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sites T1 and T2, the latter being the normal silicon site. Fe 3+ enters the melilites
mainly the T1 site. The hyperfine parameters are d Fe & 0.2 mm/s, D & 0.9 mm/s
for Fe 3+ and d Fe & 1.00 mm/s, D & 2.3 mm/s for Fe 2+ . Although it was sug-
gested that a small amount of Fe 3+ enters also the T2, it is finally found in ferric
melilites that Fe 3+ actually occupies two differently distorted T1 sites with
d Fe & 0.19 mm/s, D = 0.75 and 1.04 mm/s [ 190 ].
3.5.3.2 Ilvaite
Ilvaite with chemical formula CaFe 2 2 Fe 3 þ Si 2 O ð Þ OO ðÞ has an orthorhombic
structure. Technically spoken, it can be considered as a subsaturated sorosilicate in
view of the additional O 2- and OH - groups. The structure can be described as
ribbons of double rows of edge-shared octahedra consisting of five oxygens and
one hydroxyl anion and designated as M1(8d) sites. Above and below the ribbons
are octahedral M2(4c) sites with solely oxygen surrounding. It was later found that
low-impurity ilvaite possesses a slightly monoclinic structure with two M1 sites,
M11 and M12, and becoming orthorhombic with increasing substitution (e.g. by
Mn 2+ ) and temperature [ 191 ]. Iron enters all the octahedral sites. The Mössbauer
spectra show globally three absorption peaks (Fig. 3.24 ) which can essentially be
analyzed with 3 doublets, one for Fe 2+
in M11 site, one for Fe 3+
in M12 site and
one for Fe 2+
in M2 [ 192 , 193 ]. Representative hyperfine parameters are listed in
Table 3.14 .
On the other hand, two additional doublets are sometimes introduced to account
for Fe 2+ -Fe 3+ electronic relaxations [ 194 , 195 ], in particular for Mn-rich ilvaites
[ 196 ]. The appearance of only two doublets in Mn-rich ilvaites at higher tem-
peratures indicates a collapse to Fe 2.5+ in the M1 site, so that in the intermediate
temperature region a relaxation process occurs which significantly broadens the
central absorption peak [ 197 ].
Fig. 3.24 Typical RT
spectrum of ilvaite (adapted
from Dotson and Evans
[ 193 ])
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