Jökull - 01.01.2005, Side 51

Stellerite from Hvalfjörður
Table 1. Compositions (cations on basis of 72[O]) of single stellerite crystals from Hvammsvík. – Efna-
samsetning stellerítkristalla frá Hvammsvík.
Sample 1 2 3 4 5
Si 28.09 28.14 28.04 27.92 28.17
Al 7.91 7.82 8.01 8.12 7.87
Fe 0.00 0.01 0.01 0.00 0.00
Mg 0.00 0.00 0.01 0.00 0.00
Ca 3.88 3.93 3.86 3.90 3.83
Sr 0.02 0.00 0.01 0.00 0.00
Ba 0.00 0.01 0.00 0.00 0.01
Na 0.06 0.03 0.03 0.12 0.02
K 0.05 0.03 0.02 0.04 0.02
P 40.01 39.98 39.98 40.10 39.92
is orthorhombic as stellerite and has a similar XRD
pattern, but is Na-rich and Ca-poor with the space
group Amma.
RELATIONSHIP TO ZEOLITE
ZONATION
Geological observations suggest that zeolite-bearing
assemblages are sensitive thermobarometric indica-
tors. The composition of stilbite-stellerite solid so-
lution in metabasalts is a function of temperature,
fluid composition, coexisting minerals and geother-
mal gradient. Stilbite-stellerite solid solution crystals
described from Iceland and Icelandic geothermal sys-
tem change systematically toward the stellerite end-
member with increasing stratigraphic depth and tem-
perature (Fridriksson et al., 2001). This observation is
also relevant for the Hvalfjörður area where stellerite
occurs in the lowest exposed part of the zeolite zona-
tion mapped in this area (Weisenberger, 2005).
CONCLUSIONS
The sample from the Hvammsvík locality in
Hvalfjörður is an orthorhombic zeolite, with space
group Fmmm, and lattice parameters of this sample
are a = 13.5961, b = 18.2114, c = 17.8615. The chem-
istry, calculated space group and the lattice parameters
for the Hvammsvík sample are in agreements with the
criteria of Coombs et al. (1997) for naming the min-
eral species stellerite.
Chemistry and the XRD data for the sample is
similar to stellerite described from Villanova Mon-
teleone Sardinia (Galli and Passaglia, 1973).
Acknowledgment
Permission to conduct field work and to collect and
export samples from Iceland was given by Icelandic
Institute of Natural History is gratefully acknowl-
edged. The analysed material is donated to the Ice-
landic Museum of Natural History, Reykjavík. The
technical staff at Mineralogisch-Geochemisches Insti-
tut is thanked for the preparation of EPMA and XRD
samples and help with the analytical work. Dr. Juraj
Majzlan is thanked for the stellerite-stilbite lattice cal-
culations. Dr. Þráinn Friðriksson, and an anonymous
reviewer greatly improved the manuscript with their
critical reviews and useful suggestions.
ÁGRIP
Höfundar hafa gert efnagreiningar og röntgen-
greiningar á zeolítum úr Hvalfirði. Hvítir stellerít
kristallar, allt að 1.5 cm langir, finnast ásamt yuga-
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