Silica phases in sinters and residues from geothermal fields of New Zealand

Abstract

Five silica phases are major components of silica sinters, deposited from both near-neutral pH alkali-chloride and acid-sulfate thermal waters, and of silica residues formed at the surface of geothermal fields in New Zealand. In all cases, the initial silica is noncrystalline opal-A deposited commonly as microspheres that possess an underlying nanospherical substructure, upon different substrate templets, including microbes living in hot springs. Deposition may also occur monomerically upon earlier deposited silica. Following microsphere growth through Ostwald ripening, silica remains mobile throughout the postdepositional history of the sinter/residue deposits, resulting in a range of textures. These include the continuing growth of microspheres, the development of secondary microspheres and silica coatings, phase transformations, a reduction in sinter porosity, dissolution features, and late-stage deposition of drusy quartz and opal-A. The sinter mass attempts to achieve thermodynamic equilibrium through stepwise phase transformations (maturation): opal-A crystallises to paracrystalline opal-CT±opal-C, which recrystallises to microcrystalline α-quartz+moganite. No intermediate silica phases are produced, but gradual changes occur among different opal-A or opal-CT/-C phases. The phase maturation produces changes in particle densities, silanol water, and in X-ray powder response of the different silica phases, although the rates of change can be perturbed by heating, weathering, and dissolution of the sinter/residue. The properties of opal-A change little in a sinter/residue mass within the first 10,000 years, but reductions occur in the densities, silanol water, and X-ray scattering bandwidth of older sinters where opal-A can persist for up to 100,000 years. Eventually, opal-A transforms to opal-CT when silanol water is reduced sufficiently for enough –Si–O–Si– linkages to produce a crude diffraction-like X-ray response. The transformation is aided by heat, as happens among residues, or by the presence of phases such as carbonate that enhance inorganic alkalinity, whereas it can be inhibited by clays and organic debris. Three-dimensional short- and long-range order (crystallinity) becomes possible with the elimination of virtually all silanol water when the density increases to >2.6 g/cm 3. This density increase is accompanied by a reduction in volume, such that additional silica must be introduced to avoid shrinkage. More silica is required to account for reductions in porosity on sinter aging. The mobilisation, transport, and deposition of the required silica are driven by the differing solubilities and relative metastabilities of the successive phases; at each stage of maturation, a less-mature phase is dissolved, transported via diffusion as monomeric silica and precipitated as a more mature phase. The process is sustained by pore fluid through a network of silica sources and sinks within the sinter/residue mass, but it does not operate uniformly at either macroscopic or microscopic scales. Silica phase heterogeneity of outcrop is the norm until thermodynamic equilibrium is achieved, and two or more phases commonly coexist. Sinter maturation can be modelled in terms of changes in phase physical properties with time. However, differences in transformation rates at different sites preclude such models being used to unravel outcrop chronologies.