Assessment of Feldspars from Central Portugal Pegmatites for Sustainable Ceramic Applications

Abstract

This study investigates the mineralogical, chemical, and fusibility characteristics of feldspar samples collected from eight pegmatitic bodies in central Portugal. The primary aim was to evaluate their suitability for use in ceramic applications, driven by the need to valorize local georesources, reduce dependence on imported raw materials, and contribute to the sustainability and competitiveness of the Portuguese ceramic sector. Samples were analyzed by X-Ray Diffraction (XRD), X-ray Fluorescence (XRF), inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). Firing tests were performed to assess fusibility, whiteness, and visible impurity behavior. Results indicate that seven of the eight samples were dominated by a combination of microcline and albite, with minor amounts of quartz and muscovite. Crystallinity indices varied across samples, reflecting differences in mineral order and thermal reactivity. Chemical compositions showed acceptable levels of SiO₂ and Al₂O₃, and total alkali contents (Na₂O + K₂O) between 10% and 16%, aligning with industrial standards for ceramic raw materials. The Fe₂O₃ contents were below 0.3% in most samples, suggesting favorable conditions for whiteness upon firing. Loss on ignition (LOI) values were generally low, except for one sample rich in muscovite. Fusibility behavior varied significantly between samples, with albite-rich samples showing lower melting points and better flow characteristics, while microcline-dominant samples required higher temperatures for vitrification but contributed to structural stability. The K₂O/Na₂O ratio presented values favoring earlier softening and fluxing. Whiteness revealed that samples with low Fe₂O₃ and TiO₂ content, particularly those with low mica content, achieved the best aesthetic outcomes post-firing.

1. Introduction

Feldspars are among the most extensively used raw materials in the global ceramic industry due to their crucial roles as fluxing agents in ceramic bodies and glazes. As aluminosilicate minerals, composed primarily of silica (SiO₂), alumina (Al₂O₃), and alkali oxides (Na₂O and K₂O), feldspars contribute to the formation of a vitreous phase during firing, enabling densification, enhancing mechanical strength, and improving the appearance and surface properties of the final ceramic products [1,2]. Their availability, chemical consistency, and relatively low cost make them essential components in a variety of ceramic products, from porcelain and sanitaryware to tiles and tableware.

The behavior of feldspar during firing is dictated by its mineralogical composition and physical characteristics. Microcline (K-feldspar) and albite (Na-feldspar) are commonly used in ceramic formulations, having distinct thermal behaviors: Na-feldspars melt at slightly lower temperatures with a more fluid glass phase, advantageous for fast-firing and energy-efficient ceramic production; K-feldspars tend to form a more viscous melt and contribute to the mechanical stability and durability of the ceramic bodies [3,4]. Therefore, a proper balance between Na₂O and K₂O is crucial in achieving optimal firing performance.

International studies reinforce the significance of feldspar compositional differences in industrial applications. Gaied and Gallala [2] studied feldspars from central Tunisian pegmatites and confirmed that a higher Na content favored the production of sanitaryware and wall tiles due to better fusibility and higher whiteness after firing. In Turkey, Kayac [5] showed that feldspar-rich formulations improved the vitrification and sintering kinetics of porcelain and tiles, especially when combined with alternative fluxes like perlite. Likewise, a study in Poland explored the effect of feldspar crystallinity and impurity content on the final ceramic product, with emphasis on Fe₂O₃ and TiO₂ levels that can cause undesirable coloration or reduce mechanical strength [6].

The mineralogical purity of feldspar deposits plays a decisive role in ceramic quality. Feldspar ores often contain impurities such as micas or Fe-bearing phases, which must be minimized to meet industrial specifications. For instance, Fe-oxides (Fe₂O₃) above 0.4 wt% can lead to discoloration and interfere with sintering [7]. Micas, like muscovite, can affect melt viscosity and reduce vitrification efficiency. Therefore, beneficiation techniques, such as magnetic separation and flotation, are frequently applied to upgrade feldspar quality and reduce unwanted minerals [8].

The crystallinity index (CI) reflects the structural order of the crystals. Lower values suggest higher crystallinity, which generally correlates with predictable and efficient melting behavior. Feldspars with poor crystallinity may exhibit erratic fusibility or form incomplete glass phases, adversely affecting the sintering process and final product performance [9,10]. Research conducted on feldspar resources in India and Brazil confirmed that crystalline and low-Fe feldspars are preferable for producing white porcelain and glazed tiles [11,12]. In addition to compositional and structural factors, the alkali ratio (K₂O/Na₂O) is often used as a functional index to assess the expected melt behavior of feldspar-bearing ceramic bodies. A high ratio typically indicates a K-dominant flux, which favors mechanical strength but may require higher sintering temperatures. Conversely, lower ratios suggest Na-rich compositions, which promote early melting and are especially useful in fast-firing processes and energy-saving ceramic production [13]. This balance is critical for tailoring ceramic body formulations to specific industrial processes.

The global shift toward sustainable manufacturing and the rising costs of imported raw materials have increased interest in evaluating and valorizing the local feldspathic georesources. European countries, such as Portugal, are currently focusing on geological mapping and mineral beneficiation studies to improve the quality and usability of raw minerals and to identify feldspar sources suitable for ceramic industry standards. These efforts aim to reduce raw material imports, cut energy costs, and enhance local supply chains [14]. Comprehensive mineralogical, chemical, and firing behavior analyses are essential for determining the application potential of these materials.

This study aligns with these global efforts by focusing on the characterization of feldspar raw materials for ceramic applications. Through an integrated approach that considers chemical composition, mineral phases and crystallinity, fusibility, and impurity levels, this study aims to determine the suitability of feldspar samples collected in central Portugal for use in different sectors of the ceramic industry, such as whiteware, sanitaryware, tiles, and glazes. Ultimately, this research contributes to the broader goal of sustainable, high-performance ceramic production by promoting the informed use of mineral resources.

2. Materials and Methods

2.1. Study Area Context and Sampling

The study area is located in Sátão, a district of Viseu (Portugal; 40°48′40″ N 7°39′40″ W; Figure 1). It iss part of the Beira region plateau [15], within the domain of Vilariça fault, with elevations ranging ~600 to 750 m. The fractures are primarily oriented N–S, NE–SW, and NW–SE, with zones of discontinuity. Within these zones exist diverse quartz and aplite-pegmatitic veins, mainly oriented NNE–SSW and NE–SW, along with dolerites. The structural pattern suggests the presence of deep fracturing, favorable to thermo-mineral systems genesis [16].

The study area is located within the Centro-Iberian Zone, part of the Iberian Massif and part of the Iberian Peninsula that extends through much of its central and western regions. The study area is essentially composed of rocks that were deformed and metamorphosed during the Variscan Orogeny and, subsequently, intruded by large volumes of granitic magmas (Upper Proterozoic to the Carboniferous) [17]. The granitic rocks are extensively exposed, being associated with the D3 Variscan deformation [18]. Tectonic, stratigraphic, metamorphic, and magmatic variations among the Iberian Massif domains, defined as six major tectono-stratigraphic zones [19], are the study areas inserted in the Central Iberian Zone (CIZ). The CIZ is located in the central part of the Iberian Massif, subdivided into three distinct domains, and the study area was located in a thick terrigenous sequence. The study area is inserted in the Schist–Greywacke Complex (SGC) [20].

Geologically, the study area consists of Hercynian to post-kinematic granitic facies, both early and late, with medium-grained calc-alkaline porphyritic granodiorites and granites, predominantly biotitic, with numerous quartz vein occurrences, aplite-pegmatitic masses, and basic rock veins (dolerites) predominantly oriented NNE–SSW and NE–SW (Figure 2). The granitic outcrops, in the study area, cover most of the surrounding region being characterized mainly by two-mica biotitic granites. According to Azevedo and Aguado [18], the granitic bodies in this region are essentially composed of mega crystals of K-feldspar (primarily perthitic microcline) exceeding 8 cm, dispersed within a medium- to coarse-grained matrix of quartz, plagioclase, and biotite, with some accessory minerals such as apatite, zircon, monazite, ilmenite, and, rarely, xenotime [18].

Within the study area, eight pegmatitic occurrences have been identified [22]. Morphologically, the pegmatites in the study area belong to the lower-volume homogeneous intra-granitic class, consisting essentially of quartz, feldspar, and micas (predominantly muscovite) with graphic texture, large crystal size, and poor in accessory minerals. They are mainly structured as benches, occurring predominantly beneath the granitic bands that exhibit pegmatitic structures [23]. Pegmatitic bodies in the study area are primarily composed of K-feldspar, with yellowish to pinkish hues crystals and some with a ferruginous appearance; milky quartz, occasionally with a ferruginous appearance; and micas, mostly muscovite. Feldspar and quartz crystals are generally coarse-grained, with occasional quartz and micas found within the feldspar matrix.

In each pegmatitic occurrence, ~1 kg of feldspars was collected (Figure 1;

Table 1. Location of collected samples.

ID	Location	Coordinates
1	Quinta da Carrasqueira de Baixo	40°47′52.6″ N 7°37′59.6″ W
2	Quinta da Carrasqueira	40°48′09.0″ N 7°37′57.0″ W
3	Quinta da Carrasqueira de Cima I	40°47′58.8″ N 7°37′39.9″ W
4	Quinta da Corujeirinha	40°48′15.6″ N 7°37′36.9″ W
5	Poço Palheiros de Cima	40°48′29.1″ N 7°37′19.6″ W
5.1	Poço Palheiros de Baixo	40°48′26.2″ N 7°37′19.6″ W
6	Quinta da Carrasqueira de Cima II	40°48′08.0″ N 7°37′19.8″ W
7	Pinheiro	40°48′26.6″ N 7°36′50.8″ W

). Samples were transported to the laboratory, dried at <40 °C, visible potential contaminants were manually removed, and samples were disaggregated. A representative homogenized portion of each sample was crushed on an agate mill for chemical and mineral phase analysis.

2.2. Samples Characterization

The chemical composition of major elements was assessed by XRF, with a Panalytical Axios PW4400/40, with Rh radiation (Malvern Panalytical, Malvern, Worcestershire, UK). Trace element content was determined by ICP-MS, with an Agilent Technologies—7700 Series (Agilent Technologies, Santa Clara, CA, USA). Mineral phases were identified by XRD, with a Philips/Panalytical powder diffractometer, model X′ Pert Pro MPD (Malvern Panalytical, Malvern, Worcestershire, UK). This equipment uses a Cu-X-ray tube operated at 50 kV and 30 mA, with data collected from 2 to 70° 2θ with a step size of 1° and a counting interval of 0.02 s [24]. Samples of chemical semi-quantification were assessed through a SEM-EDS Hitachi Schottky SU-70 (Hitachi High-Tech, Tokyo, Japan), with images acquired through secondary and back-diffused electron detectors. The accuracy and precision of the methods were determined using duplicate sample analyses in each analytical set and internal standard materials.

The crystallinity index (CI) is a semi-quantitative measure of how well-ordered (crystalline) a mineral is. It is commonly derived from XRD data, being calculated as CI = area of broad peak/area of sharp peak [9]. The CI is inversely proportional to the crystallinity of a mineral. High crystallinity feldspars (low CI) are suitable for ceramics, especially in glass-ceramics, glazes, and porcelain, promoting better fluxing behavior during firing, more uniform melting etc.; quartz with high crystallinity improves thermal expansion control, reducing cracking, and muscovite, and even in small amounts, it can influence plasticity and thermal behavior when poor crystallinity is undesirable [25,26].

To assess, in each sample, fusibility upon firing, whiteness, and contamination levels, a fusibility test was performed. To assess fusibility upon firing, homogenized grounded samples were dried at 110 °C, up to constant weight; 2 g of homogenized feldspars was placed on a ceramic container; and the assembly was placed in a muffle at 1350 °C. To assess whiteness and potential contamination, the same procedures were followed, adjusting the sample quantity to 4 g and using a porcelain dish as the ceramic support. Fusibility tests were carried out at the Ceramics and Glass Technology Centre (CTCV, Coimbra, Portugal; https://www.ctcv.pt/) following the internal standard PE 311.146.

3. Results and Discussion

Mineral phases identified in the collected samples were microcline (KAlSi₃O₈), albite (NaAlSi₃O₈), quartz (SiO₂), and muscovite (KAl₂(Si₃Al)O₁₀(OH)₂) (Figure 3a). Microcline was the main mineral phase, ranging from 81 to 97% of the total mineral content, except in sample 6, where albite showed a ~85% content and microcline ~3%. Quartz was not identified in samples 2, 3, and 6. The microcline–albite–quartz ternary diagram (Figure 3b) confirmed the similarities between all samples with higher microcline content and low quartz, apart from sample 6 with albite as the main feldspar and almost absent microcline and quartz content. Muscovite was identified in samples 1, 4, 5, 5.1, and 6, with 2.1, 10.6, 6.9, 4.6, and 8.1%, respectively.

The microcline crystallinity index (CI) varied from 0.19 to 0.23 in samples 1 to 5.1 and 7, indicating a moderate crystallinity, while sample 6 with CI = 0.02 suggests a very well-crystallized microcline or, possibly, a low abundance that influences peak broadening (Figure 4). Albite minerals showed low CI (0.01–0.08) revealing high crystallinity, while sample 6 (CI = 0.27) indicated lower crystallinity or secondary alteration/weathering. Quartz minerals with low CI (0.007–0.013) suggested high crystallinity, which is ideal for ceramic applications, and muscovite, detected in samples 1, 4, 5, 5.1, and 6, presented a CI varying from 0.021 to 0.04, indicating relatively crystalline minerals.

The chemical analyses showed that major oxide content, in general, was ranked SiO₂ > Al₂O₃ > K₂O > Na₂O > Fe₂O₃ (

Table 2. Chemical composition of the collected samples (in %) and required chemical composition of feldspars to be used in ceramics (Ref).

Variables	Samples ID								Ref. *
	1	2	3	4	5	5.1	6	7
SiO2	62.782	62.694	61.038	63.280	66.760	62.849	57.841	67.559	65–75
Al2O3	18.334	19.211	21.378	18.893	16.955	18.841	21.997	17.060	17–24
K2O	16.887	13.764	12.788	13.773	12.610	13.637	2.165	11.208	3–15
Na2O	0.456	2.983	2.243	3.014	1.987	2.959	7.064	2.796	3–12
Fe2O3	0.121	0.049	0.107	0.041	0.253	0.100	0.530	0.111	<0.4
P2O5	0.204	0.545	0.391	0.354	0.314	0.395	0.548	0.337	<2
SO3	0.021	0.009	0.011	0.009	0.010	0.007	0.013	0.006	-
Cl-	nd	nd	0.009	nd	0.006	nd	nd	0.010	-
MgO	0.229	0.137	0.487	0.099	0.117	0.098	0.246	0.167	<0.5
CaO	0.140	0.066	0.041	0.062	0.230	0.071	0.405	0.064	<2.5
TiO2	nd	nd	0.009	nd	nd	nd	0.012	nd	<0.2
LOI	0.620	0.370	1.320	0.360	0.600	0.890	9.060	0.570	<2
K2O/Na2O	37.03	4.61	5.70	4.57	6.35	4.61	0.31	4.01	3.1
Na2O + K2O	17.34	16.75	15.03	16.79	14.60	16.60	9.23	14.00	≥11
MgO + CaO	0.369	0.203	0.528	0.161	0.347	0.169	0.651	0.231	<2

nd—not detected; LOI—loss on ignition; * refs. [27,31,32,33].

). Samples 1, 2, 3, 4, and 5.1 exhibited higher alkali (Na₂O + K₂O > 15%), alumina (Al₂O₃ > 18%), and silica (SiO₂ > 60%) content; sample 7 presented a higher SiO₂ content; and sample 6 presented the higher Al₂O₃ and Na₂O content, despite the lower alkali value, which is related to the abundance of Na-feldspar (albite). The Na-feldspars act as a strong flux in ceramics, promoting early vitrification, lowering firing temperature, saving energy, and improving whiteness once albite contains less Fe and Ti oxides [27]. Low iron oxide content aids in avoiding discoloration that can affect ceramic product aesthetics. Nevertheless, excessive Na₂O can cause over-fluidity of the glass phase by lowering viscosity significantly, leading to warping or bloating and reducing the control of the final microstructure [3]. Also, it might adversely disturb processes that rely on a well-controlled melt structure, such as glazing or glass-ceramic formation where the balance between flow and network stability is critical [4,13]. When K₂O predominates, due to larger ionic radius and lower field strength, K⁺ can stabilize the aluminosilicate framework, resulting in a more polymerized structure, leading to higher viscosity and greater activation energy for viscous flow [13,28]. Similarly, all samples, except sample 6, showed an alkali ratio (K₂O/Na₂O) > 3:0, displaying a K₂O enrichment and the prevalence of K-feldspars (microcline), that can improve ceramic products refractoriness [2,29]. Despite an alkali ratio within the reference, sample 1 presented slightly higher K₂O and very low Na₂O content, which if blended with Na-feldspar, to balance the combined alkali content, can promote the melting behavior [2,30].

The high LOI of sample 6 revealed a significant volatile loss, likely related to muscovite content, which can cause cracking, bloating, or porosity during firing in ceramic products by adding impurities, reducing thermal stability, and affecting vitrification [33]. The Fe₂O₃ content is slightly above the typical value of 0.4% (Table 2) only in sample 6, which can lead to unwanted coloration and formation of secondary phases, adversely affecting strength and durability, especially in glass ceramics [7]. The SEM-EDS analysis of sample 6 revealed a homogeneous structure, with semi-quantification of points 6.1 and 6.2 revealing a particle constituted by Si, K, and high Al, with less Na and Fe (Figure 5b).

The Al₂O₃ content falls within the acceptable values desired by the ceramic industry to enhance the mechanical strength and resistance to chemical attack, except in sample 5 where it was slightly below, meaning that sample is more suitable for low-grade ceramics, such as bricks, and not ideal for porcelain or sanitaryware. Sample 6, for its use in these types of ceramics, needs beneficiation by increasing K₂O (minimum 10%–11%) and SiO₂ (minimum 65% to improve glassy phase formation), which will improve sintering; removing muscovite, through flotation or electrostatic separation, and controlling Fe₂O₃ and P₂O₅, (e.g., froth flotation) to avoid discoloration and glaze defects [34]. All samples, except 7, presented a SiO₂ content slightly below the lower end of the reference range, which might influence the ceramic structural integrity and thermal stability. Additionally, the alkaline content (MgO + CaO) was below the reference values for application in ceramic products (Table 2), a combination that can promote over-sintering, even at relatively low temperatures [35].

The low concentration of trace elements (

Table 3. Trace element chemical compositions of the studied samples (in mg/kg).

Variable	1	2	3	4	5	5.1	6	7
Ba	400.0	25.1	59.1	110.0	290.0	41.3	9.1	34.8
Br	2.7	2.4	4.2	1.5	3.8	4.9	4.2	1.8
Cr	5.3	4.3	8.8	4.3	8.1	12.9	6.1	16.7
Cs	28.7	34.2	44.1	12.7	19.3	14.8	35.5	25.4
Cu	7.5	3.2	3.8	4.6	nd	8.2	31.2	nd
Ga	11.9	18.6	19.3	17.3	17.3	12.7	33.9	15.2
Mn	41.3	19.5	26.6	15.5	29.3	53.6	260.0	25.2
Pb	19.9	29.9	38.2	46.6	49.5	9.8	11.2	28.6
Sn	7.1	17.7	17.1	12.5	14.2	9.9	34.9	12.1
Sr	32.7	8.5	8.1	31.7	44.4	14.4	21.3	11.9
Tl	8.9	8.5	7.6	4.2	5.9	7.0	nd	4.7
U	1.9	3.0	1.8	2.5	3.3	1.7	5.9	3.0
Zn	9.5	1.4	nd	nd	2.7	4.9	15.9	1.6
Zr	nd	nd	nd	nd	nd	8.0	25.5	0.9

nd—not detected.

) suggested the absence of secondary minerals, with no late-stage hydrothermal alteration. The low trace elements concentration in ceramics improves stability and prevents inclusions in glazes, among other things [36].

The fusibility upon the firing assay, at 1350 °C, allowed us to determine the flowability degree and qualitative assessment of the whiteness and contamination levels. The flowability degree is related to how easily a ceramic slip or glaze flows, linked to viscosity or melting behavior during firing; the qualitative whiteness degree is the perceived brightness of a material, which is important for aesthetic and purity evaluation, especially in white ceramics; the qualitative contaminations related to the presence of impurities (e.g., Fe, organic matter) that can affect color and/or performance. Concerning flowability, the results showed that samples 2, 3, and 4 (Figure 6a) presented an excellent degree, while samples 1 and 5 (Figure 6b) demanded higher firing temperature to achieve equivalent results, which will raise production costs to a level that is not classified as acceptable by the ceramic industry, where a small difference of 20 °C is technologically significant [3].

Compared to microcline, Na-feldspar has higher fusibility, as a strong flux requires a lower firing temperature (~1170 °C), melting more easily, assisting in the formation of an early glassy phase promoting densification and fast firing. Sample 6 showed low viscosity and high fluidity (Figure 7a), which can be related to the lower alkali ratio, and high alkaline oxides and LOI, contributing to the release of volatiles and fluids from the crystal structure [37].

The presence of impurities was low in sample 2, but even lower in 3, presenting a uniform white color, revealing the sample to have better properties for ceramics compared to the others (Figure 8a). The SEM-EDS analysis of sample 3 showed a homogeneous structure with the exception of the outlined grain (Figure 5a), despite that analysis revealing a similar composition. Samples 1 and 5, despite a good whiteness level and low contamination content, present less potential for the ceramic industry, due to the low fusibility degree, which will raise the firing temperature (Figure 8b). Sample 6 presented the highest impurities content, such as Fe oxides (Figure 7b). Associated with the very low viscosity, this sample cannot be used in ceramic products, unless subject to beneficiation. However, for wall or floor tiles, products are sintered at lower temperatures, mainly to form a small amount of glassy phase to ensure bonding between particles and increase mechanical strength, what turns these feldspars suitable. In this case, the Na-feldspar is more adequate due to its lower viscosity and melting temperature compared to K-feldspar. Additionally, the laboratory technique to separate feldspars was not industrial, which could lead to a refined final product with fewer impurities.

4. Conclusions

This study investigated the mineral phases, chemical composition, and fusibility of feldspar samples collected from eight pegmatitic occurrences in central Portugal, aiming to evaluate their potential for ceramic applications. The results revealed a predominance of microcline and albite in seven of the samples, with varying crystallinity indices and alkali oxide content. Quartz and minor amounts of muscovite were also identified in some samples, which can influence the behavior of the ceramic body. The sample richest in albite exhibited a lower melting point and high fusibility (low viscosity), making it well-suited for fast-firing tile production. In contrast, microcline-dominant samples, while requiring higher firing temperatures, contribute to improved mechanical strength and structural stability. Samples 2 and 4, and particularly Sample 3, demonstrated strong potential for the ceramic industry, due to their low levels of contamination, higher fusibility, and enhanced whiteness. These properties make them suitable for key ceramic sectors such as whiteware and sanitaryware. Additionally, with proper beneficiation, samples 1 and 5 may also be effectively utilized in ceramic products.