Original paper

Taseqite, a new member of the eudialyte group from the Ilimaussaq alkaline complex, South Greenland

Petersen, O.V.; Johnsen, O.; Gault, R.A.; Niedermayr, G.; Grice, J.D.

Neues Jahrbuch für Mineralogie - Monatshefte Jg. 2004 Heft 2 (2004), p. 83 - 96

7 references

published: Feb 5, 2004

DOI: 10.1127/0028-3649/2004/2004-0083

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ArtNo. ESP156200402003, Price: 29.00 €

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Abstract

Taseqite is a new mineral species found in small cavities in a single hydrothermal, albitite vein on the top of the Taseq slope, in the northern part of the Ilímaussaq alkaline complex, South Greenland. Besides albite the associated minerals include aegirine, analcime, ancylite-(La), calcite, catapleiite, dolomite, ferrobustamite, fluorapatite, hemimorphite, pectolite, sphalerite and strontianite. Taseqite forms clusters of crystals or individual crystals ranging in size from 0.05 by 0.25 to 0.5 by nearly 3 mm. The crystals show combinations of {0001}, {0001}, {1120}, {1010}, {0110}, the latter two only represented by very narrow faces, {1011}, {0112}, {0111}, {1012}, all nearly equally developed, and occasionally {0221} and {2021}; the crystals are platy on {0001}. The colour varies from clove to yellowish brown and lemon yellow; taseqite is transparent and has a vitreous lustre. The Mohs hardness is 51ı2; taseqite is brittle and shows fair cleavage on {0001}/{0001}. Dmeas. = 3.24(1)g/cm3, Dcalc. = 3.20 g/cm3. Taseqite is uniaxial, optically negative, with nε = 1.6378(5) and nω = 1.6494(3) for λ = 589 nm. The infrared spectrum shows the presence of OH, H2O and CO3. The microprobe analyses gave: Na2O 7.71, K2O 0.23, CaO 8.19, SrO 13.98, MnO 3.02, FeO 3.92, Y2O3 0.28, Ce2O3 0.08, SnO2 0.13, ZrO2 9.89, HfO2 0.32, Nb2O5 4.38, Ta2O5 0.24, SiO2 41.64, Cl 1.91, H2O 0.59 (by stoichiometry determined from crystal structure analysis), O ≡ Cl = -0.43, total = 96.08. The empirical formula, based on 78.14 anions as found in the crystal structure analysis is: (Na8.81St4.78K0.17Ce0.02)Σ13.78(Ca5.17Mn0.59Y0.09)Σ5.85(Fe1.93 Mn0.92)Σ2.85(Zr2.84Nb0.11Hf0.05)Σ3.00 (Nb1.06Ta0.04Sn0.03)Σ1.13Si24.55O73(O1.65OH0.75(H2O)0.74)Σ3.14(Cl1.91OH0.09)Σ2.00; Z = 3. The ideal formula is: Na12Sr3Ca6Fe3Zr3NbSi25O73(O,OH,H2O)3Cl2. The strongest lines on the X-ray powder-diffraction pattern [d in à (I) (hkl)] are: 9.51 (9) (012), 3.43 (9) (131, 223), 3.19 (8) (208), 2.98 (10) (315), 2.86 (10) (404). The unit cell parameters refined from powder data are: a = 14.286(6) and c = 29.99(2) à , c:a = 2.0993, V = 5300.7à 3. The crystal structure was determined by single-crystal X-ray diffraction and refined to R = 0.037. Taseqite crystallizes in the trigonal system, space group R3m, a = 14.2828(6) and c = 30.0222(12) à , V = 5303.9(4)à 3. The compatibility index 1 - Kp/Kc is equal to 0.0005, which is rated as superior. Taseqite belongs to the eudialyte group. It is primarily distinguished from other members of the group by the unique combination of dominance of Sr in N (4) and Nb in M(3).

Keywords

taseqite • new mineral species • crystal structure • ilimaussaq alkaline complex • greenland