Colour After Firing Versus Mineral Composition of Feldspar Raw Materials from the Sobótka Region

This article reviews the mineralogical and petrographic characteristics of granitoids occurring in four deposits extracted by the StrzeblowskieMineral Mines of Sobótka, placing particular emphasis on minerals containing iron and other elements which affect the colour of the raw material after firing. For this purpose, microscopic examinations in transmitted light, chemical analyses, and observations in scanning electron microscope (SEM/EDS) were performed. The last of these methods proved to be crucial for the complete identification of mineral phases in the rocks in question. These studies have shown that the main iron-bearing minerals are biotitic-origin chlorite and biotite, light micas (sericite, muscovite), as well as garnets and other heavy minerals (rutile, apatite, epidote, monazite, zirconium, pyrite, titanium-magnetite, sphalerite) present in the form of small grains dispersed in the rock or as larger clusters and fillings in cracks and fissures. The examinations also found that the structure of mineral phases identified in the studied granitoids also contain other colouring elements such as manganese (garnets, chlorites, and micas), thorium and uranium (monazite, xenotime, and zirconium), cerium (monazite), neodymium (monazite, xenotime), and titanium (rutile, titanium-magnetite). The occurrence of these elementsmay be the cause of differences in L*a*b* colour parameters measured by spectrophotometer for the fired samples, despite a similar or identical content of iron oxide. Confirmation of this hypothesis, however, would require examinations of the rocks’ chemical composition for minor and trace elements, in particular the determination of Mn and REE (Ce, Pr, and Nd). Minerals carrying these elements occur in small quantities in the examined raw materials.