Cut Zoisite (Tanzanite) gem, fluorescence & transmission

The gem, 5mm long, is photographed through a light yellow filter that removes most - but not all - of the scattered/reflected 404nm laser beam that excites the green-orange fluorescence.

The orange line spectrum above is the white light transmission of the crystal showing the blue bump at 435nm that gives the stone its beautiful light blue colour. The red bump, peaking at 680nm gives it a slightly lavender tint.

In the fluorescence spectrum, the bright peaks between 670 and 800nm are due to Vanadium (V^2+) while the narrow emissions between the green and the orange red are from the Lanthanide rare earths. The two fluorescence spectra shown result from the excitation by 404nm (violet line) and 532nm (green line) lasers. Only the violet laser significantly excites the Lanthanide lines. The V^2+ emission is analgous (isoelectronic) to the Cr^3+ fluorescence seen in ruby, emerald and many other minerals. There should be a similar behaviour with Mn^4+ which is in the same isoelectronic series and of similar size.

The mineral Zoisite is a variety of Epidote which is a crystal comprising Ca_2 Al Al_2 O(OH)(Si_2 O_7)(SiO_4). The stone is pleochroic with the colours light blue, dark blue and purple.



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