Cation Ordering in Amphiboles

This is my part III project, submitted as part of my Minerals Science degree course at the Department of Earth Sciences in the University of Cambridge. I've put it here just in case anyone might find it useful. Well, you never know, someone might.

1. Abstract
2. Full text
3. Figures
4. Computer files
5. Calculated structure

Abstract

A model of glaucophane, Na₂(Mg₃Al₂)Si₈O₂₂(OH)₂, was adapted to represent the tschermakite (Ca₂(Mg₃Al₂)(Si₆Al₂)O₂₂(OH)₂) structure. The model was tested to determine its appropriateness for modeling the disordering of aluminium and silicon atoms over the T1 and T2 tetrahedral sites. It was found that the model is suitable as a basis for the calculation of lattice energies for tschermakite. Computer calculations of the equilibrium lattice energies were carried out, using the General Utility Lattice Program to calculate energies based on empirical potentials. The lattice energy for tschermakite was modelled using a Hamiltonian of the form:

Values for the exchange energies J_i were then calculated for the tschermakite structure. These values were used to produce data suitable for use in a Monte Carlo simulation of disordering in the mineral. An infra red spectrum was produced of the OH bond stretching. The J_i values were used to build a model of the structure, and the ordering pattern of aluminium and silicon on the tetrahedral sites determined. It was seen that the ordering process is essentially two-dimensional.

Full text

• Download the full write-up in PDF format here (161K).

Figures

• Figure 7a
• Figure 7b
• Figures 8, 9, 10
• Figures 11, 12

Computer files

• Assorted computer files used in the calculations

Calculated structure

The calculated structure of the (Si, Al)O₄ framework of tschermakite: