**2.2.3. Carlgieseckeite-(Nd)**

Carlgieseckeite-(Nd)13 (NaNdCa3(PO4)3F [31],[72],[73]) was named according to the mineralo‐ gist and polar explorer GIESECKE with respect to the content of Nd that is higherthan the content of other REE. The mineral was found in Kuannersuit (formerly Kvanefjeld) Plateau, north‐ ern section of the Ilímaussaq alkaline complex, South Greenland, Denmark. It is associated with albite (NaAlSi3O8 [74]), analcime (NaAlSi2O6·H2O [75]) and fluorapatite in the cavities of 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 epitactical‐ ly overgrown on prismatic crystals of fluorapatite. A phase with idealized formula Na1.5Nd1.5Ca2(PO4)3F epitactically overgrows some crystals of carlgieseckeite-(Nd).

The structure of the mineral (**Fig. 21**) is representative of the structure type of belovite *sensu stricto*. In this structure, large M cations occupy three sites with different coordination numbers: 9-fold polyhedra M(1) (average distance <M(1)–O> is 2.522 Å), 6-fold polyhedral M(1)′ (reduced 9-fold polyhedra, <M(1)′–O> with average interatomic distance 2.445 Å) and 7-fold polyhedra M(2) (<M(2)–O,F > 2.486 or 2.560 Å in the case of F1) [72].

**Fig. 21** The structure of carlgieseckeite -(Nd) (perspective view according to the *c*-axis).

Mineral carlgieseckeite-(Nd) is trigonal, from the space group P3 ¯ with crystallographic cell parameters *a* = 9.4553, *c* = 6.9825 Å, *a*:*c* = 1:0.738, *V* = 540.62 Å3 and *Z* = 2. Carlgieseckeite-(Nd) is the isostructural Ca- and Nd-dominant analogue of belovite-(Ce) and belovite-(La). The mineral carlgieseckeite-(Nd) is colorless, 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 about 5. The mineral is brittle with no observed cleavage [72].

<sup>12</sup> Sheetlike body of minerals which crystallize within the rock.

<sup>13</sup> The holotype material is deposited in the Fersman Mineralogical Museum of Russian Academy of Sciences, Moscow [72].
