OCCURRENCE OF LOW-Ti AND HIGH-Ti FREUDENBERGITE IN ALKALI SYENITE

Abstract

1609 Freudenbergite (Na2Fe 3+ 2Ti6O16) occurs in peralkaline alkali syenite dikes at Katzenbuckel, southwestern Germany, in association with lorenzenite, pyrochlore, landauite and Na–Zr silicates as rare, late-stage mineral phases. Freudenbergite shows a broad range in chemical composition; we recognize two varieties: 1) a “low-titanium ” type, with low concentrations of Ti and all iron in the trivalent state, as established by Mössbauer spectroscopy; this is the normal type of freudenbergite found in a syenite from the Katzenbuckel; 2) a “high-titanium ” type, with elevated concentrations of Ti and high contents of divalent cations (Mg 2+, Mn 2+, Zn 2+) substituting for Fe 2+. Increased concentrations of Nb are the cause of cation deficiency of sodium at the A site. Freudenbergite, up to 0.75 mm in size, is a sector-zoned mineral with selective enrichments in (Nb + Zr) and Ti in sectors (001) and (100), respectively. In one dike (Kb 45), freudenbergite metasomatically replaces primary ilmenite as well as thin exsolutionlamellae within hematite grains. Pseudobrookite, ilmenite, hematite or magnetite occur as Fe–Ti oxides in the freudenbergitebearing syenite samples. Ilmenite–hematite mineral pairs show large miscibility-gaps, and indicate conditions of formation in the range of 750–550°C at low pressures of about of 0.2–0.3 kbar. The unique occurrence of primary freudenbergite is mainly due to an efficient process of fractionation of the peralkaline syenitic melt, leading to a Na- and Ti-rich, but Ca-poor melt accompanied by a distinct enrichment in niobium and zirconium