Summary: Reyerite is found in close association with gyrolite in the metamorphic aureole of the volcanic plug of 'S Airde Beinn, northern Mull, and has apparently been formed by metamorphism of the gyrolite. The gyrolite, with a composition close to Ca₁₆Si₂₄O₆₀(OH)₃.14H₂O, closely approaches trigonal symmetry with a 9·76±0·02 Å and c 3 × 22·26±0·1 Å; biaxial with a small 2V and α 1·537, β 1·547, γ 1·549. The reyerite is trigonal with a 9·76±0·02 Å, c 19·10±0·04 Å and space group P3¯; ω 1·568 and ε 1·563, and the unit cell contains about 14 atoms of Ca, 24 of Si (with some substitution by 4(OH)⁻), and 14H₂O as (OH)⁻ and H₂O.