CRYSTAL-CHEMISTRY OF SPINELS FROM XENOLITHS OF THE ALBAN HILLS VOLCANIC REGION

Twenty-one spinel crystals from xenoliths found in the volcanic produc ts of the Alban Hills (Roman Comagmatic Region) were studied by electr on microprobe and X-ray single-crystal diffraction. The crystals belon g 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 Angst rom and the oxygen coordinate from 0.2624 to 0.2616, the latter value being particularly low for natural spinels of this composition and occ urrence. The distribution of unit cell parameters and oxygen coordinat e values allows two series to he distinguished in spite of the small v ariations in bulk chemistry. The cation distribution, obtained by a mi nimization procedure which takes into account chemical and structural data, shows a high degree of disorder, with Al-IV up to 0.22 afu, Fe-I V(3+) up to 0.06 afu; Fe2+ is almost exclusively ordered in the tetrah edral site. The variations in tetrahedral bond distance in both series are due to two contrasting effects: substitution of Fe-IV(2+) for Mg- IV, which causes T-O lengthening, and substitution of Al for Mg-IV (in version), which results in T-O shortening. The octahedral bond distanc es are linked to the cooperative effects of inversion and substitution of Fe-VI(3+) for Al-VI.