Chemistry Reference
In-Depth Information
Fig. 3.26 RT Mössbauer
spectra of synthetic samples
of the enstatite-ferrosilite
series showing a gradual
increase of Fe
2+
on M1 sites
(from Dyar et al. [
209
])
Ca-Fe pyroxenes with compositions between hedenbergite and clinoferrosilite
(Ca,Fe
2+
)(Fe
2+
)Si
2
O
6
show structure in their spectra which requires more than two
ferrous doublets for an adequate fit [
214
]. The observed subspectra may arise from
the different numbers of iron and calcium neighbors on the three M3 sites. Even a
more distinct structure is observed in the omphacites in which the distribution of
the charges among the neighboring M2 sites (Ca
2+
and Na
+
) has a large effect on
the Fe
2+
located in the M1 sites [
215
].
Aegirine NaFe
3+
Si
2
O
6
shows a ferric doublet with d
Fe
= 0.39 mm/s and
D = 0.30 mm/s (Fig.
3.27
b). Replacing Na by Li causes a change in coordination
number from eight to six for M2 sites, but has no effect on the hyperfine
parameters. Systematic studies of solid solutions of aegirine with diopside,
hedenbergite, kosmochlor and LiFeSi
2
O
6
showed that neither d nor D changes
markedly as long as the valences of the neighboring cations of Fe remain unal-
tered. Whenever ions of different valences are mixed on the same site electron
