Description
Detailed Description
This is a very unusual goethite pseudomorph from the White Desert. Most of these specimens are pseudomorphs after marcasite, and some have had quite a star-like appearance, with pointed spires radiating out from a centre. This one, however, is very different – many of these were pyrite crystals before they were goethite. There is more to it, however. At first inspection, this specimen looks like it’s al pyrite, but upon a closer look a the centre of the radiating arms, one can see two very short stubby spires on each side of the piece that are clearly goethite after marcasite, and although they are rounded, one does display the “Phillips screwdriver head” cross-section which is a signature look of some of the marcasite specimens from here. I believe that the shape of this specimen is attributable to underlying marcasite spires, and the pyrite aggregates then grew over the underlying marcasite. (And then it was al. pseudomorphed to goethite.)
This specimen is dark brown, with slightly sharper crystals on one side and slightly more polished and rounded crystals on the other side. It looks great displayed with either side facing forward! In excellent condition. From the Tracy Kimmel Collection, and formerly in the collection of Martin Zinn (the latter label is included). A very cool specimen.
Please Note: the colour representation in the still photograph has been carefully calibrated and adjusted to show an accurate rendition of this specimen in daylight (shade). The colours in the video are not similarly adjusted.
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). 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 usually 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.
