Gramaccioliite-(Y): paragenesis chemistry, and structure in a new occurrence, Samos Island, Greece

We describe a second occurrence of the recently defined mineral species gramaccioliite-(Y), a member of the crichtonite group, on Samos Island, Greece. The mineral occurs in form of centimetre-sized crystals in brecciated metabauxite, associated with diaspore, hematite, muscovite, chloritoid, calcite, rutile, monazite-(Ce), and REE carbonate minerals (bastnäsite-(La) and parisite-(Ce)). Petrological observations indicate that the coarse-grained vein assemblage containing gramaccioliite-(Y), diaspore and chloritoid was succeeded by calcite, rutile, monazite-(Ce) and hematite/limonite, suggesting a chronological relation such as: gramaccioliite-(Y) + fluid —> monazite + rutile + hematite. Latest stage alteration is characterized by the formation of fine-grained kaolinite + calcite + limonite. The petrographical observations suggest that gramaccioliite-(Y), because of the contemporaneous formation with chloritoid and diaspore, was formed at a temperature of about 400 °C, i.e. under metamorphic conditions.

A high-pressure experiment was conducted to examine the P-T stability of gramaccioliite-(Y). Neither phase transformations nor formations of other phases were observed up to 50 kbar, 1100 °C under dry conditions, indicating that this mineral may be stable down the Earth’s mantle.

An electron-microprobe examination reveals that gramaccioliite-(Y) from Samos Island contains a significant amount of REE, and has accordingly a distinct miscibility towards davidite-(La). In contrast, gramaccioliite-(Y) from the type locality contains a relatively high amount of crichtonite (s.s.) component. A characteristic feature of the examined Samos material is growth zoning within single crystals, mainly concerning Pb being enriched and REE being depleted in the cores.

A single-crystal structure refinement of gramaccioliite-(Y) was performed to R₁ = 0.0308 in space group R3̅, with a = 9.1814(5) Å and α = 68.820(4)°, Z = 1. The crystal structure is topologically identical to those of minerals of the crichtonite group, with M0 occupied by Pb and La, M1 occupied by Y, Sc, Ce, Zr, Dy and Mn²⁺, M2 occupied by Fe²⁺ and Zn, M3 occupied by Fe³⁺, Ti and Al, M4 and M5 occupied by Ti.