Abstract
Natural magnetic black sands are known from several sites often located in areas of volcanic origin. Their elemental and mineral composition provides information on the geology of their territory and depends on several factors occurred during their formation. A sample of black sand was collected on the seashore of the island of Mayotte in the Indian Ocean and its magnetic part was investigated by means of energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD), and MOssbauer spectroscopy at room temperature. The mineral composition is dominantly magnetite, in good agreement with samples collected in other sites of volcanic origin. Contrary to pure magnetite, a relevant fraction of Ti was detected by EDS. The 16% Ti and 1% Mn content increase the magnetite lattice parameter to 8.4312 (25) A. The broadening of XRD lines pointed towards a significant degree of disorder. This was confirmed by MOssbauer spectroscopy and is attributed to the presence of Ti replacing Fe in the magnetite lattice. The presence of Ti modifies the local magnetic field on the Fe sites, leading to a broader and more complex MOssbauer transmission spectrum with respect to the one of pure magnetite. To study the effect of temperature, samples were heated for 12 hours to 600°C and 800°C in argon and to 1000°C in air. Annealing in argon did not improve the crystallinity while annealing in air caused a complete decomposition of magnetite into hematite and pseudobrookite.
Highlights
The natural occurrence of strongly magnetic black sands is known from several sites, e.g. in Italy [1], the Black Sea [2], or Costa Rica [3]
A sample of black sand was collected on the seashore of the island of Mayotte in the Indian Ocean and its magnetic part was investigated by means of energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD), and Mössbauer spectroscopy at room temperature
The broadening of XRD lines pointed towards a significant degree of disorder. This was confirmed by Mössbauer spectroscopy and is attributed to the presence of Ti replacing Fe in the magnetite lattice
Summary
The natural occurrence of strongly magnetic black sands is known from several sites, e.g. in Italy [1], the Black Sea [2], or Costa Rica [3]. Often these sands are of volcanic origin. The chemical structure of natural samples depends on several factors and conditions occurred during their formation. The presence of elements different from iron, pressure and time of solidification determine changes in the structure that can be put in evidence and quantitatively studied by means of the experimental methods used in this work
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