# Transforming Petrochemical Processes: Cutting-Edge Advances in Kaolin Catalyst Fabrication
## 2 Classification of Kaolin Based on the Origin: A Geological Perspective  
Kaolins can be classified based on their origin into primary categories of pure aluminum silicate minerals with hydroxyl groups, such as kaolinite, dickite, halloysite, and nacrite. These minerals share the chemical formula [Al\({}_{2}\)(OH)\({}_{4}\)Si\({}_{2}\)O\({}_{5}\)], are predominantly found as fine particles, small sheet-like crystals, or clusters of white earth (<2 \(\upmu\)m), and are seldom observed as macroscopic crystals [23]. A summary of the historical milestones in the classification of kaolin minerals is provided below [24; 25; 26]. The term \"kaolinite\" was first applied by [PERSON] and [PERSON] in 1867 based on samples from Kau Ling (High Ridge), a hill near Jauchau Fu, China [27; 28]. \"Dickite\" was named by [PERSON] in 1908 from specimens found on the Island of Anglesey, Wales, UK [27; 28]. The mineral \"nacrite\" was identified by [PERSON] in 1807 at the Einigkeit mine in Brand-Erbisdorf, Freiberg, Saxony, Germany [27; 28]. Lastly, \"halloysite\" was first described by [PERSON] in 1826 from samples collected in Liege, Belgium [29; 30]. Kaolin minerals, which are crucial for various industrial applications, are distributed globally, but only a few deposits meet high quality standards. The grade classification of raw kaolin typically influences its price and suitability for different industrial uses. The main constituents of kaolin are SiO\({}_{2}\), which makes up about 46%, and Al\({}_{2}\)O\({}_{3}\), which makes up around 38% [27]. The quality of natural kaolin fillers is regulated by ISO 3262, Part 8, which specifies that high-quality kaolin must contain at least 90% kaolinite (a hydrated aluminum silicate). Commercial kaolin typically comprises kaolinite as the primary component, along with other minor minerals that vary depending on the deposit's characteristics. This material, which is commonly referred to as kaolin or China clay [28], is extensively used in various industries. Notably, dickite and nacrite are not subject to large-scale exploitation, and this overview does not cover other industrial minerals that also contain significant amounts of kaolinite. The mineral kaolinite, which imparts the characteristic white hue to kaolin, is prevalent in sedimentary formations akin to kaolin deposits found worldwide. Kaolin is found as sedimentary rock, and its deposits are broadly classified into primary (residual) and secondary (redeposited) types [7]. Primary kaolin forms in situ from the weathering of feldspar-rich crystalline rocks, such as granite (feldspar, muscovite, and quartz) [29]. This process, known as \"kaolinization\", disintegrates the rocks into smaller fragments, which are subsequently transported by wind or water. Then, these primary deposits remain at their original location of formation. The modification may involve hydrothermal transformation or weathering under humid, acidic climatic conditions (involving CO\({}_{2}\)  
Figure 2: Number of publications containing the words “kaolin” and “kaolin and catalysis” per year in the period (2005–2024) in the Web of Science (WOS) database. Columns and lines correspond to searches with words in the title, abstract, or keywords, as reported in the inset. containing waters). Primary deposits often contain 15% to 30% kaolin, with the remainder consisting of unaltered granite components like feldspar, muscovite, and quartz. In contrast, secondary kaolin deposits result from the erosion of primary deposits, with the eroded materials being transported and subsequently deposited in sedimentary environments like streams, lakes, and basins. Smaller and lighter kaolin particles can travel longer distances, eventually settling in lakes, estuaries, and lagoons to form secondary deposits. In other words, secondary kaolin deposits are generated when existing kaolin deposits undergo reworking through alteration or other geological processes, leading to the accumulation of new kaolinite layers over older ones.