Rit (Vísindafélag Íslendinga) - 01.06.1984, Page 250

246 PÁLLIMSLAND
iron is here treated as FeO as recorded by the microprobe. This in fact
slightly affects the calculated value of H20.
Representative analyses are given in Table 39 including the calculated
water content.
The compositions of the kaersutites and the basaltic hornblendes are
quite different regarding nearly all the analysed elements. The element
showing the most similar concentrations in both amphiboles is Mg. Relative
to the kaersutites the basaltic hornblendes show a wider range of concentra-
tions regarding all analysed elements except for A1 and Ti. This may be
expected of a late crystallizing mineral.
The average MgO content of the basaltic hornblendes is 12.52 wt. per
cent and 12.64 per cent in the case of the kaersutites. The range is, however,
about 1 per cent broader in the basaltic hornblendes or 11.44 to 14.04 per
cent compared to 11.89 to 13.48 per cent in the kaersutites. Iron, on the
other hand, shows a much wider range. In the basaltic hornblendes which
are much higher in FeO' than the kaersutites, it ranges from 15.39 to 19.70
per cent with an average of 17.15 per cent. In the kaersutites the FeO1 range
is from 10.28 to 12.60 with an average of 11.74 per cent. In each sample both
minerals show increasing Fe as Mg decreases. These concentrations give a
Mg/Fe ratio ranging from 1.06 to 1.63 for the basaltic hornblendes and 1.68
to 2.24 for the kaersutites (see the abscissa of the variation diagrams, Figs.
120 to 122). Between these two compositional types of the amphiboles (a
ferromagnesian mineral) there is thus no overlap in Mg/Fe ratio in spite of
identical Mg contents.
Si02 (Fig. 120). The silica content of the kaersutites ranges from 38.8 to
42.6 wt. per cent and has an average of 40.4 per cent. In the basaltic
hornblendes the Si02 content is about 6 per cent higher. The range is of a
similar width or from 45.1 to 48.4 wt. per cent and the average is 46.5 per
cent. Thus, of the two amphiboles, the Si rich one is the one crystallizing
from the late evolved liquid, as could be expected. Neither of the amphiboles
has enough Si to fill the tetrahedral position.
Al203 (Fig. 120) is much higher and shows a wider range in the kaersutites
than in the basaltic hornblendes. In the kaersutite the range is from 12.17
wt. per cent to 14.42 and the average 13.36, while in the basaltic
hornblendes the range is from 5.03 to 6.42 per cent and the average is 5.75.
The basaltic hornblendes need all the A1 in the tetrahedral (Z) position
together with the Si ions and in fact some more, while the kaersutites have
some A1 in excess of the tetrahedral need.
Ti02 (Fig. 120). These basaltic hornblendes are relatively Ti poor when
compared to the analyses given by Deer et al. (1963). The Ti02 range is
very small or from 1.11 to 1.65 wt. per cent and the average is 1.47. The
kaersutites have a much higher Ti02 content and a wider range, 4.69 to
6.24, and an average of 5.60 per cent. In the case of the basaltic hornblendes
the tetrahedral position, unfilled by Si and Al, may take some of this Ti,