Rit (Vísindafélag Íslendinga) - 01.06.1984, Síða 254

250
PÁLL IMSLAND
while Ti is not needed in tetrahedral positions in the kaersutites.
CaO (Fig. 120) in the kaersutites is higher than in the basaltic
hornblendes. On the average the basaltic hornblendes contain 10.12 wt. per
xent CaO and the range is relatively broad, í'rom 8.85 to 11.38 per cent. The
kaersutites, on the other hand, have an average value of 11.90 per cent and a
narrow range from 11.20 to 12.27 per cent.
Na^O (Fig. 121) in the kaersutites ranges from 2.08 to 2.80 per cent and
has an average of 2.37. In the basaltic hornblendes it ranges from 2.56 to
3.33 per cent and the average is 2.96 per cent.
K20 (Fig. 121) in the kaersutites ranges from 1.18 to 1.84 per cent and is
on the average 1.46 per cent. The basaltic hornblendes have a range from
0.99 to 1.89 and an average of 1.38 per cent. One kaersutite analysis
contains an exceptionally high K20 or 2.56 per cent which is almost as high
as the Na20 content of that analysis. The Na20/K20 ratio is greater than
one in all cases and calculated on the average figures it gives 1.57 for the
kaersutites and 2.14 for the basaltic hornblendes. The late crystallized
basaltic hornblendes are thus K poor relative to the kaersutites. This is in
accord with the fact that the basaltic hornblendes crystallize after consider-
able crystallization of K rich feldspars has taken place and is still going on.
MnO (Fig. 121) is very low in the kaersutites, ranging from 0.14 to 0.29
per cent, with an average of 0.22. In the basaltic hornblendes, on the other
hand, Mn has greatly increased. The generally close association of Fe and
Mn is quite apparent here. The range of MnO in the basaltic hornblendes is
very wide, or from 0.79 to 1.66, and the average is 1.37 per cent. Individual
samples show a relatively great difference in the Mn content of the basaltic
hornblendes. On a sample basis the average for example varies from 0.87
(for Jan 81) to 1.59 (for Jan 175). Late crystallizing amphiboles thus are
clearly capable of taking up the generally accumulated Mn of late liquids as
do for instance the late crystallizing iron-titanium oxides.
F (Fig. 122) shows the contrasting pattern of being nearly absent in the
kaersutites, where it occupies only from 1.3 to 5.4 per cent of the hydroxy-
halogen space, to íilling, on the other hand, 40 to 53 per cent of the hydroxy-
halogen positions in the basaltic hornblendes. The concentration range in
the kaersutites is narrow or from 0.06 to 0.23 wt. per cent and the average is
0.12 per cent. In the basaltic hornblendes the concentrations are high but
the range is still relatively narrow, or from 1.76 to 2.24 wt. per cent, and the
average is 2.02 per cent. This high F content of the basaltic hornblendes is in
agreement with late crystallization from an evolved liquid.
Cl (Fig. 122) is present in trace amounts only in both amphiboles.
Nevertheless it shows a clear tendency to be higher in the basaltic
hornblendes, where the range is from 0.05 to 012 wt. per cent and the
average 0.09. In the kaersutites, on the other hand, it only ranges from 0.01
to 0.06 wt. per cent with an average of 0.03 per cent. In the case of the
kaersutites, this means that C1 occupies only from 0.1 to 0.6 per cent of the