Relationships between mineralogical and textural

72-LEZZERINI 394-403_GEOLOGIA 28/11/11 12.56 Pagina 394

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Ital.J.Geosci. (Boll.Soc.Geol.It.), Vol. 130, No. 3 (2011), pp. 394-403, 6 figs., 7 tabs. (DOI: 10.3301/IJG.2011.16) © Società Geologica Italiana, Roma 2011

Relationships between mineralogical and textural factors in respect to hydric dilatation of some sandstones and meta-sandstones from the Northern Apennine ANNA GIONCADA (*), LEONARDO LEONI (*), MARCO LEZZERINI (*) & DOMENICO MIRIELLO (**)

ABSTRACT This paper presents the results of a study of the linear hydric dilatation of some sandstones and meta-sandstones and its relations with their mineralogy, texture and physical properties. The ultimate aim is to identify the main factors and mechanisms responsible for hydric dilatation in silicoclastic rocks. The study has been conducted on various rock types selected for their different mineralogical, textural and physical properties: Macigno, Monte Senario and Manciano sandstones, and Pietra del Cardoso and Monte Pisano Quartzite meta-sandstones. In the selected samples, linear hydric dilatation measured perpendicular to rock bedding or foliation ranges from 0.15 to 1.01 mm/m. The results herein obtained indicate that the main factors influencing the degree and kinetics of dilatation upon water uptake are the amount, size and size distribution of the open pores, the pore shape and spatial distribution, the preferential orientation of phyllosilicates and the presence of significant amounts of expansible clayey components. These factors combine and interact in various ways to generate the different hydric behaviours observable in these stones. In particular, the linear hydric dilatation depends at first on the pore dimensions in respect to the absolute water absorption.

KEY WORDS: Hydric dilatation, sandstone, meta-sandstone, clay minerals, physical properties.

INTRODUCTION

Linear hydric dilatation is a measure of the elongation of a rock specimen in response to water absorption. This physical property is particularly appreciable in sandstones, which commonly undergo dilatation on the order of several tenths of a millimetre per meter (in rare cases even as high as 3-4 mm/m; see WEISS et alii, 2004; SEBASTIAN et alii, 2008). Indeed, sandstones generally exhibit greater hydric dilatation than thermal expansion (FRANZINI et alii, 2008). The role of hydric dilatation in the decay of sandstones has recently been discussed by several authors (BARGOSSI et alii, 2002; FRANZINI et alii, 2007a,b; SEBASTIAN et alii, 2008; BENAVENTE et alii, 2008; WANGLER & SCHERER, 2008; SIEDEL, 2010), who have evidenced a relation in some sandstones between hydric dilatation and the presence of expansible clay minerals.

(*) Dipartimento di Scienze della Terra, Università di Pisa, Italy. Corresponding author: Dr. Marco Lezzerini, Dipartimento di Scienze della Terra, Via S. Maria, 53 - 56126 Pisa, Italia. E-mail: [email protected]. Phone number: 39 - 050 2215705. Fax number: 39 - 050 2215800. (**) Dipartimento di Scienze della Terra, Università della Calabria, Italy.

Damage due to the presence of small proportions of swelling clays has also been recognized in various other stones (e.g. limestone, RODRIGUEZ-NAVARRO et alii, 1997). As hydric dilatation favours physical and chemical weathering, it may well represent an important cause of sandstone decay. Thus, defining any relations existing between this property and other physical properties (such as real and apparent densities, open and close porosities), as well as chemical, mineralogical and textural characteristics, may provide valuable criteria for identifying those stone varieties exhibiting greater durability. This paper reports on the results of a study about the relations between hydric dilatation, water absorption, mineralogy (including the content of clay minerals and related expansible phases) and the texture of some sandstones and low-grade meta-sandstones differing significantly in their characteristics and physical properties. The selected sandstones and meta-sandstones come from quarries in Tuscany (Central Italy) and were widely employed as building materials. Previous data on hydric dilatation are available only on Macigno sandstones, in which FRANZINI et alii (2007a,b) measured appreciable elongation after water absorption and found a strong correlation between the measured dilatation values and the amount of a smectitic component associated to chlorite/smectite mixedlayer phases (Chl/S).

MATERIALS AND METHODS

The sandstones and the low-grade meta-sandstones selected for the study are listed in tab. 1. The Macigno sandstones (upper Oligocene-lower Miocene) belong to the stratigraphic sequence of the Tuscan Nappe Unit. These rocks are grey to light bluish-grey in colour when unaltered and mainly consist of quartz, feldspars, micalike minerals and chloritic material (e.g., VALLONI, 1978; VAN DE KAMP & LEAKE, 1995). The physical properties of such rocks were investigated by DI BATTISTINI & RAPETTI (2003) and FRANZINI et alii (2007a,b) on samples mainly from Lunigiana area outcrops (north-western Tuscany, Massa province). With regard to the examined sandstones, historically Macigno is the most extensively employed stone in Tuscany for monumental buildings, churches, castles and towers, ornamental elements (pilasters, pillars, columns, window and door frames) and since the 19th century also for railways and as paving stones for city centre squares and streets. Despite its good technical properties, when used outdoors this sand-


RELATIONSHIPS BETWEEN MINERALOGICAL AND TEXTURAL FACTORS IN RESPECT TO HYDRIC DILATATION

stone often undergoes a typical process of decay, consisting predominantly of detachment of fragments parallel to the rock surface, which in many cases may be exacerbated by the occurrence of significant amounts of expansible clayey components (FRANZINI et alii, 2007a,b; DI PETTA, 2008). The examined samples were drawn from the Gonfolina quarries (GF1-GF2, shown in fig. 1 with A), which in the past furnished excellent building materials for many towns and cities along the Arno River from Florence to Pisa (RODOLICO, 1953) and from the Greve in Chianti quarry (GR1, shown in fig. 1 with B), where the Macigno sandstone is still actively extracted. The Monte Senario sandstones (Lower Oligocene) belong to the Canetolo Complex. They consist of a turbiditic sequence with macroscopic characteristics somewhat similar to Macigno sandstone (CHIOCCHINI & CIPRIANI, 1996; CIPRIANI et alii, 2005). The uses to which this stone is put are also similar to Macigno. The selected samples come from the working quarry at Santa Brigida, near Florence, and are representative of the two quarried varieties, Pietra Bigia and Pietra Serena (SBB and SBS, shown in fig. 1 with C). The Manciano sandstones (Miocene) are calcareousquartz-feldspathic sandstones (MARTINI et alii, 1995). This stone has begun to be used only rather recently and it is now widely employed for paving city streets and as building stones. The samples studied herein come from the Pietra Dorata quarry (SF1 and SF3, in fig. 1 shown as D) and from the Santa Fiora quarry (SF2, also shown in fig. 1 as D). These varieties mainly differ in their colour, which may vary from reddish to yellow-orange. Regarding metamorphosed sandstones, Pietra del Cardoso and Monte Pisano Quarzites have been selected for study. The Pietra del Cardoso is a quartz-feldspathic metasandstone belonging to the Pseudomacigno Formation (CARMIGNANI et alii, 2005) of the Tuscan metamorphic Unit. The rocks of this formation represent the counterpart of the Macigno sandstones in the un-metamorphosed Tuscan sequence. They are quarried near the village of Cardoso (Lucca province), in the southern Apuan Alps (sample CD20, in fig. 1 as E). Nowadays they are widely used as ornamental stones for outdoor and indoor structures (including staircases). The Monte Pisano Quartzites are meta-sandstones of the Monte Serra Formation, which belongs to the Triassic Verrucano meta-sedimentary sequence of the Northern Apennines (RAU & TONGIORGI, 1974). The selected rock samples were drawn from the “white-pink quartzite” member (samples QZ1QZ2, in fig. 1 shown as F). The Monte Pisano Quartzites were largely used as building material during the Middle Ages in Pisa and nearby villages until the 15th century, after which they were replaced by the more workable Macigno (FRANZINI et alii, 2001). Chemical analyses of the selected samples were carried out via XRF for major elements, according to the procedure suggested by FRANZINI et alii (1975). The total volatile components were determined as L.O.I. at 950°C on powders dried at 105 ± 5°C. CO2 content was determined by the calcimeter method (LEONE et alii, 1988) and the difference between L.O.I. and CO2 was ascribed entirely to H2O+. The qualitative mineralogical compo sition of bulk samples and clay fraction was obtained by X-ray powder diffraction analysis (XRPD) using a Bragg-Brentano geometry and Ni-filtered CuKa radiation

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TABLE 1 Provenance of the sandstones and meta-sandstones.

obtained at 40 kV and 20 mA. Clay minerals of both sandstones and meta-sandstones were studied on oriented specimens of the 63%). The highest values of porosity and water absorption are observed in samples SF1, SF2 and SF3, while the lowest ones occur in the CD20, QZ1 and QZ2 meta-sandstones. Concerning the Macigno sandstones, the physical properties measured on samples GF1, GF2, and GR1 are similar to those reported by FRANZINI et alii (2007a,b) on samples from the Lunigiana area (Massa province) and Filettole (Pisa province). The physical properties of Monte Senario sandstones do not differ significantly from those of the Macigno samples. Overall, the data reported in tab. 7 indicate that the examined Macigno and Monte Senario sandstones are characterized by medium porosity, the Manciano sandstones by rela-



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TABLE 4 Normative mineralogical composition of the sandstones and meta-sandstones (wt %).

Qz = quartz; Ab = albite; An = anorthite; Kf = K-feldspar; Cc = calcite; Micas = illite + biotite; Chl s.l. = chlorite s.s. + chlorite/smectite mixed layers.

TABLE 5 Chlorite s.s., Chl/S mixed layer and smectite contents (wt %) in the studied Macigno sandstones. Mean data with ranges (in italics) in Macigno samples from others localities of Tuscany are also reported.

Chl = chlorite; S = smectite; Chl/S = chlorite/smectite mixed layers; 1 = VENTRE, 2006; 2 = DI PETTA, 2008; 3 = FRANZINI et alii, 2007a (n = 19 samples); 4 = FRANZINI et alii, 2007b (n = 5 samples); 5 = FRANZINI et alii, 2007b (n = 4 samples).

tively high porosity and the meta-sandstones by low porosity. HYDRIC DILATATION The linear hydric dilatation (a, mm/m) of water-saturated samples shows a wide range of variation and its value is higher along the direction perpendicular (a^) to the sandstone stratification or the meta-sandstone foliation than along the direction parallel (a//) to them. The values of a^ and a// range from 0.15 to 1.01 mm/m and from 0.06 to 0.63 mm/m, respectively. Despite such anisotropy, a^ and a// are roughly correlated (r = 0.65), the correlation being considerably higher (r = 0.96) when the highly foliated CD20 sample is excluded. The value of Da = 100 . (a^ – a//)/am, where am = (a^ – a//)/2, varies from 0% (sample QZ2) up to +171% (sample CD20)

Fig. 4 - Back-scattered electrons SEM photo of GR1 Macigno sandstone (A). Close-up of Chl/S mixed layers minerals in the matrix (B) and in detrital grains (C).

(tab. 7). The cause of this difference can be attributed mainly to the textural features (including pore shape and distribution) of each specific stone, particularly the


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TABLE 6 Chemical composition of the main phyllosilicates in the sandstones (framework minerals and authigenic Chl-S minerals). Mean data with ranges (in italics); total iron as FeO. Data from FRANZINI et alii (2007a) and LEONI et alii (2010) are also reported.

(*) Mean composition used in the calculation of the normative mineralogical composition (tab. 4).

TABLE 7 Main physical properties of the sandstones and meta-sandstones.

rs = real density (g/cm3); rb = apparent density (g/cm3); CA = water capillarity coefficient (mg/cm2 . s-1/2); Ww = water absorption at atmospheric pressure (%); P = total porosity (% vol.); SI = Saturation Index (%); a = linear hydric dilatation (mm/m). Specimens were cut with their longest dimension normal (^) and parallel (//) to bedding or foliation. Da = 100 . (a^ – a//)/am, where am = (a^ – a//)/2. The degree of mineral orientations 1/Vc is also reported (Vc = circular variance).

existence of more or less marked layering. Excluding samples CD20 and QZ1, no correlation exists between the calculated Da values and the degree of phyllosilicate orientation 1/Vc (r = 0.33). The influence of preferential orientation of the phyllosilicates is above all evident in the highly foliated sample CD20 (a^ =0.78 mm/m and a// = 0.06 mm/m), characterized by a serried alternation of quartz-feldspathic granoblastic and phyllosilicate lepido blastic beds, preferentially oriented parallel to S1 foliation.

Fig. 5 illustrates the variation in hydric dilatation as a function of the square root of water immersion time separately for each type of sandstone (fig. 5A, B, C) and meta-sandstones (fig. 5D). In general, as expected, the dilatation measured perpendicular and parallel to the stratification or foliation increases with increasing water absorption, even though there is a significant delay (exemplified in fig. 5B for sample SBS). For sandstones, with increasing water absorption, the dilatation develops through three main distinct stages: a first phase characterized by a negligible increase; a second rapidly rising phase; then a final stage, during which dilatation asymptotically approaches its maximum value (see inset in fig. 5A). Sandstones reach 95% maximum hydric dilatation in a time varying from 6 to 60 hours. The time necessary to reach 50% dilatation is in general much lower; for example the Manciano sandstone SF2 takes 60 hours to reach 95% final dilatation, but only 12 hours to reach 50%. Similarly, 50% water absorption (Ww, %) is generally attained after a relatively short period of water immersion, compared to the time needed for complete saturation (i.e., 30′ to 2.5 hours compared to 8-48 hours). The variation in dilatation measured in the two main directions is plotted against the absorbed water in fig. 6, which clearly shows that the increase in dilatation upon water uptake follows different kinetics in each type of stone. It proceeds rather quickly in the foliated meta-sandstones, moderately fast in Macigno and Mt. Senario sandstones, and more slowly in the Manciano sandstones. At water saturation, however, the final values of both a// and a^ appear unrelated to the water absorption (Ww) of



401

Fig. 5 - Time course of linear hydric dilatation and water absorption in the studied Tuscan sandstones (A, B, C) and meta-sandstones (D). Vertical bars indicate the relative standard deviation.

Fig. 6 - Relations between linear hydric dilatation and corresponding water absorption. A and B: linear hydric dilatations measured in the direction normal and parallel to rock bedding or foliation, respectively.

the stone (fig. 6). For example, Santa Fiora sandstones (samples SF1, SF2 and SF3), which exhibit high water absorption (1.82-2.69%), instead yield low dilatation values (a^= 0.19-0.25 mm/m; a// = 0.13-0.18 mm/m), while the Monte Senario and Macigno sandstones, characterized by lower water absorption (1.13-1.70%), undergo significantly higher dilatation (a^ = 0.45-1.01 mm/m; a// = 0.300.63 mm/m). QZ1 meta-sandstone also shows relatively high dilatation (a^=0.53 mm/m; a// = 0.35 mm/m) compared to its water absorption (0.51%). When sandstones and meta-sandstones are considered together, hydric dilatation at water saturation is not significantly correlated with most of the measured physical properties, suggesting that there are many factors influencing this physical property. Hydric dilatation results are roughly correlated only with the saturation index (a^– SI; r = 0.70), although this correlation becomes insignificant when the saturation index is correlated with a//. The absence of the correlation between a// – SI is probably due to the influence of the highly foliated CD20 meta-sandstone, which though characterized by a high saturation index (92%) dilates very little along this direction (a// = 0.06 mm/m). When this sample is disregarded, a^ and a// show rough and similar correlations with saturation index (a^– SI, r = 0.69; a// – SI, r = 0.70). The Hg-porosimetry data collected on SBS, GR1, SF1 sandstones and CD20 meta-sandstone samples provide some insight into the role of the size and the size distribution of the open pores (fig. 3). Some indications regarding

the influence of these factors can be drawn by comparing the porosimetry data on Mt. Senario (sample SBS) and Macigno (samples GR1 and GF1) sandstones with those of Manciano (sample SF1). The latter rock, though characterized by a relatively moderate dilatation anisotropy, (see tab. 7) differs significantly in hydric dilatation and SI values. The Mt. Senario (SBS, a^= 0.45 mm/m; a// = 0.30 mm/m; SI = 83%) and Macigno (GR1, a^ = 1.01 mm/m, a// = 0.63 mm/m; SI = 96% and GF1, a^ = 0.72 mm/m, a// = 0.42 mm/m; SI = 78%) sandstones form a relatively homogeneous group of samples characterized by relatively high hydric dilatation and saturation index in comparison to the measured SF1 Manciano sandstone, which on the contrary, exhibits lower hydric dilatation and saturation index (a^ = 0.19 mm/m; a// = 0.13 mm/m; SI = 63%). Concerning open pore size and size distribution, the group including the Mt. Senario (SBS) and Macigno (GR1 and GF1) samples exhibit a mean pore diameter (0.07 ± 2.49 μm; 0.06 ± 2.27 μm, and 0.04 ± 1.47 μm, respectively for SBS, GF1 and GR1 samples), which is lower than that of the SF1 sample (0.1 ± 0.76 μm). They moreover differ considerably in pore size distribution, which in SF1 sample appears strongly asymmetric with a tail degrading toward the lower pore dimensions. Since dilatation is due to the pressure acting on the open pore surface by the infilling water, and pressure increases considerably with decreasing pore dimensions, the Mt. Senario (SBS sample) and Macigno (GR1 and GF1 samples) can undergo higher dilatation than the Manciano (SF1


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sample) sandstone, despite their lower water absorption. The low dilatation value observed in sample SF1 would be also due in part to its lower saturation index (63%) compared to the others samples (ranging from 78 to 96%). In Pietra del Cardoso meta-sandstone, instead, the response to dilatation upon water absorption is mainly influenced by the pore shape and spatial distributions, as suggested by the strong difference in dilation between a^ (0.78 mm/m) and a// (0.06 mm/m). In this highly foliated stone, the open pores exhibit a substantially unimodal distribution and high mean diameter (17.03 ± 0.76 mm). Thus, they would be situated predominantly parallel to the superficies of foliation and would appear as coarse pores along the plane parallel to the foliation, but as very fine-sized pores along the perpendicular direction. Regarding the role of mineral composition, on the whole, no apparent significant relation between bulk mineral composition and hydric dilatation has been observed, with the one exception of the observed positive correlation between dilation and chlorite s.l. (including chlorite and chlorite-rich chlorite/smectite mixed layers) (chlorite s.l. – a^, r = 0.81; chlorite s.l. – a//, r = 0.63). As expected, this correlation is strengthened (chlorite s.l. – a^, r = 0.81; chlorite s.l – a//, r = 0.80) by disregarding the highly foliated CD20 meta-sandstone and, obviously, the quartzites, which do not contain detectable amounts of chlorite. It seems that such correlation is influenced above all by the presence of a smectitic component in GF1, GR1 and GR2 Macigno sandstones. According to FRANZINI et alii (2007a,b) and DI PETTA (2008), most of the hydric dilatation affecting these sandstones is in fact caused by the occurrence of significant amounts of smectite. In the studied samples, the effect of this component is relevant only in sample GR1 (smectite content = 2.5 wt.%), whereas it is negligible in samples GF1 and GF2 (smectite content = 0.5 wt.%), as can be seen from the significant difference in dilatation between these samples (tab. 7).

meta-sandstones, the layering or foliation is attributable mainly to the preferential orientation of the sheet silicates. Therefore, a variable degree of anisotropy in hydric dilatation, caused by phyllosilicate orientation, is to be expected even within each specific rock type, thereby leading to variations not only between samples from different quarries, but also when comparing blocks extracted from the same quarry. Although porous materials, including sandstones, may undergo hydric dilatation upon water absorption irrespective of the presence of clay minerals, the present data confirm that in Macigno sandstones, the presence of expansible clayey components may significantly enhance the effects of decay related to hydric dilatation, consisting prevalently in the flaking off of the stone surface with detachment of rocks fragments (FRANZINI et alii, 2007a,b). The collected data point out that hydric dilatation may also occur in low-grade meta-sandstones characterized by a pronounced foliation, representing a possible cause of decay in these stones, too. Therefore, when sandstones and meta-sandstones are used as building materials in outdoor structures, hydric dilatation, together with other physical and chemical factors related to stone water absorption, must be taken into account as an important cause of decay, and determinations of hydric dilatation values are fundamental in identifying those stone varieties that are less subjected to decay and, consequently, more durable. ACKNOWLEDGMENTS We wish to dedicate this paper to the memory of Prof. Marco Franzini recently passed away. He held for many years the chair of Mineralogy at University of Pisa and for the last three decades his field of research was focused on Science and Technology for Cultural Heritage of the western Tuscany. The linear hydric dilatation measurements, herein reported, were performed with an apparatus realised on his project. Three anonymous reviewers are acknowledged for their useful suggestions, which greatly improved the manuscript.

CONCLUDING REMARKS

The measurements of linear hydric dilatation performed on the sandstone and meta-sandstone samples for this study indicate that this physical property is influenced by many factors, for the most part related to the textural and mineralogical characteristics of the individual stone. The main determining factors appear to be: the stone saturation index; the amount, size and size distribution of open pores; pore shape and spatial distribution; the preferential orientation of phyllosilicates; and the presence of expansible clayey components. These same factors also control the kinetics of dilatation upon water uptake. The combined effects of the above factors and their interactions lead to different hydric behaviours of the stones examined. The porosimetry data (fig. 3) indicate that the lower the mean size of the open pores, and the more abundant fine pores (about 0.01-0.03 mm) are with respect to coarse ones (about > 0.1 mm), the greater the hydric dilatation will be. In highly foliated meta-sandstones, the shape and spatial distribution of the open pores appear to be the dominant factors. In such highly foliated rocks, exemplified by the Pietra del Cardoso, the pores are mostly flattened and arranged along the rock foliation. In phyllosilicate-bearing sandstones and

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Manuscript received 30 May 2011; accepted 5 July 2011; editorial responsability and handling by S. Conticelli.