Growth Marks of titanian-andradite crystals from Colli Albani(Italy)

摘要

Growth defects and chemical zonings of Ti-rich andradite (melanite) crystals from Colli Albani were studied using X-ray Diffraction Topography and Electron Probe Microanalysis in order to determine the distinctive features, "growth marks", characterising minerals grown directly from high temperature solution in an open system (volcanic chamber). The samples, cut in (110) and (001) slices, showed a number of primary and, above all, multiphasic melt inclusions, as well as, an unusual colour zoning consisting of a darker colour in the {211} growth sectors with the respect to the {110} sectors. The analysis of X-ray topographic contrasts of growth defects as growth bands, sector boundaries and bundles of dislocations developed parallel to the growth directions has enabled the reconstruction of the morphological evolution vs. time of the samples. The results also suggest that from their onset the crystals were faceted with {211}and predominant {110} faces. Moreover, the topographic images indicated that most of the examined dislocations were nucleated from inclusions. By using the extinction criterion, it was possible to determine that these defects are characterised by a strong edge component. The faceted morphology of garnets found in this paper and the fact that edge dislocations were only characterised suggest a layer spreading growth mechanism by two-dimensional nucleation. The reconstruction of morphology was also particularly useful in distinguishing two different types of chemical zoning: concentric and sector zoning. The results of chemical analyses confirmed that the optically observed colour zoning coincided with sector zoning because of a small but meaningful enrichment of Ti0_2 and a decrease of A1_20_3 in correspondence with {211} growth sectors. Instead, concentration variations, chiefly of Ti0_2 and Si0_2, were recorded crossing successive growth stages. This event appeared to be associated with the physio-chemical evolution of the growth environment and was defined as concentric zoning. Comparing the results found in our study with those of previous studies on garnets from hydrothermal and metasomatic environments, the meaning of the growth marks in the Ti-rich andraditic garnets from Colli Albani could be assigned to primary multiphase melt inclusions, concentric and sector zoning, sector boundaries, growth bands and edge dislocations since these defects characterise and distinguish our samples from those grown in different genetic environments.