No abstract

Three natural spinels of different places of occurrence and compositions were investigated by means of microprobe chemical analysis, single crystal X-ray diffraction and MOssbauer spectroscopy. All cation distributions between T and M sites were calculated from microprobe and XRD experimental data, by means of a mathematical model with appropriate assumptions. Fe 2 § and Fe 3 § assignments calculated in this way were compared with those observed in MOssbauer spectra. The satisfactory agreement found proves, at least in the samples studied, the reliability of the model and the assumptions used. In the spinels examined, such results show Fe 2 § and Fe 3 § virtually ordered in T and M sites respectively. M6ssbauer data also revealed Fe 2 § in different tetrahedral sites due to the next-nearest neighbour effect, probably as a consequence of spinel genetic conditions.

A natural Mg-A1-Fe spinel from the Balmuccia peridotite (Italian Western Alps) was annealed at T between 650 and 1150~ under controlled oxygen activity, and quenched in H20. Twenty-three cation distributions were calculated from XRD structural refinements in tandem with microprobe analysis, and verified by Mrssbauer spectroscopy in the case of unheated samples.Unheated crystals showed essentially ordered distribution of Fe 3+ in octahedral and Fe 2+ in tetrahedral sites, the only intracrystalline disorder being represented by ~0.12 atoms per formula unit of [41AI and 161Mg. Thermal runs and quenching maintained substantially ordered distribution of Fe 2 § and Fe 3+ up to ~ 990~ and produced continuous [41Mg-[6IAI exchange. Between 990 and 1150~ the previous order of Fe2 § 3+ appeared to change slightly, t61Fe 2+ reaching ~0.04 afu and [6]Mg ---I41AI ~ 0.24 afu at the highest T. After quenching from this temperature, Fe 2+ still resided mainly in the T site. Some previously heated crystals underwent reordering on lowering of the temperature.Experimental data, integrated with existing literature, enabled cation-oxygen distance in this structure to be improved. Results from annealed samples allowed the formulation of an experimental thermometric function based on Mg-AI intracrystalline disorder.

Microprobe analyses and X-ray crystal structure refinement data of Mg-Fe-Al-Cr spinels from different environments are compared. The investigated crystals represent both restitic and recrystallized spinels from an Alpine peridotitic massif, and restitic spinels from four suites of ultramafic xenoliths. Within each suite the crystals represent different steps of an increasing melting process, which causes a strong increase in Cr3+ and a moderate increase in Fe2+, with depletion of Al3+ and Mg2+. Within each suite, in spite of relevant bulk chemistry changes, the ratio of the octahedral to tetrahedral coordination distances, and consequently the oxygen positional parameterx, are kept constant. Conversely,x may differ in suites with similar bulk chemistry, mainly due to different Mg-Al ordering between octahedral and tetrahedral sites. This suggests that thex parameter is strongly affected by physical environment, and that consequently, within the range of the investigated compositions, it could be used as a marker

The structural changes occurring in the clinopyroxenes with composition Di100, Di90En10 and Di80En20, due to the Ca-Mg substitution in the M2 site, have been studied. Evidence is given that with increasing Mg content a small percentage of the atoms converts from the M2 position to a new M2′ position which is solely occupied by Mg. The maximum conversion of M2 to M2′ found in this study is 7%. The closest parallel to the M2′ geometry is found in the ZnSiO3 pyroxene (C2/c). The presence of this new site causes significant changes in the tetrahedral configuration, because the M2′ atoms are not bonded to 03. The intermediate compositions, Di90En10 and Di80En20, may be thought of as the coexistence of two structural models: diopside and ZnSiO3 pyroxene (C2/c). © 1982 Springer-Verlag