*Editorial* **Natural vs. Synthetic Zeolites**

## **Magdalena Król**

Faculty of Materials Science and Ceramic, AGH University of Science and Technology, 30 Mickiewicza Av., 30-059 Krakow, Poland; mkrol@agh.edu.pl

Received: 14 July 2020; Accepted: 15 July 2020; Published: 17 July 2020

**Abstract:** This brief review article describes the structure, properties and applications of natural and synthetic zeolites, with particular emphasis on zeolites obtained from natural or waste materials. Certainly, such short work does not exhaust the complexity of the problem, but it sheds light on some outstanding issues on this subject.

**Keywords:** natural zeolite; zeolite synthesis; zeolite characterization; zeolite application

#### **1. What Are Zeolites?**

Zeolites can be defined in two ways. They are hydrated tectoaluminosilicates with the general formula [1,2]:

$$\mathrm{M\_{x}M\_{y}'N\_{z}\left[T\_{m}T\_{n}'\dots\right.\left.\left.O\_{2(m+n)-\varepsilon}\left(OH\right)\_{2x}\right]\left(OH\right)\_{br}\left(aq\right)\_{p}\cdot qQ}\tag{1}$$

where M, M' are exchangeable and non-exchangeable cations, respectively; N are non-metallic cations (generally removable on heating); (aq) represents chemically bonded water (or other strongly held ligands of T-atoms); Q are sorbate molecules; T, T' are Si and Al, but also Be, B, Ga, Ge and P, among others. This formula is particularly useful when describing natural zeolites [2], but also those synthesized from natural or waste materials, due to their complex chemical composition. where M, M' T, T'

On the other hand, zeolites can be more graphically, as shown in Figure 1, defined as crystalline inorganic polymers consisting of [SiO4] and [AlO4] tetrahedra, having the structure filled ions and water molecules, having great freedom of movement.

**Figure 1.** Scheme of zeolite structure.

The specific structure of the zeolites gives them a number of unique properties. The most important regarding potential uses include [3,4]:


These properties, caused by zeolites, arouse great interest among chemists, technologists and mineralogists. Although it may seem that the years of the most intensive research on this group of minerals have already passed, interest in them is not decreasing. Constantly conducted research on the specific properties of zeolites show comprehensive possibilities of using this type of mineral, as shown in Figure 2. Of course, all applications of zeolites cannot be mentioned (only examples will be presented in this paper), but even those presented, for example, give a picture of the great importance of zeolites in various applications. In addition, other properties and possibilities of practical use are being discovered. 

**Figure 2.** Zeolite applications.

#### **2. Natural Zeolites**

Natural zeolites are hydrothermal and of mainly volcanic origin. They can occur both in crystallized forms found in igneous and metamorphic rocks, as well as in grains of smaller diameters accumulated in sedimentary rocks [5]. Ocean bottom sediments are relatively huge and rich in zeolites, but these deposits are so far inaccessible to humans. However, these minerals may also constitute important components of tuffs or clay. Such surface retention of zeolite sediments, and therefore relatively simple mining using the opencast method, creates perfect conditions for their wider use. It should be mentioned here that the zeolites naturally occurring in nature, possessing operational significance, are: clinoptilolite; mordenite; chabazite.

#### *Application Examples*

– Natural zeolites have a high selectivity for heavy metal ions [6–8] and ammonium ions [9]. Thus, zeolites have found important uses in environmental protection and agriculture. Wastewater treatment from heavy metal ions [10,11] or radioactive isotopes [12] can take place in sorption columns filled with zeolite. Ammonium ions contained in municipal, industrial and agricultural wastes can also

be removed in a similar way [13]. In agriculture, zeolites can be used as carriers of agrochemical compounds, in the treatment of soil and fish ponds and as a feed additive [2,14]. Attempts to modify the structure to give them catalytic [15] or antibacterial [16] properties are also made. After all, they are also widely used in many households as pet litter.

On the other hand, natural zeolites have limited applications in industry because, as already mentioned, their properties are strictly dependent on their crystal structure. The main disadvantage is that the channel diameters are too small (in the case of clinoptilolite, which is the most common in nature, it is 0.30–4 nm [17]), which do not allow for the adsorption of larger gas molecules and organic compounds. In addition, zeolite deposits are a non-renewable resource. The need for the synthesis of molecular sieves and adsorbents with very specific parameters means that numerous attempts were made to obtain zeolites in the laboratory.

#### **3. Synthetic Zeolites**

Zeolites have been recognized as minerals of natural origin, but currently more than one hundred different types of zeolite structures are known which can be obtained synthetically [17]. Under natural conditions zeolites were formed as a result of the reaction of volcanic ash with the waters of the basic lakes. This process lasted several thousand years. In laboratory conditions, an attempt can be made to imitate hydrothermal processes using elevated temperature or pressure and using natural raw materials and/or synthetic silicates. The synthesis reaction requires appropriate equipment, clean substrates and energy. As a result, the price of the product may be much higher than the price of natural zeolite. Therefore, research often focuses on the search for cheaper and available substrates for the production of zeolites, while striving to reduce the cost of the reaction itself. The current trends in research on the synthesis of zeolites are shaped by environmental aspects, which implies the use of natural or waste raw materials for this purpose.

#### *3.1. Substrates and Products*

Various natural silica carriers, such as clay minerals (kaoline [18–20], haloisite [20,21]), volcanic glasses (perlite [22–24], pumice [25]) or diatomites [26], are used in zeolite synthesis. On the other hand, zeolites are widely obtained from aluminosilicate waste materials, such as fly ash [27–29], rice husk [30] or expanded perlite waste [31].

Of course, synthesis using raw materials with a complex chemical composition will not give the product 100% purity and zeolites obtained in this way are excluded from many important commercial applications; however, the use of natural raw materials for the production of zeolites has economic advantages when compared with the use of synthetic substrates.

#### *3.2. Synthesis Methods*

The oldest of the works on the synthesis of aluminosilicates under hydrothermal conditions dates back to the 1950s of the last century [32]. They show that, by heating the aluminosilicate raw materials in the presence of alkaline solutions within a few hours or days, depending on the type of raw materials and process conditions (temperature, pressure), a final product can be obtained. Today, many different methods for the synthesis of zeolites are known. The most important of them should be mentioned:


The first method is relatively commonly used. This process generally reflects the natural conditions in which rocks containing zeolite minerals were formed. Hydrothermal (80–350 ◦C) synthesis of zeolites requires the supply of components that are a source of Si and Al, followed by treatment with an alkaline solution (pH > 8.5). The reactions, during which processes such as dissolution, condensation, gelatinization and crystallization take place [40] are carried out in autoclaves, are often at elevated pressure. The appropriate control of process parameters favors the formation of desired products.

The estimated cost of zeolite material obtained by the above methods is between natural and synthetic zeolite. However, taking into account the fact that fees for the storage and utilization of waste will probably increase, the implementation of one of these technologies may prove to be the most cost-effective solution.

#### *3.3. Zolitization Products*

Synthetic zeolites, as with natural ones, are diverse in structure and properties. This is influenced by the composition of the raw materials and the synthesis conditions. As mentioned above, the crystallization of individual types of zeolites is a function of such parameters as reaction time, temperature and pressure, as well as the chemical composition of the reaction mixture, including the concentration of reagents [41]. The type of phases formed in such systems is relatively well known. At high temperatures and at elevated pressure, mainly analcime, as shown in Figure 3a, zeolite Na-P1, as shown in Figure 3b, and hydroxysodalite, as shown in Figure 3c, in various quantitative proportions are obtained, depending on the synthesis parameters. At temperatures <100 ◦C, zeolite X, as shown in Figure 4a, Na-P1 zeolite, as shown in Figure 4b, and hydroxysodalite can be obtained. In addition, zeolite A can be obtained in reaction systems with a high proportion of aluminum (Si/Al < 1), as shown in Figure 4c. A less frequently used fusion method leads to the synthesis of sodalite and cancrite.

**Figure 3.** Microstructures of zeolites obtained in hydrothermal conditions at elevated pressure: (**a**) analcime; (**b**) zeolite Na-P1; (**c**) hydroxysodalite.

**Figure 4.** Microstructures of zeolites obtained in hydrothermal conditions at low temperatures (<100 ◦C): (**a**) zeolite X; (**b**) zeolite Na-P1; (**c**) zeolite A.

It is worth noting that, when comparing the microstructures of materials obtained under different conditions, it can be stated that, at low temperatures, crystallites of smaller sizes and less developed morphology are formed.

From a practical point of view, zeolites with a larger pore size are more useful. Hence, in order to obtain material with good sorption properties, it is important to choose process parameters that promote the formation of zeolite X or zeolite Na-P1. The possibility of obtaining other zeolite phases can be regulated by the addition of organic templates, which generate higher process costs.

An important ecological approach is planning the synthesis conditions in such a way as to limit the production of highly alkaline wastewater.

#### *3.4. Benefits of Synthetic Zeolite*

Numerous scientific studies confirm the benefits of synthetic zeolites compared to natural ones. The efficiency of natural zeolites in the removal of radioactive waste from the environment (Cs and Sr radionuclides) was found to be lower than that of synthetic zeolites [42,43]. Synthetic zeolites also show much higher adsorption capacity for heavy metal ions (eg. Cd2+, Pb2+, Zn2+, Cu2+, Ni2+, Cr3+) than the natural zeolites [44,45].

Another advantage of synthetic zeolites over natural ones is the significantly larger pore size. This allows for the sorption of larger molecules, which extends the range of potential applications. For example, it was found that synthetic zeolites have two-times higher oil sorption capacities than the natural clinoptilolite, so synthetic zeolites are a promising alternative for natural mineral sorbents for land-based petroleum spills cleanup [46]. Furthermore, zeolites with smaller pore sizes, used as catalysts, suffer from pore blockage and ultimately poisoning and deactivation, while zeolites with large interconnected channels remain stable much longer in reactions [47].

During the synthesis of zeolites, the Al content can be adjusted. Zeolites with low Si/Al ratio are much more polar and thus exhibit stronger sorption capacities. Zeolites with high silicon content are also characterized by greater power of active centers, which promotes them for catalytic applications [48]. On the other hand, high silica zeolites have a more homogeneous surface characteristic and exhibit hydrophobic properties. They have been used in reactions in which resulting water poisons the acid sites of the catalysts [49].

It should be noted that the synthetic zeolites are not without disadvantages. The worrisome problem is that synthetic zeolites are mostly in the form of finely grained crystalline and highly dispersive powder (single crystals have a size to a few microns), which certainly limits their use. For natural zeolites, after mining the deposit, isolation processes, such as crushing and pellet formation, are usually used to obtain zeolites in usable form. In contrast, the synthetic ones can be used in the form of hard, wear-resistant granules. There are many reports related to receiving zeolite granulates, but for the most part they are not yet used in practical applications.

#### **4. Summary**

In agreement with the scientific evidence presented in the related literature so far, it can be generally stated that both sedimentary rocks containing zeolites and zeolites synthesized from aluminosilicate raw materials may be regarded as useful for many industries, including agriculture and environmental protection. Synthetic zeolites are the major alternate materials to natural ones. Synthetic zeolites can tailor physical and chemical characteristics to serve many applications more closely and they are more uniform in quality than their natural equivalent. However, conclusive statements on the exact applications and benefits of clay- and waste-based zeolites should be carefully investigated and analyzed for each specific material as the mechanisms of action correlate with the specific material's physical and chemical properties.

Due to the different origins of substrates and different methods of synthesis, the products have various chemical and phase compositions. Constantly conducted research on the specific properties of zeolites show comprehensive possibilities of using this type of mineral. Further research is necessary to tackle yet undetermined topics, such as:

• Accurate analysis of used raw material composition;


Additional comprehension of the above problems will allow advances in the applicability of zeolites in providing new low cost, eco-friendly solutions to industrial, agricultural or everyday applications.

**Funding:** This work was partially supported by The National Science Centre Poland under grant no. 2016/ 21/D/ST8/01692.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Article* **First Occurrence of Willhendersonite in the Lessini Mounts, Northern Italy**

**Michele Mattioli \* and Marco Cenni**

Department of Pure and Applied Sciences, Campus Scientifico Enrico Mattei, University of Urbino Carlo Bo, Via Cà le Suore 2/4, 61029 Urbino, Italy; marco.cenni@tiscali.it

**\*** Correspondence: michele.mattioli@uniurb.it

**Abstract:** Willhendersonite is a rare zeolite, with very few occurrences reported globally (Terni Province, Italy; the Eifel Region, Germany; Styria, Austria). Moreover, the data available from these sites are very limited and do not allow a detailed picture of this zeolite's mineralogical and chemical characteristics. In this work, a new willhendersonite occurrence is reported from the Tertiary volcanic rocks of the Lessini Mounts, northern Italy. Morphology, mineralogy and chemical composition of selected crystals were studied by scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray Diffraction (XRD), and electron probe microanalyser (EPMA). Willhendersonite occurs within basanitic rocks as isolated, colorless, transparent crystals with prismatic to flattened morphologies. Individual crystals often grow together to form small elongated clusters and trellis-like aggregates. The diffraction pattern exhibits 33 well-resolved diffraction peaks, all of which can be indexed to a triclinic cell with unit cell parameters *<sup>a</sup>* = 9.239(2) Å; *<sup>b</sup>* = 9.221(2) Å; and *<sup>c</sup>* = 9.496(2) Å, <sup>α</sup> = 92.324(2)◦ , β = 92.677(2)◦ , γ = 89.992◦ (Space Group P1). The chemical data point to significant variability from Ca-rich willhendersonite (K0.23Na0.03)Σ=0,26Ca1.24 (Si3.06Al3,00Fe3+ 0.01)Σ=6,07 O12·5H2O) to Ca-K terms (K0.94Na0.01)Σ=0,95Ca0.99 (Si3.07Al2.93Fe3+ 0.00)Σ=6,00O12·5H2O). Willhendersonite from the Lessini Mounts highlights the existence of an isomorphous series between the Ca-pure crystals and Ca-K compositions, possibly extended up to a potassic end-member.

**Keywords:** willhendersonite; chabazite; zeolites; Lessini Mounts

#### **1. Introduction**

Willhendersonite was first described by Peacor et al. [1], who studied specimens from mafic potassic lava at San Venanzo (Terni, Italy) and from a limestone xenolith within basalt at Mayen (Eifel area, Germany). Both these samples correspond to the schematic chemical formula CaKAl3Si3O12·5H2O, with space group P1 and unit cell parameters *a* = 9.206; *b* = 9.216; and *<sup>c</sup>* = 9.500 Å, α = 92.34, β = 92.70, γ = 90.12◦ [1,2]. Structural investigations [2,3] on single crystals from the German locality revealed the close structural relationship to chabazite, CaAl2Si4O12·6H2O, which has the same framework topology with framework type code CHA (chabazite, willhendersonite [4–6]). However, the Si/Al ordering leads to a reduction of the framework symmetry from topological R3m to topochemical R3. Moreover, the presence of different degrees of ordering (Si,Al) and the position of extra-framework cations and water molecules further reduce the real symmetry to P1 [2,7,8]. A further occurrence was reported by Walter et al. [9] from Wilhelmsdorf (Styria, Austria), and a chemical and crystallographic description of a Ca-rich willhendersonite from a melilitite plug at Colle Fabbri, a second locality within the volcanic rocks of the Terni province, was given by Vezzalini et al. [10]. Recently, single crystals of willhendersonite from Bellerberg (eastern Eifel District, Germany) were studied by X-ray diffraction methods between 100 and 500 ◦K, showing a phase transition from triclinic to rhombohedral symmetry [11]. These changes in the framework are accompanied by migration of cations, partly assuming unfavorably low coordinations in the high-temperature structure due to the loss of H2O molecules. Re-

**Citation:** Mattioli, M.; Cenni, M. First Occurrence of Willhendersonite in the Lessini Mounts, Northern Italy. *Crystals* **2021**, *11*, 109. https://doi.org/10.3390/cryst11020109

Academic Editor: Magdalena Król Received: 31 December 2020 Accepted: 24 January 2021 Published: 26 January 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

hydration at room temperature yields the triclinic structure of willhendersonite, although the single crystals become polysynthetically twinned [11].

Willhendersonite from Terni was chemically analyzed by Peacor et al. [1]. The data yield the empirical formula, based on 12 oxygens of K0.90Ca1.01Al2.93Si3.08O12·5.43H2O, which compares favorably with the ideal formula KCaAl3Si3O12·5H2O (Z = 2). A large number of crystals from Colle Fabbri were chemically analyzed by Vezzalini et al. [10]. The obtained data showed chemical variability ranging from the holotype sample composition (CaKAl3Si3O12·5H2O), through intermediate compositions, to a Ca-pure term. The most common Ca/Ca + K ratio among the different varieties ranges from 1.0 to 0.9 and represents Ca-pure terms. The ratio 0.6 to 0.5, which is similar to that reported in the literature for willhendersonite from San Venanzo, is also quite common. The rarest compositions are the intermediate ones. The value of R is always close to 0.50, with calcium as the dominant extra-framework cation (0.98–1.54 apfu) [10]. The potassium content is relevant in samples from San Venanzo and Mayen (0.9 apfu and 0.74 apfu, respectively) but is very low (0.03 to 0.30 apfu) in the sample from Colle Fabbri [1,10]. In any case, all the data available in the literature for this zeolite come from only three areas (Terni, Styria, and Eifel). For this reason, it is of great importance to increase the amount of data, finding crystals of willhendersonite also in other locations.

The Tertiary basalts of the Lessini Mounts (Veneto Volcanic Province, Northern Italy) are known as suitable host rocks for the growth of secondary mineral associations [12]. Zeolites such as gmelinite, analcime, chabazite, and phillipsite have already been observed in the basaltic rock of Monte Calvarina [13]. Chabazite, phillipsite, harmotome, and analcime were discovered in the vugs of volcanite outcropping near Fittà [14]. Fibrous erionite and offretite with potential toxicological implications have been recently discovered in several localities of the Lessini Mounts [12,15–18]. Among these common zeolites, rare species such as willhendersonite and yugawaralite were recently found in northern Italy [12]. This study aims to present a mineralogical and chemical characterization of willhendersonite, found for the first time within vesicles of the basanitic rocks in the Lessini Mounts.

#### **2. Geological Background**

The Veneto Volcanic Province (Northern Italy) covers an area of about 2000 km<sup>2</sup> (Figure 1) and is the result of extensive volcanic activity that occurred from the Tertiary [19–21]. Several magmatic pulses occurred between the Late Paleocene and the Miocene, with the most significant part of the eruptions taking place in submarine environments [19–22]. The Veneto Volcanic Province can be subdivided into four main volcanic districts based on different tectono-magmatic features (Figure 1). They are the Lessini Mounts, the Marostica Hills, the Berici Hills, and the Euganean Hills. The main products are volcaniclastic rocks, hyaloclastites, pillow lavas, and lava flows of a mafic to ultramafic composition. Rocks of more acidic compositions are rare and only occur in the Euganean Hills.

The Lessini Mounts are located within an NNW-trending extensional structure, namely the Alpone-Agno graben [23,24], bounded to the west by the NNW-SSE Castelvero normal fault (Figure 1). The volcanic sequence of the Lessini Mounts has a thickness of up to 400 m and is mainly represented by tuffs and lava flows, with several column-jointing eruptive necks and subordinate hyaloclastites and pillow lavas. The most abundant rock-types are basanites and alkali olivine basalts, while transitional basalts, tholeiites, nephelinite, hawaiites, trachy-basalts, and basaltic andesites are less abundant [20,21,25]. Petrological and geochemical data [20,21,25,26] indicate a within-continental-plate character for the magmatism in agreement with regional geodynamics, which places this magmatism in a context dominated by a tensional system.

Most of the Lessini volcanic rocks are often deeply weathered and show cavities and vugs of variable sizes that are almost always lined by a thin, microcrystalline crust, which is the substratum of well-shaped, secondary minerals [12]. The secondary phases are mainly zeolites and clay minerals, which represent ~90 vol.% of the total secondary

minerals; other silicates (apophyllite, gyrolite, prehnite, pectolite) are very rare, as are oxides (quartz) and carbonates (calcite, aragonite). Clay minerals are generally the first minerals that precipitated along the walls, whereas the core of the vesicles commonly contains well-shaped zeolites. The coating thickness is usually less than 0.5 mm, while the zeolite crystals range in size from <1 mm to 1 cm. Typical vesicle infillings consist of chabazite, phillipsite-harmotome, analcime, natrolite, gmelinite, and offretite, and these are the most represented types. Heulandite, stilbite, and erionite are less common, whereas willhendersonite and yugawaralite are very rare zeolite species. mainly zeolites and clay minerals, which represent ~90 vol.% of the total secondary minerals; other silicates (apophyllite, gyrolite, prehnite, pectolite) are very rare, as are oxides (quartz) and carbonates (calcite, aragonite). Clay minerals are generally the first minerals that precipitated along the walls, whereas the core of the vesicles commonly contains wellshaped zeolites. The coating thickness is usually less than 0.5 mm, while the zeolite crystals range in size from <1 mm to 1 cm. Typical vesicle infillings consist of chabazite, phillipsite-harmotome, analcime, natrolite, gmelinite, and offretite, and these are the most represented types. Heulandite, stilbite, and erionite are less common, whereas willhendersonite and yugawaralite are very rare zeolite species.

is the substratum of well-shaped, secondary minerals [12]. The secondary phases are

*Crystals* **2021**, *11*, x 3 of 10

**Figure 1.** Simplified geological map of the Veneto Volcanic Province (modified from de Vecchi et al. [20]) showing the Lessini Mounts and the locations of the sampling site. **Figure 1.** Simplified geological map of the Veneto Volcanic Province (modified from de Vecchi et al. [20]) showing the Lessini Mounts and the locations of the sampling site.

#### **3. Materials and Methods 3. Materials and Methods**

The investigated crystals were obtained from a suite of more than 300 samples of volcanic rocks from Lessini Mounts, Italy. The zeolite willhendersonite can be found in small cavities of basanitic rocks predominantly filled by chabazite and clay minerals. The investigated crystals were obtained from a suite of more than 300 samples of volcanic rocks from Lessini Mounts, Italy. The zeolite willhendersonite can be found in small cavities of basanitic rocks predominantly filled by chabazite and clay minerals.

Willhendersonite crystals were initially recognized from their physical properties using a binocular microscope, (Nikon TK-1270E, Tokyo, Japan) while powder X-ray diffraction (XRD) was used to confirm their mineralogical composition. Pure crystals were selected from each sample and extracted from the matrix under a binocular microscope. The separated crystals were carefully cleaned and repeatedly treated in an ultrasonic bath to remove any impurities and the microspheres of clay minerals already observed during the microscopic investigations. After cleaning, an aliquot of the separated crystals was carefully pulverized in an agate mortar. All of the powder samples were prepared by sideloading an aluminum holders to obtain a quasi-random orientation. The XRD patterns were recorded using a Philips X'Change PW 1830 X-ray diffractometer (Philips X'PERT, Malvern Panalytical, Almelo, The Netherlands); Cu Kα radiation), with a monochromator on secondary optics. The samples were run between 2° and 65° 2θ. The analytical conditions were a 35 kV accelerating potential, a 30 mA filament current, a 0.02° step, and a counting time of 1 s/step. Willhendersonite crystals were initially recognized from their physical properties using a binocular microscope, (Nikon TK-1270E, Tokyo, Japan) while powder X-ray diffraction (XRD) was used to confirm their mineralogical composition. Pure crystals were selected from each sample and extracted from the matrix under a binocular microscope. The separated crystals were carefully cleaned and repeatedly treated in an ultrasonic bath to remove any impurities and the microspheres of clay minerals already observed during the microscopic investigations. After cleaning, an aliquot of the separated crystals was carefully pulverized in an agate mortar. All of the powder samples were prepared by side-loading an aluminum holders to obtain a quasi-random orientation. The XRD patterns were recorded using a Philips X'Change PW 1830 X-ray diffractometer (Philips X'PERT, Malvern Panalytical, Almelo, The Netherlands); Cu Kα radiation), with a monochromator on secondary optics. The samples were run between 2◦ and 65◦ 2θ. The analytical conditions were a 35 kV accelerating potential, a 30 mA filament current, a 0.02◦ step, and a counting time of 1 s/step.

Morphological observations and semi-quantitative chemical compositions were performed by Scanning Electron Microscopy (SEM) and Energy Dispersion Spectroscopy

(EDS) using a Philips 515 equipped with EDAX 9900 (Eindhoven, The Netherlands) and a Jeol 6400 (Jeol, Japan) with an Oxford Link Isis. The operating conditions were a 15 kV accelerating potential and a 2 to 15 nA beam current. A defocused electron beam and a shortened accumulation time (from 100 s down to 50 s) were used to minimize the alkaline metals' migration. The standards used were natural minerals and synthetic phases. a Jeol 6400 (Jeol, Japan) with an Oxford Link Isis. The operating conditions were a 15 kV accelerating potential and a 2 to 15 nA beam current. A defocused electron beam and a shortened accumulation time (from 100 s down to 50 s) were used to minimize the alkaline metals' migration. The standards used were natural minerals and synthetic phases. Quantitative chemical compositions were determined by an Electron Probe Microanalyser (EPMA), a CAMECA Camebax 799 (Cameca sas, cedex, France), on the other ali-

Morphological observations and semi-quantitative chemical compositions were performed by Scanning Electron Microscopy (SEM) and Energy Dispersion Spectroscopy (EDS) using a Philips 515 equipped with EDAX 9900 (Eindhoven, The Netherlands) and

*Crystals* **2021**, *11*, x 4 of 10

Quantitative chemical compositions were determined by an Electron Probe Microanalyser (EPMA), a CAMECA Camebax 799 (Cameca sas, cedex, France), on the other aliquot of the previously separated and cleaned crystals, fixed with epoxy resin on polished thin sections. Operating conditions were 15 kV and 15 nA using the wavelength-dispersive method; errors were ±2–5% for major and ±5–10% for minor components. Standards comprised a series of pure elements, simple oxides, or simple silicate compositions. The analyses were selected on the basis of their low E% value (E% is the balance error of the electrical charges: [Al − (K + 2Ca)]/(K + 2Ca) [27]). Zeolites with an E% > 10 were rejected. quot of the previously separated and cleaned crystals, fixed with epoxy resin on polished thin sections. Operating conditions were 15 kV and 15 nA using the wavelength-dispersive method; errors were ± 2–5% for major and ± 5–10% for minor components. Standards comprised a series of pure elements, simple oxides, or simple silicate compositions. The analyses were selected on the basis of their low E% value (E% is the balance error of the electrical charges: [Al − (K + 2Ca)]/(K + 2Ca) [27]). Zeolites with an E% > 10 were rejected. **4. Results** 

#### **4. Results** In the Lessini basanitic rocks, willhendersonite generally occurs as euhedral to sub-

In the Lessini basanitic rocks, willhendersonite generally occurs as euhedral to subhedral small crystals (about 0.1–0.2 mm) with a prismatic, tabular, and flattened morphology on {001}, often with rectangular sections. Individual crystals often grow together to form small elongated clusters of characteristic twinned combinations (Figure 2). In these twinned aggregates, the crystals grow in three diverse orientations, each with faces nearly perpendicular to those of the others, which typically leads to the so-called "trellis-like" aggregates [1]. The crystals are colorless and perfectly transparent, with vitreous luster on crystal faces. The cleavage is perfect, parallel to {100}, {010}, and {001}. These three cleavage planes are equivalent to the rhombohedral cleavage of isostructural chabazite. Willhendersonite from the Lessini Mounts grows on a substrate mainly consisting of clay minerals. These latter typically form layers along the walls of pore spaces with botryoidal habits and appear in a wide range of colors varying from white, pink, yellow, brown, and green to black. Clay minerals are also present as micrometric spherules of a characteristic green-blue to red color, corresponding to saponite composition with a tri-octahedral structure [12]. hedral small crystals (about 0.1–0.2 mm) with a prismatic, tabular, and flattened morphology on {001}, often with rectangular sections. Individual crystals often grow together to form small elongated clusters of characteristic twinned combinations (Figure 2). In these twinned aggregates, the crystals grow in three diverse orientations, each with faces nearly perpendicular to those of the others, which typically leads to the so-called "trellis-like" aggregates [1]. The crystals are colorless and perfectly transparent, with vitreous luster on crystal faces. The cleavage is perfect, parallel to {100}, {010}, and {001}. These three cleavage planes are equivalent to the rhombohedral cleavage of isostructural chabazite. Willhendersonite from the Lessini Mounts grows on a substrate mainly consisting of clay minerals. These latter typically form layers along the walls of pore spaces with botryoidal habits and appear in a wide range of colors varying from white, pink, yellow, brown, and green to black. Clay minerals are also present as micrometric spherules of a characteristic green-blue to red color, corresponding to saponite composition with a tri-octahedral structure [12].

**Figure 2.** Willhendersonite from the Lessini Mounts: (**a**) stereomicroscopic image of colorless, transparent individual crystals of willhendersonite (WIL) associated to form small elongated clusters and grown on clay minerals; (**b**) SEM photomicrograph of willhendersonite twinned crystal aggregates (WIL) on a substrate of clay minerals with botryoidal shape and micrometric spherules of tri-octahedral saponite. **Figure 2.** Willhendersonite from the Lessini Mounts: (**a**) stereomicroscopic image of colorless, transparent individual crystals of willhendersonite (WIL) associated to form small elongated clusters and grown on clay minerals; (**b**) SEM photomicrograph of willhendersonite twinned crystal aggregates (WIL) on a substrate of clay minerals with botryoidal shape and micrometric spherules of tri-octahedral saponite.

Powder X-ray diffraction data of separated willhendersonite crystals are listed in Table 1, while their diffraction pattern is shown in Figure 3. The unit-cell parameters of willhendersonite were determined by least-squares refinement of powder-diffractometer Powder X-ray diffraction data of separated willhendersonite crystals are listed in Table 1, while their diffraction pattern is shown in Figure 3. The unit-cell parameters of willhendersonite were determined by least-squares refinement of powder-diffractometer data, which was carried out in the space group P1 starting from the positional parameters of the framework atoms by Tillmanns et al. [2]. High-purity quartz (a = 4.9137, c = 5.4053 Å) has been used for the refinement as an internal standard. The diffraction pattern exhibits

Å) has been used for the refinement as an internal standard. The diffraction pattern exhibits 33 well-resolved diffraction peaks, all of which can be indexed to a triclinic cell with unit cell parameters *a* = 9.239(2) Å; *b* = 9.221(2) Å; and *c* = 9.496(2) Å, α = 92.324(2)°, β =

data, which was carried out in the space group P1 starting from the positional parameters of the framework atoms by Tillmanns et al. [2]. High-purity quartz (a = 4.9137, c = 5.4053

33 well-resolved diffraction peaks, all of which can be indexed to a triclinic cell with unit cell parameters *a* = 9.239(2) Å; *b* = 9.221(2) Å; and *c* = 9.496(2) Å, α = 92.324(2)◦ , β = 92.677(2)◦ , γ = 89.992◦ (Space Group P1). The diffractogram perfectly fits that of the holotype willhendersonite and does not show other peaks relating to impurities, testifying to the purity of the analyzed crystals. 92.677(2)°, γ = 89.992° (Space Group P1). The diffractogram perfectly fits that of the holotype willhendersonite and does not show other peaks relating to impurities, testifying to the purity of the analyzed crystals. **Table 1.** X-ray powder diffraction data for willhendersonite from the Lessini Mounts. **2 Theta I/I<sup>o</sup>** *d* **(Å)** *h k l* **2 Theta I/I<sup>o</sup>** *d* **(Å)** *h k l* 

**Table 1.** X-ray powder diffraction data for willhendersonite from the Lessini Mounts. 9.32 34.77 9.4789 0 0 1 27.36 6.15 3.2601 −2 2 0

*Crystals* **2021**, *11*, x 5 of 10


**Figure 3.** X-ray powder diffraction pattern for willhendersonite from the Lessini Mounts. All the diffraction peaks refer to willhendersonite. **Figure 3.** X-ray powder diffraction pattern for willhendersonite from the Lessini Mounts. All the diffraction peaks refer to willhendersonite.

A representative number of crystals from different specimens were extracted, carefully cleaned, and analyzed for chemical composition. The resultant analyses are presented in Tables 2 and 3. The collected chemical data show significant variability, mainly A representative number of crystals from different specimens were extracted, carefully cleaned, and analyzed for chemical composition. The resultant analyses are presented in Tables 2 and 3. The collected chemical data show significant variability, mainly in the extra-framework cation content. Based on the Ca/(Ca + K) ratio, two different groups of crystals can be distinguished.

The first group of crystals correspond to a Ca-rich willhendersonite (Table 2), with an average chemical composition of (K0.29Na0.03)Σ=0,32Ca1.24(Si3.06Al3,00Fe3+ 0.01)Σ=6,07O12·5H2O). The Ca/(Ca + K) ratio varies from 0.78 to 0.84 (average 0.81) and calcium is the dominant extra-framework cation (1.21–1.29 apfu), while potassium is very low (0.24–0.34 apfu) and the sodium is always <0.1 apfu.

Crystals of the second group are Ca-K willhendersonite (Table 3) and are characterized by an average chemical composition of (K0.94Na0.01)Σ=0,95Ca0.99(Si3.07Al2.93Fe3+ 0.00)Σ=6,00 O12·5H2O). They have a Ca/(Ca + K) ratio in the range of 0.49–0.54 (average 0.51), with significant calcium (1.04–0.96 apfu) and potassium (0.9–0.98 apfu) contents, while the sodium is always <0.1 apfu. For all samples, the tetrahedral content R (R = Si/(Si + Al)) is near to 0.50 (average 0.51).

**Table 2.** Representative chemical compositions of Ca-rich willhendersonite from Lessini Mounts. \* The H2O content is calculated by difference to 100; Mg, Sr, and Ba were checked but were always below the detection limit; E% is the balance error of the electrical charges: [Al − (K + 2Ca)]/(K + 2Ca); R is the tetrahedral content (Si/(Si + Al)) and St-d is standard deviation.


**Table 3.** Representative chemical compositions of Ca-K willhendersonite from Lessini Mounts. \* The H2O content is calculated by difference to 100; Mg, Sr, and Ba were checked but were always below the detection limit; E% is the balance error of the electrical charges: [Al − (K + 2Ca)]/(K + 2Ca); R is the tetrahedral content (Si/(Si + Al)) and St-d is standard deviation.



**Table 3.** *Cont.*

#### **5. Discussion and Conclusions**

Alteration phenomena in the basaltic rocks from the Lessini Mounts resulted in distinctive secondary mineral assemblages representing a multi-stage hydrothermal alteration process [12]. The occurrence of secondary minerals, their frequency, and their associations may be significantly different on both the outcrop- and sample-scale, with variability that can be in the order of a few centimeters. In the earliest stages, clay and silica minerals precipitate along the vesicles' inner walls, followed by the fine-grained zeolites erionite, offretite, analcime, natrolite, heulandite, and stilbite. The final stage is marked by the large, well-shaped zeolites (phillipsite-harmotomo, gmelinite, chabazite) associated with rare zeolites yugawaralite and willhendersonite. In particular, as regards willhendersonite, it is important to remember that, in the world, only three finds of this zeolite are reported (Terni, Eifel, Styria), and the data available in the literature are very few (practically absent for that of Styria). The morphological and chemical data presented here aim to implement this rare zeolite knowledge, found for the first time in northern Italy.

Willhendersonite from the Lessini Mounts is morphologically very similar to that of the other three known localities. Crystals are colorless, transparent, and mainly tabular and flattened, and they are often twinned to form trellis-like intergrowths. The cleavages are perfect, well developed, and parallel to {100}, {010}, and {001}, as indexed on the triclinic cell.

The calculated cell parameters' values were found to be relatively consistent with literature data (Table 4). In particular, unit-cell values of willhendersonite from Lessini are very similar to the Mayen and Bellberg samples, while those from Terni and Styria show slightly lower unit-cell parameters. No significant differences were found in the unit-cell parameters with respect to the holotype willhendersonite studied by Tillmanns et al. [2].

**Table 4.** Unit-cell parameters of willhendersonite from the Lessin Mounts, compared with literature data. Values of estimated standard deviations are given in parentheses.


Regarding the chemical composition, two different groups of crystals were distinguished based on the extra-framework cation content (Figure 4). The Ca-rich group (red circles in Figure 4) has a composition that straddles those observed in the Colle Fabbri

samples [10]. In particular, willhendersonite from the Lessini Mounts has a composition that includes the intermediate terms from Colle Fabbri (Will-int, [10]) but extends towards the Ca-richest terms, expanding the compositional range so far known in the literature. *Crystals* **2021**, *11*, x 9 of 10

**Figure 4.** Compositional ternary diagram illustrating major variations of willhendersonite compositions. Red and blue circles are crystals from the Lessini Mounts. Components are: S4O8 (molecular proportions [Si − 2(Ca + Mg + Sr + Ba) − 2(Na + K)]/4, (Ca,Mg,Sr,Ba)Al2Si2O8 (molecular proportions Ca + Mg + Sr + Ba), and (Na,K)2Al2Si2O8 (molecular proportions (Na + K)/2). **Figure 4.** Compositional ternary diagram illustrating major variations of willhendersonite compositions. Red and blue circles are crystals from the Lessini Mounts. Components are: S4O<sup>8</sup> (molecular proportions [Si − 2(Ca + Mg + Sr + Ba) − 2(Na + K)]/4, (Ca,Mg,Sr,Ba)Al2Si2O<sup>8</sup> (molecular proportions Ca + Mg + Sr + Ba), and (Na,K)2Al2Si2O<sup>8</sup> (molecular proportions (Na + K)/2).

According to the literature data [12], the sampling area of willhendersonite (southwestern sector of the Alpone valley, Figure 1) is characterized by the occurrence of other zeolites, all of which are of Ca-rich type (e.g., Ca-chabasite, Ca-phillipsite, Ca-gmelinite, Ca-erionite, heulandite), followed by extensive crystallization of calcite. The presence of Ca-rich species in this area of the Lessini Mounts might suggest that the zeolite-forming The Ca-K crystals (blue circles in Figure 4), on the other hand, are characterized by comparable quantities of K and Ca and, consequently, are distributed in the central sector of the ternary diagram of Figure 4. Their composition comprises the San Venanzo samples [1] but extends towards richer terms in K, defining greater chemical variability in extra-framework cation content.

fluids could have interacted with the underlying calcareous marine sedimentary rocks through which the basalts erupted. Hydrothermal fluids may have permeated the sedimentary and volcanic pile, leaching Ca out of the calcareous sedimentary rocks. Furthermore, as suggested by experimental works [28], with a likely high concentration of carbonate ions in solution, the zeolites' growth rate would increase in response to the increased dissolution of silica from the surrounding rocks. In this context, the composition of the underlying rocks through which the volcanics erupted seems to play an important role in zeolite formation. In the willhendersonite from the Lessini Mounts, the K content is low in the Ca-rich group (average of 0.29 apfu) and it is notably high in the Ca-K group (average 0.94 apfu). In comparison, the K content is significant in the samples from San Venanzo and Mayen (0.90 and 0.74 apfu, respectively) and very low in the sample from Colle Fabbri (0.03–0.30 apfu). In the Lessini samples, the Ca content has an average of 1.24 apfu in the Ca-rich group and 0.99 apfu in the Ca-K group, whereas the Ca content is notably high in the Colle Fabbri sample (1.43–1.28 apfu) and it is low in the samples from San Venanzo and Mayen (1.01 and 1.06 apfu, respectively).

**Author Contributions:** Conceptualization, methodology, and analysis, M.M. and M.C.; writing original draft preparation, M.M.; writing—review and editing, M.M. and M.C.; supervision, project administration, and funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript. **Funding:** This work was funded in the framework of the 2018 research programs of the Department of Pure and Applied Sciences of the University of Urbino Carlo Bo (project "New asbestiform fibers: mineralogical and physical-chemical characterization", responsible M. Mattioli). **Data Availability Statement:** Data is contained within the article. The data presented in this study can be seen in the content above. **Acknowledgments:** We would like to thank Amleto Longhi for giving us the idea of studying The new chemical data of willhendersonite from the Lessini Mounts, together with the literature data on Mayen [1,2] and the occurrence of a Ca-pure willhendersonite reported in Vezzalini et al. [10], highlight the presence of a considerable compositional variability (probably not yet fully defined) of this zeolitic species. The existence of a continuous isomorphous series between Ca and K end-members, suggested by Vezzalini et al. [10], has been reinforced by the description of Lessini samples with intermediate compositions. This suggests a redefinition of willhendersonite as a series extending from a Ca end-member to compositions with equal proportions of Ca and K. According to [10], the absence of the K-pure willhendersonite end-member means that, at present, the series is incomplete. Further analyses are planned to investigate the distribution of this rare zeolite in the Lessini secondary minerals and to better understand the chemical variability of willhendersonite to define the existence of a Ca-K isomorphous series.

secondary minerals in the Lessini basalt. We are also grateful to Franco Bressan, Dino Righetti, and Guglielmino Salvatore for their help in the field and for providing us with high-quality mineral specimens. Field work was supported by the Mineralogical and Geological Association of Verona AGMV. Special thanks go to E. Salvioli Mariani and L. Valentini for their kind help and advices on According to the literature data [12], the sampling area of willhendersonite (southwestern sector of the Alpone valley, Figure 1) is characterized by the occurrence of other zeolites, all of which are of Ca-rich type (e.g., Ca-chabasite, Ca-phillipsite, Ca-gmelinite,

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the man-

uscript, or in the decision to publish the results.

Ca-erionite, heulandite), followed by extensive crystallization of calcite. The presence of Carich species in this area of the Lessini Mounts might suggest that the zeolite-forming fluids could have interacted with the underlying calcareous marine sedimentary rocks through which the basalts erupted. Hydrothermal fluids may have permeated the sedimentary and volcanic pile, leaching Ca out of the calcareous sedimentary rocks. Furthermore, as suggested by experimental works [28], with a likely high concentration of carbonate ions in solution, the zeolites' growth rate would increase in response to the increased dissolution of silica from the surrounding rocks. In this context, the composition of the underlying rocks through which the volcanics erupted seems to play an important role in zeolite formation.

**Author Contributions:** Conceptualization, methodology, and analysis, M.M. and M.C.; writing original draft preparation, M.M.; writing—review and editing, M.M. and M.C.; supervision, project administration, and funding acquisition, M.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was funded in the framework of the 2018 research programs of the Department of Pure and Applied Sciences of the University of Urbino Carlo Bo (project "New asbestiform fibers: mineralogical and physical-chemical characterization", responsible M. Mattioli).

**Data Availability Statement:** Data is contained within the article. The data presented in this study can be seen in the content above.

**Acknowledgments:** We would like to thank Amleto Longhi for giving us the idea of studying secondary minerals in the Lessini basalt. We are also grateful to Franco Bressan, Dino Righetti, and Guglielmino Salvatore for their help in the field and for providing us with high-quality mineral specimens. Field work was supported by the Mineralogical and Geological Association of Verona AGMV. Special thanks go to E. Salvioli Mariani and L. Valentini for their kind help and advices on the SEM observations and EDS analysis, and R. Carampin for his great expertise with the EMP.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

#### **References**

