Arfvedsonite, an uncommon sodic amphibole, occurs in basalt dykes at Buchans, Newfoundland. Stability and textural relations suggest that the mineral grew during subsolidus cooling of basalt within the submarine geothermal system which persisted during contemporaneous massive sulfide ore formation. It is only sporadically preserved, readily altering to chlorite during prehnite-pumpellyite facies metamorphism or associated hydrothermal alteration. Sodic amphiboles may occur more widely in similar parageneses than has previously been recognized.

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