Abstract
The geology of the area near Job’s Hill, St. Mary, Jamaica, is reviewed with reference to the occurrence of dickite, which appears to have been formed by hot ascending waters or emanations from igneous bodies along fractures and through brecciated zones in at least three stages. Angular fragments of the initially-formed dickite, mainly pink, are embedded in a matrix of subsequently-formed dickite, mainly green; narrow veins and coatings of dickite, often white, also occur.
X-ray diffraction patterns of dickites show a range of order-disorder; the pink angular fragments are often highly disordered, the green matrix material disordered to various extents, and the white coatings may be well-ordered or variously disordered. Disorder weakens or eliminates diffractions with the index k ≠ 3n, and the most highly disordered dickites are difficult to distinguish from highly disordered kaolinites. The structural relations of dickite and kaolinite and the probable nature of the disorders are reviewed.
DTA patterns of the Jamaican and other dickites exhibit endotherms ranging from broad almost-double peaks extending from 500-700°C to sharp peaks near 700°C. There is a tendency to pass from broad to sharp endotherms with increasing structural order, but exceptions are found, so that no firm rule can be claimed. The highly anomalous result given by the well-ordered Keokuk geode kaolinite, namely a DTA pattern resembling those of disordered dickites, is confirmed, but no obvious explanation is forthcoming.
