Composition, structure and origin of Lightning Ridge black opal

Submission note: A thesis submitted in total fulfilment of the requirements for the degree of Doctor of Philosophy to the School of Engineering and Mathematical Sciences, Faculty of Science, Technology and Engineering, La Trobe University, Bundoora.

Gemstone opal from multiple regions around the world has been investigated in comparative studies and via in depth studies focussed on single or related regions. The composition, structure and origin of Lightning Ridge Black Opal have been considered by a number of research teams from the 1960’s to the present time. Major questions remain as to the nature of these gemstones. The complexity and heterogeneity of the material, even within a single opal deposit in a small local region, and the formation mechanisms have not been adequately documented and explained. This study aims to provide a description of a broad set of opal material whose provenance is meticulously documented. From this description an integrated view of the origin of Lightning Ridge opal will be presented. Lightning Ridge black opal (32 discrete samples) has a typical composition comprising approximately 91.79 wt percent silica, approximately 5.96 wt percent volatile materials and traces of other elements, assembled in an X-ray amorphous structure, thereby classifying the material as OpalA. The role of water and silanol groups in the amorphous structure have been explored together with the structural transformation into cristobalite. Infrared studies were used to identify of the silica components comprising the ubiquitous silica spheres in common opal and gemstone opal and the surrounding silica cement. Previous publications have illustrated the differences between silica cement and silica spheres through the variations in the etching behaviour of the both components with hydrofluoric acid. In this work, the role of molecular water and silanol groups as a potential catalyst to accelerate the hydrofluoric acid reaction was outlined. This indicates that the appearance of nutshell-like structures in some etched opal samples arises from a greater concentration of hydroxyl and water in the centre of these spheres. Opal found at Lightning Ridge shows a variety of colours, ranging from honey translucent to dark grey and black. The composition variation of trace elements shows an increased concentration of SO2, F e2O3, P b, Cu and U (and possibly pyrite and other trace elements) with increasing darkness of the sample. Optical microscopy studies showed an increased level of pigmentation with a swirly or sponge-like distribution within the translucent opal material. Microbial organisms were observed with optical microscopy in minor quantities and categorised as filamentous bacteria, such as Actinomycetes. In addition, carbon concentrations were detected in the order of 0.03 wt percent with radiocarbon ages corresponding to the Pleistocene epoch. These radiocarbon ages are in contrast to the proposed formation of Lightning Ridge opal in the Holocene epoch (less than 11,700 yrs BP ago). This core finding was verified with more than 20 radiocarbon dating experiments and provides strong evidence for carbon of Late Pleistocene age or potentially of earlier formation.