The crystal structure of cayalsite-(Y), with idealized composition CaY6Al2Si4O18F6, has been determined on the basis of single-crystal X-ray diffraction data. The mineral occurs in several polytypic modifications arising from the combination of three non-equivalent structural layers. Two maximum degree of order (MDO) polytypes have been identified in cayalsite-(Y). The orthorhombic MDO polytype, cayalsite-(Y)-1O, has lattice parameters a = 15.993(1), b = 5.5306(3) and c = 9.6590(7) Å. It is described in space group symmetry Pban and refines to Robs = 2.2%. The monoclinic MDO polytype, cayalsite-(Y)-1M, a = 11.0602(7), b = 5.5280(2), c = 16.0195(9) Å, β = 118.925(3)° is described in space group symmetry P2/c and refines to Robs = 3.5% in a two-phase mixture with cayalsite-(Y)-1O. Predominantly heavy lanthanide elements substitute for Y and Ca in cayalsite-(Y), with some Ca also substituting for the rare-earth elements (REE). A typical empirical composition, normalized to 24 anions, is  
Cayalsite-(Y) forms colourless to faintly pink, prismatic crystals in cavities of yttrian fluorite in two granitic pegmatites located in Tysfjord, Nordland, Norway. Cayalsite-(Y)-1O crystals are optically biaxial (+), with indices of refraction α = 1.730(5), β = 1.740(5), γ = 1.760(5) and 2Vmeas = 56.5(5). The crystal structure of cayalsite-(Y) is composed of linear chains of edge sharing [AlO6] octahedra with isolated [SiO4] tetrahedra. The REE and Ca-cations occur in eight-fold coordination by O and F anions. Apart from substitutional disorder of REE and Ca atoms, the cayalsite structure is characterized by substitutional and positional disorder affecting the local position of [SiO4] tetrahedra in one of its layers. Avoidance of close Si-F contacts causes the partial splitting of O and F positions. The possible interplay of layer stacking and cation ordering is discussed.
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