Allanite-(Y), ideally CaY(Al2Fe2+)(Si2O7)(SiO4)O(OH), is a valid species with the type locality in the Åskagen pegmatite, Värmland, Sweden. The mineral occurs as an accessory phase in the blocky zone of the NYF granitic pegmatite near Åskagen, Värmland, Sweden. It forms rims together with iimoriite-(Y), gadolinite-(Y) and allanite-(Nd) around altered crystals of thalénite-(Y). Allanite-(Y) replaced primary thalénite-(Y) during an episode of early post-magmatic hydrothermal activity. Allanite-(Y) forms euhedral crystals with size up to 1 mm, black with a vitreous lustre, conchoidal fracture and greyish brown streak. It has a Mohs hardness of ca. 6, the calculated density of 3.945 g.cm–3 and is biaxial (−) with α = 1.760(3), β = 1.799(2) and γ = 1.784(3) in 589 nm light; pleochroism is weak pale yellowish brown in all directions. Allanite-(Y) has monoclinic symmetry, with the space group P21/m, a = 8.8520(8) Å, b = 5.6959(5) Å, c = 10.0543(9) Å, β = 115.510(2)°, V = 457.52(7) Å3 and Z = 2. Crystal-chemical analysis resulted in the empirical formula: A1(Ca0.900Mn0.090Na0.010)Σ1.000A2(Y0.323Ca0.260Nd0.118Sm0.087Gd0.098Dy0.044Ce0.034Pr0.014Tb0.012Er0.005La0.003Ho0.002Yb0.001)Σ1.001M1(Al0.921Fe2+0.070Ti0.003)Σ0.994M2(Al1.000)M3(Fe2+0.638Fe3+0.262Al0.072Mg0.028)Σ1.000T1(Si1.000)T2(Si1.000)T3(Si1.003)O12.000(OH)1.000.

Allanite-(Y) belongs to the allanite group of the epidote supergroup. The closest end-member compositions of valid allanite group species are allanite-(Ce), allanite-(La) and allanite-(Nd) related via the simple exchange mechanism Y ↔ Ln. The allanite-(Y) origin during metasomatic replacement of the thalénite-(Y) was mainly affected by local system composition and structural constraints rather than Ln+Y fluoride complexation in hydrothermal solution.