Introduction

Stone monuments and ruins are one of the most important artefacts of ancient civilizations, especially at archaeological sites. They provide valuable information about past cultures and their practices and activities (ICOMOS 1993). Losses in stone material lead to the loss of traces from the past. Historical outdoor monuments and archaeological structures exposed to the weather undergo various deterioration processes related to successive wetting–drying and freezing–thawing cycles, changes in temperature and relative air humidity, salt crystallization, air pollution, wind, rain and living organisms in the environment (Camuffo 1995; Doehne and Price 2010; Hatır et al. 2021; Cozzolino et al. 2022; Krisztina and Török 2022). These processes can result in the loss of stone material, discoloration/soiling, detachment and fissures/deformation (Fitzner and Heinrichs 2001; Hatır 2020). In this respect, weathering problems and preservation of stone monuments are critical issues in the field of conservation. Proper diagnosis of stone weathering and appropriate conservation can minimize the loss of stone at archaeological sites.

Andesites are the most commonly used the building stones for structural and ornamental purposes in both ancient and modern times due to their high resistance to weathering. They are fine-grained volcanic rocks, usually light to dark grey in colour. They consist of the plagioclase minerals such as andesine and oligoclase, together with mafic minerals (Winkler 1973; Schaffer 2004).

Beside their high resistance to weathering, the andesites can be decayed by physical, chemical and biological processes (Columbu et al. 2014, 2019; Columbu et al. 2020a, b (a)). Physical weathering of andesites is generally described by changes in density, porosity, and pore volume of internal parts and external surfaces (Columbu et al. 2018a, b; Columbu et al. 2021). Several studies show a significant increase in porosity and pore volume and a decrease in density values in the external parts of andesites (Oguchi and Matsukura 2000; Yavuz 2006; Sak et al. 2010; Jamtveit et al. 2011; Kaplan et al. 2013; Korkanç 2013).

Chemical and physical weathering of andesites and other common from basic to acid volcanic rocks occurs mainly as a result of hydrolysis of paragenetic minerals and precipitation of new minerals (Press et al. 2003). The most common new minerals formed after a long geological time, are clay minerals and iron and aluminum oxides (Mulyanto et al. 1999; Orhan et al. 2006; Lee and Yi 2007; Sak et al. 2010).

The growth of living organisms such as bacteria, fungi, or algae causes both physical and chemical deterioration of the andesite surface. High surface roughness and porosity, the presence of secondary phases formed from andesite, warm and humid climate accelerate the generation of living organisms and the degradation of the andesites (Korkanç and Savran 2015; Columbu et al. 2018a, b; Cozzolino et al. 2022).

The deterioration of buried stones differs from that of outdoor standing ones (TS EN 17652 2022). In the burial environment, oxygen, water, temperature changes, pH, salinity and carbon dioxide concentration are the main factors in the degradation of most archaeological materials in the archaeological deposits (TS EN 17652 2022). Oxygen accelerates the decomposition of organic materials and oxidation of metals in the deposits. Water leads to microbial attack, corrosion and leaching of stone, wood, bones and metals. Degradation of buried materials can be slowed by very wet or dry conditions. Temperature affects the rate of chemical reaction and microbial degradation. pH of soil solution affects both chemical and microbial degradation of buried materials. Salts accelerate the physical deterioration of archaeological materials in accordance with the type and composition of the salt (Doehne 2002; Oguchi and Yu 2021). In the burial environment, carbon dioxide with water promotes the chemical decomposition of organic materials such as wood and textiles (Tjeerdsma and Militz 2005). Carbon dioxide also causes the dissolution of stone, especially the carbonatic rocks and the subsequent precipitation of secondary minerals (Columbu et al. 2017; Columbu et al. 2020a, b; Mortatti and Probst 2003; Press et al. 2003).

As mentioned above, there are several studies focused on the problems of deterioration of standing monuments formed from volcanic rocks. However, only a few studies have dealt with buried and later excavated andesite monuments. Of these, Curran et al. (2002) examined the problems of deterioration of the buried and later excavated stone circle complex composed of quartz porphyry and andesite at Copney (Ireland). They observed visible deterioration as splitting/fracturing of stones, development of patina, surface exfoliation and granular disintegration, and in some cases complete disintegration of stones into sandy regolithic material. In another study conducted by Kaplan et al. (2013) on the buried and later excavated andesite monuments at the Aigai archaeological site (Turkey), the main deterioration problem observed was the formation of microbial colonization depending on environmental factors.

At the archaeological sites of Aigai and Assos (Turkey), the standing outdoor andesites monuments have been exposed for up to 2500 years (Wescoat 2012; Sezgin 2023). Despite the long exposure to the atmosphere, moderate weathering damage has been observed on the surfaces of the standing andesites, such as disintegration of the stones into smaller fragments, increasing microcracking from the inner parts to the surfaces, deposition of iron oxides and microbiological colonisation (Kaplan 2015). However, the extent of weathering damage on the excavated andesites differs from that on the standing parts. In the excavated andesites, whitish crust, accumulation of clay minerals and slight microbiological growth were observed as the main weathering forms on the stone surfaces. In this study, the mineralogical, chemical and microstructural characteristics of the whitish crust formed on the surface of the buried and excavated andesite monuments in the archaeological sites of Assos and Aigai (Fig. 1a) were determined in order to constitute a conservation approach at the archaeological sites.

Fig.1
figure 1

(Modified from Google Earth pro maps (version 7.3.6.9345), eastern wall of the Agora of Aigai (b) and the Theater of Assos (c)

Location of Aigai and Assos (a)

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The hypothesis of this study is the possibility of calcite formation by alteration of plagioclase by carbon dioxide in the soil during the burial of andesites at archaeological sites. This study shows that this hyphotesis is most likely possible in the cases of buried and excavated andesites.

The Archaeological Site of Aigai and Assos

The ancient inland site of Aigai was constructed in eighth century BC settled till the end of the third century AD. It was then resettled during the Byzantine period in the twelfth and thirteenth centuries (Sezgin 2023). It is located on the top of Mount Yunt in the northwestern part of Turkey. The main monuments of the site are a three-story Agora (Fig. 1b) with stoas in front, the Bouleuterion, the Theatre and the Gymnasion (Ramsay 1960). All the monuments and roads were built using the local andesite and soil was used as binding and infill material.

Assos is an ancient coastal site in the northwestern part of Turkey. It has been settled and inhabited from the Early Bronze Age to the present without interruption in the intervening centuries (Wescoat 2012). The site is 234 m above sea level and was built on a hilltop and the foothills of a mountain composed of andesites. As at Aigai, all the buildings were constructed within the site and the roads were paved with andesite and soil was used as binding and infill material. Within the site, from the hilltop to the foothills, there are several monumental buildings such as the Temple of Athena, the Agora, the Stoa, the Bouleuterion, the Gymnasium and the Theatre (Fig. 1c).

Sampling

This study was initiated by collecting whitish-colored crust samples from andesite surfaces that were buried and later excavated at the Aigai and Assos archaeological sites (Turkey) (Fig. 2). Excavated andesite samples with whitish crusts were collected by chipping pieces of stone and scraping whitish crusts from the stone surfaces (Table 1). Additionally, soil samples were also collected from a depth of 30 cm below the surface where the andesites were excavated (Table 1, Fig. 2).

Fig. 2
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The whitish crust on the excavated andesite wall of the Assos (a) and Aigai (b) sites

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Table 1 Samples of andesite, whitish crusts and soils from Aigai and Assos (Codes: Ai: Aigai; As: Assos)
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Methods

This study determined the density and porosity (RILEM 1980), mineralogical and chemical characteristics of the andesite, mineralogical, chemical and microstructural characteristics of the whitish crust and the soluble salt content of the soils and andesites.

The mineral phases of whitish crust, inner cores of andesite and soils samples were determined by X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy. XRD analyses were performed using a Philips X-Pert Pro X-ray diffractometer. Analyses were performed on finely ground samples with a grain size < 53 μm. XRD was performed using CuKα radiation with a Ni filter set at 40 kV and 40 mA in the selected range of 2–60 degrees from 2 theta at a scan rate of 1.6 per minute, incident beam radius 240 mm, soller slits 0.04 rad, diffracted beam monochromator and X'celerator detector. A Philips X’Pert Graphics and Identity software was used to indicate the mineral phases in each XRD spectrum.

FT-IR analyses were carried out using a Spectrum BX II FTIR spectrometer (Perkin Elmer). For analysis, 0.3–0.5 mg of 105 °C preheated powder samples were dispersed in approximately 80 mg of 500 °C preheated spectral grade potassium bromide (KBr) and pressed into pellets at approximately 10 tons/cm2 pressure. FTIR spectrometer was operated in the absorbance mode in the range 400–4000 cm−1 at a resolution of 4 cm−1. They were all corrected with respect to the background spectrum.

Petrographic analysis was carried out to identify common minerals and textural features of andesites by examining their thin sections under the light polarizing microscope (Leica DM750P). Thin sections were prepared by cutting a thin slice from the andesite samples using a diamond saw (Buehler Isomed Low Speed Saw). They were then glued to a glass slide using Canada Balsam and ground to the desired thickness using progressively finer grits of abrasive paper.

The chemical composition of the inner cores of andesite was determined by X-ray fluorescence spectroscopy (XRF; Spectro IQ II instrument). The analyses were carried out on powdered samples (grain size < 53 μm) diluted with lithium tetraborate.

The elemental composition of the whitish crust was determined by using scanning electron microscopy equipped with energy dispersive spectrometry (SEM–EDS). Analyses were carried out on chipped small samples coated with gold using an FEI Quanta 250 FEG scanning electron microscope (SEM) equipped with an X-ray energy dispersive system (EDS).

The microstructural properties of the samples were determined by scanning electron microscopy (Philips XL 30S FEG) coupled with an X-ray energy dispersive system (EDS). The samples were fixed on aluminum stubs using carbon adhesive discs and coated with gold for analysis.

The soluble salt content of the soils and andesites was determined by ion chromatography to estimate the damage of the stone by soluble salts from the soils. In this study, the amounts of chloride, sulphate, phosphate and nitrate ions were determined according to the TS EN 16455 (2014) standard using ion chromatography (Thermo Scientific Dionex, Model ICS-5000 +).

Results and Discussion

Mineralogical, Physical and Chemical Characteristics of the Andesite

Petrographic thin section analysis indicated that plagioclase, clinopyroxene and orthopyroxene are the major phenocrystalline phases while quartz and hornblende are subordinated in the andesites (Fig. 3). XRD analysis confirms that andesites were composed mainly of plagioclase, pyroxene (clinopyroxene and orthopyroxene) and with rare quartz (Fig. 4). The andesites found in Mt. Yunt, where the ancient city of Aigai is located, have similar mineral compositions (Seghedi et al. 2015). This may indicate that the andesites used in the ancient city were obtained from the surrounding ancient quarries. The andesite used in the Aigai and Assos archaeological sites had a density of 2.4 g/cm3 and low porosity (10%) values.

Fig. 3
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Petrographic thin section micrographs (in cross-polarized light) for samples Ai.2 (a) and As.3 (b) (Pl:Plagioclase, Px:Pyroxene, Hbl:Hornblende)

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Fig. 4
figure 4

XRD patterns of inner andesites (a) Aigai and (b) Assos

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The chemical composition of the andesites is shown in Table 2. The andesites were composed mainly of silica (62–65%), aluminium (16–20%) and iron (3–4%) with minor amounts of alkali metal oxides. Elemental composition expressed as percent oxides was evaluated to classify and determine the chemical character of the andesite using a total alkali versus silica (TAS) diagram based on Le Bas et al. (1986) and Irvine and Baragar (1971).

Table 2 Major oxide content (%) and loss on ignition (LOI) of the andesites determined by XRF
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The diagrams show that six samples fall within the andesite field (Fig. 5a) and one sample falls within the trachy-andesite field and has been classified as latite according to le Maitre (2010) on the basis of Na2O < K2O content. All the samples are subalkaline according to Irvine and Baragar (1971) (Fig. 5b).

Fig. 5
figure 5

Total alkali vs silica (TAS) diagrams of the andesite samples according to (a) Le Bas et al. (1986) and (b) Irvine and Baragar (1971)

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Mineral Composition and Soluble Salt Content in Soils

Mineral composition of the soils from Assos and Aigai sites was very similar to the mineral composition of the andesites. The main minerals occurring in the soils were plagioclase, quartz and pyroxene (clinopyroxene and orthopyroxene) (Fig. 6). This indicated that the soils were formed from the andesites. The samples did not contain calcite. Therefore, the soils where the andesites were excavated could not be the source of the whitish crusts on the stones.

Fig. 6
figure 6

XRD patterns of soil samples (a) Ai.So.1 and (b) As.So.1

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The migration of soluble salt anions such as chloride, sulfate, phosphate, and nitrate from the soil to the andesite can cause substantial damage of the andesites through repeated crystallization and dissolution, depending on temperature and humidity. The results of the ion chromatographic analysis showed that the soil and stone samples from Aigai and Assos contained low amounts of anions (Table 3). Thus, the soluble salt content of the soil may be less effective than factors such as pH of the soil solution, temperature, moisture, carbon dioxide in the deterioration of the buried Aigai and Assos andesites.

Table 3 Chloride, nitrate, phospate and sulphate contents (ppm) in the soils and stone samples
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Mineralogical, Chemical and Microstructural Characteristics of the Whitish Crust

The main mineral phases of the whitish crusts identified in the XRD patterns were calcite, quartz and kaolinite (Fig. 7 a,b). FTIR analysis also confirmed the presence of these minerals in samples. In the FTIR spectrum of whitish crust, the main CaCO3 bands at 1,435 cm−1 (C–O stretching), 873 and 713 cm−1 (C–O bending) and Si–O stretching (1,100 cm−1), Si–O bending (470 cm−1) and O–H stretching (3440 cm−1) were observed (Fig. 7 c,d) (Gadsden 1975).

Fig.7
figure 7

XRD patterns (a) As.WCr2, (b) Ai.WCr3 and their FTIR spectra (c,d) of whitish crusts on excavated andesite surfaces

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Microstructural and Chemical Characteristics of the Whitish Crust

SEM–EDS analysis revealed that the whitish crusts varied in thickness from 15 to 300 microns and contained large amounts of calcium, along with smaller amounts of aluminium and silicon (Fig. 8). The crusts consisted mainly of broken, etched and needle-shaped calcite crystals with a few species of diatoms (Fig. 9). Diatoms are the skeletal shells of many species of unicellular algae (Korunic 2013). They are siliceous organisms that are abundant in all climates and waters (Elmas and Bentli 2013). The formation of needle-shaped calcite crystals and the presence of diatoms may indicate that the excavated andesites at the archaeological sites have been in waterlogged conditions for a long time (Curran et al. 2002; Kuznetsova and Khokhlova 2012).

Fig. 8
figure 8

Backscattered electron image of whitish crust (a) and EDS maps of calcium, sodium, silicon and aluminium (b) for sample Ai2

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Fig.9
figure 9

Secondary electron image of broken and etched (a) and needle-shaped calcite crystals (b), centric (c) and pennate diatoms (d) in the whitish crusts for sample Ai.4

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The whitish crusts can be formed by the alteration of plagioclase (Fig. 10) by carbon dioxide in the waterlogged soils during the burial of andesites (Blum and Stillings 1995; Curran et al. 2002; Gaus et al. 2005; Oelkers et al. 2008; Hangx and Spiers 2009). In the waterlogged soils, CO2 dissolves in the water and reacts with plagioclase to produce kaolinite and calcium bicarbonate solution during burial (Reaction 1). After excavation, water and carbon dioxide are released from the calcium bicarbonate solution and calcium carbonate is precipitated on the andesite surfaces (Reaction 2).

Fig. 10
figure 10

Backscattered electron image of altered plagioclase (a) and its EDS spectrum (b) for sample Ai.3

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$$\begin{array}{lc}{\text{CaAl}}_2{\left({\text{SiO}}_4\right)}_{2\left(\text{s}\right)}+{2\text{CO}}_{2\left(\text{g}\right)}+{3\text{H}}_2{\text{O}}_{\left(\text{l}\right)}\rightarrow&{\text{Al}}_2{\text{Si}}_2{\text{O}}_5{\left(\text{OH}\right)}_{4\left(\text{s}\right)}+\text{Ca}{\left({\text{HCO}}_3\right)}_{2\left(\text{aq}\right)}\\\text{Plagioclase}&\text{Kaolinite}\end{array}$$
(Reaction 1)
$$\begin{array}{c}\text{Ca}{\left({\text{HCO}}_3\right)}_{2\left(\text{aq}\right)}\leftrightarrow{\text{CaCO}}_{3\left(\text{s}\right)}+{\text{H}}_2{\text{O}}_{\left(\text{l}\right)}+{\text{CO}}_{2\left(\text{g}\right)}\\\mathrm{Calcite}\end{array}$$
(Reaction 2)

The alteration of plagioclase with dissolved CO2 in the water leads to the development of secondary porosity and increases with the dissolution of the plagioclase (Curran et al. 2002; Gaus et al. 2005). As can be seen in Fig. 10 (Ai.3), the plagioclases are altered both at the edges and within the crystals beneath the whitish crust of andesite. They show numerous deeply etched nanofractures. An increase in porosity due to the formation of cracks beneath the whitish crust of the andesite surfaces indicates the higher degree of alteration of the plagioclase and subsequent precipitation of calcite (Fig. 11).

Fig. 11
figure 11

Backscattered electron images of whitish crust and the cracks beneath the crust for sample Ai.3 (a, b)

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Conclusions

The Aigai and Assos andesites consist mainly of plagioclase, clinopyroxene and orthopyroxene with rare quartz and hornblende.

The main forms of weathering observed on the standing andesites were mainly the detachment of individual grains, the increase of cracks from the inner parts to the surface.

On the excavated andesites, whitish crusts associated with slight microbiological growth were the main weathering forms observed at the Aigai and Assos archaeological sites. The whitish crusts, composed mainly of calcite crystals with small amounts of kaolinite and quartz, contain freshwater diatom species. Calcite is most likely precipitated by the alteration of plagioclase by carbon dioxide in the soil during the burial of the andesites. The presence of diatoms in the whitish crusts indicates that the excavated andesites may have been in waterlogged conditions in the soil for a long time.

The soils from which the andesites were excavated are composed mainly of plagioclase, quartz, and may also contain clay minerals. They contain very low amounts of soluble salts and do not contain calcite. At the archaeological sites of Aigai and Assos, lime mortar was not used in the construction of the andesite monuments. Therefore, the source of calcite formation on andesite surfaces can not be related to lime mortar or soil.

Whitish crusts composed of calcite with diatoms should not be cleaned from the stone surfaces as they are evidence of the burial history of the monuments and form a protective layer against weathering.