Tuesday, 2 October 2018

Seeing the effect of radiation damage

The rock matrix contains quartz, plagioclase feldspar, microcline feldspar and biotite mica. The greenish brown flake of biotite shown in the centre exhibits many dark brown circular features resembling cigarette-type burns. These features are known as pleochroic halos and are a diagnostic feature of biotite. These halos are interesting as they are isotropic compared to the surrounding material which is anisotropic. The halo in the centre of field shows a small brightly coloured crystal of zircon which causes the dark brown effect due to a process called metamictization. This process is the radioactive decay and destruction of the crystal structure surrounding the zircon and the affected area becomes amorphous and hence isotropic.

Zircon is a well-known mineral of many colours with a brilliant lustre with good hardness to make it a desirable gem. In its natural setting it contains slight traces of radioactive elements which cause metamictization. The level of radioactivity is so small and poses no risk when used in jewellery.

MOTIC equipment used: B1 Series microscope with attachable rotating stage and polarising/analyser accessories - Plan 4X achromatic objective with 10X WF 18mm eyepieces and Moticam X camera.

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