Abstract: Jeppeite, a new mineral, similar in composition to and overgrown on priderite, has been found in the lamproite plug of Walgidee Hills (18° 19′ S, 124° 51′ E), Western Australia. The mineral is named for the discoverer, Dr J. Jeppe. It is monoclinic, C2/m, a 15.453 b 3.8368 c 9.123 Å β 99.25°, strongest powder lines 4.50(002) (4), 3.07(310) (10), 2.99(003,311¯) (10), 2.961(203¯) (4), 2.812(311,112) (10), 2.091 (6), 2.074 (6), 1.919 (8) similar to artificial K₂Ti₆O₁₃. The sparse eluvial crystals are black, elongated along b, bounded by {100}, {201¯} faces (Λ 45°) and {010}; perfect 100 and good 201¯ cleavages or partings, submetallic lustre, pale-brown streak, brittle, and cleave into (100) flakes. D_obs 3.94, D_calc 3.98. Colour values for illuminant C from reflectance spectra for R_p, R_b, and R_θ are: Y% 13.3, 14.4, 16.6; λ_d 474, 473, 475; and P_e% 5.1, 4.5, 4.3. Refractive indices from reflectances at 590 nm in air are 2.13, 2.21 and 2.35. In thin section, αΛα10° blue, β = b dark greenish brown almost to black, γ = c brown. Bireflectance and birefringence positive. H 5–6, VHN₁₀₀ orientation dependent; for indentations normal to b 664–773.

Jeppeite is common in the lamproite as prismatic to acicular aggregates associated with priderite, richterite, shcherbakovite, wadeite, perovskite, and apatite in a green and white celadonite and chlorite matrix, with a little calcite and sphene, after olivine, pyroxene, and leucite.

Electron probe analysis, using Fe, Ti, nepheline, and benitoite standards, gave K₂O 8.47, BaO 17.35, TiO₂ 69.29, Fe₂O₃ (total Fe) 4.74, sum 99.85%; (Mg, Na, Zr detected). This analysis calculates to (K_1.15,Ba_0.73)_Σ1.88 (Ti_5.56, Fe³⁺_0.38)_Σ5.94O₁₃, or ideally, (K,Ba)₂(Ti,Fe)₆O₁₃.