Jökull - 01.01.2005, Blaðsíða 49

Reviewed research article
Stellerite from the Hvalfjörður area, Iceland
Rune S. Selbekk and Tobias Weisenberger
Mineralogisch-Geochemisches Institut, Albert-Ludwigs-Universität Freiburg, Albertstr. 23b,
D - 79104 Freiburg, Germany; Corresponding author: rune.selbekk@minpet.uni-freiburg.de
Abstract – Stellerite (a zeolite) occurs as tabular crystals up to 1.5 cm on veins in the Hvammsvík area,
Hvalfjörður, together with yugawaralite and laumontite. The stellerite has the space group Fmmm, and the
lattice parameters for our sample are: a = 13.5961, b = 18.2114, c = 17.8615. The average composition
of 5 analyses calculated on the basis of 72 oxygen atoms is: ( Ca 3.88Na0.05 K0.03Sr0.01 )
P
=3,97 Al7.95
Si28.07O72×28H2O. The chemistry, calculated space group and the lattice parameters are in agreement with
the criteria for the mineral species stellerite.
INTRODUCTION
Iceland is famous for an abundance of zeolite miner-
als, especially from localities such as Teigarhorn, the
Western fjords and Hvalfjörður. Although more than
20 different zeolites have been described (e.g. Sæ-
mundsson and Gunnlaugsson 2002), Stellerite (sensu
stricto) has not to the authors’ knowledge been doc-
umented from Iceland before. Stilbite-stellerite solid
solution has been documented by Fridriksson et al.
(2001). In this note, we present compositional and
XRD data for stellerite from in the Hvalfjörður area.
OCCURRENCE
Stellerite forms transparent to white tabular stellerite
crystals (Figure 1) up to 1.5 cm length with well-
developed (100) (010) (001) and (111) faces (Figure
2). It occurs with yugawaralite as veins rather than
as vug fillings in hydrothermally-altered greenish to
grey basalt. Laumontite is also found in the area,
but in vugs and not in veins. This mode of occur-
rence suggests that the stellerite-yugawaralite associ-
ation formed during a hydrothermal alteration event
overprinting regional laumontite formation.
ANALYTICAL METHODS
Mineral analyses were performed using a CAMECA
SX100, electron microprobe equipped with three
WD spectrometers and one ED detector at the
Mineralogisch-Geochemisches Institut, University of
Freiburg, Germany. Natural and synthetic mineral
standards were used for calibration, and raw data cor-
rected according to the PAP method (Pouhcou and Pi-
chior, 1991). Operating conditions were 10 nA and
15 kV with a defocused beam, in order to minimize
sodium loss. Na and K were counted first and Na was
counted for 10 s. All other elements were counted for
20 s. XRD analyses was performed using a BRUK-
ERaxs D8 ADVANCE X-ray diffractometer with a
Cu-anode at 40 mA and 40 kV at the Mineralogisch-
Geochemisches Institut, University of Freiburg.
Lattice parameters were determined by a profile
fit of the XRD pattern using the software GSAS (Lar-
son and von Dreele 1994). The initial structure model
was taken from Galli and Alberti (1975). H2O was
not determined.
COMPOSITION, SPACE GROUP AND
LATTICE PARAMETERS
The chemical composition in terms of structural for-
mula calculated on the basis of 72 oxygen atoms of
JÖKULL No. 55 49