Abstract : This study deals with the crystal-chemistry of manganese in the ferro-magnesian silicates, clino-amphiboles and 2:1 phyllosilicates, supported by an experimental approach under various T, P, and fO2 conditions which control its the valence state. The phases resulting of the experimental work have been characterized by X-ray diffraction, microprobe analyses, scanning electron microscopy, UV-visible optical absorption, thermogravimetric analyses, vibrational spectrometries (FTIR and Raman), and EXAFS.
Manganese, which possesses several valence states, inducing several ionic radii, is able to occupy different sites in silicates, oxides and many other compounds.
In clino-amphiboles, of tremolite and hornblende type as well as in 2:1 phyllosilicates, talc, phlogopite and clintonite, this study demonstrates the importance of dimensional factors on the extent of solid solutions involving manganese. The variety of available sites in clino-amphiboles structures, the octahedra M1,2,3 and the pseudo-cubic antiprism M4, as well as the M1 and M2 octahedra plus, possibly, the interlayer site in clintonite, allows multiple possibilities of incorporation of manganese, under different valence states, depending on experimental conditions.
This study demonstrates that divalent manganese can effectively occupy all M sites in manganocummingtonite, the Mn equivalent of tremolite. In the sodi-calcic clino-amphibole richterite, chosen as a model, and synthesized under oxydizing conditions, an increase of the valence state of manganese from Mn2+ to Mn3+, has been observed. This oxydation of manganese leads to the local, or total, absence of proton in the structure, such as in ungarettiite NaNa2( )Si8O22O2, whose stability conditions has been determined in the present work.
Similarly, in trioctahedral 2:1 phyllosilicates, this study has demonstrated the possibility of occupancy of octahedral sites by manganese, replacing magnesium, and, as a minor element in the interlayer space of clintonite, replacing calcium. In talc, the extent of the Mg2+ - Mn2+ replacement is restricted to less than twenty atomic percents, owing to dimensional constraints, concretely, a misfit between tetrahedral and octahedral layers. At the opposite, the structural adaptability of the trioctahedral mica structure allows a much larger incorporation of divalent manganese. Under oxidizing conditions, trivalent manganese contributes to the stabilization of the structure, also for dimensional reasons, like in norrishite, K( Li)Si4O12. As well as in clino-amphiboles, the presence of trivalent manganese at the octahedral sites, leads to a deprotonotation of the mica.
The association of the large divalent cation with smaller elements, like aluminium, magnesium or zinc, for example in trioctahedral micas, allows a further stabilisation of structures, also, for dimensional reasons.