Alteration of Volcanic Rocks and Genesis of Kaolin Deposits in the Şile Region, Northern Istanbul, Turkey. I: Clay mineralogy

Abstract

The S°ile Region contains discontinuous, cyclic, thin coal beds and industrial clay deposits that were accumulated in lacustrine basins which received extensive volcanoclastic sediments due to transport of highly weathered calc-alkaline volcanic rocks. The Sü lü klü area has the largest kaolin deposit in this region. Cyclic kaolinization depended on the degree of leaching of Si and alkalis in cyclic swamp environments and, therefore, kaolinite contents vary in each discontinuous lens-shaped clay bed and underclay within the basin. The kaolins comprise disordered kaolinite, illite, smectite, gibbsite, quartz, pyrite, anatase, K-feldspar and goethite. Depth-related changes in the distribution of clay minerals, associated with coal beds, are indicative of organic acid-mineral reactions. Kaolinite crystallization initiated at the edges of sericitic mica sheets in the form of composite kaolinite stacks. The small size (<1 mm), morphology and poor crystallinity of kaolinite crystals suggest that kaolinization post dated transportation. Primary or secondary origins of particles can be determined from the stacking sequences of kaolinite particles using high-resolution transmission electron microscopy images. Kaolinite stacks always contain a small amount of illite, but smectite is only present in the middle and upper levels. Gibbsite is a main constituent of refractory bauxitic clays locally found as discontinuous lenses and exploited from the lower level of the basin.Genesis of kaolin deposits took place in two stages: first, there was in situ weathering of the oldest andesitic agglomerates, tuffs and ashes at the base of the lacustrine basin coupled with discharge of shallow thermal waters which were initiated by local hydrothermal alteration; second, surface weathering enhanced transportation of altered rocks from the surrounding hills into the lacustrine basin. Kaolinization took place in cyclic swamp environments, as indicated by the presence of cyclic thin-to thick-bedded coals that provided necessary humic and fulvic acids for the post-depositional alteration of altered volcanic rocks to kaolin in dysaerobic, relatively low-pH conditions in saturated groundwater zones.

Fourier transform infrared spectroscopy and thermal analyses of kaolinitic clays from South Africa and Cameroon

“…The OH bands in kaolin typically display four sharp stretching bands between 3600 and 3700 cm -1 namely; 3697, 3670, 3652/50 and 3620 cm -1 (Ekosse and Mulaba, 2008;Ekosse, 2008;Saikia and Parthasarathy, 2010;Kiros et al, 2013;Djomgoue and Njopwouo, 2013). The strong band at 3697 cm -1 arises from surface hydroxyls and produces an in-phase vibration perpendicular to the 1:1 layers (Ece et al, 2003). The two bands at 3670 cm -1 and 3652 cm -1…”

“…The DSC curves show endothermic and exothermic reactions during calcination, such as desorption of surface water (H 2 O), dehydroxylation (structural OH-groups) and the transformation to mullite and cristobalite, with resultant temperature effects of dehydroxylation and polymorphic transformation strongly dependent on kaolinite structural order (Ece et al, 2003;Ekosse, 2008).…”

“…The exothermic peak corresponds to the destruction of the 1:1 kaolinite structure and is typically observed between 943 °C and 968 °C (Ece et al, 2003;Ekosse, 2008). The dehydroxylation temperature of most kaolinites ranges from 498 °C to 605 °C, whereas the dehydroxylation of dickite and nacrite occurs at 680 °C.…”

“…The dehydroxylation temperature of most kaolinites ranges from 498 °C to 605 °C, whereas the dehydroxylation of dickite and nacrite occurs at 680 °C. According to Ece et al (2003) and Saviano et al (2005) well-ordered kaolinites have broad endothermic peaks at approximately 590 °C however, reported endothermic peaks for well-ordered kaolinites from Mayouom at 520 -The positions and assignments of the transmittance bands of representative samples from South Africa and Cameroon compared against that of theoretical kaolinite is summarized in Table 1. Three characteristic peaks at 3688/3687 cm -1 , 3651/3652°C°C cm°C -1 and 3690/3692 cm -1 were observed in ZEB 1, ZEB 2, DUT 1 and DUT 2 ( Fig.…”

Synthesis of zeolite-A using kaolin samples from Darazo, Bauchi state and Ajebo, Ogun state in Nigeria

“…The inset on the right is an enlargement of the -OH bands that are typical in kaolin samples. The strong characteristic peaks at 3695.73 cm -1 arises from surface hydroxyls and produces an in-phase vibration perpendicular to the 1:1 layers in kaolin [39]. The two weak peaks at 3670 cm -1 and 3652 cm -1 are due to stretching vibrations that are sub-parallel to the 1:1 layers whereas the low frequency 3620.51 cm -1 is assigned to the fourth OH inner group.…”