Self-Assembly in the Growth of Precious Opal

TL;DR

This paper proposes a model for the growth of natural precious opal, emphasizing electrostatic self-assembly of silica microspherulites and the role of chemical and thermodynamic conditions in forming characteristic structures.

Contribution

It introduces a new self-assembly mechanism driven by electrostatic interactions and specific chemical conditions for the formation of precious opal.

Findings

Electrostatic self-assembly facilitates microspherulite ordering.

High pH and surface charge density are crucial for nucleation.

Sequential salt deposition explains concentric shell structures.

Abstract

It is proposed that primary nucleation of amorphous microspherulites of hydrated silica in natural proto-precious-opal can be followed by a long range superlattice ordering process by means of electrostatic self-assembly. Necessary conditions in the thermodynamics are a high surface charge density on microspherulite surfaces, a long Debye length and an appropriate number density of nucleation centres. A further chemical requirement is a high alkaline environmental pH from 9 to 10. It is also proposed that the characteristic concentric spherical shell-like structure of spherulites, centred on primary nuclei, are due to sequential deposition of intrinsic salts which precipitate out when the corresponding solubility limits in the liquid are successively exceeded. It can be that the better-known sedimentation of microspherulites under gravity only plays part in the final stabilization period of overall growth.