A crystal chemical insight into sector zoning of a titanian andradite ('melanite') crystal

A crystal chemical study was undertaken on a melanite crystal from the pyroclastic deposits of Colli Albani (Lazio, Italy), a Middle-Late Pleistocene volcanic complex belonging to the Roman Perpotassic Province. Previous investigations by electron-probe microanalysis and X-ray topographic methods have shown that melanite from Colli Albani is affected by both sector and concentric zoning. In the present study, we focus on the crystal chemistry of the sector zoning using a combination of Mossbauer spectroscopy, electron probe microanalysis and single crystal X-ray diffraction methods. Electron probe microanalyses show the occurrence of minor chemical differences, especially in the Al and Ti content between non-equivalent {121} trapezohedron and {110} rhombic-dodecahedron growth sectors. Structural analyses are carried out on three crystals fragments taken from the ((1) over bar 10), (01 (1) over bar) and ((1) over bar2 (1) over bar) sectors of the same macrocrystal. We find that the slight chemical difference between the growth sectors produce detectable effects on bond distances: the Y-O distance is significantly shorter and the Z-O distance greater in {121} than in the {110} sector. Two doublets are fitted to both RT and LNT Mossbauer spectra and assigned to octahedral Fe3+ (similar to 93%) and to dodecahedral Fe2+(similar to 7%). As far as the crystal chemical behaviour is concerned, two kinds of heterovalent substitutions seem to affect the melanite structure, i.e. M2+(Y) + M4+(Y) <----> 2M(3+)(Y), morimotoite substitution, with M2+ = Mg, Mn; M3+ = Al, Fe; M4+ = Ti, Zr, and Ti4+(Y) + Fe3+(Z) <----> Si4+(Z) + Al3+(Y), schorlomite substitution. The former occurs in both growth sectors, {121} and {110}, the latter only in the {121} sector. These results seem consistent with the more rapid growth of {121} sector according to a "layer by layer" mechanism, as revealed by the X-ray topographic investigation.