Mineralogy - Origin of Potassium-rich Silica-deficient Igneous Rocks - page 30

and a concomitant decrease in Al from the centre to the edge. They are charac-
terised by de cit in the tetrahedral site [e.g. (Si + Al + Cr) < 4 atoms].

The presence of Fe 3+ in the tetrahedral sites is well-established. Tetra-ferriph-
logopite is not only found in nature, but has also been synthesised. In these micas,
Si 4+ and Mg 2+ can substitute for 2 Al 3+ in the tetrahedral site (Seifert and Schreyer
1971). The presence of tetrahedral Mg in micas from Smoky Butte lamproites has
been demonstrated by Robert (1981). He further showed that in such micas Ti is
exclusively located on the octahedral sites.
Different types of substitution in micas have been suggested by different min-
eralogists as summarised by Wagner and Velde (1986):

2Mg ¼ Ti þ h Forbes and Flower 1974 ð Þ ð 2 : 1 Þ

3Mg ¼ Ti þ Ba þ h Velde 1979 ð Þ ð 2 : 2 Þ

Mg þ 2Si ¼ Ti þ 2Al Robert 1976 ð Þ : ð 2 : 3 Þ

Mg þ Si ¼ Ti þ Mg Robert 1976 ð Þ ; and ð 2 : 4 Þ

Mg þ 2OH ¼ Ti þ 2O Arima and Edgar 1980 ½ ð Þ ð 2 : 5 Þ

The substitution of type ( 2.1 ) can be demonstrated in Fig. 2.9 a and b, where Ti
has been related to the total number of cations. In this substitution it has been
assumed that both the tetrahedral and interlayer sites are filled, and no Fe 3+ is

present in the structure. Authigenic micas in lamproites differ from phlogopites
from ultrabasic xenoliths by their higher TiO 2 contents, but xenocrystal micas are
found to be similar even in their trace element contents with phlogopites from
peridotites.
The micas of wyomingite and olivine orendites (Leucite Hills, U.S.A.) have
cores generally enriched in Al 2 O 3 and impoverished in TiO 2 than those from the
micas of madupites (Kuehner et al. 1981). In comparison to phlogopites from
madupites and olivine orendites, those from wyomingites have higher Cr 2 O 3 con-
tents. The phlogopite cores from the olivine orendites are enriched in Ti, Ba, Al and
Fe and depleted in Mg and Si in contrast to the microphenocrystal phlogopites of
the same rock type (Table 2.5 ).
The Highwood Mountains minettes have phlogopite phenocrysts (up to 10 cm
long) characterised by high Fe/(Mg + Fe) ratio (0.1 0.2) with Cr 2 O 3 content

-

ranging up to 2.2 wt%. Biotite micro-phenocrysts in ma c phonolites from the same

locality have Fe/(Mg + Fe) ratio <0.3, but are more enriched in Ti, Fe, Mg, Ca, Ba,
Na and F relative to the phlogopites. As a matter of fact, some of the biotites have
the BaO content as high as 7.5 wt% with (Al + Si) <0.8 per 22 oxygen.
The biotites (up to 4 mm long) of the Brown Leucitic Tuffs of Roccamon

na
(Italy) have apatite inclusions with Fe/(Mg + Fe) ratio varying between 0.38 and
0.36. In CaO-poor but Fe-rich samples, this ratio is as low as 0.60 (Luhr and
Giannetti 1987).