Single-Crystal X-Ray Diffraction and Raman Spectroscopic Studies of Miyahisaite Sr2Ba3(PO4)3F from the Graulai Quarry, Germany

Miyahisaite, ideally Sr2Ba3(PO4)3F, a member of the apatite supergroup, was identified in pegmatite-like veins hosted by melilite–nephelinite volcanic rocks at the Graulai quarry, Hillesheim, West Eifel, Germany. This represents the second occurrence of this rare mineral, previously known only from the hydrothermally altered chert at the Shimoharai mine, Japan. At Graulai, miyahisaite occurs as colorless anhedral crystals, up to 300–400 μm in size, within a complex late-stage mineral assemblage that includes leucite, nepheline, diopside, fluorapatite, götzenite, delhayelite, lamprophyllite-group minerals, magnetite, fresnoite, oxyphlogopite, and perovskite. Electron-microprobe analyzes yielded the empirical formula (Sr0.97Ca0.87Ba0.12Na0.04)Σ2.00(Ba3.00)(P2.87S0.06V0.04Al0.04)Σ3.01(F1.00Cl0.03)Σ1.03 confirming a Sr- and Ba-dominant fluorophosphate member of the hedyphane group. Previously unavailable, single-crystal X-ray diffraction data show that miyahisaite has an apatite-type structure and crystallizes in the hexagonal P63/m, with unit-cell parameters a = 9.8876(5) Å, c = 7.4142(3) Å, and V = 627.74(5) Å3. Structure refinement indicates Sr dominance at the M1 site and Ba at M2 site. The X anion site, occupied by F, shows positional disorder along the channel, reflecting structural adjustment to large divalent cations, a feature typical of apatite-supergroup members. Raman spectroscopic data, obtained for the first time, revealed that the spectrum is dominated by PO43− vibrations, with the most intense band at 948 cm−1 assigned to the ν1 symmetric stretching mode. The absence of OH-related bands confirms F dominance at X site. Genetically, the Graulai occurrence of miyahisaite differs fundamentally from the sediment-hosted hydrothermal origin reported from Japan. Unlike the previously described setting, miyahisaite in the Eifel volcanic system crystallized during a late-magmatic, pneumatolytic stage from volatile-rich fluids enriched in incompatible elements. This discovery expands the known genetic environments of miyahisaite and refines its characterization within the apatite supergroup.