Coupled substitutions involving REEs and Na and Si in apatites in alkaline rocks from the Ilimaussaq Intrusion, South Greenland, and the petrological implications^*

Apatites from the Ilimaussaq augite syenite, the sodalite foyaite, and a quartz-bearing peralkaline pegmatite have been analyzed using an electron microprobe for the major elements plus Na, Si, and selected rare-earth elements (REEs). In all three rock types, the maximum REE₂O₃ content in the apatite exceeds 16.0 wt%. Apatite from the augite syenite and primary apatite from the sodalite foyaite are characterized by a low Na content that varies independently of the REE content. The composition of these apatites approaches 25 mol% of the end-member Ca₂REE₃(SiO₄)₃(OH,F,Cl) (lessingite) in response to the substitution REE³⁺ + Si⁴⁺ ⇌ Ca²⁺ + P⁵⁺. REE- and Na-enriched apatites from the quartz-bearing peralkaline pegmatite are almost Si free, and REEs and Na are positively correlated. The composition of these apatites approaches 25 mol% of the hypothetical end-member Na₅REE₅(PO₄)₆(OH,F,Cl)₂, in response to the substitution Na⁺ + REE³⁺ ⇌ 2Ca²⁺. The compositional trend of late-crystallizing apatite from the sodalite foyaite reflects almost equal contributions of the two substitutions. The relationship between these substitutions in a given apatite and its associated mineral assemblage indicates that the Na component is only present in considerable amounts when apatite crystallizes in a strongly peralkaline environment.