Jökull - 01.01.2005, Side 50

Rune S. Selbekk and Tobias Weisenberger
Figure 1. Stellerite crystals from the Hvalfjörður
area, Iceland. – Stellerít úr Hvalfirði.
Figure 2. Miller indexes for the stellerite from the
Hvammsvík area. –Kristalgerð stelleríts.
5 essentially homogeneous stellerite crystals is pre-
sented in Table 1.
The average composition is:
(1) ( Ca3.88Na0.05 K0.03Sr0.01 ) P=3,97 Al7.95
Si28.07O72×28H2O,
similar to stellerite from Villanova Monteleone, Sar-
dinia (Galli and Passaglia, 1973) as is the XRD pat-
tern (Galli and Alberti, 1975). The XRD pattern of
the stellerite from Hvammsvík can be indexed with
the space group Fmmm; the refined lattice parame-
ters of a = 13.5961, b = 18.2114, c = 17.8615 that
fall within the range of other published lattice param-
eters for stellerite a = 13.507–13.605, b = 18.198–
18.270 and c = 17.823–17.863 Å (Passaglia et al.,
1978: Coombs et al., 1997). Alternatively, the pat-
tern can be described by a model of the monoclinic
mineral stilbite-Ca. We have refined the model with
the F2/m space group according to Passaglia et al.
(1978), giving the following results: a = 13.5937, b =
18.2074, c = 17.8519, β = 90.13. The statistics for the
fit for both space groups are similar. A slightly better
fit for the orthorhombic system indicates that the min-
eral is probably stellerite. The symmetry reduction
between stellerite and stilbite is continuous and does
not involve a phase transition since there is a near lin-
ear correlation between Na+ content and change in β
angle (Passaglia et al., 1978). Single crystal diffrac-
tometer measurements instead of powder diffractom-
etry will give better data for the refinement but has not
been obtained during this study.
Chemically stellerite and stilbite-Ca can be rather
similar, so the formal mineral name stellerite is
restricted to specimens of nearly stoichiometric
Ca2Al4Si14O36×14H2O. It is distinguished from
stilbite-Ca by containing low Na2O and high SiO2
(Table 1). The maximum amount of Na, K,Mg and Fe
in stellerite is suggested to be up to 0.2 atoms per for-
mula unit (apfu) (Coombs et al., 1997), and the stel-
lerite has in average 0.04 apfu on the basis of 36 (O).
The highest concentration of Na, K,Mg and Fe in stel-
lerite is 0.08 apfu, and this is well inside the criteria
of Coombs et al. (1997).
Comparing apfu Na (0.03) to apfu Al (3.98) per 36
(O) clearly shows that the mineral is stellerite not stil-
bite which normally has 5 apfu Al and 1 apfu Na on a
36 (O) basis (e.g. Fridriksson et al., 2001). Barrierite
50 JÖKULL No. 55