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Microstructural changes accompanying the opal‐A to opal‐CT transition: new evidence from the siliceous sinters of Geysir, Haukadalur, Iceland
Author(s) -
JONES BRIAN,
RENAUT ROBIN W.
Publication year - 2007
Publication title -
sedimentology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.494
H-Index - 108
eISSN - 1365-3091
pISSN - 0037-0746
DOI - 10.1111/j.1365-3091.2007.00866.x
Subject(s) - geology , sedimentary rock , diagenesis , geothermal gradient , mineralogy , quartz , calcite , scanning electron microscope , geochemistry , paleontology , materials science , composite material
Opaline silica (opal‐A) has formed in marine, lacustrine and geothermal environments throughout geological time. During diagenesis opal‐A normally changes to opal‐CT, then opal‐C, and finally to quartz. Such changes commonly destroy the original fabrics and any fossils that opal‐A contained. The physical changes that accompany the opal‐A to opal‐CT transition, however, are known poorly. X‐ray diffraction analyses, electron microprobe analyses and high‐resolution, high‐magnification scanning electron microscope imagery of siliceous sinters from the Geysir geothermal area in Iceland show that opal‐A is formed of heterometric arrays of randomly packed microspheres (up to 5 μ m diameter) with neighbouring spheres commonly being joined by small connection pads. In contrast, enlarged spheres, lepispheres, inverse opal (two types) and spindle frameworks with hexagonal motifs characterize opal‐CT. The textures in opal‐CT, which vary on a microscale, reflect the complex interplay between dissolution (e.g. inverse opal) and precipitation (e.g. enlarged spheres, spindle frameworks) that probably was mediated by groundwater in a near‐surface environment. The processes deciphered from these young rocks should, however, be applicable to sedimentary opal‐A and opal‐CT of all ages, irrespective of their origin.