Other peaks appears the consequence of establishing of equilibrium composition, i.e. the formation of strontian as the single phase via reactions between the different intermediates (see Eq. (9)) and intermediates with single oxides (Eqs. (8) and (9)). The establishment of equilibrium composition leads to the formation of secondary strontian. The reaction mechanism of formation is different, but no structural difference between primary and secondary formed strontian is observed. According to the results of heating microscopy (Fig. 3(b)), the strontian starts to melt at the temperature of 1660 °C.
The scanning electron microscopy results of raw meal treated to temperatures marked in Fig. 3(b) are shown in Fig. 5. The results of HT-XRD (Fig. 2) indicate that the diffraction lines of strontian appear at the temperature of 1150 °C. The SEM analysis of sample treated to this temperature (a) shows the growth of nuclei of strontian. At the temperature of 1300 °C (b) the plate-like particles of strontian can be recognized. The structure consists of many narrow layers of twinned feldspar crystals (multiple lamellar [010] contact twins according to the “albite law” [79] and [80]), which endow the parts of the crystal with faintly striped appearance. The melt formed with increasing temperature enables the recrystallization and growth of large plate-like particles of tertiary strontian (c and d) which are stable up to the melting temperature. The process is accompanied by the expansion of specimen (see Fig. 3(b)).