Crystal Chemistry of Barian Titanian Phlogopite from a Lamprophyre of the Gargano Promontory (Apulia, Southern Italy).

This study is focused on a barian titanian phlogopite found in an alkaline ultramafic dyketransecting Mesozoic limestones of the Gargano Promontory (Apulia, Italy). The rock containingthe barian titanian phlogopite, an olivine-clinopyroxene-rich lamprophyre with nepheline and freeof feldspars, has been classified as monchiquite. The present study combines chemical analyses,single crystal X-ray diraction and Raman spectroscopy. Chemical variations suggest that the entryof Ba into the phlogopite structure can be explained by the exchange Ba + Al = K + Si. The crystalstructure refinement indicates that the Ti uptake is consistent with the Ti-oxy exchange mechanism.The structural parameters associated with the oxy substitution mechanism are extremely enhanced andrarely reported in natural phlogopite: (a) displacement of M2 cation toward the O4 site (~0.7); (b) M2octahedron bond-length distortion (~2.5); (c) very short c cell parameter (~10.14 Å). Raman analysisshowed most prominent features in the 800-200 cm?1 region with the strongest peaks occurring at773 and 735 cm?1. Only a weak, broad band was observed to occur in the OH-stretching region.As concerns the origin of the barian titanian phlogopite, the rock textural features clearly indicate thatit crystallized from pockets of the interstitial melt. Here, Ba and Ti enrichment took place after majorcrystallization of olivine under fast-cooling conditions, close to the dyke margin.