[1] Naturally occurring freudenbergite is a solid solution of two end members Na2Fe +32Ti6O16 and Na2Fe +2Ti7O16, which were referred to as low-Ti and high-Ti freudenbergite, respectively[2] Both varieties were found in the course of close examination of samples from the Katzenbuckel Complex, Germany, in which this mineral was documented for the first time[3] As a possible mineral Fe2+ analog was named UM1983-07-O:FeNaTi [4] Monoclinic with straight extinction and high birefringence.

: 375-384; in: Dunn, P.J., Cabri, L.J., Ferraiolo, J.A., Grice, J.D., Jambor, J.L., Mueller, W., Shigley, J.E., Puziewicz, J., Vanko, D.A.

Canadian Mineralogist: 45: 983-1055; http://nrmima.nrm.se(2016) Stahle V.; Koch M.; McCammon C. A.; Mann U.; Markl G. (1 December 2002).

"Occurrence of low-Ti and high-Ti freudenbergite in alkali syenite dikes from the Katzenbuckel volcano, Southwestern Germany".

Taylor L. A. and PokhilenkoN., “Ferrous freudenbergite in ilmenite megacrysts: a unique paragenesis from the Dalnaya Kimberlite, Yakutia,” Am.

Zaitsev, V. A., Kogarko, L. N. and Senin, V. G. (2013) ‘Phase equilibria in the system lamprophyllite-nepheline’, Geochemistry International, 51(11), pp.