The crystallite size and density of agates from three Western Australian host rocks aged 1.84, 2.72 and 3.48 Ga have been determined. Infrared and Raman spectra from these ancient agates have respectively shown minimal water and zero moganite content. Data comparison of agates from Western Australian and younger hosts showed an advanced crystallisation in the Western Australian samples with, in some instances, the crystallite size comparable to macrocrystalline quartz. Low-grade metamorphism is present in the host rocks and localised heating is one possible cause of the increased crystallite growth. Micrographs of one agate sample from the Warrawoona Group (3.48 Ga) show a breakdown in the fibrous chalcedony; regional variations within the agate were supported by data from infrared and X-ray diffraction. It has been shown that part of the sample had undergone a transformation of the fibrous chalcedony to granular microcrystalline quartz. The Crystallinity Index for microcrystalline quartz, based upon the degree of resolution of the (212) quartz reflection, offers a rapid method to identify abnormal crystallite growth that could serve as a palaeoindicator of host-rock metamorphism.
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Moxon et al. (2006) studied this question.