Opalisation of the Great Artesian Basin (central Australia): an Australian story with a Martian twist

Silica Colloid Ordering in a Dynamic Sedimentary Environment

“…All of these models agree that gravity and short-range particle interaction potentials create structural order when uniform spheres form in a gel and sediment from or through it at quiescent conditions in sealed environments. This classical theory contrasts considerably with the abundant presence of opal-bearing hydraulic fractures, viscous colloid flow textures, opal reactivation structures, and nanoscale replacement processes in the subsurface precious opal deposits [1,6,7,11]. These characteristics demonstrate the highly dynamic environment that produces ordered arrays of uniform, X-ray amorphous silica spheres.…”

“…Several authors have suggested that opal forms from a gel [1,4,5,10,11]. At quiescent conditions in sealed environments, uniform spheres are expected to grow inside this gel and settle gravitationally to form a regular sphere array, i.e., precious opal.…”

“…From a total of 82 samples from Andamooka and 13 samples from Mintabie, we selected 22 and 6 samples, respectively, based on textural and mineralogical characteristics, for in-depth analyses. Andamooka opal samples are associated with the deeply weathered early Cretaceous sediments of the Marree Subgroup [1,12]. Samples were extracted from the bleached Early Cretaceous Bulldog Shale (~20 m beneath the surface) in the Teatree Flat field, which is located about 15 km northwest of the Andamooka Township.…”

“…In precious opal-A, uniform spheres form a regular three-dimensional array that diffracts visible light, giving the characteristic play-of-color [8,9], which is absent in common opal-A. So far, the formation of ordered sphere lattices in fractures and pores of the host rocks and during mineral replacement has been attributed to gravitational sphere settling [1,4,5,10,11]. All of these models agree that gravity and short-range particle interaction potentials create structural order when uniform spheres form in a gel and sediment from or through it at quiescent conditions in sealed environments.…”

“…The hydrated mineralogical assemblages of the Great Artesian Basin (central Australia) record the acidic oxidative weathering of volcaniclastic sediments on a multimillion-year time scale [1]. Chemical weathering of the silicate rocks induces the in situ formation of amorphous silica at silicate mineral surfaces [2] and the release of silica into solution, followed by amorphous nanoparticle precipitation through inorganic processes [3].…”