T. S. Ercit

Cesstibtantite—a geologic introduction to the inverse pyrochlores

SummaryThe crystal structure of cesstibtantite has been solved from diffractometer data collected on samples from Leshaia, Russia and the Tanco pegmatite, Manitoba. Cesstibtantite from the Leshaia pegmatite (type locality) hasa 10.515(2) Å, space groupFd3m, composition Cs0.31(Sb0.57Na0.31Pb0.02Bi0.01)ΣO.91(Ta1.88Nb0.12)Σ2(O5.69[OH, F]0.31)Σ6(OH, F)0.69, Z 8; its structure was refined toR 3.8,wR 4.3% using 96 observed (F > 3σ[F]) reflections (MoKα). Cesstibtantite from the Tanco pegmatite hasa 10.496(1) Å, space groupFd3m, composition (Cs0.22K0.01)Σ0.23(Na0.45Sb0.39Pb0.14· Ca0.06Bi0.02)Σ1.06(Ta1.95Nb0.05)Σ2(O5.78[OH,F]0.22)Σ6(OH,F)0.55,Z 8; its structure was refined toR 3.9w R 3.7% using 104 observed reflections. Cesstibtantite differs from the normal pyrochlores in that it contains significant amounts of very large cations such as Cs. As these cations are too large (VIII[r] > 1.60 Å) for the conventional [8]-coordinated A site, they occupy the [18]-coordinatedϕ site, which normally contains monovalent anions. Natural cesstibtantite samples are non-ideal in that both Cs and monovalent anions occur at theϕ site; thus cesstibtantite is intermediate to thenormal pyrochlores (with only monovalent anions at theϕ site) and theinverse pyrochlores (with only large cations at theϕ site).ZusammenfassungDie Kristallstruktur von Cesstibtantit wurde auf der Basis von Diffraktometerdaten von Proben von Leshaia, Russland and dem Tanco Pegmatit, Manitoba, gelöst. Cesstibtantit aus dem Leshaia Pegmatit (Typlokalität) hat a 10.515(2) Å, RaumgruppeFd3m, die Zusammensetzung CS0.31(Sb0.57Na0.31Pb0.02Bi0.01)Σ0.91(Ta1.88Nb0.12)Σ2· (O5.69OH, F0.31)Σ6(OH, F)0.69Z 8; die Struktur wurde aufR 3.8,wR 4.3% verfeinert unter Benützung von 96 beobachteten Reflexen. Cesstibtantit vom Tanco Pegmatit hat a 10.496(1) Å, RaumgruppeFd3m, die Zusammensetzung (Cs0.22K0.01)Σ0.23(Na0.45· Sb0.39Pb0.14Ca0.06Bi0.02)Σ1.06(Ta1.95Nb0.05)Σ2(O5.78OH,F0.22)Σ6(OH,F)0.55,Z 8; seine Struktur wurde aufR 3.9wR 3.7% auf der Basis von 104 beobachteten Rettexen verfeinert. Cesstibtantit unterscheidet sich von normalen Pyrochloren insofern, als er signifikante Mengen von sehr großen Kationen, wie z.B. Cs enthält. Da these Kationen zu groß sind (VIIIr 1.60 Å) für eine konventionelle [8]-koordinierteA Stelle, nehmen she die [18]-koordiniertenϕ Positionen ein, welche normalerweise monovalente Anionen enthalten. Natürliche Cesstibtantitproben sind nicht ideal insofern als sowohl Cs als auch monovalente Anionen in derϕ Position vorkommen. Somit ist Cesstibtantit intermediär zu den normalen Pyrochloren (mit nur monovalenten Anionen auf derϕ Position) and den inversen Pyrochloren (mit ausschließlichen großen Kationen an derϕ Position).

Structures of zinc selenite and copper selenite

Les deux composes cristallisent dans le systeme orthorhombique, groupe PcaB. Coordination bipyramidale rhomboedrique et pyramidale rhomboedrique autour de En et Cu respectivement

Reinstatement of stibiomicrolite as a valid species

Abstract The name stibiomicrolite was first used by Quensel & Berggren (1938) to describe a hypothetical primary mineral at the Varutrask pegmatite in Sweden. The name was later applied to a microlite found as a breakdown product of stibiotantalite at Varutrask, but Fleischer (1942) discredited stibiomicrolite because of a lack of reliable compositional data. We have re-examined the stibiotantalite ← microlite + antimony assemblage, and our results show that the microlite composition satisfies the specifications for naming new minerals in the pyrochlore group (Hogarth 1977), as Sb constitutes 24.5 to 44.6 at.% of the A-site population, surpassing the required 20% minimum. Our proposal to re-instate the mineral species under its original namne stibiomicrolite has been approved by the International Mineralogical Association.