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

A non-stoichiometric frondelite from the Hagendorf Süd pegmatite, Bavaria, has been studied using chemical and thermal analyses, electron diffraction (ED), Mössbauer spectroscopy and synchrotron single-crystal structure analysis. The structural formula, with occupancies of the M1, M2 and M3 sites given in sequence, is (Fe³⁺)(Mn²⁺_0.67Zn_0.20Fe²⁺_0.08Fe³⁺_0.70)_Σ1.65(Fe³⁺_2.30)(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 ∼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.