Amorphous silica maturation in chemically weathered clastic sediments

作者：Liesegang Moritz^*; Milke Ralf; Berthold Christoph

来源：Sedimentary Geology, 2018, 365: 54-61.

DOI：10.1016/j.sedgeo.2018.01.001

• Silica
• Diagenesis
• Opal-A
• Opal-CT
• Australia
• X-ray diffraction (XRD)

A detailed understanding of silica postdepositional transformation mechanisms is fundamental for its use as a palaeobiologic and palaeoenvironmental archive. Amorphous silica (opal-A) is an important biomineral, an alteration product of silicate rocks on the surface of Earth and Mars, and a precursor material for stable silica phases. During diagenesis, amorphous silica gradually and gradationally transforms to opal-CT, opal-C, and eventually quartz. Here we demonstrate the early-stage maturation of several million year old opal-A from deeply weathered Early Cretaceous and Ordovician sedimentary rocks of the Great Artesian Basin (central Australia). X-ray diffraction, scanning electron microscopy, and electron probe microanalyses show that the mineralogical maturation of the nanosphere material is decoupled from its chemical properties and begins significantly earlier than micromorphology suggests.
Non-destructive and locally highly resolved X-ray microdiffraction (mu-XRD2) reveals an almost linear positive correlation between the main peak position (3.97 to 4.06 angstrom) and a new asymmetry parameter, AP. Heating experiments and calculated diffractograms indicate that nudeation and growth of tridymite-rich nanodomains induce systematic peak shifts and symmetry variations in diffraction patterns of morphologically juvenile opal-A. Our results show that the asymmetry parameter traces the early-stage maturation of amorphous silica, and that the mineralogical opal-A/CT stage extends to smaller d-spacings and larger FWHM values than previously suggested.

• 出版日期2018-3