Effect of growth conditions on the birefringence of mixed crystals revealed in alum solid solutions

Abstract

An experimental study of optical anomalies in solid solutions of alums in relation to their composition, growth temperatures and rates, and hydrodynamic regime was carried out. Theoretical analysis of the data showed that they can be understood in terms of a mechanism of kinetic ordering of the isomorphous atoms (the phenomenon of so-called 'growth dissymmetrization'). A theoretical model is presented which is supported by data for a number of minerals and synthetic compounds, which are known for their optical anomalies due to growth dissymmetrization.

Kinetic ordering of atoms in sodium chlorate–bromate solid solutions

“…Summarizing all the data, one can conclude that the Hal11 and Hal12 sites (Cl(1) and Cl(3) in the paper [12]) have similar occupancies x 11 and x 12 and always contain more Br than the two other sites Hal21 and Hal22 (Cl (2) and Cl(4) in the paper [12]) with the similar occupancies x 21 and x 22 . This scheme allows us to use the averaged (monoclinic) occupancies x 1 ¼ ðx 11 þ x 12 Þ=2 and x 2 ¼ ðx 11 þ x 12 =2Þ instead of the initial triclinic values.…”

“…The effect of crystal composition on growth ordering has been already considered in [2]. Let us discuss it in more detail.…”

“…For example, ordering of the trivalent cations in ugrandite garnets and related optical anomalies were studied in detail [4,5,8,9], however, reliable data on the effect of crystal composition and growth conditions are still unavailable. On the contrary, these growth aspects were examined for the alum crystals, but the structural reasons for the anomalous birefringence remains unclear so far [2,3,10]. The main objective of the present paper is to demonstrate all the principal aspects of the growth dissymmetrzation phenomenon for one and the same object.…”

“…Kinetic ordering was discovered and studied in at least 20 compounds, e.g. in quartz [1], alums [2,3], garnets [4,5], topaz [6], Na 2 CdðSO 4 Þ 2 ÁH 2 O [7], etc. Unfortunately, different aspects of the phenomenon were studied for different compounds that led to some ambiguity in the data interpretation.…”

Optical properties and crystal structure of triclinic growth sectors in vesuvianite

“…alum in Shtukenberg et al, 2000). It is thought that the sectors result from cation (AI3C /Si4+, Ae+/Fe3+) or anion (OH-/F-) ordering or preferred orientation of the OHdipole during crystal growth.…”