Holtite, a new mineral allied to dumortierite, occurred as pebbles with stibiotantalite and tantalite on an alluvial tin lease near Greenbushes, Western Australia, and is named after the late H. E. Holt, Prime Minister of Australia.

The mineral is orthorhombic with a 11·905 Å, b 20·355 Å, c 4·690 Å, space group Pmcn, weak supercell 2a, 2b, c developed. Crystals are elongated along c, D 3·90 ± 0·02, hardness 8½, fluorescent. Optical properties α 1·743−1·746, mainly yellow, ‖ [001], β 1·756−1·759, colourless, γ 1·758−1·761, colourless, 2V_α 49−55°, r < v. X-ray powder data are given.

Chemical analysis gave SiO₂ 20·30, Sb₂O₅ 4·61, Al₂O₃ 46·43, Ta₂O₅ 11·24, Nb₂O₅ 0·76, Fe₂O₃ 0·27, MnO 0·05, TiO₂ 0·09, BeO 0·05, B₂O₃ 1·82, Sb₂O₃ 13·89, H₂O⁺ 0·38, H₂O⁻ 0·08, sum 99·97%. On a water-free basis the unit cell contains Al24·5Sb2·56‴Ta1·36Sb0·76vNb0·16Fe0·10‴Be0·05Ti0·03Mn0·02B1·40‴Si9·09O66·85. Compared with dumortierite, 4[(A1,Fe)₇BSi₃O₁₈] or 4 (X₁₁O₁₈), the holtite unit cell contains approximately 4(X₁₀O₁₇).

Type material is preserved at the Government Chemical Laboratories, Perth, Western Australia.