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Distribution changes of Mn2+ and Fe3+ on weathered marble surfaces measured by EPR spectroscopy
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Atmospheric Environment 38 (2004) 3617–3624
Distribution changes of Mn2+ and Fe3+ on weathered marble surfaces measured by EPR spectroscopy Kyriaki Polikreti*, Yannis Maniatis Laboratory of Archaeometry, Institute of Materials Science, N.C.S.R. ‘‘Demokritos’’, Aghia Paraskevi Attikis 153 10, Greece Received 4 August 2003; received in revised form 25 February 2004; accepted 15 March 2004
Abstract The behavior of Mn2+ and Fe3+ on weathered marble surfaces was studied by electron paramagnetic resonance spectroscopy (EPR). The paper is concentrated in Mn2+ and Fe3+ replacing Ca2+ in the CaCO3 of marble. Nineteen marble samples of different types and exposed to different environmental conditions were analyzed: Samples of various grain sizes and manganese concentrations, samples from quarry fronts, excavations and monuments exposed in rural or polluted environments. Mn2+ does not show a systematic behavior but Fe3+ decreases up to 100% on the surface, compared to the marble bulk. The depletion starts at a depth around 4 mm from the outer surface. The phenomenon is explained by dissolution and re-crystallization of calcite together with microbe-mediated reduction of Fe3+ to Fe2+. Higher decrease is observed on samples of low-grade metamorphosis, on excavated samples and samples exposed to heavily polluted urban environment. Unexpectedly, this depletion extends deeper than the actual weathering depth (measured by SEM), i.e. the decrease in Fe3+ concentration extends beyond the patina–marble interface, up to a few millimeters into the healthy marble. This additional depletion may be explained by solid-state diffusion of iron. Such a hypothesis has already been reported in literature for other ions in calcite crystals. r 2004 Elsevier Ltd. All rights reserved. Keywords: Marble; Weathering; EPR spectroscopy; Ion distribution; Iron; Manganese
1. Introduction The majority of studies on marble weathering use total concentrations of trace elements to describe ionic balance on marble surfaces, i.e. concentrations of ions in the CaCO3 lattice together with ions in impurities, diagenetic or contaminants (Del Monte et al., 1987; Margolis and Showers, 1988; Ulens et al., 1995; Moropoulou et al. 1998; Polikreti and Maniatis, 2003). This macroscopic approach results in useful, practical weathering models but leaves the actual *Corresponding author. E-mail address: [email protected] (K. Polikreti).
alterations induced in ‘‘healthy’’ marble lattices poorly understood. The present work aims to reveal and understand the ionic changes induced in the crystal lattice of marble surface layers, in atomic scale and estimate the contribution of these changes to weathering rates and final condition of the crystalline material. The technique of electron paramagnetic resonance spectroscopy is ideal for this purpose, due to its capability to distinguish ions in different environmental symmetries and record their concentration changes in various weathering conditions. EPR spectroscopy has been used for 15 years in our laboratory, on marble characterization and provenance investigation (Polikreti and Maniatis, 2002),
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.03.048
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but has never been used before in marble weathering studies. The manganese and iron balance on marble surfaces is mainly affected by the following factors: ionic concentrations of the marble itself, dissolution–re-crystallization cycles, deposition and infiltration of clay minerals and iron oxides and biological action. Manganese (Mn2+) and iron (Fe3+) ions, typically replace calcium (Ca2+) in the CaCO3 lattice. Mn2+ fits into the calcite lattice more easily without causing significant disruption, because its radius (0.91 A˚) is closer to that of Ca2+ (1.00 A˚). Fe3+ (ionic radius: 0.67 A˚) on the contrary, causes much more crystallographic distortion and prefers to enter non-lattice sites in calcite instead of substitutional ones (Rimstidt et al., 1998). Re-crystallization results in newly formed calcite crystals, which are expected to show lower impurity concentrations than the marble ones (Bathurst, 1979). Water penetration causes re-crystallization but also helps manganese and iron to infiltrate in marble surfaces in different oxidation states and crystal environments, usually in the form of manganese oxides and iron oxides or hydroxides in clay minerals (Camuffo et al., 1982). On the other hand, the presence of lichens, fungi, algae or bacteria, accelerates the infiltration of water and crystal dissolution by mechanical (root penetration) and chemical action (dissolution of the grains in the acidic environment created by the microorganisms) (Diakumaku et al., 1995). Besides, microbial oxidation or reduction of Fe and Mn are common phenomena on rock surfaces (Brown et al., 1994). All the previous factors create a very complicated net of simultaneous, not independent processes, which result in ionic changes. In marble weathering literature, ion concentration changes are usually reported in terms of total concentrations. Mn and Fe occasionally show dramatic increase in the outer altered marble layers. Ulens et al. (1995) for example, report a change in total iron from 30ppm in the bulk to 11 000 ppm on the outer surface. The profile of total Mn and Fe concentrations is typically mapped by electron dispersive X-ray (EDX) microanalysis under a scanning electron microscope (SEM) (Kouzeli et al., 1996). Proton induced X-ray emission spectroscopy (PIXE, Margolis and Showers, 1988) and laser induced breakdown spectroscopy (LIBS, Maravelaki et al., 1997) have also been used in measuring total concentrations. However, no attempt has been made to distinguish manganese or iron originating from the marble bulk from that at the alteration layer. We have to note here that the previous methods cannot be used alternatively to EPR spectroscopy. On one hand, the concentrations of Mn and Fe in white, well-crystallized healthy marbles (20–200 ppm for Fe) are below the detectable limit of EDX microanalysis. On the other hand, it has been reported that the use of Q-switched Nd:YAG laser in marble cleaning, oxidizes Fe2+ to Fe3+ (Eichert et al., 2000).
2. Experimental 2.1. Samples Nineteen marble samples showing various types of weathering were analyzed by EPR spectroscopy in an attempt to check if different types of weathering affect differently ionic distribution. The samples were collected from the sample bank of the laboratory which numbers more than 1500 marble samples by now. A short description of marble type and weathering characteristics of the analyzed samples are given in Table 1. The description includes information obtained by X-ray diffraction analysis and SEM of the surface samples, i.e. the main mineralogical constituents found in the crust, the micro morphology and traces of biological action. Powder from the weathered layer and the healthy marble bulk (2 cm below surface) were analyzed for each sample of Table 1. For the samples of appropriate dimensions, the detailed profiles of ion concentration from the bulk to the weathered surface were drawn. A cylindrical specimen (diameter of 2 cm, thickness of several centimeters depending on the sample) was drilled out of the marble and then cut in slices (thickness of 2 mm). Powder of each slice, obtained by agate mortar, was analyzed by EPR spectroscopy, resulting in the profiles given in the following. This procedure restricts the number of sampled depths compared to direct drilling of marble powder. But we avoided drilling because the drilling action induces high local pressures that distort lattice symmetries, move ions out of their sites and alter EPR spectra (Maniatis and Mandi, 1992). 2.2. EPR measurements The EPR instrument used is a BRUKER ER-200DSRC with an NMR Gaussmeter ER-035 M and a microwave frequency counter Anritsu MF76A. The microwave frequency is 9.5 GHz (X-Band). The quantity of the sample needed is 220 mg. More details on the experimental procedure may be found at Polikreti and Maniatis (2002). Figs. 1a–c show typical EPR spectra of Mn2+ in calcite (Kikuchi and Matarese, 1960), Fe3+ in calcite (Marshal and Reinberg, 1963) and Fe3+ in oxides and silicates, correspondingly (Pinnavaia, 1980). Fe2+ in relevant-to-calcite lattice symmetries cannot be detected by EPR spectroscopy. Intensity values of the measured peaks are expressed in relative units and are measured by the height of the first peak derivative, normalized to a gain of 1 106. Relative units actually mean centimetres, measured on the plotted spectrum. In the case of Mn2+, the intensity corresponds to the first low-field doublet
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Table 1 Marble type and weathering characteristics for the Sample code
Location-environment
(a) Quarry front samples EP1 Ancient quarry front on Pentelikon mountain EP2 Quarry front on Pentelikon mountain EP3 Quarry front on Pentelikon mountain EP4 Quarry front on Pentelikon mountain EP5 Ancient quarry front on Pentelikon mountain YM7 Quarry front on Hymettus mountain YM13 Quarry front on Hymettus mountain MU1 Quarry front at Thassos Murgena (countryside) D6 Quarry front at Doliana (countryside) (b) Ancient samples NA2 Temple of Athina Althea, Peloponnese (building block – countryside) ME24 Ancient Mesimvria, Thrace (excavated object) ME20 Ancient Mesimvria, Thrace (excavated object) ME34 Ancient Mesimvria, Thrace (excavated object) (c) Samples from building stones KP6 Kapnikarea church, Athens, (building block-facing south) KP12 Kapnikarea church (building block-facing west) KP27 Kapnikarea church (building block-facing east) KP30 Kapnikarea church (building block-facing east) KP67 Kapnikarea church (interior) KP69
Kapnikarea church (interior)
Marble type
Surface micro-morphology and composition
White, medium grained, Pentelic marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble Light gray, fine grained, Hymettian marble Light gray, fine grained, Hymettian marble White, medium grained, dolomitic, Thassian marble White, medium grained, Doliana marble (local)
Pitting–lichens–oxalates Crust of dead and alive lichens and oxalates Pitting–lichens–oxalates Pitting–lichens Sugar-like surface–pitting Pitting. Clay aggregates Thin layer of clay mineral depositions Lichens, fungi. Clay aggregates Thin, black crust covering a brown layer
White, medium grained, Doliana marble (local)
Clay mineral depositions. Fungi, lichens
White, coarse grained, Proconnesian marble White, coarse grained, Proconnesian marble White, medium grained, uknown origin
Sugar-like surface. Clay mineral depositions penetrating the bulk Sugar-like surface. Clay mineral depositions penetrating the bulk Thin layer of clay mineral depositions
White, medium grained, Pentelic marble Light gray, fine grained, Hymettian marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble Light gray, fine grained, Hymettian marble
Thin layer of clay mineral depositions covering a gypsum layer Thin gypsum layer
of the Mn2+ sextet (h1 in Fig. 1a). For spectrometer tuning procedures and stability tests, an internal laboratory standard was used: a calcitic marble sample from the famous ancient quarries of Pentelikon mountain. The total error of peak height measurements was calculated at 10%. This includes both measurement reproducibility and spectrometer stability variation.
Heavily weathered surface with gypsum and clay minerals Black crust Thin gypsum layer Thin gypsum layer
3. Results and discussion 3.1. Mn2+ and Fe3+ distribution changes In order to avoid confusion when referring to ions in various lattices, we are going to use the term ‘‘substitutional’’ for ions replacing Ca2+ in the CaCO3 lattice and ‘‘non-calcite’’ for ions in clay minerals and oxides.
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Fig. 1. Typical EPR spectrum of calcitic marble: (a) Mn2+ sextet due to Mn2+ replacing Ca2+ in the CaCO3 lattice. (b) The peak at g ¼ 4:31 is due to Fe3+ in silicates and oxides and the four rest peaks are due to Fe3+ replacing Ca2+ in the CaCO3 lattice. (c) The peak at g ¼ 2:1 is due to Fe3+ in silicates and oxides. The triangles show the positions of the Mn2+ sextet peaks.
Comparative results for substitutional Mn2+ and Fe3+ peak intensities obtained by EPR spectroscopy, for bulk and surface samples are given in Table 2. Fig. 2 shows the percentage of change for each ion on the surface. The samples in Fig. 2 are classified in three groups depending on the environmental conditions they are exposed to: urban, rural and burial conditions. The intensity of Mn2+ does not show any systematic change or tendency. The observed changes do not seem
to depend on the bulk concentrations or the grain sizes of the samples. In samples exposed to urban environment, Mn2+ shows a tendency to decrease, while for quarry samples exposed to rural environment it shows small but not systematic variations. On the contrary, the concentration of substitutional Fe3+ clearly decreases in the surface layers, in percentages from 7% to practically 100%. The highest decrease is observed in the two excavated, ancient pieces, ME20 and 24. The surface of these samples shows a sugar-like texture, with several loose grains and clay depositions penetrating up to 3 mm. The main weathering factor, which has caused this dramatic depletion of iron, seems to be the dissolution and consequent re-crystallization of CaCO3. Re-crystallization typically causes a reduction of the elemental contaminants in solid solution, relative to the primary crystals (Bathurst, 1979). The newly formed calcite is expected to show lower substitutional Fe3+ concentrations. Recrystallized calcite is the most typical mineral on excavated marble surfaces (Margolis and Showers, 1988), in abundances depending on the presence and acidity of the underground water. The third excavated sample (ME34) shows a lower depletion but it is far more well preserved than the other two, bearing only a thin layer of clay depositions and no signs of extended re-crystallization. The samples exposed to urban environment also show severe iron depletion, especially KR6, 12, 27 and 30, which are covered by thick gypsum crusts. Gypsum crusts need wet surfaces to develop (Camuffo, 1998), a condition indicating again dissolution and re-precipitation cycles. Samples from rural environment (EP) follow next, showing moderate depletion. These samples show traces of biological action (bodies, hyphaes, etc. of the microorganisms), another factor that accelerates dissolution. Additionally, the microbe-mediated reduction of Fe3+ to Fe2+ may cause further substitutional Fe3+ decrease, but unfortunately Fe2+ cannot be detected by EPR spectroscopy (at least in this crystal symmetry). In the case of iron-reducing bacteria for example, the reduction takes place on the biofilm surfaces, where the iron ions are absorbed. Fe3+ ions are released from marble by local micro-dissolution and subsequently taken up by the bacteria (Brantley et al., 2001). The samples from the mount Hymettus, although exposed in a rural environment, show a very high value in iron depletion, most probably due to the low grade of metamorphosis of the marble and presence of numerous veins and impurities, which accelerate dissolution and microbial action. The samples from Pentelikon mountain show dramatic depletion when exposed in urban environment compared to those exposed to rural environment. The dolomitic sample (MU1) shows practically no depletion due to the substantially different rates of dissolution of calcite and dolomite (Martinez and White, 1999). Dolomitic samples have also been
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Table 2 Concentration changes and depletion depths for Mn2+ and Fe3+ on the analyzed marble sample surfaces Sample code
EP1 EP2 EP3 EP4 EP5 YM7 YM13 MU1 D6 NA2 ME24 ME20 ME34 KP6 KP12 KP27 KP30 KP67 KP69
Mn2+ in calcite (r.u.)a
Fe3+ in calcite (g ¼ 14:25)(r.u.)a
Bulk
Surface
Bulk
Surface
2250 3360 5437 6600 2500 45 88 184 56 387 32 26 1056 4031 182 3100 3175 3000 93
1890 3750 4921 5093 3140 67 100 143 47 415 40 27 988 2087 390 2680 325 2500 85
8.4 17.2 20.5 14.1 7.7 8.3 8.0 1.7 7.0 0.3 12.9 7.3 5.0 16.6 12.0 8.1 8.0 6.2 10.3
2.6 14.0 7.7 8.7 4.4 1.2 1.0 1.5 4.0 0.2 0.1 0.1 2.0 3.2 3.0 1.3 0.1 4.6 4.1
Fe3+ depletion depthb (mm)
Alteration depthc (mm)
Fe3+ in silicates (g ¼ 2:1)(r.u.)a
4 4 4 4 5 3 3 0 — 4 — 7 — — — 4 — 4 —
0.25 2.00 0.15 2.00 0.15 0.02 0.05 0.05 0.03 0.10 2.00 3.00 0.03 0.15 0.05 0.12 1.00 0.05 0.10
90 265 0 514 0 180 475 875 1000 800 150 328 320 5320 1746 1800 3875 0 3400
a
Values expressed in relative units of EPR peak intensities. Estimated from the EPR profiles (dashes correspond to small samples inadequate for profile drawing). c Measured by SEM. b
found more resistant to fungal attack than the calcitic ones (Bogomolova et al., 2003). Obviously, humidity (groundwater, atmospheric humidity, etc.) is the critical environmental factor in substitutional Fe3+ depletion occurring on marble surface layers. As for the type of marble, we can only say that dolomitic marbles show increased resistance to depletion compared to the calcitic ones and that wellcrystallized marbles also show higher resistance. 3.2. Substitutional Fe3+ depletion depths The maximum depth at which substitutional Fe3+ is observed was estimated by drawing detailed EPR peak profiles from the marble bulk to the surface. The profile of three representative marble pieces is presented in Fig. 3. Together with the values of substitutional Fe3+ peak, the Mn2+ and non-calcite Fe3+ peak intensities were also drawn for comparison. The first sample (Fig. 3a) is a weathered quarry front from the Pentelikon mountain, the second one is an excavated piece (Fig. 3b) and the third one a building stone from a Byzantine church in the center of Athens, Greece (Fig. 3c). The quarry sample (EP4) and the building element (KR27) show a decrease in Mn2+ concentration (25% and 45% correspondingly), while the excavated sample (ME20) shows no decrease. The change (if any) is evident mainly
in the outer 2 mm.The depletion of substitutional Fe3+ though extends in larger depths, up to around 4 mm for the quarry front and the building stone and around 7 mm for the excavated sample. At the same time, noncalcite Fe3+ shows a rapid increase in the outer 3 mm of the weathered marble surface. The depletion depths for the rest of the samples are given in Table 2. No value is given for very small or disintegrating pieces, where no profile could be drawn. The depletion in Fig. 3 extends deeper than the point where the increase of non-calcite Fe3+ starts. But it should not extend further than the marble alteration depth. If we accept that the only possible depletion mechanisms are all types of physicochemical process that cause chemical, mineralogical or mechanical alteration to the surface layers, this depletion depth should determine the marble ‘‘patina’’ thickness. In our samples, this alteration depth was estimated by SEM and given in Table 2. These depth values represent the maximum distance from the outer surface, at which foreign particles, re-crystallization products, biological remnants, loose crystals or other types of alteration can be identified under the SEM. However, none of these values is higher than 2 mm, while Fe3+ depletion extends up to 4 mm and in one occasion up to 7 mm from the surface. This observation means that substitutional Fe3+ also depletes in healthy marble crystals.
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substitutional Mn
2+
7000
18
Mn peak intensity (r.u.)
Fe peak intensity (r.u.)
6000 16 5000 14 4000
12 3+
3000 2000
8
non-calcite Fe
6
3+
1000
4 0.0
0.2
0.4
(a)
0.6
0.8
1.0
1.2
1.4
1.6
0 1.8
Distance from surface (cm) 10
substitutional Mn
2+
35 30
8 25
7 6
20
3+
substitutional Fe
5
15
4 3
non-calcite Fe
2
10
3+
2+
3+
Fe peak intensity (r.u.)
9
Mn peak intensity (r.u.)
3+
10
2+
substitutional Fe
5
1 0
0
0.0
0.2
0.4
(b)
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Distance from surface (cm) 3500
substitutional Mn
Re-crystallization of CaCO3 or microbe induced Fe3+ reduction cannot be considered valid interpretations at a depth of a few millimeters in an undisturbed crystalline material. A possible explanation for this additional depletion could involve diffusion of Fe3+ through the marble bulk. Dry calcite was considered static under the Earth’s ambient surface conditions till recently, but it has been demonstrated on single crystals (Stipp et al., 1997; Stipp, 1998), that monovalent ions from the calcite bulk (Na+, K+ and Cl), move out via solid-state diffusion and accumulate in small crystals on the surface. According to these results, the trace ions were at equilibrium when they were trapped at the time of calcite formation. Now at Earth’s surface conditions of temperature and pressure, they are no longer stable, so they move out by solid-state diffusion, along trajectories where defects are in higher proportion, through the single crystal to the surface, where their free energy is lower.
2+
2000
8
1500 1000
3+
non-calcite Fe
4
substitutional Fe
500
3+
0
0
0.0
(c)
2+
2500
12
Mn peak intensity (r.u.)
3000
3+
Fig. 2. Surface change of substitutional (a) Mn2+ (b) and Fe3+ concentration, measured by EPR spectroscopy. Different environments of exposure are indicated. Reproducibility of measurements: 10%.
Fe peak intensity (r.u.)
16
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Distance from surface (cm)
Fig. 3. Profile of Mn2+ and Fe3+ with distance from the weathered marble surface: (a) Weathered quarry front (sample EP4); (b) ancient excavated piece (sample ME20) and; (c) building element exposed to urban environment (sample KR27).
If we suppose that a similar process is valid for Fe3+ in marble, the following hypothetical model would apply: iron is entrapped in Ca2+ sites during metamorphosis. However, it does not form a stable carbonate in the +3 oxidation state because Fe3+ (0.67 A˚) has a quite small radius and higher charge than Ca2+, causing significant crystallographic distortion. Consequently, at Earth’s surface conditions this sensitive equilibrium is destroyed and Fe3+ ions start diffusing through the lattice, in a search for lower free energy positions, that is
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the surface, grain boundaries and micro fractures. At their new positions, the iron ions probably form oxides, with a lattice structure more suitable for trivalent ions. If we suppose that the diffusion rates are in the scale of nanometers in weeks or months, as reported for Cd2+ (0.95 A˚) and Zn2+ (0.74 A˚) (Stipp et al., 1998), we can explain why the phenomenon can be detected by EPR spectroscopy only on surfaces exposed for very long periods. We have to note here that the depletion has never been observed on samples kept in the lab for up to 10 years. However, the presence of iron oxides at the grain boundaries would have been detected by EPR. We should have an increase at the iron oxide EPR peaks, which was not observed. That means either that the hypothesis is not valid or that it needs modification. The problem is that, to isolate the hypothetical diffusion effect, we need marble surfaces exposed for very long periods but not weathered. We could of course accelerate the phenomenon by heating the samples, but we might trigger other processes this way. For short periods when the depletion will have reached a depth of nanometers or microns, another high-resolution technique like auger electron spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS) might be more suitable. As a last comment, we have to note here that the same phenomenon of additional depletion was not observed also for manganese ions probably because Mn2+ forms a nearly complete solid solution with calcite. It fits into the calcite lattice more easily without causing significant disruption, because its radius is closer to that of Ca2+.
4. Conclusions The results presented in here prove that EPR spectroscopy is a very useful tool in measuring ionic changes on weathered marble surfaces. One of the advantages of the technique is the discrimination between ions hosted in different lattices. The use of the EPR spectroscopy on different types of weathered marble surfaces illustrates the behavior of Mn2+ and Fe3+ replacing Ca2+ in the CaCO3 of marble. Mn2+ may decrease or increase on the surface, but without any obvious trend. On the contrary, Fe3+ shows a dramatic depletion, varying from 20% to 100% of the bulk value. The phenomenon is explained by dissolution and re-crystallization of calcite together with microbe-mediated reduction of Fe3+ to Fe2+. Higher decrease is observed on excavated samples and samples exposed to heavily polluted urban environment. This depletion extends deeper than the actual weathering depth as measured by SEM. The results show that the depletion extends up to a few millimeters inside the healthy marble. More work is ahead to be done in order to verify if this additional depletion is a result of solidstate diffusion or not.
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Distribution changes of Mn2+ and Fe3+ on weathered marble surfaces measured by EPR spectroscopy Kyriaki Polikreti*, Yannis Maniatis Laboratory of Archaeometry, Institute of Materials Science, N.C.S.R. ‘‘Demokritos’’, Aghia Paraskevi Attikis 153 10, Greece Received 4 August 2003; received in revised form 25 February 2004; accepted 15 March 2004
Abstract The behavior of Mn2+ and Fe3+ on weathered marble surfaces was studied by electron paramagnetic resonance spectroscopy (EPR). The paper is concentrated in Mn2+ and Fe3+ replacing Ca2+ in the CaCO3 of marble. Nineteen marble samples of different types and exposed to different environmental conditions were analyzed: Samples of various grain sizes and manganese concentrations, samples from quarry fronts, excavations and monuments exposed in rural or polluted environments. Mn2+ does not show a systematic behavior but Fe3+ decreases up to 100% on the surface, compared to the marble bulk. The depletion starts at a depth around 4 mm from the outer surface. The phenomenon is explained by dissolution and re-crystallization of calcite together with microbe-mediated reduction of Fe3+ to Fe2+. Higher decrease is observed on samples of low-grade metamorphosis, on excavated samples and samples exposed to heavily polluted urban environment. Unexpectedly, this depletion extends deeper than the actual weathering depth (measured by SEM), i.e. the decrease in Fe3+ concentration extends beyond the patina–marble interface, up to a few millimeters into the healthy marble. This additional depletion may be explained by solid-state diffusion of iron. Such a hypothesis has already been reported in literature for other ions in calcite crystals. r 2004 Elsevier Ltd. All rights reserved. Keywords: Marble; Weathering; EPR spectroscopy; Ion distribution; Iron; Manganese
1. Introduction The majority of studies on marble weathering use total concentrations of trace elements to describe ionic balance on marble surfaces, i.e. concentrations of ions in the CaCO3 lattice together with ions in impurities, diagenetic or contaminants (Del Monte et al., 1987; Margolis and Showers, 1988; Ulens et al., 1995; Moropoulou et al. 1998; Polikreti and Maniatis, 2003). This macroscopic approach results in useful, practical weathering models but leaves the actual *Corresponding author. E-mail address: [email protected] (K. Polikreti).
alterations induced in ‘‘healthy’’ marble lattices poorly understood. The present work aims to reveal and understand the ionic changes induced in the crystal lattice of marble surface layers, in atomic scale and estimate the contribution of these changes to weathering rates and final condition of the crystalline material. The technique of electron paramagnetic resonance spectroscopy is ideal for this purpose, due to its capability to distinguish ions in different environmental symmetries and record their concentration changes in various weathering conditions. EPR spectroscopy has been used for 15 years in our laboratory, on marble characterization and provenance investigation (Polikreti and Maniatis, 2002),
1352-2310/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.atmosenv.2004.03.048
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but has never been used before in marble weathering studies. The manganese and iron balance on marble surfaces is mainly affected by the following factors: ionic concentrations of the marble itself, dissolution–re-crystallization cycles, deposition and infiltration of clay minerals and iron oxides and biological action. Manganese (Mn2+) and iron (Fe3+) ions, typically replace calcium (Ca2+) in the CaCO3 lattice. Mn2+ fits into the calcite lattice more easily without causing significant disruption, because its radius (0.91 A˚) is closer to that of Ca2+ (1.00 A˚). Fe3+ (ionic radius: 0.67 A˚) on the contrary, causes much more crystallographic distortion and prefers to enter non-lattice sites in calcite instead of substitutional ones (Rimstidt et al., 1998). Re-crystallization results in newly formed calcite crystals, which are expected to show lower impurity concentrations than the marble ones (Bathurst, 1979). Water penetration causes re-crystallization but also helps manganese and iron to infiltrate in marble surfaces in different oxidation states and crystal environments, usually in the form of manganese oxides and iron oxides or hydroxides in clay minerals (Camuffo et al., 1982). On the other hand, the presence of lichens, fungi, algae or bacteria, accelerates the infiltration of water and crystal dissolution by mechanical (root penetration) and chemical action (dissolution of the grains in the acidic environment created by the microorganisms) (Diakumaku et al., 1995). Besides, microbial oxidation or reduction of Fe and Mn are common phenomena on rock surfaces (Brown et al., 1994). All the previous factors create a very complicated net of simultaneous, not independent processes, which result in ionic changes. In marble weathering literature, ion concentration changes are usually reported in terms of total concentrations. Mn and Fe occasionally show dramatic increase in the outer altered marble layers. Ulens et al. (1995) for example, report a change in total iron from 30ppm in the bulk to 11 000 ppm on the outer surface. The profile of total Mn and Fe concentrations is typically mapped by electron dispersive X-ray (EDX) microanalysis under a scanning electron microscope (SEM) (Kouzeli et al., 1996). Proton induced X-ray emission spectroscopy (PIXE, Margolis and Showers, 1988) and laser induced breakdown spectroscopy (LIBS, Maravelaki et al., 1997) have also been used in measuring total concentrations. However, no attempt has been made to distinguish manganese or iron originating from the marble bulk from that at the alteration layer. We have to note here that the previous methods cannot be used alternatively to EPR spectroscopy. On one hand, the concentrations of Mn and Fe in white, well-crystallized healthy marbles (20–200 ppm for Fe) are below the detectable limit of EDX microanalysis. On the other hand, it has been reported that the use of Q-switched Nd:YAG laser in marble cleaning, oxidizes Fe2+ to Fe3+ (Eichert et al., 2000).
2. Experimental 2.1. Samples Nineteen marble samples showing various types of weathering were analyzed by EPR spectroscopy in an attempt to check if different types of weathering affect differently ionic distribution. The samples were collected from the sample bank of the laboratory which numbers more than 1500 marble samples by now. A short description of marble type and weathering characteristics of the analyzed samples are given in Table 1. The description includes information obtained by X-ray diffraction analysis and SEM of the surface samples, i.e. the main mineralogical constituents found in the crust, the micro morphology and traces of biological action. Powder from the weathered layer and the healthy marble bulk (2 cm below surface) were analyzed for each sample of Table 1. For the samples of appropriate dimensions, the detailed profiles of ion concentration from the bulk to the weathered surface were drawn. A cylindrical specimen (diameter of 2 cm, thickness of several centimeters depending on the sample) was drilled out of the marble and then cut in slices (thickness of 2 mm). Powder of each slice, obtained by agate mortar, was analyzed by EPR spectroscopy, resulting in the profiles given in the following. This procedure restricts the number of sampled depths compared to direct drilling of marble powder. But we avoided drilling because the drilling action induces high local pressures that distort lattice symmetries, move ions out of their sites and alter EPR spectra (Maniatis and Mandi, 1992). 2.2. EPR measurements The EPR instrument used is a BRUKER ER-200DSRC with an NMR Gaussmeter ER-035 M and a microwave frequency counter Anritsu MF76A. The microwave frequency is 9.5 GHz (X-Band). The quantity of the sample needed is 220 mg. More details on the experimental procedure may be found at Polikreti and Maniatis (2002). Figs. 1a–c show typical EPR spectra of Mn2+ in calcite (Kikuchi and Matarese, 1960), Fe3+ in calcite (Marshal and Reinberg, 1963) and Fe3+ in oxides and silicates, correspondingly (Pinnavaia, 1980). Fe2+ in relevant-to-calcite lattice symmetries cannot be detected by EPR spectroscopy. Intensity values of the measured peaks are expressed in relative units and are measured by the height of the first peak derivative, normalized to a gain of 1 106. Relative units actually mean centimetres, measured on the plotted spectrum. In the case of Mn2+, the intensity corresponds to the first low-field doublet
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Table 1 Marble type and weathering characteristics for the Sample code
Location-environment
(a) Quarry front samples EP1 Ancient quarry front on Pentelikon mountain EP2 Quarry front on Pentelikon mountain EP3 Quarry front on Pentelikon mountain EP4 Quarry front on Pentelikon mountain EP5 Ancient quarry front on Pentelikon mountain YM7 Quarry front on Hymettus mountain YM13 Quarry front on Hymettus mountain MU1 Quarry front at Thassos Murgena (countryside) D6 Quarry front at Doliana (countryside) (b) Ancient samples NA2 Temple of Athina Althea, Peloponnese (building block – countryside) ME24 Ancient Mesimvria, Thrace (excavated object) ME20 Ancient Mesimvria, Thrace (excavated object) ME34 Ancient Mesimvria, Thrace (excavated object) (c) Samples from building stones KP6 Kapnikarea church, Athens, (building block-facing south) KP12 Kapnikarea church (building block-facing west) KP27 Kapnikarea church (building block-facing east) KP30 Kapnikarea church (building block-facing east) KP67 Kapnikarea church (interior) KP69
Kapnikarea church (interior)
Marble type
Surface micro-morphology and composition
White, medium grained, Pentelic marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble Light gray, fine grained, Hymettian marble Light gray, fine grained, Hymettian marble White, medium grained, dolomitic, Thassian marble White, medium grained, Doliana marble (local)
Pitting–lichens–oxalates Crust of dead and alive lichens and oxalates Pitting–lichens–oxalates Pitting–lichens Sugar-like surface–pitting Pitting. Clay aggregates Thin layer of clay mineral depositions Lichens, fungi. Clay aggregates Thin, black crust covering a brown layer
White, medium grained, Doliana marble (local)
Clay mineral depositions. Fungi, lichens
White, coarse grained, Proconnesian marble White, coarse grained, Proconnesian marble White, medium grained, uknown origin
Sugar-like surface. Clay mineral depositions penetrating the bulk Sugar-like surface. Clay mineral depositions penetrating the bulk Thin layer of clay mineral depositions
White, medium grained, Pentelic marble Light gray, fine grained, Hymettian marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble White, medium grained, Pentelic marble Light gray, fine grained, Hymettian marble
Thin layer of clay mineral depositions covering a gypsum layer Thin gypsum layer
of the Mn2+ sextet (h1 in Fig. 1a). For spectrometer tuning procedures and stability tests, an internal laboratory standard was used: a calcitic marble sample from the famous ancient quarries of Pentelikon mountain. The total error of peak height measurements was calculated at 10%. This includes both measurement reproducibility and spectrometer stability variation.
Heavily weathered surface with gypsum and clay minerals Black crust Thin gypsum layer Thin gypsum layer
3. Results and discussion 3.1. Mn2+ and Fe3+ distribution changes In order to avoid confusion when referring to ions in various lattices, we are going to use the term ‘‘substitutional’’ for ions replacing Ca2+ in the CaCO3 lattice and ‘‘non-calcite’’ for ions in clay minerals and oxides.
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Fig. 1. Typical EPR spectrum of calcitic marble: (a) Mn2+ sextet due to Mn2+ replacing Ca2+ in the CaCO3 lattice. (b) The peak at g ¼ 4:31 is due to Fe3+ in silicates and oxides and the four rest peaks are due to Fe3+ replacing Ca2+ in the CaCO3 lattice. (c) The peak at g ¼ 2:1 is due to Fe3+ in silicates and oxides. The triangles show the positions of the Mn2+ sextet peaks.
Comparative results for substitutional Mn2+ and Fe3+ peak intensities obtained by EPR spectroscopy, for bulk and surface samples are given in Table 2. Fig. 2 shows the percentage of change for each ion on the surface. The samples in Fig. 2 are classified in three groups depending on the environmental conditions they are exposed to: urban, rural and burial conditions. The intensity of Mn2+ does not show any systematic change or tendency. The observed changes do not seem
to depend on the bulk concentrations or the grain sizes of the samples. In samples exposed to urban environment, Mn2+ shows a tendency to decrease, while for quarry samples exposed to rural environment it shows small but not systematic variations. On the contrary, the concentration of substitutional Fe3+ clearly decreases in the surface layers, in percentages from 7% to practically 100%. The highest decrease is observed in the two excavated, ancient pieces, ME20 and 24. The surface of these samples shows a sugar-like texture, with several loose grains and clay depositions penetrating up to 3 mm. The main weathering factor, which has caused this dramatic depletion of iron, seems to be the dissolution and consequent re-crystallization of CaCO3. Re-crystallization typically causes a reduction of the elemental contaminants in solid solution, relative to the primary crystals (Bathurst, 1979). The newly formed calcite is expected to show lower substitutional Fe3+ concentrations. Recrystallized calcite is the most typical mineral on excavated marble surfaces (Margolis and Showers, 1988), in abundances depending on the presence and acidity of the underground water. The third excavated sample (ME34) shows a lower depletion but it is far more well preserved than the other two, bearing only a thin layer of clay depositions and no signs of extended re-crystallization. The samples exposed to urban environment also show severe iron depletion, especially KR6, 12, 27 and 30, which are covered by thick gypsum crusts. Gypsum crusts need wet surfaces to develop (Camuffo, 1998), a condition indicating again dissolution and re-precipitation cycles. Samples from rural environment (EP) follow next, showing moderate depletion. These samples show traces of biological action (bodies, hyphaes, etc. of the microorganisms), another factor that accelerates dissolution. Additionally, the microbe-mediated reduction of Fe3+ to Fe2+ may cause further substitutional Fe3+ decrease, but unfortunately Fe2+ cannot be detected by EPR spectroscopy (at least in this crystal symmetry). In the case of iron-reducing bacteria for example, the reduction takes place on the biofilm surfaces, where the iron ions are absorbed. Fe3+ ions are released from marble by local micro-dissolution and subsequently taken up by the bacteria (Brantley et al., 2001). The samples from the mount Hymettus, although exposed in a rural environment, show a very high value in iron depletion, most probably due to the low grade of metamorphosis of the marble and presence of numerous veins and impurities, which accelerate dissolution and microbial action. The samples from Pentelikon mountain show dramatic depletion when exposed in urban environment compared to those exposed to rural environment. The dolomitic sample (MU1) shows practically no depletion due to the substantially different rates of dissolution of calcite and dolomite (Martinez and White, 1999). Dolomitic samples have also been
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Table 2 Concentration changes and depletion depths for Mn2+ and Fe3+ on the analyzed marble sample surfaces Sample code
EP1 EP2 EP3 EP4 EP5 YM7 YM13 MU1 D6 NA2 ME24 ME20 ME34 KP6 KP12 KP27 KP30 KP67 KP69
Mn2+ in calcite (r.u.)a
Fe3+ in calcite (g ¼ 14:25)(r.u.)a
Bulk
Surface
Bulk
Surface
2250 3360 5437 6600 2500 45 88 184 56 387 32 26 1056 4031 182 3100 3175 3000 93
1890 3750 4921 5093 3140 67 100 143 47 415 40 27 988 2087 390 2680 325 2500 85
8.4 17.2 20.5 14.1 7.7 8.3 8.0 1.7 7.0 0.3 12.9 7.3 5.0 16.6 12.0 8.1 8.0 6.2 10.3
2.6 14.0 7.7 8.7 4.4 1.2 1.0 1.5 4.0 0.2 0.1 0.1 2.0 3.2 3.0 1.3 0.1 4.6 4.1
Fe3+ depletion depthb (mm)
Alteration depthc (mm)
Fe3+ in silicates (g ¼ 2:1)(r.u.)a
4 4 4 4 5 3 3 0 — 4 — 7 — — — 4 — 4 —
0.25 2.00 0.15 2.00 0.15 0.02 0.05 0.05 0.03 0.10 2.00 3.00 0.03 0.15 0.05 0.12 1.00 0.05 0.10
90 265 0 514 0 180 475 875 1000 800 150 328 320 5320 1746 1800 3875 0 3400
a
Values expressed in relative units of EPR peak intensities. Estimated from the EPR profiles (dashes correspond to small samples inadequate for profile drawing). c Measured by SEM. b
found more resistant to fungal attack than the calcitic ones (Bogomolova et al., 2003). Obviously, humidity (groundwater, atmospheric humidity, etc.) is the critical environmental factor in substitutional Fe3+ depletion occurring on marble surface layers. As for the type of marble, we can only say that dolomitic marbles show increased resistance to depletion compared to the calcitic ones and that wellcrystallized marbles also show higher resistance. 3.2. Substitutional Fe3+ depletion depths The maximum depth at which substitutional Fe3+ is observed was estimated by drawing detailed EPR peak profiles from the marble bulk to the surface. The profile of three representative marble pieces is presented in Fig. 3. Together with the values of substitutional Fe3+ peak, the Mn2+ and non-calcite Fe3+ peak intensities were also drawn for comparison. The first sample (Fig. 3a) is a weathered quarry front from the Pentelikon mountain, the second one is an excavated piece (Fig. 3b) and the third one a building stone from a Byzantine church in the center of Athens, Greece (Fig. 3c). The quarry sample (EP4) and the building element (KR27) show a decrease in Mn2+ concentration (25% and 45% correspondingly), while the excavated sample (ME20) shows no decrease. The change (if any) is evident mainly
in the outer 2 mm.The depletion of substitutional Fe3+ though extends in larger depths, up to around 4 mm for the quarry front and the building stone and around 7 mm for the excavated sample. At the same time, noncalcite Fe3+ shows a rapid increase in the outer 3 mm of the weathered marble surface. The depletion depths for the rest of the samples are given in Table 2. No value is given for very small or disintegrating pieces, where no profile could be drawn. The depletion in Fig. 3 extends deeper than the point where the increase of non-calcite Fe3+ starts. But it should not extend further than the marble alteration depth. If we accept that the only possible depletion mechanisms are all types of physicochemical process that cause chemical, mineralogical or mechanical alteration to the surface layers, this depletion depth should determine the marble ‘‘patina’’ thickness. In our samples, this alteration depth was estimated by SEM and given in Table 2. These depth values represent the maximum distance from the outer surface, at which foreign particles, re-crystallization products, biological remnants, loose crystals or other types of alteration can be identified under the SEM. However, none of these values is higher than 2 mm, while Fe3+ depletion extends up to 4 mm and in one occasion up to 7 mm from the surface. This observation means that substitutional Fe3+ also depletes in healthy marble crystals.
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substitutional Mn
2+
7000
18
Mn peak intensity (r.u.)
Fe peak intensity (r.u.)
6000 16 5000 14 4000
12 3+
3000 2000
8
non-calcite Fe
6
3+
1000
4 0.0
0.2
0.4
(a)
0.6
0.8
1.0
1.2
1.4
1.6
0 1.8
Distance from surface (cm) 10
substitutional Mn
2+
35 30
8 25
7 6
20
3+
substitutional Fe
5
15
4 3
non-calcite Fe
2
10
3+
2+
3+
Fe peak intensity (r.u.)
9
Mn peak intensity (r.u.)
3+
10
2+
substitutional Fe
5
1 0
0
0.0
0.2
0.4
(b)
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Distance from surface (cm) 3500
substitutional Mn
Re-crystallization of CaCO3 or microbe induced Fe3+ reduction cannot be considered valid interpretations at a depth of a few millimeters in an undisturbed crystalline material. A possible explanation for this additional depletion could involve diffusion of Fe3+ through the marble bulk. Dry calcite was considered static under the Earth’s ambient surface conditions till recently, but it has been demonstrated on single crystals (Stipp et al., 1997; Stipp, 1998), that monovalent ions from the calcite bulk (Na+, K+ and Cl), move out via solid-state diffusion and accumulate in small crystals on the surface. According to these results, the trace ions were at equilibrium when they were trapped at the time of calcite formation. Now at Earth’s surface conditions of temperature and pressure, they are no longer stable, so they move out by solid-state diffusion, along trajectories where defects are in higher proportion, through the single crystal to the surface, where their free energy is lower.
2+
2000
8
1500 1000
3+
non-calcite Fe
4
substitutional Fe
500
3+
0
0
0.0
(c)
2+
2500
12
Mn peak intensity (r.u.)
3000
3+
Fig. 2. Surface change of substitutional (a) Mn2+ (b) and Fe3+ concentration, measured by EPR spectroscopy. Different environments of exposure are indicated. Reproducibility of measurements: 10%.
Fe peak intensity (r.u.)
16
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
2.0
Distance from surface (cm)
Fig. 3. Profile of Mn2+ and Fe3+ with distance from the weathered marble surface: (a) Weathered quarry front (sample EP4); (b) ancient excavated piece (sample ME20) and; (c) building element exposed to urban environment (sample KR27).
If we suppose that a similar process is valid for Fe3+ in marble, the following hypothetical model would apply: iron is entrapped in Ca2+ sites during metamorphosis. However, it does not form a stable carbonate in the +3 oxidation state because Fe3+ (0.67 A˚) has a quite small radius and higher charge than Ca2+, causing significant crystallographic distortion. Consequently, at Earth’s surface conditions this sensitive equilibrium is destroyed and Fe3+ ions start diffusing through the lattice, in a search for lower free energy positions, that is
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the surface, grain boundaries and micro fractures. At their new positions, the iron ions probably form oxides, with a lattice structure more suitable for trivalent ions. If we suppose that the diffusion rates are in the scale of nanometers in weeks or months, as reported for Cd2+ (0.95 A˚) and Zn2+ (0.74 A˚) (Stipp et al., 1998), we can explain why the phenomenon can be detected by EPR spectroscopy only on surfaces exposed for very long periods. We have to note here that the depletion has never been observed on samples kept in the lab for up to 10 years. However, the presence of iron oxides at the grain boundaries would have been detected by EPR. We should have an increase at the iron oxide EPR peaks, which was not observed. That means either that the hypothesis is not valid or that it needs modification. The problem is that, to isolate the hypothetical diffusion effect, we need marble surfaces exposed for very long periods but not weathered. We could of course accelerate the phenomenon by heating the samples, but we might trigger other processes this way. For short periods when the depletion will have reached a depth of nanometers or microns, another high-resolution technique like auger electron spectroscopy (AES) or X-ray photoelectron spectroscopy (XPS) might be more suitable. As a last comment, we have to note here that the same phenomenon of additional depletion was not observed also for manganese ions probably because Mn2+ forms a nearly complete solid solution with calcite. It fits into the calcite lattice more easily without causing significant disruption, because its radius is closer to that of Ca2+.
4. Conclusions The results presented in here prove that EPR spectroscopy is a very useful tool in measuring ionic changes on weathered marble surfaces. One of the advantages of the technique is the discrimination between ions hosted in different lattices. The use of the EPR spectroscopy on different types of weathered marble surfaces illustrates the behavior of Mn2+ and Fe3+ replacing Ca2+ in the CaCO3 of marble. Mn2+ may decrease or increase on the surface, but without any obvious trend. On the contrary, Fe3+ shows a dramatic depletion, varying from 20% to 100% of the bulk value. The phenomenon is explained by dissolution and re-crystallization of calcite together with microbe-mediated reduction of Fe3+ to Fe2+. Higher decrease is observed on excavated samples and samples exposed to heavily polluted urban environment. This depletion extends deeper than the actual weathering depth as measured by SEM. The results show that the depletion extends up to a few millimeters inside the healthy marble. More work is ahead to be done in order to verify if this additional depletion is a result of solidstate diffusion or not.
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