Nabesite, Na2BeSi4O10ž4H2O, a new mineral species from the IliŽmaussaq alkaline complex, South Greenland

Autor(en)

Ole V Petersen, Gerald Giester, Franz Brandstätter, Gerhard Niedermayr

Abstrakt

Nabesite is found in complex tugtupite-bearing albitites on the Kvanefjeld Plateau, in the northwesternmost part of the lliŽmaussaq alkaline complex, South Greenland. It occurs in cavities, covered with albite crystals, in association with gmelinite, neptunite, analcime, gonnardite, lovdarite, trona(?) and opal(?). Nabesite forms aggregates of thin platy crystals, in parallel or subparallel orientation, the individual crystals show the pinacoids {100}, {010}, {001}, and {111} and {111}, both sphenoids, and range in size from 0.05 × 0.5 × 0.5 to 0.2 × 5 × 5 mm. The mineral is colorless and transparent, it has a white streak, and the luster is vitreous. The Mohs hardness is 5-6, it is brittle and shows good cleavage on (110) and (001), and the fracture is uneven. Nabesite is biaxial negative with na = 1.499(1). nß = 1.507(1) and n? = 1.511(1), 2Va (measured) 65(5)°, 2Va (calculated) 70°, and has X = a, Y = c and Z = b. Dmeas = 2.16(2) g/cm3, Dcalc = 2.21 and 2.22 g/cm3 for V derived from powder and single-crystal diffraction data, respectively. Data from EMP, SEM-EDS and results of the refinement of the crystal structure gave: Na2O 13.8 K2O 0.34, BeO 6.26, CaO 0.13, SiO2 62.4, H2O 18.05, total 100.98 wt%. The ideal formula is Na2BeSi4O10ž4H2O. The strongest lines on the X-ray powder-diffraction pattern [d in A°(1)(hkl)] are: 6.11(80)(111). 5.97(100)(002), 3.09(70)(310), 3.06(50)(222) and 2.988(60)(311). The unit-cell parameters refined from the powder data are: a 9.722(1), b 10.142(1), c 12.030(1) A°, V 1186.2 A°3. The structure was determined by single-crystal X-ray diffraction. Nabesite crystallizes in the orthorhombic space-group P212121. a 9.748(1), b 10.133(1), c 11.954(2) A°. V 1180.8(2) A°3, Z = 4. A total of 5704 unique reflections (at 293 K) gave a final R1 factor of 0.028 (wR2 = 0.078). The [SiO4] tetrahedra form sheets consisting of a combination of 4- and 8- membered rings, which are interconnected via the [BeO4] tetrahedra (a spiro-5 arrangement as secondary building unit) to form a framework. Channels house octahedrally coordinated sodium atoms and H2O molecules with a well-established system of hydrogen bonds. Nabesite is a zeolite with a new type of framework; the framework density is 16.94 T/1000 A°3. Relationships to the crystal structure of weinebeneite are evident.

Organisation(en)

Institut für Mineralogie und Kristallographie

Externe Organisation(en)

Geologisk Museum

Journal

The Canadian Mineralogist

Band

40

Seiten

173-181

Anzahl der Seiten

9

ISSN

0008-4476

Publikationsdatum

2002

Peer-reviewed

Ja

ÖFOS 2012

1051 Geologie, Mineralogie

Link zum Portal

https://ucris.univie.ac.at/portal/de/publications/nabesite-na2besi4o104h2o-a-new-mineral-species-from-the-ilimaussaq-alkaline-complex-south-greenland(ad912920-a6b0-4c5e-aad7-b74001dbdfd3).html