, efflorescences of many

soluble salts have been identified: rock salt, KN03, NaN03, feather alums and many other complex sulphates of K, Na, Al and Fe, some not yet described from nature. The impregnation of the stone surface with insoluble minerals leads to accumulation of high amounts of soluble salts beneath it and to the spallation of the indurated surficial layer, followed by pronounced mechanical erosion of freshly exposed surface. The source of constituents of neogenic minerals are portlandite and calcite (mostly in mortars), minerals containing alkalis (feldspars, mica) and iron (glaukonite, hematite, biotite). The stones and binding materials for repairs should be low in them.

1. INTRODUCTION

At first sight, the Charles Bridge appears to be a solid, homogeneous lithic structure. Close observation reveals a variety of materials used therein. During its history, the bridge has been damaged by several floods and wars. Stones and mortars used for repairs and reconstructions differed from the original ones in many respects, and have resulted in a considerable material diversity of the bridge. Owing to changes in their provenance, petrographic character (colour, structure, porosity, mineral composition etc.), composition of binding mortar (lime or cement) , age of embodiment in the bridge, their location in the structure, orientation with respect to wind , sun and other factors, individual ashlars display different stages of degradation. Some of the bridge arches suffered from the formation of fractures, running parallel to the longer axis of the bridge, and in some cases also cutting the ashlars crosswise. The scope of deterioration differs from arch to arch. Generally, 5 to 70% of their surface is to some extent corroded, the depth of total corrosion exceeding s mm in places. The weathering of the building stone of Charles Bridge has been studied by several authors (Konta 1988, Lang 1989, Sramek 1987). Nevertheless. the still increasing degree of deterioration since the general reconstruction in the 1970's made it imperative that a complex re assessment of its condition was completed prior to the presumed reconstruction of the bridge. An integral part of this study (Wiczany 1994) has been the mineralogical, petrological, geochemical (Sulovsky, Gregerova; Pospisil, Locker - Wiczany 1994.) and microbiological (Wasserbauer - ibid) study. 30

2. METHODS OF INVESTIGATIONS

For the purpose of petrographical and mineralogical study, 21 core samples (diameter of 20 and 35 mm, about 220 mm long) were taken. The selection of sampling points included all main rock types composing the bridge body, ashlars of different age of inclusion in the bridge structure and of different degrees of deterioration. Besides these samples, surface samples of efflorescences and crusts were also taken. The rocks from drill cores were studied in polished thin sections. Rock slices, cut along the core axis, document the surficial, most weathered layer of the stone (40 - 50 mm thick). For comparison, thin sections from deeper levels (from the depth of 100 - 150 mm) of the examined blocks were prepared and observed too. Moreover, a part of the outermost portion of each drillcore (the first 3 mm from the surface) was cut off parallel to the surface, mounted in epoxy resin and carefully lapped until the section's surface revealed grains of secondary minerals covering the rock surface and filling rock pores in the thin surficial layer. The same procedure was applied in case of efflorescences and crusts. The sections were polished and examined with an electron microprobe, together with the polished thin sections. Stone surfaces with the most highly developed efflorescences were also examined, but without mounting in resin, in a scanning electron microscope and identified by EDX spectra; their identification was complemented with XRD and IR spectroscopy.

3. PETROGRAPHIC CHARACTERISTIC OF BUILDING STONES USED

Among 21 samples taken from Charles Bridge, four types of sandstone were identified. The most abundant type is si/icarenite, composed of elastic quartz grains (92 - 98 %), smaller amounts of K feldspar and rock fragments of quartzite and metaquartzite, the matrix being formed of clay minerals, Fe-oxidohydroxides and silty quartz particles. Gypsum rarely occurs in the pores near the surface. Three samples of eleven silcarenites were strongly porous and friable. Green (glaukonite) sandstone has been used less frequently. It is macroscopically compact, gray-brown and greenish in colour, and has massive structure. In its composition, quartz (90 - 93 %) dominates over K-feldspar, glaukonite, and clay minerals. Binding material is mostly formed of silty quartz, and locally of clay minerals. Clastic psammitic grains are closely packed, empty pores are therefore rare. The subsurface zone is highly friable. The weathering crusts on these stones usually contains not only gypsum, but also jarosite. The least often used rock type is sandstone with Fe-oxidohydroxide cement. It is rusty-brown to dark brown, and very porous. This sandstone is highly friable. The clasts are formed predominantly of quartz (over 95 %), rock fragments of quartzite and metaquartzite. Typical feature is the basal Fe oxidohydroxide cement. Close to the ashlar surface, the cement remains preserved only in fragments. All above described sandstone types are Cretaceous, while the arcose sandstone to po/ymict conglomerate (provenance: mostly Kamenne Zehrovice), the second most common rock type used in 1 Prague for building purposes from the 14th till the beginning of the 20 h century (Sramek 1989), is of Carboniferous age. Among the elastic components, quartz grains (85 - 87 %) prevail over K-feldspar (locally up to 15 %), and minor portions of rock fragments (quartz porphyry, sericite schist, metaquartzite and mudstone), plagioclase, altered muscovite and biotite. The matrix consists of silty quartz, clay minerals, calcite, Fe-oxidohydroxides, and in subsurface zones sometimes gypsum.

4. CHARACTERISTIC FEATURES OF ROCK WEATHERING

In all the above rock types, the material of the surficial zone differs from that of deeper levels (15-20 cm) of the ashlars in colour, mineral composition, compression strength, in the degree of disintegration, porosity and character of pores. Feldspars in the sub-surface samples are strongly kaolinized and fractured (Sramek and Tolar 1987). The outer zone is usually friable, strongly weathered, with individual elastic grains easily spalling off the surface. Generally, the rock porosity is much higher there than inside 31 the ashlar (see Tab. 1). In some blocks, the depth porosity profile starts with a few millimetres of tightly packed layer encrusted with neogenic minerals, followed by a zone of extremely high porosity 3 - 15 mm thick (see Fig. 1 ). '

Table 1: Physical properties of selected rock samples

No Porosity Absorption Volume pH of No of bacte- Rock (mm3.g.1) capacity mass water riae x 103 x) (%) (kg.m-3) extract xx) 6 11, 1 1789 7,5 (0)- 251/0,5/13 sandstone w. 8,0 (I) limonite cement 8 80,8 386/1320/0,9 arcose sandstone to conglomerate 10 13,8(1) 3,5 2191 7,5 (0)- 1000/980/13 quartz sandstone 13,910) 8,5 (I) 11 34,3 (I) 5,0 (0) - 39/15/2 quartz sandstone 27,5 (0) 8,0 (I) 14 41,8 (I) 9,5 1798 5,5 (0)- 1/0,5/2 quartz sandstone 38,0 (0) 4,5 (I) 15 8,8 1838 4,5 (0)- 26/36/52 quartz sandstone 5,0 (I) 16 48 (I) 4,3 2210 glaukonite 34,9 (0) sandstone 17 7,0 (0)- 291/19/1462 sandstone w. 6,0 (I) limonite cement 18 6,9 1982 quartz sandstone 19 11 ,8 (I) 11 ,8 2011 4,5 (0)- 0,710,21- quartz sandstone 2,0 (0) 4,5 (I) 20 31 ,2 (I) 3,4 2235 glaukonite 32,5 (0) sandstone 21 46,9 (I) 6,7 2031 arcose sandstone 31 ,2 (0) to conglomerate

x> Porosity measurement performed with mercury porosimeter Carlo Erba AG60 (measurement range of the pore diameter 7500 - 7,5.10-3 mm) xx> Number of bacteriae present in 1 gramm of the rock taken from the depth of 2, 5, and 1O cm - data of VVasserbauer(1994) In the surficial parts, the pore spaces are often fully or partially filled with secondary minerals - gypsum and various other sulphates, gelous silica, chlorides, nitrates. The increase in content of insoluble neo formed minerals towards the surface and the opposite behaviour of soluble mineral species has been noticed already by Kaiser (1929) in the case of glaukonite sandstone of the Regensburg Cathedral. This trend was also confirmed by our observations. It is most pronounced in the case of weathering profiles with jarosite/silica - Fe oxidohydroxides - gypsum zonation, observed especially in glaukonite sandstone.

Mineralogy of weathering crusts The most surficial part of crusts is formed either solely of gypsum, or of complex sulphates of the jarosite-alunite group, interpenetrating or overlying the gypsum zone (Fig.1). The third most common neogenic mineral - amorphous silica - fills the pores or replaces the matrix, mostly in the less surficial parts. It has formed by decomposition of various minerals, above all feldspars: 2 2KAISi 0 + 2H• + S0 2- + H20 Al2Si20 s(OH)4 + 2K• + 4Si02 + S04 - 3 8 4 ~ lts occurrence is not so conspicuous as that of another abundant compound - calcium sulphate. According to the equilibrium calculations, it should precipitate in ambient conditions as dihydrate - 32 gypsum. The electron microprobe study of outer zones of stones used in Charles Bridge also revealed the presence of other two mineral forms - anhydrite (anhydrous calcium sulphate) and bassanite (calcium sulphate hemihydrate). Their presence can be explained by the high content of other salts dissolved in pore solutions, e.g. KCI or NaCl, which decreases the formation temperature of anhydrite from 66°C to almost 3ooc. This explanation is also supported by the finds of syngenite K2S04.CaS04.H20, which formed from pore solutions high in potassium. According to Van't Hoff (1912), if K2S04 concentration exceeds 3,3 milimole, gypsum transforms to syngenite; the process can be reversible. Syngenite was reported to occur in another Prague monument - St. Vitus Cathedral, this time as product of weathering of glass mosaic (Pe0ina, Chab, Jurek 1994) and in efflorescences on another bridge in Switzerland - Blauer (1992).

mm~ jarosite

~ Fe-oxidohydroxide

#.': fresh cement

J jarosite +silica zone with minor gypsun G gypswn zone I Fe-oxidohydroxide ± gypswn zone F fresh sandstone with minor gypsum

Fig. 1: Schematic view of the mineral zonation developed in ashlarsof ferruginous sandstones

Other sulphate minerals detected in surficial zones and efflorescences belong to several mineral groups with wide compositional ranges: simple hydrated sulphates of sodium, potassium and ammonium

(syngenite, hydrated glaserite); sulphates of monovalent (Na,K, NH4) and trivalent (Fe, Al) ions - amarillite, mendozite and their analogues; feather alums (pickeringite, halotrichite); minerals of the jarosite - alunite group, and other, yet unnamed minerals (see Tab. 2). Minerals of the jarosite-alunite group occur usually at the very surface of the stone, outlining, as the least soluble among observed neo-formed minerals, the current position of the evaporation front. Jarosite crust functions as moisture trap. In places sheltered from precipitation, the surficial crust composed of more or less insoluble minerals (if any) is overgrown with efflorescences of water-soluble minerals: nitrates (of K, less often Na and NH ), whisker-forming alums of the halotrichite group, up-to-now undescribed hydrated sulphates of alkalis a~d ferric iron and Al , and alkali chlorides. An overview of neo-formed minerals determined in the studied samples is given in Table 2 33

Table 2. Neo-formed minerals found in weathering crusts of stones in Charles Bridge

Mineral species Idealised Formula Empirical Formulae Identification method ~vine KCI A halite NaCl A nitrokalite KN03 D _IDJ>SUm CaS04.2H20 D bassanite 2CaS04.H20 A anhydrite CaS04 A opal Si02. nH?O B scawtite Ca5(Si30 9)2.CaC03. 2H20 A jarosite KFe3[(0H)d(S04)2] (NH4)0,01s(Ko.a2Nao.11)(Fe2. gAlo1 )[(OH)d(S04)2] c astrachanite Na2Mg[S04]2. 4H20 c,, s~enite K2Ca[_S04)2 . H20 A amarillite NaFe(S04)2 . 6H20 Na11(Feo.gsAlo,os)(S04)2. 6.4 H20 A unnamed Na2(Fe,Al)4[S04]7 . 16-18H20 Na21 MQ01(Fe3,4aA102a)[S04]1. 15,7 H20 c mineral X Na203( Fe405 Al om)[S04]7 . 18,5H20 Na 21 MQ01(F~ . 4aA102a)[S04]7 . 15,7 H20 3 Na2.14(Fe +3,aaA10.1a)(S04)7 . 18,2 H20 halotrichite group: c bflinite (Feo,31Nao,32MQ0,31 )(Fe,Al)2[S04]4 . 14.2H20 2 3 halotrichite Fe +Fe +2[S04]4 . 22H2 0 (Mgo,48Feo,34Nao,12)(Ah,1aFeo.22)[S04]4 . 17.8H20 2 pickeringite Fe +A12[S04]4. 22 H20 (Nao,49MQo,3aFeo,18)(Ah,9Feo,1)[S04]4 . 18. 7H20 K-halotrichite MgAl2[S04]4 . 22H20 (Mgo s3Feo32)Al19a[S04]4 . 17.2 H20 Na-bflinite K2Al2[S04]4 . 22 H20 (Mgo45Feo 2aNao25)(Alu9Feo 21)[S04]4 . 16.4 H20 Na2Fe3+ 2[S04]4 . 22 H20 (Mgo.3s Ko.31 Feo.21 )Ah 91[S04]4 . 17.6H20 glaserite (hydrated) ~Na[S04)2 . n H10 A ungemachite ~Na9Fe[OH/(SQ4)2h . 9H20 A c 1) tschermigite NH~l[S04]2 . 12H 2 0 mendozite NaAl[S04fa . 11 H20 (NH4)01(NaK,Mg)o3(Alo,s3Feo,oo)[S04)2 .11.4 H20 A (ammonium (NH4)0.aNao 2(Alo,9sFeom)[S04fa . 12.4 H20 mendozite) ungemachite ~Na9Fe3+ [0H/(S04)2]3 . 9H20 c humbersonite ~Na1Mg2[N03'(S04)3fa . 6H20 A Nf-!4. - kalinite Kx( NH4)1 .•Al[S04fa . 11 H20 . . ldent1f1cat1on methods used: A= EDX + WDX B = EDX + WDX, IRA C = EDX+WDX, XRD D = EDX+WDX, XRD, IRA

5. GEOCHEMICAL CONSIDERATIONS

The potential supply of elements composing the neogenic minerals can be: a) primary rock-forming minerals; b) mortars; c) chemical solutions leaking from the bridge body, d) atmospheric aerosol; e) water taken up from Vltava river; f) salts formerly used for de-icing of the bridge road (halite, urea) Alkalies some aluminium and a smaller proportion of calcium, released from altered feldspars and ' micas serve as source to secondary minerals precipitating in pores of the surficial layer. The markedly 34 increased gypsum impregnation of ashlar parts adjacent to mortar clearly indicate that the latter are the most important source of calcium. Owing to low content in the fresh rocks used, carbonates (calcite, dolomite) are only minor source of calcium. Iron is supplied either by remobilization of primary limonitic cement, iron disulfides, or, together with magnesium, from decomposition of mafic minerals (biotite, glaukonite). Anions necessary to compose the secondary minerals forming efflorescences on the rock surface, filling the pores, replacing the cement or pervading the rock along weakened layer boundaries are for the most part introduced from outside - by acid rain, aerosols, and road salting. Nevertheless, some part of sulfate and chlorine ions may have been present in the original fresh rocks: the S content in North Bohemian Cretaceous sandstones is 0.024%, Cl - 1O ppm (Konta ???). The origin of sulfur of the neo-formed sulfates was checked by determination of its isotopic composition. The various sulfates of efflorescences and crusts proved to differ only little in d34 (-3,2 to -4, 1). Of values characterising the Prague environment (precipitaiton - 3,5 %o, atmospheric aerosol - 2,9%o, air - 4,3 %o - Buzek 1990) the range observed in neo-formed sulfates is closest to that of precipitation. Present values of neo-formed sulfates, measured by the above author, are in good accordance with the results obtained from the gypsum of various Prague monuments ten years ago (Buzek 1990). The differences, though minor, are caused rather by varying sulfate concentration in analysed samples, than by the mineral chemical composition (sulfate assemblage) of the specimen. The lower d34 values were determined in samples with a rather low sulfate content, which can be explained by the fact they had more time for equilibration. The "health" of the building stone is closely related to the climate and its variations. The most important environmental factor controlling the weathering processes is water. The rock takes in water and releases it. Water acts as a leachant as well as the transport medium in the form of ionic and/or colloid solutions, supports microbial processes, and concurs in physical weathering processes (freeze-thaw effects etc.). In our case, the moisture content increases inwards: in the arch No. 4, in the depth of 2 cm it makes 10.2%, in the 5 cm depth level = 12.7%, in 10 cm - 14.07%. In the arch No. 13, the corresponding values are 7.6, 10.0, and 11 .8% , respectively. This may indicate that the moisture source is inside the bridge - uptake from the river by capillary action, seepage from the bridge road. The composition of the atmosphere in Prague varies strongly with climatic conditions. In the second half of this century, sulfur dioxide has usually been present in the atmosphere of the historical centre in concentrations exceeding 180 microgrammes per cubic metre. Peak values observed in the last decade were much higher, maintaining in periods of climatic inversion 500 - 800 g.m- (Eerveny et al. 1984). Concentration of nitrous gases averages 30 - 80 g.m-3. The total volume of emissions in Prague amounted in 1980 to 60.500 t of S02 and 23.060 t of NOx. These gases can enter the stone either directly, or with rain water. The precipitation total in the environs of the Charles Bridge is around 500 litres per square meter a year. The yearly average composition of rainwater is as follows: pH value = 3.8, N03 - 3.81 mg/I, S03 - 18.3 mg/I, c1- 1.05 mg/I, NH4 - 2.55 mg/I. Part of the chlorine present in the neogenic mineral assemblage, appears to be of atmospheric origin, as the concentrations of chloride ion as high as 0,25% were found in the stone of buildings even 50 meters above the ground (Kotlik et al. 1983). The acid pore solutions dissolve less resistant components of the matrix and elastic minerals of the sandstone and mortars, becoming enriched in alkalis, calcium, alumina, ferric oxide, and silica. Increase in acidity of pore solutions enhances decomposition of clayeous sandstone cement and some elastic minerals. Alteration of feldspar grains (kaolinization) starts at pH values = 4 - 5, leading to the release of colloid silica and alkali metals or earths. Evaporation of pore solutions at the stone surface in dry weather supports the osmotic and capillary movement towards the surface and solution thickening. The increase of ferric ions content in pore solutions containing sulphuric acid leads to the formation of a buffered system, with partial hydrolysis of iron compounds. Concentration increase at the thickening front leads to t~e precipitation of less soluble .basic sulphates of the jarosite group and of Fe-oxidohydroxides, representing the most stable form of Fe m such an environment (Borovec 1990): 3Fe2(S04)3 + 12H2 0~ 2(HFe(S04)2.2Fe(OH)3 + 5H2S04 35

In some places, the following zonation occurs (from the surface inwards): thin cover of iron oxidohydroxides - zone with jarosite I gelous silica cement - zone with gypsum cement - zone with iron oxidohydroxide occurrences - see Fig. 1. The mineral speciation is very similar to that described by Tazake, Mori, and Nonaka (1992) in a concrete sewage pipe. It is plausible that also in the case of Charles Bridge, bacteriae like Thiobacillus ferrooxidans may have played an important role in the formation of jarosite. Its presence defines the pH of the parent solution as being lower than 3, in cases of simultaneous occurrence with alunite » 3 (Rodriguez-Clemente and Hidalgo-Lopez 1984). Above this value, iron hydroxide precipitates. The 3 subsequent hydrolysis of Fe • in jarosite may also result in the formation of Fe(OHh It is possible, that Fe-rich zone occurring sometimes deeper inside the weathering profile may correspond to the former jarosite precipitation front. Temperature is another very important participating factor in physical weathering, and it substantially influences the types and rates of reactions in chemical weathering. The surface layers of stone blocks on the sunny side often reach much higher temperatures than usually expected (up to 50 °c - Hosek and Skupin 1978); progressive staining of the rock surfaces by mineral efflorescences, micro-organisms, soot and other solid particles of atmospheric aerosol promotes the temperature rise, thereby enhancing the reaction rates. Many authors - in case of Prague's monuments e.g. Sramek (1989) - held the effects of ice-water transitions to be most strongly influencing the level of stone degradation. Our results, supported by Lang 's (1989) determinations indicate the salt contents of pore solutions, especially in the surficial layer of stone blocks, may be high enough (0.8 - 1.1% sol . 0.1 - 0.6 % er) to decrease the freezing point of pore solution by at least 5 °c, as pointed out already by Niesel (1979). Together with effects of thermal radiation of river water and the bridge body mass, it can in relatively mild winters decrease the number of freeze-thaw cycles considerably. The commonest secondary minerals differ both mutually and from the rock-forming minerals in solubility (water solubility product of quartz = -2.6, amorphous silica = -4, gypsum = -4.12, alunite = -83.4), thermal properties (coefficient of thermal expansion of sulfates= 30 - 40.10-6, quartz 14.10-6, feldspars 12.1 o-6) and volume changes accompanying crystallisation and hydration/ dehydration. Pressures, induced by thermal expansion of crust minerals, crystallisation of ice and neo-formed minerals, rehydration of species partially dehydrated in dry seasons, and capillary action can reach values comparable to or exceeding the mechanical strenghth of the stone. Though the crystallisation pressures of the common neo-formed minerals and ice (theoretically ranging between 7 and 222 MPa - see e.g. Winkler E.M., Wilhelm E.J. 1970) as well as hydration pressures (according to Winkler 1987, the transition from calcium sulfate hemihydrate to dihydrate in ambient conditions invokes pressures exceeding 200 MPa) hardly achieve their theoretical values in reality (Everett 1962), they are often high enough (up to 12 MPa) to be deleterious to the integrity of the stone. In addition to them, the capillary pressures in the micropores completely filled with concentrated salt solutions can also achieve several Mpa. While the dehydration/rehydration of hydrous salts is a rather slow process, hygroscopic soluble salts may often repeatedly become deliquescent and, when relative humidity drops below the critical hygroscopic point, recrystallise. In effect, the combination of soluble salts within the surficial layer of the ashlars and the formation of a moisture trap by crust of insoluble salts covering them represents a severe threat. The precipitation of gelous silica (in some rocks jointly with basic sulphates of the jarosite-alunite group) and/or replacement of clayeous sandstone cement by opal CT may seem to be the least harmful process. Nevertheless, extensive closing of pore spaces has undesirable effect, as it causes increased capillary pressures, and more pronounced effect of pore solution crystallisation or gelation. The occurrence of varied sulphates, which must have formed under different conditions, can be explained by distinct changes in the rock environment (composition of pore solutions, great daily and seasonal variations of temperature etc.). Owing to the fact that the bridge is a very complex structure, we find places with widely varying physical conditions, depending on the block's position in the bridge construction, i.e. its orientation to the sun, whether the surface is sheltered from rain, and the distance 36 from river level and from paths draining rainwater collected at the road surface). The spectrum of mineral species depends also on the supply of elements available for neogenic mineral formation (controlled by the presence of rather easily decomposable minerals in the rock, and also by the accessibility to the acid rainwater intake), which varies from stone to stone. Globally, temporal and spatial variations of physico-chemical weathering conditions and the formation of secondary minerals lead to their uneven distribution. While gypsum crusts prefer to form in rather shaded places where the pore solutions thicken slower than in sunlit areas, in suitably sheltered niches highly soluble mineral species occur, too: halite. potassium chloride, nitrokalite. The source of soluble secondary minerals can be sought not only in the atmosphere (nitrogen), but also in road salting in broader environs (NaCl, KCI and urea sprinkled over nearby roads and streets may in dry spells be transported by wind and deposited on the stone surface).

6. CONCLUSIONS We have detected the formation of neogenic minerals at the surface of ashlars known to have been replaced in the bridge structure during the reconstruction in 1970's, indicating the weathering process is still running at rather high rate. It is therefore recommended that the most seriously damaged building stones are replaced as soon as possible. It is absolutely necessary to avoid using rocks containing minerals that may serve as source of secondary minerals formation, above all carbonates, feldspars and iron minerals. In addition to it, road salting should not be applied on the Charles Bridge or in its vicinity.

ACKNOWLEDGEMENTS The work has been a part of No. 103/93/1191 Project sponsored by the Grant Agency of the Czech Republic. The authors wish to express their thanks to F. Buzek of the Czech Geological Survey in Prague for providing analyses of sulphur isotopes.

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