Dalyite (K₂ZrSi₆O₁₅), a new mineral, occurs in ejected blocks of alkali-granite from Ascension Island. It forms small colourless triclinic crystals with a:b:c = 0·958:1:0·899, α 106° 19′, β 112° 30′, γ 99° 08′. Absolute parameters: a 7·51, b 7·73, c 7·00 Å., Class 1¯, space-group P1¯. Sixteen forms: {100}, {010}, {001}, {1¯01}, {110}, {11¯0}, {12¯0}, {1¯11}, {011}, {101}, {11¯1}, {12¯1}, {1>1¯1}, {01¯1}, {1¯02}, {01¯2}. Cleavages (1¯01) and (010). Normal twinning with (100) as composition plane. Optical constants: β 1·575, β 1·590, γ 1·601, 2V_α 72°, α:c = 7°, axial plane to (100)= 18°. Unit cell [K₂ZrSi₆O₁₅]. Density 2·84. Hardness 7½.