Crystal chemistry of spinels from xenoliths of the Alban Hills volcanic region
Lucchesi, Sergio; Amoriello, Monica; Giusta, Antonio Della
European Journal of Mineralogy Volume 10 Number 3 (1998), p. 473 - 482
published: Jun 22, 1998
manuscript accepted: Jan 16, 1998
manuscript received: Jul 18, 1997
ArtNo. ESP147051003007, Price: 29.00 €
Abstract Twenty-one spinel crystals from xenoliths found in the volcanic products of the Alban Hills (Roman Comagmatic Region) were studied by electron microprobe and X-ray single-crystal diffraction. The crystals belong to the Mg-rich side of the MgAl2O4-FeAl2O4 binary with Fe2+ up to 0.15 atoms per formula unit (afu). Fe3+ is always present, ranging from 0.05 to 0.13 afu. Unit cell parameters range from 8.098 to 8.115 Å and the oxygen coordinate from 0.2624 to 0.2616, the latter value being particularly low for natural spinels of this composition and occurrence. The distribution of unit cell parameters and oxygen coordinate values allows two series to be distinguished in spite of the small variations in bulk chemistry. The cation distribution, obtained by a minimization procedure which takes into account chemical and structural data, shows a high degree of disorder, with IVA1 up to 0.22 afu, IVFe3+ up to 0.06 afu; Fe2+ is almost exclusively ordered in the tetrahedral site. The variations in tetrahedral bond distance in both series are due to two contrasting effects: substitution of ,IVFe2+ for IVMg, which causes T-O lengthening, and substitution of Al for IVMg (inversion), which results in T-O shortening. The octahedral bond distances are linked to the cooperative effects of inversion and substitution of VIFe3+ for VIAl.