Pleochroic halo

<p>A pleochroic halo, or radiohalo, is a microscopic, spherical shell of discolouration (<a href="/facts/Pleochroism/EJJclyzk">pleochroism</a>) within minerals such as <a href="/facts/Biotite/9LuUBXm3">biotite</a> that occurs in <a href="/facts/Granite/G2evuVKo">granite</a> and other <a href="/facts/Igneous_rocks/xeMJhUah">igneous rocks</a>. The halo is a zone of radiation damage caused by the inclusion of minute <a href="/facts/Radioactive/Ya6FVjt4">radioactive</a> crystals within the host crystal structure. The inclusions are typically <a href="/facts/Zircon/84APfbLt">zircon</a>, <a href="/facts/Apatite/f5tYvl1n">apatite</a>, or <a href="/facts/Titanite/h9l6Xqt8">titanite</a> which can accommodate <a href="/facts/Uranium/n2sGWBzU">uranium</a> or <a href="/facts/Thorium/ysdWH470">thorium</a> within their <a href="/facts/Crystal_structure/bJ4bCeHd">crystal structures</a>.  One explanation is that the discolouration is caused by <a href="/facts/Alpha_particle/dzNMs9WP">alpha particles</a> emitted by the nuclei; the radius of the concentric shells are proportional to the particles' energy.
</p>

Pleochroic halo open-in-new

Pleochroic halo