Computer simulation of Al-Mg ordering in glaucophane and a comparison with infrared spectroscopy

The ordering of Mg and Al over the octahedral sites in glaucophane, ^[A]□^[8]Na₂^[6](Mg₃Al₂)^[4]Si₈O₂₂(OH)₂, has been studied by Monte Carlo simulation using a model Hamiltonian parameterised using both empirical interatomic interactions and ab initio calculations. It is found that Al is fully ordered at the M(2) site, with disorder beginning to appear for temperatures above ∼1000 K. Infrared spectra of three synthetic high-PT glaucophane-nyböite amphiboles were also collected and the OH-stretching frequencies used to infer the state of Al-Mg ordering. The spectra of all three amphiboles comprise only two peaks at ∼3662 cm⁻¹ and ∼3720 cm⁻¹, corresponding to MgMgMg-OH-^[A]□ and MgMgMg-OH-^[A]Na, respectively. These infrared spectra show unequivocally that M(1) and M(3) sites are fully occupied by Mg, and, therefore, ^[6]Al is fully ordered at M(2), in agreement with the behaviour predicted by the computational studies and bond-valence considerations.

Such a highly ^[6]Al-ordered state for alkali amphiboles contrasts starkly with calcic amphiboles synthesized under similar pressure-temperature conditions, which have a high degree of ^[6]Al disorder over M(2) and M(3) sites. This difference between alkali and calcic amphiboles shows the major influence that the M(4) cation (monovalent versus divalent) has, via its bonding relations to O(4), in controlling the ordering of trivalent cations over the octahedral sites in amphiboles.