Thermodynamic study of ordering/clustering in pyroxenes and feldspars

Project Details

Description

Abstract

Thermodynamic study of ordering/clustering in pyroxenes and feldspars

• Theoretical framework
We recently reinvestigated the thermodynamics of the disordering reaction in Cu3Au and found that the vibrational entropy behaved differently compared to the enthalpy and to the configurational entropy. At the temperature of the phase transition, there was a sudden increase in all quantities, the enthalpy, the configurational entropy and the vibrational entropy. A further increase in temperature generated an increase in enthalpy and configurational entropy, because the degree of short-range order (SRO) was reduced. However, such a behaviour was not observed for the vibrational entropy. Precise measurements showed that the vibrational entropy did not depend on the degree of SRO. This experimental result was also confirmed by simulation studies using the density functional theory (DFT), if large enough cells were used. A hypothesis was formulated in our recent publication on Cu3Au to explain such a behaviour.
• Hypotheses/Research question
In this project, similar investigations are planned for the petrologically important silicate solid solutions omphacitic pyroxenes and feldspars. It is intended to clarify, if the observed disordering phenomena in Cu3Au are generally applicable, i.e., if the vibrational entropy does not depend on the degree of SRO also in these silicates, in contrast to the enthalpy and the configurational entropy.
• Approach/methods
We will investigate the thermodynamics of ordering in omphacite and of clustering/unmixing in alkali feldspars. The pyroxenes and feldspars will be equilibrated at different temperatures producing samples with different degrees of ordering/clustering. The enthalpy of these intracrystalline reactions will then be determined via line broadening in IR spectra and the vibrational entropy via low temperature calorimetry. The applicant has already in his possession natural omphacites and alkali feldspars, both of which are chemically and structurally best characterised. These experimental investigations will be complemented by DFT calculations, thereby answering the question of the necessary size of the supercells used for simulating the vibrational entropy successfully.
• Level of originality/innovation
The planned investigations on the already excellently characterised materials will essentially improve the thermodynamics of these in the geosciences important phases and will improve our understanding of the thermodynamics of ordering/clustering in general, because no such investigations were carried out on silicate solid solutions so far.
Short titleThermodynamics of ordering/clustering in silicates
StatusActive
Effective start/end date1/03/2129/02/24