Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 223

mineral chemistry and relationships 219
but are absent from the most mafic rocks. They do not form a continuous
mineral suite through the rock suite.
Two xenolith types contain apatites. These are the syenitic xenoliths and
the hydrous mineral xenoliths. As in the case of the phenocrysts these
apatites are closely associated with titanomagnetites. In the case of the
syenitic xenoliths, where the oxides have suffered an intense secondary high
temperature oxidation, the apatites have suffered some secondary alteration
as well. This expresses itself in a slight modification of the euhedral crystal
form and a distinct brownish colouring, caused by minute opaque iron rich
needles. A tendency towards this same colouring may be seen in some of the
apatite phenocrysts of the lavas.
II. The composition of the apatites
The major variables of igneous apatite composition are the volatile
components OH, F and Cl. In the analyses presented here, I and C1 were
determined and the water content calculated assuming the sum of these
three volatile components to be 2.000 in the structural formula containing
26 anions. This treatment shows the Jan Mayen apatites to be generally low
in C1 but ranging from F rich to OH rich, or rather to group into a f rich
and an OH rich group.
I he analyses of the apatites show a low total, which in cases is too low to
be raised to satisfactory values by adding H2O in the described manner. A
histogram of the sums (before addition of H20 but after subtraction of I and
Cl in excess of O) is given in Fig. 113. Fig. 114 shows the analysed wt. per
cent of CaO and P205 plotted against this sum. '1 he figure reveals a clear
correlation between the sum and the amount of these oxides, which both
decrease with decreasing total. Other determined elements were plotted in
the same way but showed no systematic correlation. 1 his suggests that the
low sum is primarily caused by lack of these two components which are the
major components of the common apatite and make up around 96 wt. per
cent of its total. Elements not determined here might possibly account for
the low sum, to some extent. The most likely elements in this respect are the
rare earth elements, which may be incorporated by the apatite structure in
considerable amounts. To calculate the H20 content of these low-sum
analyses in the above described manner would give slightly distorted values.
I o minimize this efiect on the H20 content the analyses have been
“corrected” as regards both the CaO and P205 contents (where necessary)
before the H20 content calculation. The analyses having the highest sums
give satisfactory totals after addition of H20 and have thus not been
“corrected”. These analyses have a satisfactory balance of Ca and P in the
calculated structural formulae. The ratio ofCaO/P205 in these analyses is
therefore “correct” and is identical to that of most published apatite
analyses. The Ca0/P205 ratio of the rest of the Jan Mayen apatite analyses