Thermal expansion and spontaneous strain associated with the normal-incommensurate phase transition in melilites |
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Authors: | S L Webb C R Ross II J Liebertz |
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Institution: | 1. Bayerisches Geoinstitut, Universit?t Bayreuth, W-8580, Bayreuth, Federal Republic of Germany 2. Institut für Kristallographie, Universit?t zu K?ln, 1, K?ln, Federal Republic of Germany
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Abstract: | The linear thermal expansions of åkermanite (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, $$\alpha _1 = \frac{1}{l}\frac{{dl}}{{dT}}$$ for åkermanite: $$\begin{gathered} \alpha _{100]} = 6.901(2) \times 10^{ - 6} + 1.834(2) \times 10^{ - 8} T \hfill \\ \alpha _{100]} = - 2.856(1) \times 10^{ - 6} + 11.280(1) \times 10^{ - 8} T \hfill \\ \end{gathered} $$ for hardystonite: $$\begin{gathered} \alpha _{100]} = 15.562(5) \times 10^{ - 6} - 1.478(3) \times 10^{ - 8} T \hfill \\ \alpha _{100]} = - 11.115(5) \times 10^{ - 6} + 11.326(3) \times 10^{ - 8} T \hfill \\ \end{gathered} $$ Although there is considerable strain for temperatures within 10° C of the phase transition, suggestive of a high-order phase transition, there appears to be a finite ΔV of transition, and the phase transition is classed as “weakly first order”. |
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