PSEUDOMORPHED MINERAL AGGREGATES OF THE KHOMAN CHALK, WESTERN DESERT, EGYPT

Pseudomorphed mineral aggregates are scattered across the Farafra region of the Western Desert of Egypt, solely within the Cretaceous Khoman Chalk. While they have been sold by collectors for many years as marcasite pseudomorphs, they have not received significant scientific study until this project. The mineral aggregates exhibit a variety of morphologies; the most common are stellate aggregates, amorphous spherical concretions, thin tabular fracture-filling aggregates, and long, cylindrical tubes. Precipitation of the aggregates happened post-deposition of the Khoman Chalk unit, possibly as a result of diagenesis and dewatering during burial of the chalk. The pseudomorphs were collected in a region of the chalk that also exhibits small scale (500-1000 m in diameter) polygonal faults. The goal of this project was to determine the original mineral composition of these aggregates, as well as the secondary replacement minerals.

After studying the samples under both reflected and transmitted light, with SEM / EDS and XRD, we determined that the aggregates are iron hydroxide (primarily goethite) pseudomorphs after both marcasite and pyrite. Additional secondary minerals present in the pseudomorphs include barite, gypsum, and calcite. The presence of marcasite and pyrite aggregates in carbonate sediments is widespread, and studies have shown that both can form at relatively low temperatures (i.e. <100^oC) (Schoonen and Barnes, 1991); unfortunately, the exact conditions of formation cannot be more tightly constrained. In order to fully understand the nature of these unusual mineral aggregates and their relationship to the development of polygonal faults within the Khoman Chalk, additional field studies that carefully document the in situ spatial distribution of these mineral aggregates are required.