Abstract: The crystal structure of gyrolite from Qarusait, Greenland, was solved and refined with the space group P1¯ and cell parameters a = 9.74(1), b = 9.74(1), c = 22.40(2)Å, α = 95.7(1)°, β = 91.5(1)°, γ = 120.0(1)°. The structure is built up by the stacking of the structural units already found in the crystal structure of reyerite (Merlino, 1972, 1988), namely tetrahedral sheets S₁ and S₂ and octahedral sheets O. The tetrahedral and octahedral sheets are connected by corner sharing to give rise to the complex layer which can be schematically described as S¯2O¯S1OS2, where S₂ and S¯2, as well as O and O¯, are symmetry-related units. Successive complex layers with composition [Ca₁₄Si₂₃AlO₆₀(OH)₈]^-5 are connected through an interlayer sheet made up by calcium and sodium cations and water molecules.

The unit cell content NaCa₁₆Si₂₃AlO₆₀(OH)₈·14H₂O, determined by the structural study, was confirmed by a chemical analysis, apart from the indication of a somewhat larger water content. The crystal chemistry of gyrolite is discussed on the basis of the present structural results and the chemical data given in the literature for gyrolite from different localities: the crystal chemical formula which accounts for most gyrolite samples is Ca₁₆Si₂₄O₆₀(OH)₈·(14+x)H₂O, with 0 ⩽ x ⩽ 3.

Stacking disorder, twinning and polytypic variants in gyrolite, as well as the structural relationships of gyrolite with truscottite, reyerite, fedorite and the synthetic phases K and Z are described and discussed.