Chemistry Reference
In-Depth Information
Fig. 3.22 SiO
4
stacking in silicates (only O atoms are shown): a Nesosilicates, b sorosilicates,
c cyclosilicates, d, e inosilicates, f phyllosilicates
will
henceforward
be
adopted
for
further
discussion
of
the
specific
silicate
minerals.
3.5.2 Nesosilicates
Nesosilicates is the class of silicate minerals in which the structure is built up by
independent tetrahedra of SiO
4
. This class contains some mineral groups like the
olivines, the silicate garnets, the epidote group and some separate minerals.
3.5.2.1 Olivines
Olivines consist of independent SiO
4
tetrahedra surrounded by six-coordinated
metal cations in two distinct sites M1 and M2. Mössbauer studies of the forsterite-
fayalite (Mg
2
SiO
4
-Fe
2
SiO
4
) and fayalite-tephroite (Fe
2
SiO
4
-Mn
2
SiO
4
) series all
revealed at RT a simple doublet with a quadrupole splitting D in the range
2.80-3.02 mm/s and an isomer shift d
Fe
= 1.16-1.18 mm/s [
171
]. The doublet is
often asymmetric with different line widths and depths indicating the presence of
two doublets, one arising from M1 and the other one from M2 sites [
172
].
However, these doublets overlap to such a large extent that they can hardly be
separated in the spectrum analysis. At high temperatures (1000 K) a separation of
both doublets can be possible. Fayalite itself orders antiferromagnetically below
66 K, and becomes towards forsterite at a lower composition-dependent temper-
ature a canted antiferromagnet [
173
-
175
]. At very low temperatures two distinct
magnetic patterns are observed with B of 12.0 and 32.3 T [
176
,
177
].
A mixed-valence iron olivine is laihunite (ferrifayalite) with ideal composition
Fe
2+
Fe
3+
(SiO
4
)
2
. In this olivine species Fe
2+
occupies M1 sites, alternated with
vacancies, whereas Fe
3+
occupies the M2 sites. Due to the difference in oxidation
state two well resolvable doublets are observed with d
Fe
= 0.39 mm/s and
D = 0.91 mm/s for Fe
3+
and d
Fe
= 1.13 mm/s and D = 2.75 mm/s for Fe
2+
[
178
].
Ferrifayalite
often
occurs
as
an
intergrowth
of fayalite
and
laihunite
[
179
]
(Table
3.11
,
3.12
, and
3.13
).
