A typical zeolitic volcaniclastic formation refers to a rock with high contents of one or more zeolite minerals. In Greece, there are many widespread zeolite occurrences across the country, the most promising of which are located in north-east Greece, especially in the regional units of Evros and Rhodope. In addition, significant occurrences are located in the Aegean Islands, specifically those of Samos, Kimolos, Polyegos, Milos and Santorini (Stamatakis et al., Reference Stamatakis, Hall and Hein1996; Kantiranis et al., Reference Kantiranis, Chrissafis, Filippidis and Paraskevopoulos2006; Tsirambides & Filippidis, Reference Tsirambides and Filippidis2012).
Natural zeolites are a group of crystalline hydrated aluminosilicate microporous minerals that have as their main property the reversible dehydration and removal of water without destroying their crystal structure (Holmes, Reference Holmes1994). Zeolites are a class of microporous materials with outstanding properties because of their large pore volume, high specific surface area and thermal stability (Meier, Reference Meier1986). Regarding their structure, they consist of tetrahedra units of silica (SiO4) and alumina (AlO4) that form three-dimensional networks. These tetrahedral units are linked by sharing all apical oxygen atoms, forming channels that contain exchangeable cations (potassium, sodium, calcium, etc.), and water molecules (Rehakova et al., Reference Rehakova, Čuvanová, Dzivak, Rimár and Gaval'Ová2004; Jha and Singh, Reference Jha and Singh2011; Król, Reference Król2020). Their crystal structure consists of distinct extra-framework positions in which cations are exchanged. The positions of these extra-framework cations and water molecules bound within the crystal framework of the mineral depend on the nature of the cations involved in the ion exchange (Armbruster & Gunter, Reference Armbruster and Gunter1991; Gunter et al., Reference Gunter, Armbruster, Kohler and Knowles1994).
In nature, 67 types of zeolites are known, while more than 200 types of zeolites have been obtained synthetically (Baerlocher et al., Reference Baerlocher, McCusker and Olson2007). Zeolite-rich rocks contain significant amounts of one or more types of zeolite and have specific mineralogical, chemical, morphological and radiological characteristics (natural radionuclides in zeolite-rich rocks). Through the process of diagenesis and under specific conditions (e.g. of temperature, pressure, pH, salinity), mainly volcanic materials can be completely or partially converted into zeolites in various environments. Under these conditions, occurrences in mafic volcanic rocks that lack economic value can be found, while deposits of sedimentary origin can be formed during diagenesis (Colella, Reference Colella2005).
Of the various zeolite species, HEU-type zeolites (heulandite–clinoptilolite) are widely used in various industrial and environmental applications. HEU-type zeolites form tabular crystals, and the presence of nano/micropores in their framework is characteristic and results in channels with 10- and 8-member rings and dimensions of 7.5 × 3.1 Å, 4.6 × 3.6 Å and 4.7 × 2.8 Å. Greek zeolite-bearing volcaniclastic rocks also contain HEU-type zeolites with similar characteristics (Misaelides et al., Reference Misaelides, Godelitsas, Filippidis, Charistos and Anousis1995; Baerlocher et al., Reference Baerlocher, McCusker and Olson2007; Filippidis & Kantiranis, Reference Filippidis and Kantiranis2007; Filippidis et al., Reference Filippidis, Apostolidis, Paragios and Filippidis2008, Reference Filippidis, Kantiranis, Papastergios and Filippidis2015a, Reference Filippidis, Papastergios, Kantiranis and Filippidis2015b, Reference Filippidis, Kantiranis and Tsirambides2016a, Reference Filippidis, Tziritis, Kantiranis, Tzamos, Gamaletsos, Papastergios and Filippidis2016b, Reference Filippidis, Mytiglaki, Kantiranis and Tsirambides2019; Filippidis, Reference Filippidis2010; Mitchell et al., Reference Mitchell, Michels, Kunze and Perez-Ramirez2012; Vogiatzis et al., Reference Vogiatzis, Kantiranis, Filippidis, Tzamos and Sikalidis2012; Papastergios et al., Reference Papastergios, Kantiranis, Filippidis, Sikalidis, Vogiatzis and Tzamos2017; Floros et al., Reference Floros, Kokkari, Kouloussis, Kantiranis, Damos, Filippidis and Koveos2018).
The use of inorganic materials as feed additives is restricted by several regulations due to their potential toxicity to humans and animals. According to Commission Implementing Regulation (EU) No. 651/2013, only zeolitic tuffs containing ≥80 wt.% clinoptilolite and ≤20 wt.% clay minerals and that are free of fibres and quartz can be used as feed additives in animal husbandry and therefore as nutrition supplements. The existence of fibrous zeolites (mordenite, erionite, roggianite and mazzite) and SiO2 polymorphs (cristobalite, tridymite, quartz) in HEU-type zeolitic tuffs prevents their use as feed additives and nutrition supplements (Filippidis et al., Reference Filippidis, Kantiranis and Tsirambides2016a, 2019), as they can be dangerous to both humans and animals (Davis, Reference Davis, Guthrie and Mossman1993; Driscoll, Reference Driscoll, Guthrie and Mossman1993; Ross et al., Reference Ross, Nolan, Langer, Cooper, Guthrie and Mossman1993).
Important properties of natural zeolites include cation exchange, adsorption, dehydration–hydration, catalytic ability and diffusion capacity. Due to these properties, natural zeolites have been used successfully in animal nutrition and biotechnology to improve the health, safety and productivity of livestock animals and their products (Mumpton & Fishman, Reference Mumpton and Fishman1977; Pond & Mumpton, Reference Pond and Mumpton1984; Elliot & Edwards, Reference Elliot and Edwards1991; Papaioannou et al., Reference Papaioannou, Katsoulos, Panousis and Karatzias2005; Nadziakiewicza et al., Reference Nadziakiewicza, Kehoe and Micek2019; Simona & Camelia, Reference Simona, Camelia, Margeta and Farkaš2019; Mondal et al., Reference Mondal, Biswas, Garai, Sarkar, Banerjee and Brahmachari2021; Souza et al., Reference Souza, García-Villén, Viseras and Pergher2023). Specifically, they prevent or reduce mycotoxin contamination in farm animals, decrease concentrations of ammonia and toxic heavy metals and improve immunity, general health and growth performance in animals of veterinary and biomedical importance (Karaca et al., Reference Karaca, Demir and Onus2004; Wu et al., Reference Wu, Wu, Zhou, Ahmad and Wang2013; Valpotic & Gracner, Reference Valpotic and Gracner2017). They have also been used as alternatives to antibiotics (Papatsiros et al., Reference Papatsiros, Katsoulos, Koutoulis, Karatzia, Dedousi and Christodoulopoulo2013 and references therein). Many researchers have demonstrated the beneficial effects of zeolites on the average daily gain and/or feed conversion in sheep, cattle, rats (Pond & Yen, Reference Pond and Yen1983; Saribeyoglu et al., Reference Saribeyoglu, Aytac, Pekmezci, Saygili, Uzun and Ozbay2011), pigs (Papaioannou et al., Reference Papaioannou, Kyriakis, Alexopoulos, Tzika, Polizopoulou and Kyriakis2004; Ly et al., Reference Ly, Grageola, Lemus-Flores and Castro2007; Prvulovic et al., Reference Prvulović, Jovanović-Galović, Stanić, Popović and Grubor-Lajšić2007) and broilers (Miazzo et al., Reference Miazzo, Rosa, Carvalho, Magnoli, Chiacchiera and Palacio2000; Fendri et al., Reference Fendri, Khannous, Mallek, Traore, Gharsallah and Gdoura2012; Mallek et al., Reference Mallek, Fendri, Khannous, Hassena, Traore, Ayadi and Gdoura2012; Wu et al., Reference Wu, Wu, Zhou, Ahmad and Wang2013). Zeolites also increase the milk yield of dairy cows (Olver, Reference Olver1997; Miazzo et al., Reference Miazzo, Rosa, Carvalho, Magnoli, Chiacchiera and Palacio2000; Papaioannou et al., Reference Papaioannou, Kyriakis, Papasteriadis, Roumbies, Yannakopoulos and Alexopoulos2002, Reference Papaioannou, Katsoulos, Panousis and Karatzias2005; Katsoulos et al., Reference Katsoulos, Panousis, Roubies, Christaki, Arsenos and Karatzias2006; Ly et al., Reference Ly, Grageola, Lemus-Flores and Castro2007; Dschaak et al., Reference Dschaak, Eun, Young, Stott and Peterson2010; Colella, Reference Colella2011). However, such performance enhancement is related to the type of the zeolite used, its purity and physicochemical properties, as well as to the supplementation level used in these diets (Olver, Reference Olver1997; Papaioannou et al., Reference Papaioannou, Kyriakis, Papasteriadis, Roumbies, Yannakopoulos and Alexopoulos2002, Reference Papaioannou, Katsoulos, Panousis and Karatzias2005; Colella, Reference Colella2011).
Clay minerals besides zeolites are also used as feed additives, and they have many beneficial effects due to their specific properties. For example, sepiolite is widely used as a feed additive supplied to broiler chickens and pigs (Rodríguez-Beltrán et al., Reference Rodríguez-Beltrán, Rodriguez-Rojas and Blazquez2013). Smectites and kaolin minerals (kaolinite, dickite, nacrite, halloysite) have been tested as dietary supplements for pigs (Trckova et al., Reference Trckova, Vondruskova, Zraly, Alex, Hamrik and Kummer2009; Slamova & Trckova, Reference Slamova and Trckova2011; Subramaniam & Kim, Reference Subramaniam and Kim2015), and attapulgite has been used as a food additive to promote the growth and health of pigs and broilers (Pappas et al., Reference Pappas, Zoidis, Theophilou, Zervas and Fegeros2010; Zhou & Tan, Reference Zhou and Tan2014).
The Pliocene zeolite-rich volcaniclastic rocks of Santorini Island (Aegean Sea, Greece) cover an area of ~1 km2 west of the village of Akrotiri, and they have a thickness of at least 220 m. These zeolites were formed by the activity of meteoric water within the pile of volcaniclastic material. Variations in the heat flow, ionic activity and permeability of the meteoric water caused the development of the various mineralogical assemblages (Tsolis-Katagas & Katagas, Reference Tsolis-Katagas and Katagas1989; Hall et al., Reference Hall, Stamatakis and Walsh1994; Francalanci et al., Reference Francalanci, Vougioukalakis, Pinarelli, Petrone and Eleftheriadis1995; Stamatakis et al., Reference Stamatakis, Hall and Hein1996; Vougioukalakis, Reference Vougioukalakis2006; Pank et al., Reference Pank, Hansteen, Geldmacher, Hauff, Jicha and Nomikou2022). The zeolitic rocks of Akrotiri have already been studied for their physicochemical characteristics as an industrial commodity that is a potential source of pozzolanic materials (Fytikas et al., Reference Fytikas, Karydakis, Kavouridis, Kolios and Vougioukalakis1990; Kitsopoulos & Dunham, Reference Kitsopoulos and Dunham1996; Kitsopoulos, Reference Kitsopoulos1997, Reference Kitsopoulos1999, Reference Kitsopoulos2001; Pank et al., Reference Pank, Hansteen, Geldmacher, Hauff, Jicha and Nomikou2022).
In this study, the physicochemical characteristics (i.e. mineralogical and chemical composition and sorption ability) of representative samples of the zeolitic rocks of Akrotiri, Santorini Island (Greece), were studied to evaluate their potential as feed additives and nutrition supplements according to Commission Implementing Regulation (EU) No 651/2013. To the best of our knowledge, this is the first time that representative total samples from the rhyodacitic zeolitic rocks of Akrotiri have been evaluated for specific livestock uses.
Materials and methods
Four representative zeolitic rock samples (S1, S2, S3, S4) were collected from an area of ~1 km2 located west of the village of Akrotiri on Santorini Island (Aegean Sea, Greece; Fig. 1). The locations of the surface rock samples were scattered throughout the zeolitic rocks of Akrotiri, so the samples were representative of the various types of zeolite occurring in the study area. In the broader area of Akrotiri, there are successive occurrences of volcanic rocks and volcaniclastic materials from the various volcanic eruptions of the Santorini volcano. This research focuses on the study of hornblende-bearing rhyodacitic volcaniclastic tuffs known as ‘Akrotiri tuffs’. The exact locations (coordinates and distance) of each sample in relation to the village of Akrotiri are given in Table 1. Each sample was ground with an agate mortar and sieved to pass through a 0.125 mm sieve. The powdered sample was then divided into halves. One half was further powdered in an agate mortar and passed through 0.063 mm a sieve. The second half (with a grain size of <0.125 mm) was used for ammonium acetate saturation (AMAS) analysis, while that of grain size <0.063 mm was used for the mineralogical (powder X-ray diffraction; XRD) and chemical (X-ray fluorescence; XRF) analysers. Furthermore, polished thin sections of the rock samples were prepared to study their morphological and chemical characteristics using scanning electron microscopy and energy-dispersive X-ray spectroscopy (SEM-EDS).
(a) Location of Santorini Island, Greece, and (b) geological map and location of zeolite sampling points in Akrotiri, Santorini Island (modified after Druitt et al., Reference Druitt, Edwards, Mellors, Pyle, Sparks and Lanphere1999).
Mineralogical composition (wt.%) of the zeolitic rocks from Akrotiri (Santorini Island, Greece).
Powder XRD was used to determine the mineralogical composition of randomly oriented samples using a Philips PW1710 diffractometer with Ni-filtered Cu-K – radiation. The counting conditions were: start angle 3°2θ, end angle 63°2θ, step size 0.02°2θ, time per step 1 s and scan speed 0.02° s–1. Mineral abundance was estimated using (1) intensity (counts) of certain reflections, (2) density of the examined mineral and (3) the mass absorption coefficient for Cu-Kα radiation. Finally, for the refining of the results, a Rietveld-based refinement routine was used (TOPAS 6.0® (2016) software). The routine is based on the calculation of a single mineral-phase pattern according to the crystalline structure of the respective mineral. The refinement of the pattern uses a non-linear least squares routine. The quantification errors were determined without an internal standard and calculated for each phase according to Bish & Post (1993). The clay minerals present were identified from air-dried, saturated in glycol and heat-treated (550°C for 2.5 h) oriented samples scanned from 3° to 23°2θ at a scanning speed of 1.2° min–1. The amorphous (volcanic glass) content of the studied samples was determined using the fitting method on the broad background hump between 10° and 20°2θ in the powder XRD trace (Kantiranis et al., Reference Kantiranis, Stergiou, Filippidis and Drakoulis2004b, Reference Kantiranis, Filippidis and Georgakopoulos2005).
The cation-exchange capacity (CEC) of the studied materials was evaluated according to their efficiency at exchange their extra-framework cations with the ammonium ion. Consequently, the ammonium-exchange capacity (sorption ability) was determined using the ammonium acetate (CH3COONH4) saturation (AMAS) method (Bain & Smith, Reference Bain, Smith and Wilson1987; Kitsopoulos, Reference Kitsopoulos1999; Kantiranis et al., Reference Kantiranis, Stamatakis, Filippidis and Squires2004a, Reference Kantiranis, Filippidis and Georgakopoulos2005).
Initially, a solution of 1 N ammonium acetate with pH value 7.0 was prepared (using a Hanna Instruments HI2002-01 Edge® Dedicated pH/ORP Meter). Each sample passed through the 0.063 mm sieve (Retsch®) was divided into four aliquots of 100–150 mg each that were placed into 15 mL test tubes. Some 10 mL of 1 N CH3COONH4 solution (Chem-Lab NV) was then added and stirred vigorously by hand for a few seconds. The tubes were then placed in a rotary stirrer (Heidolph Reax20) for 24 h before being centrifuged at 1500 rpm for 4 min. The supernatant was decanted and 10 mL of fresh 1 N CH3COONH4 solution added following the same procedure. A 10 day saturation procedure was followed to achieve complete sorption of ammonium ions by the studied zeolite-rich materials (Bain & Smith, Reference Bain, Smith and Wilson1987; Kitsopoulos, Reference Kitsopoulos1999; Kantiranis et al., Reference Kantiranis, Stamatakis, Filippidis and Squires2004a). After the 10 day saturation procedure, the samples were rinsed with 99% isopropyl alcohol (Chem-Lab NV) to remove excess NH4+. Specifically, 10 mL of isopropyl alcohol was added to each test tube, which was stirred vigorously and centrifuged at 2500 rpm for 5 min (Rotanda 460, Hettich Zentrifugen). The washing process was repeated six times. After the sixth washing cycle, the supernatant was collected in a beaker and checked for precipitates by adding Nessler reagent (alkaline solution K2[HgI4]; Chem-Lab NV) and concentrated NaOH solution (Chem-Lab NV). The formation of a brown precipitate or brownish-yellow solution (Bain & Smith, Reference Bain, Smith and Wilson1987) indicates an excess of NH4+ ions, meaning that washing needs to be repeated. Finally, the samples were allowed to dry at room temperature. A JENWAY 3340 ion/pH meter combined with an ORION ammonia electrode was used to measure sorption ability. Four measurements were taken from each dry sample, and the mean average sorption ability was calculated. The method was evaluated using standard mixtures of amorphous material and crystalline phases and the standard deviation was found to be 5 meq 100 g–1 (Drakoulis et al., Reference Drakoulis, Kantiranis, Filippidis and Stergiou2005).
Chemical analysis of the Akrotiri samples was performed using a Bruker S4-Pioneer XRF wavelength-dispersive spectrometer equipped with a Rh tube and six analysing crystals, a gas-flow proportional counter (P10 gas, a mixture of 90% argon and 10% methane), a scintillation detector or a combination of the two detectors. A fused glass bead was used for the analysis of the major elements at tube-operating conditions of 60 kV and 45 mA. Theoretical alpha factors and measured line overlap factors for the measured raw intensities were applied to correct the matrix effects in the samples. The standards used to calibrate the major element analyses were AGV-1 (andesite), JG-1 (granodiorite), JB-1 (granodiorite), NIM-G (granite), GA (granite) and GH (granite).
A JEOL JSM-840A SEM device equipped with an X-ray EDS micro-analytical system (SEM-EDS; with a LINK 10000 AN energy dispersion analyser) was used for the morphological study and microanalysis of the Santorini samples. Corrections were made using ZAF-4/FLS software provided by LINK. To reduce the volatilization of alkali metals from the zeolite framework, the spot size of the electron beam was enlarged whilst the counting time was decreased (accelerating voltage 15–20 keV, beam current 0.4 mA, spot size 50–60 μm, EDS live time 60–80 s). Minerals such as micas, feldspars and carbonates, as well as pure metals, were used as probe standards. The average chemical formula for the studied zeolites (clinoptilolite and mordenite) was calculated from the chemical microanalysis based on the formulas (Na,K)6(Al6Si30O72)⋅20H2O for clinoptilolite and (Na3KCa2)(Al8Si40O96)⋅28H2O for mordenite (Gottardi & Galli, Reference Gottardi and Galli1985).
Results and discussion
The mineral and amorphous phase contents of the studied samples are listed in Table 1, whereas representative powder XRD traces of the clinoptilolitic sample (S1), mordenitic sample (S4) and a representative powder XRD trace of the mixed samples (S2 and S3) are shown in Fig. 2. Samples 2 and 3 have comparable mineralogical compositions. As a consequence, only sample S2 was selected for further analysis. Mineral phases were identified using the PDF-4+ database with SIeve+ search indexing software from the International Centre for Diffraction Data.
Representative XRD traces of (a) the clinoptilolite-rich sample (S1), (b) the mixed clinoptilolite–mordenite sample (S2) and (c) the mordenite-rich (S4) sample. GL = glycol saturated; HT = heat treated; OR = air dried; WR = whole rock.
The zeolitic rocks contain up to 56 wt.% HEU-type zeolite (clinoptilolite), up to 69 wt.% mordenite, 4–36 wt.% feldspars (both K-feldspar and plagioclase), 6–8 wt.% clay minerals, 3–6 wt.% quartz, 2 wt.% opal-CT, 2–4 wt.% amphibole and 4–7 wt.% amorphous material (volcanic glass).
The main clay mineral phase in all samples is smectite, and kaolinite is also present in minor amounts.
Concerning zeolite types (Table 1), sample S1 contains only HEU-type zeolite (clinoptilolite; 46 wt.%), samples S2 and S3 contain HEU-type zeolite (clinoptilolite; 54 and 56 wt.%, respectively) and mordenite (10 and 11 wt.%, respectively) and sample S4 contains only mordenite (69 wt.%). The high alkali content of the Akrotiri rhyodacitic–dacitic (Tsolis-Katagas & Katagas, Reference Tsolis-Katagas and Katagas1989; Kitsopoulos et al., Reference Kitsopoulos, Scott, Jeffrey and Marsh2001) volcaniclastic tuffs favours environments with high pH that allow increased silica activity from the volcanic glass that, in turn, leads to the formation of clinoptilolite instead of heulandite (Gottardi & Galli, Reference Gottardi and Galli1985). However, under active geothermal conditions, extensive alteration of the Akrotiri dacitic tuffs further favours not only the formation of clinoptilolite but also the formation of mordenite (Seki, Reference Seki1970).
Tsolis-Katagas & Katagas (Reference Tsolis-Katagas and Katagas1989) and Kitsopoulos et al. (Reference Kitsopoulos, Scott, Jeffrey and Marsh2001) studied the altered dacitic pre-caldera pyroclastic rocks of the Santorini volcano to the east of the village of Akrotiri and observed that K-rich and (K,Ca)-rich clinoptilolite, mordenite and opal-CT authigenic minerals and clay minerals are abundant. Inhomogeneities in the chemical composition of dacitic materials control HEU-type mineral formation, whereas mordenite presence follows the formation of HEU-type zeolites and opal-CT.
Filippidis et al. (Reference Filippidis, Kantiranis, Stamatakis, Drakoulis and Tzamos2007) studied zeolitic samples taken near the village of Akrotiri and found that they contained clinoptilolite between 33 and 57 wt.% and mordenite between 15 and 56 wt.%, while the total microporous mineral content varied between 47 and 86 wt.%. Additionally, CEC of the samples measured using the AMAS method varied between 118 and 177 meq 100 g–1.
The CEC of the zeolitic rock samples from Akrotiri varied between 104 (sample S1) and 158 meq 100 g–1 (sample S4; Table 2). The CEC of zeolitic rocks mainly depends on the total zeolite content (clinoptilolite and mordenite), as these are the most typical microporous minerals, as well as on the total content of clay minerals and amorphous materials (Tables 1 & 2).
CEC values of the zeolitic rocks of Akrotiri and their correlation with zeolite content, microporous minerals and microporous minerals plus amorphous materials.
The CEC (104–158 meq 100 g–1) of the studied zeolitic rocks shows a strong correlation (R 2 = 0.9982) with the total zeolite (clinoptilolite + mordenite) content (46–69 wt.%). Similar behaviour is observed when the CEC is compared to the total content (52–77 wt.%) of microporous minerals (zeolites + clay minerals) and the total content (56–84 wt.%) of microporous minerals plus amorphous materials (Fig. 3).
Variation in CEC (meq 100 g–1) with mineral and amorphous matter content (wt.%).
The chemical compositions of the studied samples are shown in Table 3. Based on SiO2 content, the samples can be classified as intermediate to acidic volcanic rocks. SEM images of the studied samples are presented in Fig. 4. Both zeolites grow in empty pores as a result of volcanic glass alteration of the andesitic fragments. More specifically, clinoptilolite grows as elongated, fine, tabular crystals (Fig. 4a) from the outer parts of volcanic glass shards towards the centre of those shards. The radial growth of the tabular clinoptilolitic crystals is clearly indicated in Fig. 4b. Mordenite grows in similar empty spaces and presents a characteristic fibrous structure (Fig. 4c).
Chemical composition (wt.%) of the zeolitic rocks of Akrotiri (Santorini Island, Greece).
LOI = loss on ignition.
SEM images of the studied Akrotiri tuffs. (a & b) Tabular crystals of clinoptilolite in altered glass shards of sample S1 are highlighted in the red circle in (a), and a zoomed-in image is provided in (b). (c) Fibrous mordenite in altered glass shards of sample S2 highlighted in the red circle.
The spot chemical analyses (average of five analyses for each phase) for clinoptilolite and mordenite are listed on Table 4. As the two zeolite phases in all four samples are mordenite and clinoptilolite, the highest and the lowest values of each oxide are shown in the chemical analyses. The clinoptilolite and mordenite have comparable SiO2 contents. It is clear that clinoptilolite is richer in Ca than mordenite. Finally, mordenite contains greater amounts of Na2O and K2O than clinoptilolite.
Chemical analysis from SEM-EDS of the clinoptilolite and mordenite.
aIron was measured as total FeO.
bCalculated by difference from 100 wt.%.
The chemical analysis was used to calculate the chemical formulae of the two zeolites. The formulae obtained were: clinoptilolite = (Fet 0.016–0.311Mg0.525–0.797Ca0.955–2.763Na0.867–2.461K0.461–1.291)(Al5.883–6.032Si28.911–29.981)O72(13.72–14.18)H2O and mordenite = (Fet 0.000–0.295Mg0.338–0.660Ca0.552–1.045K2.001–2.229Na2.544–6.092)(Al7.795–8.805Si39.362–39.471)O96(13.17–13.19)H2O.
Mercurio et al. (Reference Mercurio, Langella, Cappelletti, De Gennaro, Monetti and De Gennaro2012, Reference Mercurio, Cappelletti, De Gennaro, De Gennaro, Bovera and Iannaccone2016) studied the mineralogical (powder XRD) and chemical (XRF) composition of a zeolite-rich tuff derived from alteration of an ignimbrite. Although the presence of SiO2 minerals (quartz, cristobalite, tridymite) was not discussed, these authors concluded that the studied phillipsite-rich tuff is suitable for use as a feed additive. By contrast, Filippidis et al. (Reference Filippidis, Kantiranis and Tsirambides2016a, Reference Filippidis, Mytiglaki, Kantiranis and Tsirambides2019) studied a large number of zeolitic tuffs from Greece, mainly originating from areas of Thrace (north Greece) and Samos Island (east Aegean Sea), and found that all of the studied zeolitic formations contain quartz and/or clay minerals in prohibited amounts (according to Commission Implementing Regulation (EU) No 651/2013). Consequently, these zeolite-rich tuffs are not suitable for use as feed additives and nutrition supplements.
Numerous studies have been conducted on the application of high-quality natural zeolites as feed additives and nutrition supplements (Mumpton & Fishman, Reference Mumpton and Fishman1977; Pond & Yen, Reference Pond and Yen1983; Pond & Mumpton, Reference Pond and Mumpton1984; Elliot & Edwards, Reference Elliot and Edwards1991; Olver, Reference Olver1997; Miazzo et al., Reference Miazzo, Rosa, Carvalho, Magnoli, Chiacchiera and Palacio2000; Papaioannou et al., Reference Papaioannou, Kyriakis, Papasteriadis, Roumbies, Yannakopoulos and Alexopoulos2002, Reference Papaioannou, Kyriakis, Alexopoulos, Tzika, Polizopoulou and Kyriakis2004, Reference Papaioannou, Katsoulos, Panousis and Karatzias2005; Karaca et al., Reference Karaca, Demir and Onus2004; Katsoulos et al., Reference Katsoulos, Panousis, Roubies, Christaki, Arsenos and Karatzias2006; Ly et al., Reference Ly, Grageola, Lemus-Flores and Castro2007; Prvulovic et al., Reference Prvulović, Jovanović-Galović, Stanić, Popović and Grubor-Lajšić2007; Trckova et al., Reference Trckova, Vondruskova, Zraly, Alex, Hamrik and Kummer2009; Dschaak et al., Reference Dschaak, Eun, Young, Stott and Peterson2010; Pappas et al., Reference Pappas, Zoidis, Theophilou, Zervas and Fegeros2010; Colella, Reference Colella2011; Saribeyoglu et al., Reference Saribeyoglu, Aytac, Pekmezci, Saygili, Uzun and Ozbay2011; Slaova & Trckova, Reference Slamova and Trckova2011; Fendri et al., Reference Fendri, Khannous, Mallek, Traore, Gharsallah and Gdoura2012; Mallek et al., Reference Mallek, Fendri, Khannous, Hassena, Traore, Ayadi and Gdoura2012; Papatsiros et al., Reference Papatsiros, Katsoulos, Koutoulis, Karatzia, Dedousi and Christodoulopoulo2013; Rodríguez-Beltrán et al., Reference Rodríguez-Beltrán, Rodriguez-Rojas and Blazquez2013; Wu et al., Reference Wu, Wu, Zhou, Ahmad and Wang2013; Zhou & Tan, Reference Zhou and Tan2014; Subramaniam & Kim, Reference Subramaniam and Kim2015; Valpotic & Gracner, Reference Valpotic and Gracner2017; Nadziakiewicza, Reference Nadziakiewicza, Kehoe and Micek2019). Although particular attention has been given to the chemical composition of these materials, the presence and abundance of SiO2 polymorphs (quartz, cristobalite, tridymite) and fibrous forms of zeolite have not been considered in sufficient detail to characterize and assess their suitability for these uses. For example, the effect of the zeolite (clinoptilolite) on meat quality has been reported for chickens (Mallek et al., Reference Mallek, Fendri, Khannous, Hassena, Traore, Ayadi and Gdoura2012), turkeys (Hcini et al., Reference Hcini, Ben Slima, Kallel, Zormati, Traore and Gdoura2018), geese (Larina et al., Reference Larina, Ezhkov, Fayzrakhmanov and Ezhkova2020), pigs (Kim et al., Reference Kim, Yang, Choi, Jung and Shim2014) and fish (Paritova, Reference Paritova2014). The use of natural zeolite (with 87% clinoptilolite) improved the growth of female broilers by increasing the digestibility of nutrients and improving their intestinal health (Wawrzyniak et al., Reference Wawrzyniak, Kapica, Stepien-Pysniak, Luszezewska-Sierakowska, Szewerniak and Jarosz2017). Osman & Soliman (Reference Osman and Soliman2021) also reported that supplementing zeolite in the feed of high-yielding lactating cows led to improved digestion coefficients, feed conversion, milk production and fat yields.
The zeolitic rocks from Akrotiri can be classified as follows: (1) sample S1 containing 46 wt.% clinoptilolite plus 3 wt.% quartz; (2) samples S2 and S3 containing 54–56 wt.% clinoptilolite plus 10–11 wt.% mordenite (fibrous zeolite) plus 3 wt.% quartz; and (3) sample S4 containing 69 wt.% mordenite (fibrous zeolite) plus 6 wt.% quartz (Table 1). All of the samples contain <20 wt.% clay minerals, but they do not contain significant amounts (≥80 wt.%) of clinoptilolite and are not quartz-free. Therefore, we conclude that the zeolitic rocks of the village of Akrotiri (Santorini Island, Greece) are not suitable as feed additives or nutrition supplements (according to Commission Implementing Regulation (EU) No 651/2013). However, Kitsopoulos & Dunham (Reference Kitsopoulos and Dunham1996), who studied heulandite- and mordenite-rich zeolitic tuffs from Santorini in the same area of Akrotiri, concluded that, following calcination, these materials may replace Portland cement at up to 4 wt.% in concrete mixtures and may increase the compressive strength of concrete.
Summary and conclusions
The zeolitic rocks of Akrotiri on Santorini Island can be grouped as follows: (1) one sample contains only clinoptilolite (46 wt.%); (2) two samples contain clinoptilolite (54–56 wt.%) plus mordenite (10–11 wt.%); and (3) one sample contains only mordenite (69 wt.%). All samples contain 8–39 wt.% feldspars (K-feldspar + plagioclase), 6–8 wt.% clay minerals (mainly smectite and minor kaolinite), 3–6 wt.% quartz, 2 wt.% opal-CT and 4–7 wt.% amorphous materials. Based on their chemical composition according to SiO2 content, the Akrotiri samples can be classified as intermediate to acidic volcanic rocks, whereas based on the total content of alkalis (K2O + Na2O) vs SiO2 content they can be classified as andesites or dacites.
The CEC values of the zeolite rocks vary from 104 to 158 meq 100 g–1 and depend primarily on the total zeolite content (clinoptilolite + mordenite) and secondarily on the content of clay minerals and amorphous materials. The sorption ability of these rocks increases with increasing zeolite content, increasing content of microporous minerals and increasing content of microporous minerals plus amorphous materials.
Due to the measured mineralogical content of the studied clinoptilolite-bearing rocks, we conclude that none of them are suitable as feed additives or nutrition supplements according to Commission Implementing Regulation (EU) No 651/2013. Specifically, three samples contain <80 wt.% clinoptilolite (46–56 wt.%) and 3 wt.% quartz, and two contain 10–11 wt.% of a fibrous zeolite (mordenite). The non-clinoptilolite-bearing sample contains 69 wt.% of the fibrous zeolite mordenite and 6 wt.% quartz, and therefore this sample cannot be used as a feed additive and nutrition supplement for any animal husbandry as well. Other uses of the studied zeolitic rocks (e.g. as a replacement for Portland cement in concrete mixes) offer alternative prospects for the exploitation of these materials.
Acknowledgements
The authors thank Professor Lambrini Papadopoulou, from Aristotle University of Thessaloniki, for her significant contributions to the SEM-EDS analysis process, as well as for her invaluable and insightful comments.
Conflicts of interest
The authors declare none.
