Summary: Emeleusite (Li₂Na₄Fe₂^IIISi₁₂O₃₀) is an orthorhombic (pseudohexagonal) mineral occurring as euhedra within a facies of a peralkaline trachyte dyke. The composition is: SiO₂ 70·75; TiO₂ 0·55; ZrO₂ 0·10; Al₂O₃ 1·34; Fe₂O₃ 12·13; MgO 0·10; MnO 0·03; Na₂O 11·98; Li₂O 2·78:99·76. The density is 2·775 g/cm³ (calculated) and 2·76(7) g/cm³ (determined); hardness 5–6. It is colourless, transparent, and has a glassy lustre. Emeleusite is biaxial negative with 2V_a, varying from near-zero to c. 30° r » v. The refractive indices are α 1·596; β 1·597; y 1·597; with α ∥ b; β ∥ a; y ∥ c. Emeleusite can occur as interpenetration triplets with {110} as twin planes. The space group is Acam or Aba2 with a = 10·073±0·002 Å; b = 17·350±0·005 Å and c = 14·010±0·005 Å. Z = 4. Prominent powder diffraction lines are 7·001(5) 002, 4·352(10) 220, 040, 3·501(7) 004, 3·209(8) 311, and 3·090(7) 124. While the structure is as yet unknown it is suggested that it is an orthorhombic associate of the milarite group.