The Mineral Zoisite

Tanzanite is crystalline form of a mineral called Zoisite. The mineral Zoisite was named after the 18th century Italian born nobleman Baron Von Edelstein, otherwise known as Sigmund Zois. This eminent businessman, natural historian, and collector extraordinaire had at the core of his various interests, mineralogy.

His interest in mineral collecting was a direct result of his participation in the family metallurgy and mining business. Meeting many related experts in his field, Zois journeyed all over Europe financing many mineral-collecting expeditions. His passion resulted in the gathering together of one of the largest and most extensive mineral collections of his time, a collection that can be seen today at the Slovenian Natural History Museum.


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During his expeditions, Sigmund Zois had the occasion to make the acquaintance and earn the respect of many prestigious natural scientists such as the German mineralogists Abraham Werner, Martin Klaproth and Friedrich Mohs: known for his infamous hardness scale. According to legend, in 1804 a fellow mineralogist by the name of Simon Presern, presented Zois with a sample of an unknown mineral from the Svinska Planina (Saualpe Mountains), Carinthia in Austria. Zois wasted no time in conferring with his mineralogist friends Werner and Klaproth, who confirmed the new minerals discovery. In 1805 the mineral Zoisite was officially recognized.


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In the strictest of mineralogical terms Zoisite is member of a silicate sub-group called sorosilicates. Within the sorosilicates, Zoisite belongs to the epidote group along with clinozoisite, piemontite, and allanite. Chemically it is a Calcium-Aluminum Silicate Hydroxide with the formula Ca2AI0AI2(SiO4)(SiO7)(OH).

Zoisite occurs as prismatic, orthorhombic dipyramidal crystals. Orthorhombic crystals are often shaped like rhombic prisms or dipyramids, resembling two pyramids that have been stuck together. Zoisite often occurs in massive form in metamorphic and igneous pegmatite rock.


Metamorphic Rocks

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Metamorphic rocks, from Greek ‘Meta’ to ‘Change,’ and ‘Morph ‘to ‘Form,’ are the changes in form of pre-existing rock types deep beneath the Earth's surface. These rocks have been subjected to extreme heat, pressure and the intrusion of molten rock, causing their physical and chemical properties to change. These rocks are then exposed to the surface either by uplift or erosion. Mineral and gemstone crystals that have formed within metamorphic rocks are sometimes eroded, and swept away by rains into alluvial deposits such as rivers, and streams from where they are sifted out by hand.

Igneous Rocks

Igneous rocks, from Latin ‘Ignis’ meaning ‘Fire,’ form as a result of magma rocks solidifying below, or on, the Earth’s surface. As the molten rocks cool, diverse minerals crystallize at varying points as the temperature descends: silicon, oxygen, aluminum, sodium, potassium, calcium, iron, and magnesium. These elements combine to form the silicate minerals, which account for over ninety percent of all igneous rocks. Pegmatites are very coarse-grained igneous rocks that are formed from magma that cools quickly. Pegmatites usually form veins within granite, in what is called a Pegmatite dike. These dikes often contain pockets of rare minerals and gemstones such as aquamarine, tourmaline, topaz, fluorite, and of course the silicate Zoisite and its Tanzanite variety.

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Zoisite occurs in colorless, white, gray, brown, yellow, green, blue, violet and pink. It has a vitreous pearly luster with a hardness of between 6.5 and 7 on the Moh’s hardness scale. It has a perfect one directional cleavage that runs parallel to the principal axis.

 

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Cleavage

A few centuries ago, a phenomenon within some gems was figuratively described as ‘Folia.’ Taken from the Latin, meaning ‘Leaves’, this described the visible layering of crystals within a gems structure: Today, this structure is known as cleavage. This leaf-like layered formation, occurring within a gem during its re-crystallization, happens by the process of applied one-directional pressure from the surrounding host rocks. This causes platy, or lengthened crystals to grow with their long axis perpendicular to the direction of the exerted force. Imagine the wind blowing from one direction on a field of long grass, then reproduce the image thousands of times over, layered one on top of the other.


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Because the pressure forms the crystals structure to lie in one directional plane, it creates a plane of cleavage, making the rock brittle. Slate is a good example of ‘Foliated’ rock having a strong cleavage, while marble which has had pressure exerted on it from all sides during its formation is a good example of ‘Non-Foliated’ rock. The cleavage defines how the rock should be cut: ‘Foliated’ rocks have to be cut perpendicular (at right angles) to their line of cleavage otherwise they will break in splinters, whereas as a ‘Non-Foliated’ can be cut at all angles.


Principal sources of Zoisite are Tanzania (Tanzanite), Kenya, Norway, Switzerland, Austria, India, Pakistan, and the USA. Apart from Tanzanite, Zoisite produces two other striking coveted varieties: Anyolite and Thulite, one found in East Africa the other in Norway.


Anyolite & Thulite

This variety of Zoisite, sourced from deposits in Kenya and Tanzania, is an apple green, red and black opaque gem-rock used in gem carvings and sculptures. Anyolite derived its name from the Maasai tribe of East Africa and their word for green: ‘Anyoli.’ Anyolite first discovered near Longido in northeastern Tanzania is also known as ‘Tanganyika Artstone,’ and ‘Ruby-Zoisite.’

Anyolites are rich metamorphic rocks comprising of red corundum, enclosed by green Zoisite with dark amphibole, generally termed as hornblende. The Zoisite in Anyolite is usually granular and appears sugary, the red corundum is a medium to low-grade ruby exhibiting fair to good hexagonal crystal outlines, the black amphibole is evident as spots or streaks throughout the Anyolite crystal structure.


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Thulite, mined from the deposits of Austria, Australia, the U.S. and Norway, is an opaque, pink variety of the mineral Zoisite. Thulite was first discovered in Lom, Norway in 1820, and was so named after the mythical island of Thule. Thule according to Greek legend was a country to the north of Britain, probably Norway, believed at that time to be the northernmost part of the world at the extreme limit of travel and discovery. Thule was in fact a region in northern Greenland populated by an ancient Paleo-Eskimo civilization: The forefathers of today’s Inuit people of Greenland.


Thulite, occurring within metamorphic and pegmatite rock, is an opaque gem-rock used in carving, with finer specimens sometimes being cut ‘En Cabochon’ for use in jewelry. Thulite gains its coloration from manganese combined with white calcite that gives it an overall pink hue, earning itself the names of ‘Pink Zoisite’ and ‘Rosaline.’ Under certain frequencies of long wave ultraviolet Pink Thulite fluoresces a beautiful yellow-orange color.

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