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Mechanisms of Calcite Replacement by Whewellite: Implications for The

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Mechanisms of Calcite Replacement by Whewellite: Implications for The macla nº 15 . septiembre 2011 revista de la sociedad española de mineralogía 187 Mechanisms of Calcite Replacement by Whewellite: Implications for the Conservation of Building Stone / ENCARNACIÓN RUIZ-AGUDO (1,*), PEDRO ÁLVAREZ-LLORET (1), CHRISTINE V. PUTNIS (2), ALEJANDRO RODRÍGUEZ-NAVARRO (1), ANDREW PUTNIS (2) (1) Departamento de Mineralogía y Petrología. Universidad de Granada. Fuentenueva s/n. 18071, Granada (Spain) (2) Institut für Mineralogie. Universität Münster. Corrensstrasse 24. 48149, Münster (Germany) INTRODUCTION. METHODOLOGY. cross-sections of reacted samples show that calcite is progressively replaced by Calcium oxalates are commonly found Replacement reactions in Teflon® calcium oxalate (whewellite, as in the alteration crusts of stone reactors. confirmed by 2D-XRD analysis). The monuments. From a macroscopic point replacement product is polycrystalline. of view, these layers have a smooth, The replacement experiments were Figure 1 shows the typical morphologies uniform appearance, although a more performed at room temperature (23 ±1 of these newly formed crystals growing detailed observation reveals a porous °C) in sealed Teflon® reactors. Two on the calcite surface during the early and irregular surface. The oxalate types of reactors were used with precipitation stages. patinas cover the original surfaces internal volumes of 50 and 250 mL, reproducing fine details such as the respectively. Fragments of optical signs of the instruments used for their quality Iceland Spar single crystals (ca. carving (Del Monte et al., 1987). 0.020 ± 0.005 mg) were weighed and Observations of natural patinas on placed into the reactors before the calcitic stone surfaces seem to suggest reaction solutions were added. 50 mL that these patinas play a preservative and 250 mL of reaction solutions were role on marbles and limestones; introduced into the Teflon reactors. calcium oxalates replace calcite in the After the reaction, the reactors were stone surface producing less reactive opened and the solids were rinsed with layers towards chemical attack due to ethanol and dried overnight at 60 ºC. fig 1. SEM secondary electron image of a calcite crystal partially replaced by whewellite. The its lower solubility (e.g. Del Monte and Experiments were performed at pH 3 whewellite crystals are clearly oriented on the Sabbioni, 1983; Del Monte et al, 1987). starting from aqueous oxalate-bearing calcite surfaces, and dissolution of the parent Thus, this natural process could be used solutions with oxalate concentration of calcite is seen by the formation of deep etch pits. in the design of conservation treatments 1 mM and with reaction times ranging that mimic the natural process taking from 1 day to 2 weeks. The product crystals appear clearly place in the environment (Doherty et al., oriented on the calcite substrate, 2007). However, the lack of an in-depth Textural and two-dimensional X-Ray forming a thick coating (Fig 1). From the mechanistic knowledge of the calcium diffraction analysis of reacted solids. analysis of the pole figures obtained carbonate-calcium oxalate from the bidimensional XRD analysis transformation has limited the A JEOL 6300F Field Emission Scanning and calculated for the main calcite and development of effective conservation Electron Microscope (FESEM) was used whewellite reflections (Figure 2), it can protocols for calcitic building stones. for surface morphology and texture be concluded that whewellite crystals Knowledge of the factors that control examinations. Solids were carbon- precipitated onto Iceland Spar crystals texture and porosity developed during coated and examined in the FESEM in with their {100} and {010} planes the replacement and the adhesion of both secondary electron and parallel to {10 4} calcite faces. Epitaxial the product layer to the substrate seems backscattered electron mode. 2D-XRD relationships will be further developed in critical before considering the analyses were performed to establish the next section. Dissolution of the application of such treatments in real the three-dimensional orientation of parent phase is seen by the cases. It is the aim of this work to calcite and Ca-oxalate overgrowth using development of deep etch pits on the investigate the mechanism of the an X-ray single crystal diffractometer substrate surface underlying the newly replacement of calcite by whewellite equipped with a CCD area detector (D8 formed phase. SEM images of cross- (CaC2O4·H2O), including the possible SMART APEX, Bruker, Germany). Pole sections show that partially replaced epitaxial relationships between the densities/figures for the main calcite crystals retain the external dimensions substrate and the product. This has and whewellite reflections were and crystallographic - characteristics of been done on experimental samples calculated from the registered frames the initial calcite, i.e. calcium oxalate using a combination of Scanning using XRD2DScan software (Rodriguez- pseudomorphically replaces calcite. The Electron Microscopy (SEM) and Navarro, 2006). replacement rim showed limited, bidimensional X-Ray Diffraction (2D- apparently non-connected porosity, with XRD). RESULTS AND DISCUSSION. most of the empty spaces observed in the reaction product related to the SEM examinations of surfaces and formation and widening of fractures. palabras clave: Calcita, Whewellita, Reemplazamiento, Piedra key words: Calcite, Whewellite, Replacement, Building stone. ornamental. resumen SEM 2011 * corresponding author: [email protected] macla nº 15 . septiembre 2011 revista de la sociedad española de mineralogía 188 These fractures are frequently observed dissolution - reprecipitation process solutions and calcite cleavage surfaces both within the replacement rim and (Putnis, 2002). As shown by Xia et al. follows the typical pattern of a within the still unaltered calcite crystals (2009), the coupling between the pseudomorphic, interface-coupled (Fig. 3). dissolution and precipitation in dissolution-reprecipitation reaction, with replacement reactions is achieved by a the dissolution of the substrate and the combination of substrate-assisted subsequent precipitation of whewellite nucleation and solution chemistry at the crystals, which grow epitaxially on the reaction interface. The fast dissolution (101 4) calcite surface. There are two of calcite at the low pH conditions of our orientations of the whewellite crystals experiments raises the activity of on the calcite surface: calcium in the solution at the interface (010)COM║(10 4)Cc, [100]COM ║[ 41]Cc, with the solid, increasing its 1 4 supersaturation degree with respect to and (100)COM ║(101 4)Cc, [001]COM whewellite and therefore enhancing its ║[010]Cc. The coupling between the dissolution and precipitation reactions fig 2. (010) pole figure from whewellite showing a nucleation. Furthermore, the structural maxima that indicates that these planes are similarities between calcite and in replacement processes is a oriented approximately parallel to the (104) calcite whewellite seem to promote epitaxial consequence of substrate-assisted plane. nucleation of the latter on calcite epitaxial nucleation and solution surfaces, decreasing the energy barrier chemistry at the reaction interface. for whewellite nucleation. Epitaxial Under the conditions of our relationships and structural matching experiments, formation of cracks and between these phases will be explored subsequent detachment of the in the next section. Our observations replacement rim was observed, most point towards a “Volmer-Weber” probably due to the increase in volume mechanism of epitaxial growth, in which occurring upon the reaction. Changes in the overgrowth precipitates as thick the chemistry of the replacement fluid three-dimensional crystals and that can modify the relative solubility of the typically occurs when there is a weak parent crystal and the product and lead fig 3. SEM-backscattered electron image showing a adhesion between substrate and to the eventual development of a cross section of a calcite crystal partially replaced porous, coherent interface. The results by whewellite. overgrowth. of this research are expected to With increasing reaction time, the Epitaxial relationships. represent a source of information for replacement product detaches from the the design of protective treatments for calcitic stones based on the calcite surface and breaks up into fine For (010) whewellite and (1014) calcite precipitation of calcium oxalate upon particles. The fragility of the reaction faces a very good matching was calcite dissolution, and highlight the product may be the combined result of observed along [101]COM and [010]Cc reaction parameters that must be its low adhesion to the substrate and the (misfit -0.05 %) and along [100]COM and presence of cracks. Fracture formation optimize before such a treatment can [ 4 41]Cc (misfit of 4.26 %), the angular and the observed limited porosity may be effectively implemented. misfit between both directions being - be the consequence of a positive volume 5.31 °. These misfit values are clearly change in the replacement reaction, REFERENCES. within the limits required for epitaxial most probably due to the higher molar nucleation from solution. Similarly, a volume of whewellite compared to Del Monte, M., Sabbioni, C., Zappia, G. good structural matching was found (1983): The origin of calcium oxalates on calcite. These fractures are critical in the along whewellite [001] and calcite historical buildings, monuments and progress of the
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