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Mineralogical ClassificationGersdorffite - what happened?

21st May 2018 15:30 UTCJeff Weissman Expert

The IMA mineral list gives three polymorphs of gersdorffite - Gersdorffite-P 213, Gersdorffite-Pa 3, and Gersdorffite-Pca 21, all with formula NiAsS, but the IMA list does not have 'gersdorffite'. MINDAT has all four, with the following crystal parameters:

gersdorffite : isometric, a= 5.66(6) A

gersdorffite-Pa3 : isometric, a= 5.7053

gersdorffite-P213 : isometric, a=5.576

gersdorffite-Pca21 : orthorhombic, a = 5.6849 ('b' value not given - an obvious error)

So, is gersdorffite sensu-stricto still a mineral, and if not, which of the three polymorphs is the common one, or is all 'gersdorffite' a mixture of these three?

21st May 2018 19:28 UTCUwe Kolitsch Manager

Gersdorffite-Pca21: b + c added (identical to a within error limits).

http://rruff.info/rruff_1.0/uploads/AM67_1058.pdf

21st May 2018 20:15 UTCJeff Weissman Expert

Thanks Uwe, interesting read. In the conclusion, Bayliss notes that "Separate species names for all three crystal structures are not necessary,

because they are minor derivatives of the pyrite-type crystal structure based upon different types of As-S ordering, which are probably temperature dependent" Yet, here we are with 3 or 4 species names for NiAsS.

21st May 2018 21:48 UTCJolyon Ralph Founder

Perhaps we should handle structural polymorphs in the same way we handle polytypes?

22nd May 2018 09:47 UTCUwe Kolitsch Manager

That fact that gersdorffite is missing from the IMA list may be an error (I hope that someone from the IMA chimes in).

Ullmannite is in the list, but none of its polymorphs

Compare also the case of zirconolite - the name is in the list, but the names of the polytypes are not.