I(1)OVER-BAR-I2 C PHASE-TRANSITION IN ALKALINE-EARTH FELDSPARS ALONG THE CAAL2SI2O8-SRAL2SI2O8 JOIN - THERMODYNAMIC BEHAVIOR/

Feldspars crystallized in the range SrF80An20-SrF60An40 of the join Ca Al2Si2O8-SrAl2Si2O8 from gels at T = 1350-1500-degrees-C for short tim e show cell parameters significantly different from those obtained fro m longer heating. This difference has been interpreted as due to highe r metastable Al/Si disorder in short heated samples and to more ordere d configurations with longer heat treatments. HT powder spectra show t hat the T(c) of the I1BAR-I2/c phase transition of feldspars changes w ith composition and with the degree of order at constant composition; the evolution of the spontaneous strain (approximately cosalpha) vers us T for the samples examined shows a variation in the order of the tr ansition from continuous (II order) in more ordered and Sr-rich sample s to a discontinuous behaviour (presumably I order) for more disordere d and Sr-poor samples. A biquadratic coupling between displacive and O D parameters is therefore suggested.