Temperature-dependent Fe²⁺-Mn²⁺ order-disorder behaviour in amphiboles

The partitioning of Fe and Mn between the large M(4) site and the octahedral sites, M(1,2,3) in the amphibole structure has been investigated in two natural manganogrunerites of compositions Ca_0.1Mn_1.9 Mg_1.25Fe²⁺ _3.56Fe³⁺ _0.38Si_7.81 O₂₂(OH)₂ and Ca_0.24Mn_1.57 Mg_2.27 Fe²⁺ _2.76Fe³⁺ _0.32Si_7.84O₂₂(OH)₂. The long-range cation distribution in the two samples has been elucidated by in situ neutron powder diffraction revealing that Mn is preferentially ordered onto M(4) ≫ M(2) > M(1) > M(3) in both samples. Partitioning of Mn from M(4) into the octahedral sites begins at 350 °C, with site exchange energies of -16.6 kJ mol^-1 and -14.9 kJ mol^-1, in samples containing 1.90 and 1.57 Mn apfu, respectively. Mösbauer and infrared spectroscopy have been used to study the samples at room temperature, and Mösbauer data agree well with the diffraction results, confirming that high-temperature cation distributions are retained during cooling. The fine structure in the hydroxyl-stretching region of the IR absorption spectra has been used to discuss qualitatively the site occupancies of the coordinating M(1)M(3)M(1) triplet, linked by O(3). On the basis of such modelling, we conclude that a degree of local clustering is present in both samples.