The Canadian Mineralogist
Volume 34, pages 631-647 (1996)
THE CRYSTAL CHEMISTRY OF THE TAPIOLITE SERIES
MICHAEL A. WISE
Department of Mineral Sciences, Smithsonian Institution, Washington, D.C. 20560, U.S.A.
PETR CERNƯ
Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2
Abstract
Ferrotapiolite-manganotapiolite-group minerals [(Fe,Mn)Ta2O6] are formed as accessory phases in rare-element granitic pegmatites exhibiting
moderate to high degrees of fractionation. Despite their relative paucity, 94 sets of unit-cell parameters and 194 chemical compositions were
compiled to characterize the crystal chemistry of the tapiolite series. X-ray-diffraction studies indicate that the degree of order and
compositional variations strongly influence unit-cell dimensions of tapiolite. Natural tapiolite shows wide ranges of structural state.
Crystallization of disordered phases at low temperatures is suggested; however, the available data are not unambiguous. Tapiolite chemistry
is typically uniform (Fe > Mn, Ta > Nb), and compositional variations are the result of limited, but effective homovalent and heterovalent
substitutions: (1) Mn2+Fe2+-1; (2) Nb5+Ta5+-1; (3) Fe3+(Ti,Sn)4+Fe2+-1(Ta,Nb)5+-1 and (4) (Ti,Sn)4+3(Fe,Mn)2+-1(Nb,Ta)5+-2. In association
with other Nb,Ta,Sn-bearing minerals, tapiolite shows a distinct preference for Fe2+ and Ta over Mn and Nb. Enrichment in Nb, Ti and Sn
appears to be common in tapiolite from moderately fractionated pegmatites, whereas extreme Mn enrichment is typical of highly
fractionated pegmatites of the petalite subtype or metasomatic units.