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Oliwiny
- Peridot
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Oliwiny
– grupa minerałów zaliczana do
krzemianów.
Mają zazwyczaj barwę zieloną w odcieniach, ale też brązową, czarną a
wyjątkowo są białe lub bezbarwne.
Ogólny wzór chemiczny oliwinów to (Mg, Fe)2 SiO4.
Oliwiny są głównie krzemianami:
magnezu
(forsteryt),
żelaza
(fajalit)
oraz
manganu
(tefroit)
i znacznie rzadziej
niklu.
Często zawierają
izomorficzne
domieszki
chromu,
glinu,
tytanu,
sporadycznie też:
wapnia
– monticellit i kirschtenit.
Większość z nich stanowi
roztwory stałe
(kryształy mieszane), będące członami szeregu izomorficznego:
forsteryt
–
fajalit
(oliwiny magnezowo -żelazowe).
wszystkie oliwiny krystalizują w
układzie rombowym.
tworzą zazwyczaj
kryształy
słupkowe i tabliczkowe
występują w
skupieniach
ziarnistych, zbitych, czasami występują jako pojedyncze ziarna,
niekiedy występują w postaci ziarnistych skupień zwanych „bombami
oliwinowymi”,
są kruche i stosunkowo twarde,
są bardzo mało odporne i szybko ulegają przeobrażeniu w różne
minerały z grupy
serpentynu,
czasami wykazują specjalne zjawiska optyczne:
efekt kociego oka
i
asteryzm.

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Przeważnie występują w zasadowych i ultrazasadowych skalach
magmowych:
perydotytach,
dunitach,
iherzolitach,
bazaltach
oliwinowych,
melafirach
i
diabazach.
oliwiny zasobne w magnez lub mangan (forsteryt i tefroit) są
składnikami różnych utworów zmetamorfizowanych (żelaza, manganu,
skarnów, metasomatytów itd.),
oliwiny występują w niektórych meteorytach,
w niektórych
serpentynitach
jako minerał reliktowy
w niektórych skałach okruchowych: piaskach i żwirach.
Miejsca występowania:
Egipt
– kryształy ponad 5 cm;
Kenia;
Tanzania;
Etiopia;
USA;
Meksyk;
Czechy;
Rosja
– kryształy
fajalitu
ok. 10 cm (Kołyma), kryształy
forsterytu
ok. 16cm (Ural);
Polska
– na Dolnym Śląsku (w trzeciorzędowych bazaltach – najczęściej jest
to
chryzolit).
Peridot

The vivid,
slightly golden shimmering green of Peridot is the ideal gemstone
colour to complement a light summertime outfit. This is no surprise
– Peridot, after all, is assigned to the summer month of August.
Peridot is an ancient and yet currently very popular gemstone. It is
so old that it can be found even in Egyptian jewellery from the
early second millennium BC. The stones used in those days came from
an occurrence on a little volcanic island in the Red Sea, about 70
km off the Egyptian coast, off Assuan, which was rediscovered only
around 1900 and has been completely exploited since. Peridot,
however, is also a very modern stone, for only a few years ago
Peridot occurrences were discovered in the Cashmere region, and the
stones from there show a unique beauty of colour and transparency,
so that the image of the stone, which was somewhat dulled over the
ages, has received an efficient polishing.
The ancient
Romans were already quite fond of the gemstone and coveted the
brilliant green sparkle, which does not change either in artificial
light. They already named the stone “Evening Emerald”. Peridot is
found in Europe in many medieval churches decorating several
treasures, like, for example, in the Cologne Cathedral. In the era
of Baroque the deep green gemstone experienced another short
flourishing, before it became forgotten.
Spectacular “Cashmere
Peridot”
But suddenly, around the middle of the 1990s, Peridot was the great
sensation on the Gemstone Trade Fairs all around the world. The
reason: In Pakistan there had been found a sensationally rich
occurrence of finest Peridot on a rough mountainside, in about 4,000
m height. The extremely hard climatic conditions only allowed mining
to go on through the summer months, and yet the unusually large and
fine crystals and rocks were brought down into the valley. These
stones were of finer quality than anything else ever seen before,
and the occurrence proved so rich that the high demand can be met
without problems at present.
In order to underline the outstanding quality of such Peridot from
Pakistan the stones have been termed “Cashmere-Peridot”, reminding
of the fine Cashmere Sapphires. Creative gemstone cutters have in
fact succeeded to create fascinating and beautiful unique stones of
over 100 karats from some of the larger and fine crystals in a deep
and breathtakingly beautiful green.
The depth of green depends on
iron
The gemstone is actually known under three names: Peridot,
Chrysolith (derived from the Greek word “goldstone”) and Olivin,
because Peridot is the gemstone variety of the Olivin mineral. In
the gemstone trade it is generally called Peridot, a name derived
from the Greek “peridona”, meaning something like “giving plenty”.
Peridot is one of the few gemstones which exist only in one colour.
Finest traces of iron account for the deep green colour with a
slight golden hue. Chemically Peridot is just an
iron-magnesium-silicate, and the intensity of colour depends on the
amount of iron contained. The colour as such can come in any
variation from yellow-green and olive to brownish green. Peridot is
not especially hard – it only achieves about 6.5 to 7 on the Mohs´
scale – and yet it is easy to care for and quite robust. Very rare
treasures indeed, however, are Peridot-Cat’s Eye and Star-Peridot.
The most beautiful stones come from the Pakistan-Afghanistan border
region. Peridot as gemstone does also exist in Myanmar, China, the
USA, Africa and Australia. Stones from East Burma, today’s Myanmar,
show a vivid green with fine silky inclusions. Peridot from the
American state of Arizona, where it is quite popular in Native
Indian jewellery, often shows a yellowish to golden brown shade.
Uncomplicated – but not for the
cutter
Peridot is cut according to its crystal structure, usually in
classical table and facetted cuts, round, antique, octagonal or oval
shaped. Smaller crystals are cut as calibrated stones, larger ones
are shaped by gemstone designers to fancy unique specimen stones.
The material which is rich in inclusions is worked as cabochons,
because this shape will provide the best effect for the fine silky
inclusions.
Gemstone cutters know that this stone is not easy to process. The
rough crystals can be devious and are easy to break. The tensions
existing inside the crystal are often quite considerable. When the
cutter has removed the most disturbing inclusions, however, Peridot
is a jewellery stone which is excellently suited to daily wear,
without requiring special care.
Ideal summer stone
Peridot is a gain for the green gemstone’ colour palette. There is
trend to use it not only as individual stone, but also in jewellery
series. And since the world of fashion has just discovered a
preference for the colour green, the popularity of this deep green
gemstone has increased accordingly.
And the rich occurrences in Pakistan and Afghanistan have provided
the market with sufficient raw material, so that the individual
taste and each budget can be met. But if the “right” stone for you
is a large and transparent one, intensely coloured, be prepared:
they are quite rare and valuable. Peridot is a gemstone which one
should definitely get to know. Its fine pistachio green or olive
green ideally complements a light summertime outfit.
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