Rit (Vísindafélag Íslendinga) - 01.06.1984, Síða 237

mineral chemistry and relationships
233
1S less intense (Jan 81), mica patches still exist but are spotted with small
opaque oxides; the secondary silicates being less prominent or invisible. In
the latter case the amount of the oxides decreases towards the core of the
altered crystal. The mica of the hydrous mineral xenolith shows a similar,
weak marginal alteration, where the mica is in contact with plagioclase or
amphibole.
II. The composition of the micas
In the micas under consideration, only two of the volatile components
have been analysed. 'l’hese are F and Cl. The amounts of OH in the
structure and H20 in the analyses have been calculated, assuming the total
of the volatile components in the micas to be 4.000 and the formula of the
perfect mica structure to contain 24 anions.
Representative analyses of the Jan Mayen micas are given in Table 37
and 38, including the calculated H20 content. The compositional trend of
these micas relative to a generalized picture of composition of igneous and
metamorphic micas is shown in Fig. 117.
The major variable components of the Jan Mayen micas are Mg and Fe,
reflecting the simple phlogopite-annite solid solution. This solid solution is
complicated by Al-Si substitution in the tetrahedral position beyond the Si/
A1 ratio of6/2, the compositions thus approaching the siderophyllite-easton-
ite end member solid solution. The octahedral Mg and Fe in these micas are
further extensively substituted by Ti and most probably A1 as well. Fi+ is
tnost probably present in the octahedral position as well, but the amount is
unknown. In order to maintain a valence balance this complex substitution
may further be responsible for K deviating somewhat from the simplified
ideal formula amount. K, on the other hand, is substituted by Na in minor
amounts only. Of the volatile components considerable F-OH substitution
is evident while C1 is present in rather constant trace amounts. The micas
are thus probably best described as Ti rich biotite-phlogopite solid solution
members.
Apart from the major elements, Na, Ca, Mn and C1 were also determined
tn the micas.
In the biotites the range of the MgO content is from 6.97 to 16.53 wt. per
cent and from 15.49 to 21.55 per cent in the phlogopites. As MgO in the
micas increases, iron (expressed as FeO') decreases. The range is from 24.77
down to 12.39 wt. per cent in the biotites and from 11.36 to 3.99 wt. per cent
in the phlogopites. The Mg/Fe ratio increases from 0.51 to 2.08 in the
biotites and from 2.67 to 9.63 in the phlogopites.
The wehrlite contains the most magnesian phlogopites. The composition
of the wehrlite phlogopites is, however, not constant. The Mg/Fe ratio of the
analysed grains ranges from 9.63 for the most magnesian one to 4.89 for the
least magnesian one. This represents a drop in MgO from 21.55 to 17.37 wt.