Description

Detailed Description

This thumbnail-sized goethite pseudomorph after marcasite is a group of bladed marcasite crystal forms in a crystal ball dominated by a couple of larger crystals, with many smaller ones in association. Dark brown, almost black, with good lustre and a metallic sheen. If not for the metallic sheen this would look like molten chocolate. In excellent condition, no damage. Very nice aesthetic form.

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About These White Desert Pseudomorphs

This locality has been known for a number of years and pseudomorph specimens have come out once in a while. The pseudomorphs occur within the Cretaceous Khoman Chalk, from which the White Desert derives its name. Most crystals have typically been fairly indistinct, and to date sharp specimens have been uncommon. These specimens are remarkable for their relatively sharp marcasite crystal forms in aesthetic crystal clusters.

Over the years, these pseudomorphs have been variously labeled hematite, goethite and limonite (the latter no longer a valid mineral species name, but is a term still used in reference to unidentified iron hydroxides, so its use has not been incorrect). Recent work by Hannah Allen at Hamilton College has confirmed that the White Desert pseudomorphs are predominantly goethite. The small white grains lodged in among the crystal blades are barite, calcite and gypsum. (Allen, Hannah M., Pseudomorphed Mineral Aggregates of the Khoman Chalk, Western Desert, Egypt, Geological Society of America Abstracts with Programs. Vol. 46, No. 2, p.66 (2014)).

Although pseudomorphs after cubic and cuboctahedral pyrite crystals have also been found in the Khoman Chalk, the pseudomorphs after marcasite are more dramatic. These pseudomorphs are excellent specimens featuring beautiful marcasite crystal morphology, showing habits and forms exhibited by the crystallized marcasite specimens from the famous occurrences at Cap-Blanc-Nez, Pas-de-Calais, France.