Nature, genesis and industrial properties of the kaolin from Masirah Island, Oman

Kaolin deposits >10 m thick overlie unconformably a Mesozoic ophiolite sequence at Jabal Humr, Masirah Island, Oman. The clay's mineralogical and chemical composition, plasticity and moisture content were measured to determine its genesis and suitability for commercial usage. The clayrich raw material contains 76-94% kaolinite and varying amounts of quartz (micro sheets coating kaolinite) and calcite as well as secondary sulfates. The mode of occurrence, an associated shallowmarine iron oolite/pisolite unit, various secondary minerals which can only form in a gossan environment (oxidation zone of a much older sulfide deposit), and minerals such as gypsum that are highly unstable within a laterite, have led to the conclusion that the Jabal Humr kaolinite deposit cannot have the lateritic origin that has been suggested previously. Rather, it must have formed in a coastal marine environment with a subsequent strong geochemical overprint from the underlying gossan environment, after being enveloped by Tertiary carbonates. A high plasticity and its light colour after firing indicate that this material is suitable for industrial use, especially in pottery. Occasional high contents of up to ∼25% extremely fine-grained quartz (sheet-like, <50 nm thick) reduce the need for quartz addition during the processing for ceramic materials; such natural kaolinite-quartz mixtures already produce a suitable blend of materials. The possible occurrence of spalling during or after firing, caused by the sporadic presence of accessory calcite, can be avoided by by further addition of quartz which leads to the formation of calcium silicate.