THERMAL-EXPANSION AND SPONTANEOUS STRAIN ASSOCIATED WITH THE NORMAL-INCOMMENSURATE PHASE-TRANSITION IN MELILITES

The linear thermal expansions of akermanite (Ca2MgSi2O7) and hardystonite (Ca2ZnSi2O7) have been measured across the normal-incommensurate phase transition for both materials. Least-squares fitting of the high temperature (normal phase) data yields expressions linear in T for the coefficients of instantaneous linear thermal expansion, alpha-1=1/l dl/dT for akermanite: alpha[100]=6.901(2)x 10(-6)+1.834(2) x 10(-8) T alpha[001]=-2.856(1)x 10(-6)+11.280(1) x 10(-8) T for hardystonite: alpha[100]=15.562(5)x10(-6)-1.478(3) x 10(-8) T alpha[001]=-11.115(5)x10(-6)+11.326(3) x 10(-8) T Although there is considerable strain for temperatures within 100-degrees-C of the phase transition, suggestive of a high-order phase transition, there appears to be a finite DELTA-V of transition, and the phase transition is classed as "weakly first order".