Fluorcaphite, a second occurrence and detailed structural analysis: Simultaneous accommodation of Ca, Sr, Na, and LREE in the apatite atomic arrangement

Fluorcaphite is a common accessory mineral in albitite developed at the contact of quartzite and peralkaline nepheline syenites of the Lovozero complex, in northwestern Russia. The rock consists predominantly of albite, aegirine, sodic amphibole (arfvedsonite-magnesio-arfvedsonite) and narsarsukite. Fluorcaphite forms euhedral prismatic crystals up to 0.3 rum in length. Most of the crystals are homogeneous, but a few contain a resorbed core relatively depleted in Sr, Na and light rare-earth elements (LREE). This pattern of zoning arose from two overprinting episodes of metasomatism. In terms of composition, both core and rim are intermediate members of a solid solution between fluorapatite and belovite-(Ce), with the belovite content increasing toward the rim. Homogeneous crystals are compositionally equivalent to the rim, and contain approximately 50 mol.% of the belovite component. The crystal structure of the Lovozero fluorcaphite was studied by high-precision single-crystal methods and compared to that of the type material from Khibina (as re-examined in the present work). In both cases, the structure is an acentric derivative (space group P6(3)) of the P6(3)/m apatite archetype. The dissymmetrization, involving loss of the center of symmetry and mirror plane, results from transformation of the Ca 1 site into two non-equivalent sites tailored to accommodate different cations as substituents. The larger Ca 1 site hosts a significant proportion of Sr, whereas the Ca F site excludes Sr in the structure. The observed symmetry-breaking is subtle and poses obvious difficulties for correct space-group assignment. Potential pitfalls of space-group analysis in ordered apatite-group phases are evaluated with the Durango fluorapatite. which has a well-established P6(3)/M symmetry, as an example. We found no evidence that enrichment of apatite (sensu lato) in Na, Sr and LREE causes phase separation and formation of intimately intergrown domains of fluorapatite and belovite.