Synthesis and Mössbauer characterization of Fe_1+xCr_2-xO₄ (0 ≤ x ≤ 2/3) spinel single crystals

Abstract

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 39-55 - DOI: 10.2451/2011PM0004
Special Issue in memory of Sergio Lucchesi

Synthesis and Mössbauer characterization of Fe_1+xCr_2-xO₄ (0 ≤ x ≤ 2/3)
spinel single crystals

Marco Quintiliani¹, Giovanni B. Andreozzi^1,* and Henrik Skogby<sup>2

1</sup>Dipartimento di Scienze della Terra, Sapienza Università di Roma, Italy
²Department of Mineralogy, Swedish Museum of Natural History, Stockholm, Sweden
*Corresponding author: gianni.andreozzi@uniroma1.it

Abstract

Spinel single crystals belonging to the FeCr₂O_4-Fe₃O₄ join were synthesized by a flux growth method (Na₂B₄O₇ as flux, temperature range of 1200-900 °C and cooling rate of 4 °C/h). Crystals with compositions corresponding to Fe_1+xCr_2-xO₄ (0 ≤ x ≤ 2/3) were obtained and successively investigated by a combined microchemical, diffractometric and spectroscopic approach.
Fe-Cr-spinel single crystals produced are of different quality (in terms of size and shape). The flux content in the starting mixture showed to have the most relevant influence on the quality of synthetic products. Electron microprobe analysis evidenced a chemical heterogeneity for crystals of the same batch, with small crystals (down to 50 μm) being more homogeneous than the large ones. Mössbauer spectroscopy was used to determine the actual Fe³⁺/Fe_tot ratios and highlighted the absence of magnetic phases.
The combined chemical and spectroscopic approach allowed to exclude any deviation from stoichiometry due to oxidation. In contrast with most of the existing literature, Mössbauer spectroscopy indicated the Fe^2.5+ valence state to be present even for iron poor samples with composition close to the end member chromite.


Key words: chromite-magnetite solid solution; synthesis, spinel single crystals; Mössbauer spectroscopy.