Metamorphic effects on agate found near the Shap granite, Cumbria, England: as demonstrated by petrography, X-ray diffraction and spectroscopic methods
Mineralogical Magazine, August 2007, Vol. 71(4), pp. 461–476
Agates from a 430 Ma host at Stockdale Beck, Cumbria, England have been characterized. The
crystallite size of the Stockdale Beck agates was found to be ~60% greater than any other agates from
five regions aged 400–1100 Ma. Raman spectroscopy identified moganite in all agates tested except
those from Stockdale Beck. Infrared spectroscopy showed that the silanol content of the Stockdale
Beck agates was near zero. The properties of agates from Stockdale Beck and the 1.84–3.48 Ga
metamorphosed hosts found in Western Australia were similar but different from agates found in other
hosts aged 400–1100 Ma. Cathodoluminescence demonstrates further differences between agates from
hosts aged 13–1100 Ma and those from Stockdale Beck and Western Australia. Agates from the latter
areas have a lower proportion of defects causing a red emission band (~660 nm) but an increased
proportion of defects causing blue (~470 nm) and orange (~640 nm) emission bands. Agates found in
hosts aged 13–1100 Ma are also differentiated from the Stockdale Beck and Western Australian agates
in a ternary plot of the relative intensities of violet to blue to orange emission bands. Single scans
producing this combination of colours are only found in the Stockdale Beck and Western Australian
agates. The properties shown by the Stockdale Beck and Western Australian agates demonstrate that an
agate or chalcedony infill can be used to identify post-deposition palaeoheating within a host rock.
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