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.