Jamaican bauxites occur as pocket or blanket deposits of red earthy material on the karst surface of the Tertiary White Limestones, mainly in the interior of the island. The contact between the bauxite and the limestone is sharp, with a strong colour contrast between the dark red bauxite and the white limestone. The bauxite deposits show no internal structural characteristics and their colour is mainly a uniform dark red with localized variations from off-white to yellow, depending on the iron content. Thickness of the bauxite cover is extremely variable, ranging from a few feet to an average of about 30 feet, up to as much as 100 feet.
Generally, Jamaican bauxites from any particular area have a consistent chemical and mineralogical composition. While the dominant chemical component is alumina (Al2O3), it is the mineralogical components which play the significant role in the processability of the bauxite. The predominant mineral is gibbsite (Al2O3.3H2O), which is invariably associated with boehmite (Al2O3.H2O). The latter in often present in insignificant amounts, but in some areas, the monohydrate form may exceed 5% by weight of the total bauxite. The proportion of trihydrate to monohydrate is of considerable importance since the processing characteristics of monohydrate bauxite is markedly different from that of trihydrate bauxite. The gibbsite content of Jamaican bauxite ranges from 34-46%, whereas boehmite can range from 1-12%. For this reason, Jamaican bauxite may be classified as `mixed’ trihydrate/monohydrate or gibbsite/boehmite bauxite. Some amount of blending can be tolerated in the processing, and this is encouraged in order to maximize use of the resource.
The silica or clay mineral content is the next most important, and is used for grade determination of the bauxite, as high silica causes problems in the processing of bauxite to alumina. The predominant clay mineral is either kaolinite or halloysite, and a high grade bauxite contains 1-3% of this mineral determined as SiO2. Lower grade (non-commercial) bauxite may contain 4-8% SiO2, and above 8% is generally regarded as clay rather than bauxite. However, as with the alumina minerals, some amount of blending can be done to improve the characteristics of a particular deposit.
Iron is another significant mineral in the bauxite assemblage, and occurs mainly in the form of haematite (Fe2O3) or its hydrated form, goethite (Fe2O3.H2O). In any of its forms, iron content ranges between 17-21%. The haematite is what gives Jamaican bauxites their characteristic red colour, whereas yellow bauxites indicate the presence of goethite.
Other minor, though important, bauxite constituents include titania (TiO2 1.7-2.8%), phosphorus (P2O5 0.5%), manganese (MnO 0.3%), zinc (ZnO 0.05%) and a considerable number of trace elements. The free moisture content of Jamaican bauxite is in the range of 22-23% H20.