Gyrolite: Its Crystal Structure and Crystal Chemistry

The crystal structure of gyrolite from Qarusait, Greenland, was solved and refined with the space group P 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 S 1 and S 2 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 1 OS 2 , where S 2 and , as well as O and , are symmetry-related units. Successive complex layers with composition [Ca 14 Si 23 AlO 60 (OH) 8 ] -5 are connected through an interlayer sheet made up by calcium and sodium cations and water molecules. The unit cell content NaCa 16 Si 23 AlO 60 (OH) 8 ·14H 2 O, 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 Ca 16 Si 24 O 60 (OH) 8 ·(14+x)H 2 O, 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.