Occurrence of fluororichterite and fluorian biotite in the In Tifar trachyte neck (Tazrouk district, Hoggar volcanic province, Sahara, Algeria)

The unusual occurrence in the In Tifar trachyte neck (Tazrouk district, Hoggar volcanic province, Sahara, Algeria) of the fluorian biotite–fluororichterite association is presented. The two mineral species were previously unknown in the Hoggar and their association is uncommon worldwide. Ti-rich biotite has 28–40% OH sites occupied by fluorine, hence the use of the modifier “fluorian”. Sodic–calcic fluororichterite has more than 55% OH sites filled by fluorine, hence the use of the prefix “fluoro”. Well-defined F–Mg affinities are documented in both cases, while Cl remains very low. Temperatures are estimated roughly at 775–700 °C at low pressures. The fluorian biotite → fluororichterite sequence of crystallisation implies increasingly high fH2F2/fH2O ratios in metaluminous H2O-dominated evolving to peralkaline F-enriched fluids.

• Fluororichterite, a rare amphibole species, is observed for the first time in trachyte of the Hoggar volcanic province.
• The fluorian biotite–fluororichterite association resulted from fluids with high fH2F2/fH2O ratios.
• Fluid compositions evolved from metaluminous H2O-dominated to peralkaline F-enriched.