Cordierite
| Cordierite |
|
| General |
| Category |
Silicate mineral |
Formula
(repeating unit) |
(Mg,Fe)2Al4Si5O18 |
| Strunz classification |
09.CJ.10 |
| Dana classification |
61.02.01.01 Cordierite group |
| Crystal symmetry |
2/m 2/m 2/m Orthorhombic - Dipyramidal |
| Unit cell |
a = 17.079 Å, b = 9.730 Å, c = 9.356 Å; Z = 4 |
| Identification |
| Color |
Blue, smoky blue, bluish violet; greenish, yellowish brown, gray; colorless to very pale blue in thin section |
| Crystal habit |
Pseudo-hexagonal prismatic twins, as imbedded grains, and massive |
| Crystal system |
Orthorhombic - Dipyramidal Space Group: C ccm |
| Twinning |
Common on {110}, {130}, simple, lamellar, cyclical |
| Cleavage |
Fair on {100}, poor on {001} and {010} |
| Fracture |
Subconchoidal |
| Tenacity |
Brittle |
| Mohs scale hardness |
7 - 7.5 |
| Luster |
Greasy or vitreous |
| Streak |
White |
| Specific gravity |
2.57 - 2.66 |
| Optical properties |
Usually optically (-), sometimes (+); 2V = 0-90° |
| Refractive index |
nα = 1.527 - 1.560 nβ = 1.532 - 1.574 nγ = 1.538 - 1.578 Indices increase with Fe content. |
| Pleochroism |
X = pale yellow, green; Y = violet, blue-violet; Z = pale blue |
| Fusibility |
on thin edges |
| Diagnostic features |
Resembles quartz can be distinguished by pleochroism. Can be distinguished from corundum by its lower hardness |
| References |
[1][2][3][4] |
Cordierite (
mineralogy) or
iolite (
gemology) is a
magnesium iron aluminium cyclosilicate. Iron is almost always present and a
solid solution exists between Mg-rich cordierite and Fe-rich
sekaninaite with a series formula: (
Mg,
Fe)
2Al3(
Si5AlO18) to (
Fe,
Mg)
2Al3(
Si5AlO18).
[2] A high temperature polymorph exists,
indialite, which is
isostructural with
beryl and has a random distribution of Al in the (
Si,
Al)
6O18 rings.
[3]
Crystal structure of Cordierite
|
Name and discovery
Cordierite, which was discovered in 1813, is named after the
French geologist Louis Cordier (1777–1861).
[2]
Occurrence
Cordierite typically occurs in contact or regional
metamorphism of argillaceous rocks. It is especially common in
hornfels produced by contact metamorphism of
pelitic rocks. Two common metamorphic mineral assemblages include
sillimanite-cordierite-
spinel and cordierite-
spinel-
plagioclase-
orthopyroxene. Other associated minerals include
garnet (cordierite-
garnet-
sillimanite gneisses) and
anthophyllite.
[4][5] Cordierite also occurs in some
granites,
pegmatites, and
norites in gabbroic magmas. Alteration products include
mica,
chlorite, and
talc. Cordierite occurs in the granite contact zone at
Geevor Tin Mine in
Cornwall.
Commercial use
Catalytic converters are commonly made from
ceramics
containing a large proportion of synthetic cordierite. The
manufacturing process deliberately aligns the cordierite crystals to
make use of the very low thermal expansion seen for one axis. This
prevents thermal shock cracking from taking place when the catalytic
converter is used.
[6]
Gem variety
As the transparent variety iolite, it is often used as a
gemstone. The name "iolite" comes from the Greek word for violet. Another old name is
dichroite, a Greek word meaning "two-colored rock", a reference to cordierite's strong
pleochroism.
It has also been called "water-sapphire" and "Vikings' Compass" because
of its usefulness in determining the direction of the sun on overcast
days, the Vikings having used it for this purpose.
[7] This works by determining the direction of
polarization
of the sky overhead. Light scattered by air molecules is polarized, and
the direction of the polarization is at right angles to a line to the
sun, even when the sun's disk itself is obscured by dense fog or lies
just below the horizon.
[8]
Gem quality iolite varies in color from
sapphire
blue to blue violet to yellowish gray to light blue as the light angle
changes. Iolite is sometimes used as an inexpensive substitute for
sapphire. It is much softer than sapphires and is abundantly found in
Australia (Northern Territory), Brazil, Burma, Canada (Yellowknife area
of the Northwest Territories), India, Madagascar, Namibia, Sri Lanka,
Tanzania and the United States (Connecticut). The largest iolite crystal
found weighed more than 24,000 carats, and was discovered in Wyoming,
US.
[9]
-
Left: rough specimen showing dichroism; right: cut stone
-
Facet cut Iolite gemstone
-
Another name for blue iolite is Steinheilite, after
Fabian Steinheil, the Russian military governor of Finland who observed that it was a different mineral from
quartz.
[10]
Fórmula Química - CuO2
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