Chemistry Reference
In-Depth Information
Table 3.14
Representative hyperfine parameters at RT for some sorosilicates
Mineral
Formula
Fe site
d
Fe
(mm/s)
D (mm/s)
(Ca,Na)
2
(Mg,Fe
2+
,Fe
3+
,Zn,Al)(Si,Al)
2
O
7
Fe
2+
Melilite
T1,T2 &1.00
2.3
(Ca,Na)
2
(Mg,Fe
3+
)(Si
2
O
7
)
Fe
3+
T1
1
0.19
0.75
Fe
3+
T1
2
0.19
1.04
Fe
2+
Ilvaite
CaFe
2
þ
2
Fe
3
þ
Si
2
O
7
M2
&1.05
2.30-2.50
ð
Þ
OO
ðÞ
Fe
2+
M11
&1
2.10-2.20
Fe
3+
M12
0.55
1.1-1.3
3.5.4 Cyclosilicates
The most common structural motif in the group of cyclosilicates is the six-membered
Si
6
O
18
ring, as been found in beryl and tourmaline. The Mössbauer spectra of these
minerals present a number of features that have remained incompletely understood
for a long time.
Beryl has a hexagonal structure composed of a stacking of rings forming
channels parallel to the c-axis. These columns are linked by Al
3+
and Be
2+
in six-
and four-fold coordination, respectively. The ideal formula is Be
3
Al
2
Si
6
O
18
. The
iron contents in these minerals are small and usually less than 1 wt %. In general,
the Mössbauer spectrum consists of a broad peak near zero velocity and a sharp
line near 2.5 mm/s. This particular spectral behavior has been attributed to an
unspecified relaxation process. At low temperatures two well resolved ferrous
doublet spectra are observed. At 4 K the hyperfine parameters are d
Fe
= 1.3 mm/s,
D = 2.7 mm/s and d
Fe
= 1.0 mm/s, D = 1.5 mm/s, respectively [
198
]. The first
doublet has been assigned to octahedral Fe
2+
whereas the second less intense one
represents substitutional Fe
2+
in the highly distorted Be
2+
tetrahedral sites.
Additionally, small amounts of Fe
3+
are located in the octahedral Al
3+
sites. In a
sample of deep-bleu beryl a fourth doublet has been resolved and assigned to Fe
2+
in the channels [
199
].
Cordierite with ideal formula Al
3
(Fe
2+
)
2
(AlSi
5
)O
18
is structurally similar to
beryl. The principal octahedral cations are Fe
2+
and some Mg instead of Al. The
spectra of both magnesium and iron cordeirites exhibit spectra consisting of a
predominant ferrous doublet with d
Fe
= 1.15 mm/s and D = 2.3 mm/s. A second,
much weaker, ferrous component is observed which was attributed to channel iron
[
200
]. However, this second has also been interpreted as being due to Fe
2+
replacing Al, whereas Na enters the center of the rings [
201
].
Tourmalines have a complex structure which includes Si
6
O
18
rings, BO
3
groups, spiral chains of AlO
5
(OH) octahedral (C) sites and edge-sharing clusters of
three MgO
4
(OH)
2
octahedral (B) sites. The ideal formula would be
X(Y
3
Z
6
)(Si
6
O
18
)(BO
3
)
3
(OH)
3
(OH,F) with X = Na, but also K, Ca or vacancies,
Y = Mg, but also Fe, Mn, Al, Fe, Cr, Ti and Z = Al, but also Cr, and V. Due to
the large variety of cations in this structure a wide variety of spectra have been
reported. The most common tourmalines are within the elbaite-schorl series
