Micas - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 207

Micas

Micas are primarily presented in magmatic rocks with high content of silica, alkaline
and volatile components, as well as in metamorphic and some metasomatic rocks.
Less frequently micas occur as a secondary phase in some rocks of mafic and ultrama-
fic composition and what is more, most often they are represented by phlogopite.
Chemical formula of mica group of minerals is as follows: K(Mg, Fe) 3 [Si 3 , AlO 10 ]

⋅
[F, OH] 2 . The content of the main components of phlogopite change in the following
ranges (wt%): MgO-21-29, FeO-up to 9, K 2 O-7-10, H 2 O-0.3-5.4. Biotites are char-
acterized by a high content of FeO (2.7-27.6 wt%) and K 2 O (6.2-11.4 wt%) and by
wider variations in the content of MgO (0.28-28.3 wt%) [Betekhtin, 1956].
Analytical data on the REE composition of micas from high-magnesium rocks
is now available in fairly limited amounts and characterizes the composition of this
mineral not from all the varieties of these rocks but mainly from kimberlites, perido-
tites, wehrlites, websterites and clinopyroxenites (Table 8.1). Total REE contents in
phlogopites from wehrlites and pyroxenites composing xenoliths from minettes of
Bearpaw province (Montana, USA) vary from 29 to 112 ppm, while the chondrite-
normalized La contents are higher (10.2-54.2 t.ch.) than Yb contents (3.6-7.2 t.ch.)
[Downes et al ., 2004]. The REE patterns of these phlogopites have the shape of lines
with a gentle negative slope and are described by the values of (La/Yb) n parameter in
the range of 2.3-7.5 (Figure 8.1).
Phlogopites from ultramafic rocks that form xenoliths in alkaline basalts from Vitim
province are characterized by total REE contents that are lower (1.4-20 ppm) than the
previous ones [Litasov, 1998]. The REE composition, which is very close to phlogopites
from Vitim province, was identified in the phlogopites from concentrate that have been
isolated from kimberlite sample from Yubileynaya pipe (Yakutia), the total REE content
of which is 10.8 ppm [Aschepkov et al. , 2004]. Phlogopites from ultramafic xenoliths of
Vitim province, as well as from the kimberlites of Yubileynaya pipe, are characterized by
a more intensive REE fractionation in comparison with the mineral from Bearpaw prov-
ince. Thus, in phlogopites from Vitim province the values of (La/Yb) n parameter vary in
the range from 14.2 to 21.8, while in the mineral from kimberlites of Yubileynaya pipe
∼ 55. On REE patterns of phlogopites from ultramafic xenoliths of Vitim province there

are negative Ce anomalies (Figure 8.1, 3), which are absent on the pattern of the min-
eral from kimberlites, while the latter show a weak positive Eu anomaly (Figure 8.1, 4).
Mica that is presented in the olivine nephelinite from Germany and Austria is character-
ized by relatively low overall REE accumulation in comparison with the mineral from
the manifestations mentioned above (Figure 8.1, 5).