The rare mineral freudenbergite (Na2Fe2Ti6O16) has been recognized only in apatite-rich syenites from Katzenbuckel, Odenwald, West Germany, but its ferrous iron analogue (Na2FeTi7O16) has been found in lower crustal garnet (almandine-rich)-clinopyroxene (jadeite-rich)-plagioclase (An75) xenoliths entrained in a kimberlite from Liberia. Analogue-freudenbergite forms mantles on rutile in an inferred reaction relationship followed by successive overgrowths (or reaction zones) of ilmenite, perovskite (in some cases perovskite precedes ilmenite) and sphene. The TiO2 (wt.%) content of each mineral zone decreases almost progressively from rutile (approx 99 wt.%) rt arrow analogue freudenbergite (approx 80 wt.%) rt arrow ilmenite (approx 53 wt.%) rt arrow perovskite (approx 58 wt.%) rt arrow sphene (approx= 37 wt.%), with each zone being element specific; i.e. Ti rt arrow NaFeTi rt arrow CaTi rt arrow FeTi rt arrow CaTiSi. The estimated thermal stability of ferrous iron freudenbergite is compatible with the type material (700-900oC), but its upper P limit is not yet known. The two minerals are contrasted by associated mineralogy, redox state, and environment of formation.-E.v.P.
