Anorthoyttrialite-(Y) occurs as inclusions in yttrian fluorite from the Stetind pegmatite, Narvik, Nordland, Norway, associated with allanite-(Ce), alnaperbøeite-(Ce), bastnäsite-(Ce), fluorite, hematite, hundholmenite-(Y), perbøeite-(Ce), rowlandite-(Y), schlüterite-(Y), synchysite-(Y), thalénite-(Y), törnebohmite-(Ce) and vyuntspakhkite-(Y). It forms translucent tabular or needle shaped crystals, which are colourless or white, yellow or brownish. It is optically biaxial (–) with α = 1.705(1), β = 1.750(1), γ = 1.756(2) and 2V calc = 39.09°. The calculated and measured density is 5.24 g·cm –3 and 5.1 g·cm –3 respectively. The empirical formula of anorthoyttrialite-(Y) based on 14 oxygen atoms per formula unit is (Y 1.561 La 0.033 Ce 0.242 Pr 0.059 Nd 0.367 Sm 0.177 Gd 0.298 Tb 0.036 Dy 0.297 Ho 0.058 Er 0.258 Tm 0.071 Yb 0.385 Lu 0.041 Ca 0.099 Mn 0.029 U 0.003 Th 0.01 ) Σ4.024 Si 4.011 O 14 . Two polytypic crystal structures have been determined in triclinic space group $P\bar 1$ . Anorthoyttrialite-(Y)-1 A is isostructural with the B-type structure known for synthetic rare earth element disilicates of composition REE 2 Si 2 O 7 , where REE = Y, Eu, Gd, Tb, Dy, Ho, Er, Tm. Unit cell parameters are a = 6.6107(4), b = 6.7139(3), c = 12.2034(9) Å, α = 94.819(3), β = 90.583(3), γ = 91.742(3)° and V = 539.42(5) Å 3 with Z = 2. The five strongest lines in the calculated powder diffraction pattern are ( d in Å (I) hkl ) as follows: 4.369 (72) $1\bar 1\bar 1$ , 3.040 (100) 004, 3.040 (71) $02\bar 2$ , 2.918 (68) $\bar 202$ , 2.814 (59) 211. Anorthoyttrialite-(Y)-2 A has unit cell parameters a = 6.6068(6), b = 6.7147(6), c = 24.218(2) Å, α = 94.435(6), β = 90.315(5), γ = 92.092(5)°, V = 1070.41(16) Å 3 and Z = 4. The five strongest lines in the calculated powder diffraction pattern are: 4.378 (75) $1\bar 1\bar 2$ , 3.027 (79) $0\bar 24$ , 3.018 (100) 008, 2.907 (68) $\bar 204$ , 2.810 (59) 212. Both polytypes contain linear trisilicate anion groups [Si 3 O 10 ] 8– as well as isolated [SiO 4 ] 4– tetrahedra, while the REE are six-, eight-, and nine-coordinated by oxygen. The crystal structures consist of essentially two types of layers with different stacking order. Similar layer stacking is discussed in relation to other rare earth disilicates like percleveite-(Ce) and thortveitite. Anorthoyttrialite-(Y) is also compared to a heat treated metamict Y-silicate from Stetind.