Carlgieseckeite-(Nd), ideally NaNdCa3(PO4)3F, a new mineral species of the belovite group of the apatite supergroup, was found at the Kuannersuit Plateau, Ilímaussaq alkaline complex, South Greenland. It is associated with albite, analcime and fluorapatite in cavities of an albite vein cross-cutting augite syenite. Carlgieseckeite-(Nd) forms hexagonal tabular crystals up to 0.25 × 1 × 1.3 mm, and their parallel intergrowth up to 0.7 × 1.3 mm is found epitactically overgrown on prismatic crystals of fluorapatite. A phase with the idealized formula Na1.5Nd1.5Ca2(PO4)3F epitactically overgrows some crystals of carlgieseckeite-(Nd). Carlgieseckeite-(Nd) is transparent and shows a distinct color-change effect, from almost colorless with a greenish hue in daylight to pink in yellow electric light. The luster is vitreous. The Mohs hardness is ca. 5. The mineral is brittle with no observed cleavage and an uneven fracture. The calculated density is 3.91 g/cm3. Carlgieseckeite-(Nd) is optically negative, uniaxial [ω = 1.655(3), ɛ = 1.632(2)] or shows anomalous biaxiality [α 1.632(2), β 1.654(3), γ 1.656(3), 2V(meas.) 15(5)°]. The average chemical composition (electron-microprobe data) is: Na2O 5.68, CaO 18.53, SrO 7.55, BaO 0.14, La2O3 1.32, Ce2O3 10.60, Pr2O3 2.62, Nd2O3 15.08, Sm2O3 2.89, Gd2O3 0.52, SiO2 0.56, P2O5 32.72, F 2.80, Cl 0.06, −O=(F,Cl)2 −1.19, total 99.88 wt.%. The empirical formula calculated on the basis of 13 O + F + Cl apfu is: Na1.17Ca2.11Sr0.46Ba0.01La0.05 Ce0.41Pr0.10Nd0.57Sm0.11Gd0.02Si0.06P2.94O12.05F0.94Cl0.01. Carlgieseckeite-(Nd) is trigonal, space group P3̄, a 9.4553(1), c 6.9825(1) Å, V 540.62(1) Å3, Z = 2. The crystal structure was refined from X-ray-diffraction data (single crystal, R = 0.0218). Carlgieseckeite-(Nd) is the isostructural Ca- and Nd-dominant analogue of belovite-(Ce) and belovite-(La). The strongest lines of the powder X-ray pattern [d in Å (I)(hkl)] are: 7.02(22)(001), 5.33(18)(101), 3.923(27)(111), 3.463(23)(002), 3.095(19)(210), 2.815(100)(211,112), 2.727(42)(300). The mineral is named in honor of Carl Ludwig Giesecke (1761–1833), a mineralogist and polar explorer, the pioneer researcher of the mineralogy of Greenland. The Levinson suffix modifier -(Nd) is appropriate to express the dominance of Nd over other REE in the mineral. The holotype material is deposited in the Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow.

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