The Guide to Rocks and Minerals

In focus: Zoisite

Chemical Composition: Ca2Al3Si3O12(OH)
Group: Epidote Group
Colour: Gray, Apple green, Brown, Blue, Rose red
Streak: white
Hardness: 6
Density: 3.2-3.4
Luster: Vitreous
Cleavage: Perfect
Acid sensitive: yes

Zoisite has a complex chemical structure. It contains single silicate tetrahedrons, SiO₄, and double silicate tetrahedrons, Si₂O₇. This chemical composition is characteristic of all rocks which are known under generic name of epidote. Other members of epidote group are: clinozoisite, epidote per se, piemontite, and allanite. Zoisite takes its name from its discoverer - Siegmund Zois, Baron von Edelstein (1747-1819), who supplied the the first specimens

Zoisite is mainly found in in medium-grade regionally metamorphosed crystalline schists formed from igneous, sedimentary, or metamorphic rocks relatively high in calcium. It is frequently associated with albite, andradite, biotite, calcite, corundum, garnet and quartz. The main sources of Zoisite come from Tanzania, USA, Switzerland, India and Austria.

Zoisite varieties

Anyolite: Anylite is often referred to as Ruby-Zoisite. It is a gem-rock variety consisting of apple-green Zoisite and Ruby-red Corundum. Ruby-Zoisite is unusual because of marked differences of hardness of its chief constituents (Ziosite - 6.5; Corundum - 9) which in turn suggests that the fusion between these two rocks is difficult. The name Anylite apparently comes from the Masai (of Tanzania) word for green.
Tanzanite: Tanzanite was discovered in 1967. It is a very rare blue gemstone, which created a storm among mineralogists and gemologists when it was discovered to be a Zoisite (non-gem rock) variety. The blue-lavender colour of Tanzanite is unique and sets it apart from other gemstones.
Thulite: Thulite is a pink variety (pink-rose to purple) of Zoisite. It occurs as large rocks and is often broken by jewellers into beads and cabochons. Thulite is named after "Thule", a former name for Norway

Mineral uses

for ornamental stonework
for jewellery (especially Tanzanite, but also Thulite)
as a jade substitute
as a collector's mineral

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