Lorenzenite, Na₂ Ti₂Si₂O_{9 ,}from nepheline syenite pegmatites at Bratthagen, Lågendalen, Vestfold county, Norway, occurs in two morphologically different habits: as well-defined orthorhombic crystals or more rarely as fibrous aggregates. Crystal forms observed:{100}, {210}, {1111, {211}, {321}. The unit cell dimensions for Bratthagen lorenzenites are (coarsely crystalline/fibrous): a= 14.488(2)/14.480(1) Å, b = 8.7061 ( 11)/8.6995(8) Å, c= 5.2308(8)/5.2277(3) Å. D_meas for coarsely crystalline lorenzenite is 3.436(1) gf/m³ . Coarsely crystalline lorenzenite is brown, while fibrous lorenzenite is colourless to pale yellow. It is nonpleochroic, and strongly birefringent. The axial dispersion is distinct r > v. Biaxial negative, 2 V = 42° ± 4°, n > 1.80. The mineral shows yellow fluorescence in short wave UV light. A cathodoluminescence spectrum of coarsely crystalline lorenzenite shows an intrinsic type of luminescence with a maximum at 500 nm. The average of seven electron microprobe analyses gave the following results for coarsely crystalline/fibrous lorenzenite: Na₂O 17.13/17.74, SiO₂ 34.20/35.03, TiO₂ 44.39/45.13, FeO 0.83/0.34, Nb₂O₅ 2.71/1.40, total 99.26/99.64. The average Fe:Nb ratios are 1: 1.80/1:2.25, indicating substitution according to the scheme Fe²⁺ + 2Nb⁵⁺- 3Ti⁴⁺.

A. O. Larsen, Norsk Hydro a.s., Research Centre Porsgrunn, N-3901 Porsgrunn, Norway;
G. Raade and P. Chr. Sæbø, Mineralogisk-Geologisk Museum, Sars Gate 1, N-0562 Oslo 5, Norway;