Since the discovery of the Mössbauer effect, numerous applications in a wide variety of scientific disciplines have been described. The hyperfine parameters, which can be determined experimentally from the line positions in a Mössbauer spectrum, provide information on the local atomic environment around the nuclei. In particular, the quadrupole splitting (QS) depends on the valence and spin state of the absorber atoms, as well as the coordination and degree of distortion of the crystallographic sites that such atoms occupy. Therefore, this parameter can potentially be used to describe the variation of structural distortion associated with phase transitions.

In Mg-rich cummingtonites [(Mg,Fe)_{7}Si_{8}O_{22}(OH)_{2}]
a displacive phase transition from *P*2_{1}/*m*
to *C*2/*m* occurs with increasing temperature. The difference
between low and high-symmetry phases is mainly in the degree of distortion
of the tetrahedral chains. Whereas in the *C*2/*m* structure
there is only one crystallographic distinct chain, in the *P*2_{1}/*m*
phase there are two non-equivalent chains, designed as A and B, with different
elongation angles. The lowering in symmetry causes a change in the coordination
of the *M*4 sites from 4+2 with four atoms (two O2 and two O4) at
~ 2 Å and two atoms (O6) at ~ 2.7 Å to 4+1+1 with one of the
O6 atoms now closer to the *M*4 cation at ~ 2.5 Å. This change
in the *M*4 coordination may, therefore, affect the Mössbauer
parameters.

Mössbauer spectra of a cummingtonite with *x*_{Fe} = 0.37 were collected in the
temperature range between 100 and 550 K. The spectra show only four peaks,
where the peaks correspond to two different types of iron. The inner two
peaks have been assigned as in previous studies to the *M*4 position
due to the higher distortion of the *M*4 octahedra in the amphibole
structure, whereas the outer two peaks have been assigned to the *M*1,
*M*2 and *M*3 positions. The *M*1, *M*2 and *M*3 doublets
overlap at all the temperatures studied. The QS of the *M*4 doublet
is shown in Figure 3.2-5 as a function of temperature. A clear change in
slope is visible at the critical temperature of the *P*2_{1}/*m*
to *C*2/*m* phase transition. If we assume that the valence

^{2+}
at the M4 sites. Open squares: P2_{1}/m
phase, open circles: C2/m phase. The temperature variation
within a single structure is approximately linear in the vicinity of the
phase transition, and can be used to extrapolate the high-symmetry QS data
to low temperature (solid line). |

contribution to the QS is similar for both *P*2_{1}/*m*
and *C*2/*m* phases, the most likely effect to cause the observed
variation is a change in the lattice contribution related to the distortion
of the *M*4 sites. The difference between the QS data of the low-symmetry
phase and the values obtained at the same temperatures by extrapolating
the QS data of the *C*2/*m* phase could be used as a measure
of the short-range order parameter associated with the phase transition.
Further work is underway to obtain a long-range order parameter described
in terms of lattice strains to compare with the short-range order parameter.

Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Deutschland

Tel: +49-(0) 921 55 3700 / 3766, Fax: +49-(0) 921 55 3769, E-mail: bayerisches.geoinstitut(at)uni-bayreuth.de

Tel: +49-(0) 921 55 3700 / 3766, Fax: +49-(0) 921 55 3769, E-mail: bayerisches.geoinstitut(at)uni-bayreuth.de