Rit (Vísindafélag Íslendinga) - 01.06.1984, Blaðsíða 243

mineral chemistry and relationships 239
of the analysed biotites CaO was not detected (i.e. less than 0.005 wt. per
cent).
MnO (Fig. 119) occurs in the micas in trace to minor amounts and follows
iron closely. It increases from less than 0.1 wt. per cent in the most
magnesian phlogopites towards around 0.6 per cent in the most iron rich
biotites.
Cl (Fig. 119) occurs in rather constant trace amounts in the micas. The
amount is mostly below 0.1 wt. per cent, except in the biotites of the hydrous
mineral xenolith, where it ranges up to 0.36 per cent. The average C1
content of the biotites of the lavas (neglecting those of the hydrous mineral
xenolith) is slightly higher, 0.066 wt. per cent, than that of the phlogopites,
0.048 per cent. C1 does therefore not seem to play a significant role among
the volatile components of these micas. This is in agreement with recent
studies of the volatile composition of micas, e.g. Parry & Jacobs (1975),
Kesler et al. (1975), and Nash (1976).
F (Fig. 119), being one of the two main volatile components in the micas,
ranges from less than 0.1 wt. per cent up to more than 3.6 wt. per cent. In
the biotites the F content is generally low, mostly below 0.5 per cent. An
exception to this is the high F content (3.52—3.66 per cent) of the biotites of
Jan 81, which are not primary phenocrysts as previously mentioned. In the
phlogopites, on the other hand, the F content is generally high, varying
between 1.5 and 3.5 wt. per cent. Within each rock sample, F decreases as
the Mg/Fe ratio decreases, and is thus enriched in the early phlogopites of
the samples relative to the late ones. This is apparently in contrast to what is
to be expected and will be further discussed below.
H20 (Fig. 119) was not directly analysed but calculated on the basis ofthe
ideal structural formula composition and known F- and C1 values. These
calculations assume the micas to be structurally ideal and all places
occupied by an ion and do not consider the possibility of hydroxyl deficient
micas (see Rimsaite, 1970). The water content of the Jan Mayen micas,
according to this, ranges from 2.5 to 4 wt. per cent. With the exception of
Jan 81 (see above) the biotites are water rich (H20=3.5—4 per cent) and
the phlogopites more water poor (2.5—3.5 per cent), as is to be expected
from the F pattern.
III. Crystallization relationships of the micas
As discussed previously, apatites crystallizing relatively early from a basic
magma tend to take up OH>F>Cl, because of the low F content of such
liquids, while the more evolved liquids are usually enriched in F (because of
the high affinity of F for melts), allowing these minerals to take up
F>OH>Cl, as is their tendency if all the ions are available in quantities.
This holds for micas and probably other magmatic minerals containing a
hydroxy-halogen group as well.