The crystal chemistry of elsmoreite from the Hemerdon (Drakelands) mine, UK: hydrokenoelsmoreite-3C and hydrokenoelsmoreite-6R

A crystallographic and chemical study of two 'elsmoreite' samples (previously described as 'ferritungstite' from the Hemerdon mine (now known as the Drakelands mine), Devon, United Kingdom has shown them to be two different polytypes of hydrokenoelsmoreite. Hydrokenoelsmoreite-3C (HKE-3C) crystallizes in space group Fd3m, with the unit-cell parameter a = 10.3065(3) Å. Hydrokenoelsmoreite-6R (HKE-6R) crystallizes in space group R3, with the unit-cell parameters a = 7.2882(2) Å and c = 35.7056(14) Å. Chemical analyses showed that both polytypes have Na and Fe/Al substitution giving the formulae:(NaCaK(HO)□)Σ2.00(WFe AlAs )Σ2.00[O(OH)]Σ6.00·(HO) (3C) and (NaCaK(HO)□)Σ2.00(WFe AlAs )Σ2.00[O(OH)]Σ6.00·(HO) (6R). The doubling of the unit cell in the 6R phase is due to ordering of Na and (□,HO) in the A site; no long-range ordering is observed between W and Fe/Al in the B site