Tanzanite Crystallography


11th October 2016


A brief look at Tanzanite crystallography and crystal habits with photography and videos being the focus. We look at various colors of Tanzanite (Zoisite) with some wonderful specimens and crystals


Tanzanite is the blue / violet variety of the mineral species Zoisite.  It is a Calcium Aluminium silicate (chemical compositionCa 2 Al 3 Si 3 O 12 (OH) ).   It forms in the Orthorhombic crystal system. Its crystal habit tends to include elongated, prismatic crystals with a prominent pinacoid with usually poorly developed terminations. Crystals can also occur in massive and granular form. Bladed crystals are also fairly common. Cleavage is good in one direction lengthwise and fracture is uneven to conchoidal.

This article will look at common crystal habits for Tanzanite using photography and videos of notable specimens we have seen over the many years we have been operating at the source.

Crystal Tanzanite006.jpg
 

This Tanzanite crystal above, shows typical striations on its side and is poorly terminated as is so often the case. Excellent color saturation. The video of this crystal in a later section of this article shows it in white and incandescent light highlighting the effect of the different spectrums on Tanzanite's trichroic nature. 

Crystal Tanzanite

This crystal is a more classic Orthorhombic shape. It also displays a typical "blade" face on one side. It is still in matrix which is just visible on the right.

Crystal Tanzanite

Poorly terminated crystal   displaying a single "blade" on one side. 

Tanzanite Crystal 050.jpg

Tanzanite / Zoisite in its  more "classic" crystal habit. Nicely terminated with typical striations. Well saturated crystal.

Pink Tanzanite Crystal.jpg

A very rare pink Tanzanite (Zoisite) crystal in classic orthorhombic form. The color is highly saturated with a slight orange modifier. 

Yellow Tanzanite Crystal.jpg

Yellow is not a color normally associated with Tanzanite / Zoisite but occasionally we get to see some really wonderful specimens as the photograph above bears witness to. Classic orthorhombic crystal habit.

Parti Colored Tanzanite Crystal.jpg


This is an extremely unusual specimen we saw recently mined in Block D in the Lelatema Mountains. It appears to show a parti color. Some green and some pink. It is necessary to angle the crystal perfectly for this effect to occur which may be as a result of twinning.

Green Tanzanite Crystal.jpg

Tanzanite (Zoisite) is sometimes seen in the market with rare green hues.  In white light, the green color comes with a tinge of blue as shown in this beautiful green Tanzanite (Zoisite) crystal.  Nicely terminated.  

TZ Huge 1.JPG

This very rare 160 gram piece was seen in our offices recently. In addition to it being such a large crystal, the fibre light does a great job in bringing out the trichroism in the stone. Some classic striations too. 


Tanzanite Crystal Videos

have created a few videos of some of the more interesting crystals we have seen recently. These are a bit more interactive and dynamic than still photography.

The video above does a great job of showing off Tanzanite's trichroism. The initial revolution shows the crystal under white light (approximately 1500 kelvin, simulating north daylight. The second revolution is the same crystal under incandescent light showing the notable change in color from blue to violet as the perpedicular pleochroic axis is "activated" by the new light source.

This video shows a very unusual Tanzanite crystal or group of crystals in a fan shape. Definitely not something you see every day! 

A classic orthorhombic Tanzanite crystal with a typical blade-like face still in its host rock, likely a mixture of Kyanite and Graphite.

In Summary

"Tanzanite is found at only one location on Earth, the western slopes of the Lelatema mountain range ~60 km south-southeast of Mount Kilimanjaro in northeastern Tanzania. The Lelatema Mountains form part of the Eastern Granulite Complex of the Mozambique Orogenic Belt. Tanzanite mineralization resulted from a prolonged geologic history, and it shows the delicate interrelationship between primary deposition, diagenesis, metamorphism, structural geology, and geochemistry to create one of nature's most remarkable gems." Source: Gems & Gemology . 2006, Vol. 42 Issue 3, p150-150. 1/2p Author(s): Scheepers, Reyno. 

I hope the pictorial display we have provided here does justice to that remark in representing the wonderful diversity of Tanzanite / Zoisite crystal habits.

 

Copyright Lapigems 2016


 

Lapigems Gem Company