Non-stoichiometry and local order in minerals with the rockbridgeite structure

A non-stoichiometric frondelite from the Hagendorf Siid pegmatite, Bavaria, has been studied using chemical and thermal analyses, electron diffraction (ED), Mossbauer spectroscopy and synchrotron single-crystal structure analysis. The structural formula, with occupancies of the M1, M2 and M3 sites given in sequence, is (Fe3+) (Mn0.672+Zn0.20Fe0.052+Fe0.703+)(Sigma 1.65)(Fe-2.30(3+)) (PO4)(3) (OH) 4.9 (H2O)(0.1). The non-stoichiometric frondelite contains an excess of 0.3 atoms in the M3 site and a deficiency of 0.35 atoms in the M2 site. The ED patterns show rows of reflections that violate the reflection extinctions for Bbmm, and which are due to a local ordering of Fe3+ and vacancies in the M3 sites. Based on the ED results, a model was developed in space group Pnma, in which (100) planes containing corner-shared octahedral dimers of M3 atoms are ordered on a scale of similar to 6 nm. The Pnma model was refined successfully against the synchrotron single-crystal data. Non-stoichiometry can be explained within the Pnma local-order model by distortions at the M3 sites in non-stoichiometric regions, where local, barbosalite-like, trimeric chains of face-sharing octahedra are formed.