Chemistry Reference
In-Depth Information
Fig. 3.21 RT Mössbauer spectra of siderite (a) and ankerite under the magic angle (b) (after De
Grave and Vochten [
166
])
same (Fig.
3.21
a). This feature was firstly explained as due to the so-called Gol-
danskii-Karayagin effect [
164
] being an anisotropy in the Mössbauer fraction.
However, measurements under the so-called ''magic angle'', i.e. the absorber placed
under an angle of 54 degrees with the gamma ray direction [
165
], yielded nearly
equal intensities for both doublet lines pointing to texture effects as the reason for the
asymmetry. At very low temperatures the magnetic transition is not sharply dis-
played in the Mössbauer spectra because of relaxation effects [
166
].
Ankerite, CaFe(CO
3
)
2
, has a similar rhombohedral structure as siderite but with
calcium and iron in alternate positions. In natural ankerites the composition deviates
always from the ideal one through the presence of considerable amounts of Mg and
Mn. The magnetic transition temperature is much lower than for siderite and in a
range of more than 10 K below the transition complicated spectra are observed due to
spin-lattice relaxation [
167
,
168
]. Anyway, a long-range order was not observed
down to 1.7 K [
169
]. The spectra at RT and 80 K consist similarly to siderite of a
single doublet (Fig.
3.21
b), which is mostly asymmetric, most probably also as a
result of preferential orientation of the crystallites in the absorber. The isomer shift is
about the same as for siderite, but, the quadrupole splitting is somewhat smaller and
amounts to 1.44-1.48 mm/s at RT, depending on the composition [
170
].
3.5 Silicates
3.5.1 Introduction
Silicate minerals are all structurally derived from the tetrahedral bonding of silicon
to oxygen. For a relatively small group of these minerals, the structure consists of
discrete orthosilicate anions SiO
4
4-
, but, in the vast majority, the SiO
4
tetrahedra
