This paper deals with transmission electron microscope experiments of Ca,Sr-åkermanite solid solutions at temperatures between 100 K and 375 K. The aim of the investigations was to study the compositional and temperature dependence of phase transitions from the normal to the incommensurately modulated structure of (Ca1-xSrx)2MgSi2O7 single crystals observed by electron diffraction. With increasing Sr-content the normal to incommensurate phase transition temperature decreases linearly from ca. 350 K (x = 0.04) to 295 K (x = 0.22). Depending on temperature and composition, the incommensurate phase is structurally not uniform, its stability field can be divided into two regions. The transition between the two regions can be recognized by the additional appearance of higher order satellite reflections in the diffraction pattern. Additionally, the transition is characterized by an abrupt change of the modulation wavelength. Within the high-temperature, low ordered region, the modulation wavelength varies between 18.5 Å and 20.5 Å, whereas no changes of the wavelength have been found in the low-temperature, higher ordering region. With increasing Sr-content the high-temperature, low ordered phase covers a steadily increasing temperature range.

The Al-coupled incorporation of REE3+ (1) dominates the D-values. The Na-coupled substitution (2) is of minor importance. Depending on the compositions investigated the D-values vary by up to a factor of 10. This range overlaps most of the published pyroxene-melt partition coefficients. Because we conducted isothermal and isobaric experiments, this overlap indicates that a wide range of D-values is a function of composition. For the coupled substitutions (1) and (2) this indicates that the DREE strongly depends on the amount of tetrahedrally coordinated Al3+ in clinopyroxenes.