Cation ordering in BaAl2Ge2O8-feldspar: implications for the I(1)over-bar <-> C(1)over-bar phase transition in anorthite

BaAl2Ge2O8-Feldspar undergoes an order-disorder phase transition I2/c<->C2/ m at T(tr)approximate to 1690 K. The thermodynamics of the Al,Ge cation ord ering process is described in terms of the compressible Ising model in mean field approximation. The mean field potential predicts a first order chara cter of the phase transition. This is compared to antiferromagnetic orderin g in a two-dimensional square Ising model with NN-pair interactions and fou r-spin interactions on alternating squares. Calculated order parameters and short range ordering are in good agreement with the corresponding properti es observed in BaAl2Ge2O8-feldspar by means of X-ray diffraction, hard mode infrared spectroscopy and TEM. Using known calorimetric data a similar mod el is postulated for Al,Si ordering in anorthite, CaAl2Si2O8, for which the derived potential describes a transition with slightly stronger first orde r character at T(tr)approximate to-1928 K.