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The influence of Al on the physical and crystallographic properties of ludwigite in three Romanian occurrences

Department of Mineralogy, Geological Institute of Romania, 1 Caransebes Str., RO-78344, Bucharest, Romania

^* e-mail: marincea{at}ns.igr.ro

Ludwigite samples from three boron-bearing magnesian skarns in Romania (Ocna de Fier, Masca-Baisoara and Pietroasa) were used to determine the influence of Al on the main physical and crystallographic constants of this mineral. The selected samples form a minor solid solution towards vonsenite (0.74 to 23.87 mol.%) and have X_Al = Al/(Al + Fe³⁺) ratios between 0.004 and 0.300. Other substitutions, such as those involving Mn, Ni, Co, Ti, Sn, Sb or V are of minor importance. Generally speaking, it is proven that ludwigite shows a negative correlation between X_Al and the density, indices of refraction, cell volume and c cell parameter. It is not possible to quantify the variations in the physical and crystallographic parameters by advancing the Al-for-Fe³⁺ substitution because both Fe²⁺-for-Mg and Al-for-Fe³⁺ substitutions are involved. However, it is clear that Al plays a major role in decreasing the cell volume, c cell parameter, density, absorption coefficient, bireflectance and refraction indices of ludwigite. Because of the influence of Al, natural compositional members of the ludwigite-vonsenite series cannot be unambiguously distinguished, neither by their cell dimensions nor by their physical parameters as previously suggested (e.g., Aleksandrov, 1968; 1982).

Key-words: ludwigite, Al-for-Fe³⁺ substitution, refraction indices, density, cell parameters.

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