DONATHITE DISCREDITED - A MIXTURE OF 2 SPINELS

Donathite from Ramberget (Hestmona), Norway, has been examined by Riet veld structure refinement, scanning electron microscopy and transmissi on electron microscopy. Testing of both single- and multi-phase models by Rietveld structure refinement gave the best fit to the pattern for a mixture of a cubic spinel (Fd3m, a = 8.3847(2)Angstrom, Fe-3O-4; F e = Fe2++Fe3++Cr) and a tetragonal spinel (P4(1)22, a = 5.2946(3), c = 8.3144(4), Mg-1.74Fe-1.26*O-4, Mg* = Mg + Al). These compositions w ere confirmed qualitatively by analytical electron microscopy. Leachin g donathite results in a boxwork of lamellae of a cubic spinel of comp osition (Mg0.53Fe0.432+Ti0.04) (Cr1.18Fe0.433+Al0.31Fe0.082+)O-4; the composition of this residual phase was modified by extensive chemical exchange with the acids used in the dissolution process. Donathite is a mixture of two phases, and its discreditation as a valid mineral spe cies has been approved by the International Mineralogical Association Commission on New Minerals and Mineral Names.