Mineralogical Effects on Plasticity Characteristics of Ceramic Clays: Insights from the Şile Neogene Basin (Türkiye)

The kaolinitic clays of the Şile Neogene Basin (Istanbul, Türkiye), constitute an important raw material resource for the ceramic industry. In the study area, the Istanbul Formation, composed mainly of Miocene clay, sand, and coal sediments, unconformably overlies the Upper Cretaceous andesitic volcanics of the Garipçe Formation. In this study, the relative influence of the Atterberg limits (AL), mineralogical compositions, specific surface area (SSA), and grain size distributions of six clay samples taken from three vertical sections of the İstanbul Formation were investigated comparatively. X-ray diffraction analyses of the whole rock and clay fractions show that, the dominated clay mineral is kaolinite (30-70%), accompanied by variable amounts of illite (5-25%) and smectite (0-25%). The illite-mica phase is almost entirely illite. Mixed layered phases and chlorite are absent. Non-clay minerals are mainly quartz (10-55%) and trace feldspar, siderite and anatase. In the samples, SSA values range 1.24-1.76 m²/g, and the grain size distributions are; d(0.1): 1.33-2.15 µm, d(0.5): 6.186-11.051 µm and d(0.9): 26.73-97.07 µm. The total and half-material grain sizes are approximately smaller than 40 µm and 10 µm, respectively. According to grain size distributions, the clays of Şile contain very fine-grained quartz (<10 µm) and larger-than-expected (>2 µm) clay mineral particles. AL values of the Şile clay samples are; The liquid limit (LL) was found to be in the range of 38-72%, the plastic limit (PL) in the range of 19.1-31.3%, and the plasticity index (PI) was between 19.6-41.5 (results are the average of three repeated experiments for each sample). Samples containing 5-25% smectite (four samples) exhibit systematically higher LL (60%), PL (27.90%), and PI (32.10%) ratios compared to smectite-free samples, reflecting the high surface activity and water adsorption capacity of smectite. In contrast, kaolinite and illite rich samples without smectite (two samples) display moderate plasticity (LL: 47%, PL: 23.72%, and PI: 23.28%) despite comparable grain size characteristics. This result can be explained by smectite's high specific surface area and its structural ability to adsorb more water. On the other hand, it is understood that the finest-grained samples do not always exhibit the highest plasticity. The fact that some of the quartz found in the Şile clays are fine-grained, similar to the clay minerals, may also have influenced the results. High SSA values ​​in some samples appear to be related to the total clay mineral content and, more specifically, the presence of smectite. However, samples with similar smectite content and similar AL values also have different SSA values. For example, two samples with similarly high LL, PL, and PI values ​​will have significantly different SSA values. It has been observed that the AL values in Şile clay samples are partially related to grain size and SSA, but more significantly related to mineralogical composition. In addition to kaolinite, the presence and amount of smectite, and also very fine-grained quartz will affect the results in the engineering and ceramic performances.

 

Key words: Atterberg Limits, Clay Mineralogy, Kaolinite, Smectite, Şile Neogene Basin