Paragenesis and origin of secondary beryllophosphates: Beryllonite and hydroxylherderite from the BEP granitic pegmatite, southeastern Manitoba, Canada

Rare occurrences of phosphates with beryllium are limited to granitic pegmatites and, exceptionally, to rare-element granites. Primary crystallization is extremely scarce; most of these phosphates form as late subsolidus phases. Paragenetic relationships of these minerals and conditions of their origin are poorly understood. Beryllonite, hydroxylherderite and carbonate-fluorapatite form thin coatings in fissures of beryl in the Archean petalite-subtype BEP pegmatite in southeastern Manitoba. The composition of beryllonite corresponds to its ideal formula, and hydroxylherderite contains only traces of fluorine. The B-type carbonate-fluorapatite shows considerably reduced P2O5 content, "excess" fluorine, and an apparently high CO2 content (estimated at similar to5 to similar to7 wt.%). Beryllonite was the first phosphate to precipitate, followed and in part replaced by hydroxylherderite; carbonate-fluorapatite rims, penetrates and, replaces both preceding phases. Low a(F) is indicated during the formation of beryllonite and hydroxylherderite, but it must have increased to stabilize carbonate-fluorapatite. The process probably involved F released during the (OH)F-1 exchange in the associated montebrasite.