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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.
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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.
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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.
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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.
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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.
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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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