Description
Detailed Description
This goethite pseudomorph after marcasite features radiating spires of crystals. Looking end-on, the spires exhibit the Phillips-screwdriver-cross-section nicely. In excellent condition. One spire, near the centre, facing the front as I would display the piece, is less developed and ends in a flat cross (it appears to have crystallized over, after a break, but the spire’s growth could have been interrupted – these are only guesses on my part). In any event it presents aesthetically as a cool cross-section. This is a great display specimen and stands perfectly as photographed.
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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 relatively uncommon. These specimens, collected in 2013 and 2014, 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.
