The spessartine–almandine garnet from Val Codera pegmatite, Central Alps, Italy : a new insight on the crystallochemistry and a 3D image analysis of its inclusions

This paper is a new crystallographic and chemical insight including REE and other trace elements of garnets from the “garnet dike”, located in Val Codera (Central Alps, Italy). The crystals may reach up to 2 cm with trapezohedral shape and show a homogeneous dark red colour; however, there are also present many eye-visible inclusions and cracks. Therefore, synchrotron X-ray computed micro-tomography was carried out on selected samples to visualise the 3D distribution and shape of those inclusions, as well as the distribution of voids and cracks within the crystals. To notice here, that because of those latter features a workability of this mineral could result difficult even though these crystals would otherwise may be interesting as gem. The analysed garnets are intermediate members of the almandine–spessartine solid-solution series, varying in composition from Mn-rich core (up to Sps64mol%) to Fe-rich rim (up to Alm54mol%). The samples exhibit a depleted LREE and an enriched HREE patterns with a strong negative Eu anomaly. The increase of total content of REE as well as of Y from core to rim suggests that their incorporation in the [X] site, mainly occupied by Mn and Fe2+, is controlled by the variation of both these cations. In the same way, the cell parameter and the bond lengths, measured both by powder and single crystal X-ray diffraction methods, result between those of pure spessartine and almandine, consistent with the chemistry and related to the variation of the ratio Mn/Fe. The chemical zoning of garnet represents its growth history and makes it a useful indicator of fractional trend of pegmatite-forming melt. Our results suggest that the spessartine-rich garnet crystallisation took place from a Mn-rich melt in presence of volatiles, which prevented the incorporation of Y and REE in the structure favouring the crystallisation of Y and REE-rich accessory minerals.