The new mineral, dyrnaesite-(La), is found in the Ilímaussaq alkaline complex, South Greenland. The holotype material originates from an arfvedsonite lujavrite sheet as an accessory mineral. Dyrnaesite-(La) is pale yellowish green, with no cleavage and an irregular fracture. Density is 3.68(2)/3.682 g/cm³ (measured/ calculated). It is biaxial, negative, 2Vα = 47(1)/48 (measured/calculated); α = 1.6226(5), β = 1.6852(10), γ = 1.6982(2); X = c, Y = a, Z = b. The average values of microprobe analyses are (wt.%) P₂O₅ 37.17, SiO₂ 0.15, CaO 0.90, Na₂O 20.06, La₂O₃ 16.44, CeO₂ 20.23, Pr₂O₃ 1.40, Nd₂O₃ 3.47, Sm₂O₃ 0.24, Dy₂O₃ 0.06, Y₂O₃ 0.06.
The crystal structure was solved from single-crystal X-ray diffraction data. Dyrnaesite-(La) is orthorhombic, Pnma; a = 18.4662(7) Å, b = 16.0106(5) Å, c = 7.0274(2) Å, V = 2077.7(2) ų, Z = 4. The structural formula calculated from the diffraction data and microprobe analysis is Na_7.89(Ce_0.94Ca_0.06)_Σ1.00(Ca_0.12La_1.14Ce_0.40Pr_0.10Nd_0.24)_Σ2.00(PO₄)₆, the simplified formula is Na8Ce4+REE2(PO4)6. The crystal structure is related closely to that of vitusite-(Ce), but is distinct from it in several aspects. The five strongest lines of the powder X-ray diffraction pattern are (d Å, (I %), (hkl)); 6.57 (100) (101), 4.62 (40) (301, 230, 400), 3.50 (40) (331), 2.80 (86) (232, 402), 2.67 (54) (060,630).