The nuclear magnetic resonance (NMR) of 27A1 in anorthite (CaAl2Si2O8) has been studied as a function of temperature. At the critical temperature Tc = 241 ± 4°C a complete reversible structural phase transition takes place: two sites each out of the eight inequivalent Al sites of the low-temperature phase become equal at Tc resulting in only four inequivalent sites in the high-temperature phase. The field gradient tensors at the Al nuclear sites have been determined for temperatures below and above the transition.
From the results follows that the (Al,Si)O2 framework becomes body-centered at Tc. The NMR results are consistent with x-ray structure investigations if one assumes for the high-temperature phase a “jumping” of the Ca atoms between the two split positions.
The existence of two kinds of domains with different Bravais lattices would contradict the NMR results.