Hiortdahlite from a fine‐grained phonolite dyke in the Jingera Rock Syenite Complex, N.S.W. is triclinic Pl or Pi,a= 11.011 Å,b= 10.922 Å,c =7.343 Å, α = 109.32°,β= 110.02°, γ = 83.49°. The unit cell contains (Ca8.38Na4.10Mn0.50Zr2.27Nb0.31Ti0.05Fe0.21Y0.14) [Si8O28] (O1.58(OH)1.12F5.30); S.G. = 3.309. The crystals are biaxial +ve, 2V= 86°,α= 1.658, β = 1.664, γ = 1.671, γ:z(100) = 35°, α:z(010) = 14.5°. Polysynthetic normal twinning on (010) is present in all crystals. Type material from Langesund Fjord, Norway has been re‐investigated and found to have very similar properties, but is twinned on (100).