Abstract. The temperature dependence of the absorption spectra of ilvaite, Ca(FeZ+,Fe3+)FeZ+Si2Os(OH), shows strongly one dimensional transport behaviour with no singularity at the Pnam-P21/a phase transition point near 335 K. Polarized single crystal transmission measurements were carried out between 300 K and 450 K in a frequency range between 600 and 23 000 cm-1. No Drude -absorption at low energies was found at any temperature. A macroscopic, thermodynamic model based on Landau-Ginzburg theory is given which accounts for the observed macroscopic properties of the structural phase transition and its coupling with the Fe 2 +-Fe 3 + ordering. This ordering scheme is discussed on an atomistic level and compared with the behaviour of magnetite and trans-(CH)~.