Original paper

The crystal structure and mineralogical description of a Na-dominant komarovite from the Ilimaussaq alkaline complex, South Greenland

Balić-Žunić, T.; Petersen, O.V.; Bernhardt, H.J.; Micheelsen, H.I.


The crystal structure of a Na-dominant komarovite is solved on a sample from Ilímaussaq, Greenland, the type locality of Na-komarovite. The small transparent yellowish crystals are orthorhombic, Cmmm, a = 7.310(1), b = 24.588(3), c = 7.402(1) Å; the main forms are three pinacoids; needle-like habit (κa); cleavage {010} perfect, {001} fair; optically biaxial (-) with 2Vα = 72(1)°, nα (calculated) (κb) = 1.809(20), nβ (κa) = 1.876(14), nγ (κc) = 1.912(16). Empirical chemical formula (microprobe, H2O from the ideal formula) is (Na5.43,K0.04)Σ5.47,Ca0.82(Ce0.09,La0.05,Fe0.03)Σ0.17Ti0.57Nb5.58[Si4O12]O13.89F2.19 · 4H2O, Z = 2, Dcalculated = 3.41g/cm3. The crystal structure contains layers κ {010} of pyrochlore composition, one pyrochlore period thick, where Na and Ca (+lanthanides) take eight-coordinated cation positions, and Nb (+Ti) six-coordinated. The positions on surfaces of layers that are occupied by a monovalent anion in pyrochlore are in komarovite replaced by water molecules coordinated to Na atoms. The layers are interconnected by [Si4O12] rings with ring planes κ {001}. In the channels formed between layers and rings additional Na (+K) atoms are situated coordinated to oxygens from silicate rings and additional water molecules. In the Cmmm structure these Na atoms and waters are fully disordered between two sets of equivalent positions. The proposed general formula for komarovite with assumed atomic substitutions is (Na,K)Na5+x-yCa1-2x+yLaxTiyNb6-ySi4O26F2 · 4H2O, where La stands for lanthanide (trivalent) elements.


na-komaroviteilimaussaqcrystal structurekomarovitepyrochlorefamilyniobium silicate