Opal -Opalville Mine
Queensland,
Australia
Mineral Color
Presentation by Katryn Wiese
for Crystal Gazers March 16, 2011
Discovered in
1989, this is one
of the largest
(2765 carats)
and finest
quality boulder
opals ever
mined. This
opal is unique
not only for its
size but also for
its quality--every color of
the spectrum is
visible which is
extremely rare,
even in the
finest of opals.
amethyst
aventurine
quartz
citrine
agate
carnelian
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Sonar
X-Ray
Color in minerals =
how visible light waves interact
with electrons or structural
features within the crystal.
Opal
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VISIBLE LIGHT
= A TOOL TO LEARN ABOUT THE CRYSTAL
STRUCTURE AND COMPOSITION OF A MINERAL
Light can be:
incoming light
reflected
scattered
refracted
produced within
mineral (fluorescence
or phosphorescence)
MINERAL X
refracted again as
transmitted
scattered + reflected = LUSTER
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SCATTERING (BLUE FIRST, THEN GREEN, THEN RED)
Volcanic Lake
New Zealand
Pacific Ocean,
Hawaii
Scattering and absorption
explain water colors.
Cenote, Yucatan
4
Water droplets act as prisms and split white
light into its different wavelengths
(as each is bent differently
REFRACTION (bends light)
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Minerals have color
because some
wavelengths of light
are absorbed.
The color we see is a
combination of the
wavelengths that make
it to our eyes.
Rhodochrosite, from the Sweet Home Mine, Colorado
Calcite
Transparent minerals
absorb NO visible light
and have no color.
They reflect and
transmit all light.
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OPAQUE
minerals
transmit no
light –
absorbing all
but the reflected
colors
Gold from the Natural History
Museum -- New York City
OPAQUE WHITE minerals transmit no light –
and with an albedo (reflection coefficient) of
about 85%, they are nearly perfect reflectors of
all light (WHITE) with little to no absorption.
White apophyllite from Mexico
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OPAQUE BLACK minerals transmit no light –
and with albedo of almost 0, they are nearly
perfect absorbers of all light (no reflection).
Pyroxene, Titanite
Mendig, Eifel Mts,
Rhineland-Palatinate,
Germany
2.12 mm group of
black Pyroxene
crystals with a yellow
Titanite group of
crystal on the
termination of the
biggest Pyroxene
crystal. Collection &
Photo M.Chinellato
Factors that can affect visible light waves (color):
•
•
•
•
•
Presence of major element essential to mineral composition
Presence of minor impurity
Occurrence of defects in crystal structure
Mechanical mixture of finely dispersed inclusions
Presence of finely spaced physical boundary
Fluorite
Uvarovite
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Factors that can affect visible light waves:
Formula (coloring
Color
Presence ofMineral
major element essential
to agent
mineral
composition
bold-faced)
malachite
dioptase
Turquoise colorAzurite
comes from Cu
green
Cu2CO3(OH)2
green
Cu6(Si6O18) 6H2O
blue
Cu3 (CO3) 2(OH)2
Cuprite
red
Cu2O
Sulfur
yellow
Rhodochrosite
pink
Rhodonite
pink
Vanadinite
orangy-red
Olivine color comes from Fe
S
MnCO3
Rhodochrosite color
comes from Mn
MnSiO3
Pb5(VO4) 3Cl
Uvarovite color comes from Cr
Minerals are made of atoms,
which are in turn made of
electrons, protons, and neutrons.
Electrons orbit the nucleus of an
atom and reside in energy shells
or levels. The most stable
configuration of an atom is to
hold all its electrons as close as
possible, and thus energy levels
are filled from the inside out.
The number of electrons can vary
from one instance of an element
to an other and the configuration
of its energy shells will affect its
COLOR.
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Atoms will share
and give/take
electrons from
neighboring atoms
to produce bonds
and make a solid
crystal.
These bonds affect the
number of electrons on
a particular atom/ion
AND the energy levels
of those electrons.
e-
e-
Energy of one wavelength (color) of visible light absorbed by
electron = orbital jump
Now visible light is missing this wavelength
and color has changed!
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What we said energy shells or “orbitals” look like.
Factors that can affect visible light waves:
Presence of minor impurity
Boron as a substitutional impurity: only one
percent of natural diamonds are of this type,
and most are blue to grey
Beryl
Color
Reason
Emerald
deep green
chromium, Cr3+
Aquamarine
light blue
Iron, Fe2+>Fe3+
Heliodor
yellow
Iron, Fe2+>Fe3+
Morganite
pink
Manganese, Mn2+
Bixbite
red
Manganese, Mn2+
diamonds rich in N3/N2 centers are
yellow in color.
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Similar colors but the wavelengths of
light that reach your eyes are
different combinations!
EMERALD
DIOPTASE
MUSCUEZ, COLOMBIA - Size is 1.2x0.7 and 1.3x0.4 cm.
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Titanium impurities lead to blue Sapphire – a form of Corundum: (Al2O3)
Chromium impurities lead to red RUBY – a form of Corundum: (Al2O3)
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Manganese causes
red and pink and
traces of Iron and
Titanium make the
stone green and blue.
ZONING caused by
changes in the
chemical
environment while
mineral is growing.
Watermelon
Tourmaline in
matrix from
Minas Gerais
Factors that can affect visible light waves:
Occurrence of defects in crystal structure
Isolated vacancy gives diamond green, or
sometimes even green–blue color
Diamond can be colored pink, red, or
brown owing to structural anomalies
arising through plastic deformation during
crystal growth
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Factors that can affect visible light waves:
Mechanical mixture of finely dispersed inclusions
milky quartz
white
minute fluid inclusions
greenish-blue chalcedony
greenish-blue
chrysocolla inclusions
carnelian chalcedony
orange
hematite or iron hydroxide, goethite
aventurine quartz
green
fuchsite (chrome bearing muscovite mica) included in colorless quartzite
moss agate chalcedony
colorless and dark green
chlorite and black manganese oxide inclusions
jasper
green or red
green or red clay mineral inclusions
fire agate chalcedony
brown with iridescence
iron oxide inclusions
Factors that can affect visible light waves:
Presence of finely spaced physical boundary
THIN FILM/LAYERS
Labradorite from Madagascar. 20cm.
DIFFRACTION GRATING
Quilpie boulder opal
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Blue Morpho Didius
Plagioclase Feldspar
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On area is grain of Plagioclase. On the top is big grain of biotite and in the button are
some Amphibole. Observed by 2 nikols. (Zoom in 100times, 1 millimetre corresponding
to 100 units on the scale)
CamScan 4 SEM. Secondary electron image. Original magnification x300. Beam 20 kV, working
distance 68 mm, tilt angle 60°. Au coated.
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Specimen of
labradorite from
Madagascar.
20cm.
Labradorite
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Moonstone
Opal
Opalville Mine
Queensland,
Australia
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Light Dispersed
by Diffraction
Grating
OPALS
Scanning electron microscope images show that
consist of transparent spheres of tightly packed silica. Spaces
between spheres contain air or water. If regularly arranged,
these spaces act as a diffraction grating, breaking visible white
light into separate colors.
Common opal
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Opal’s colors depend on the size of the
spheres and the voids. If you move the
stone, light hits the spheres from different
angles and you see different parts of the
interference pattern.
Blues and violets: smaller spheres -- less
than about 150 nm or 6.5 million lined up
over 1 millimeter.
Oranges and reds: larger spheres -- no
larger than about 350 nm or about 3
million lined up over 1 millimenter.
Precious opal
The more uniform the size of the spheres,
the more intense, brilliant, and defined the
color will be.
http://www.webexhibits.org/causesofcolor/15F.html
Section of fossilized tree limb that is now completely replaced by black opal.
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Precious Opal: Houston Museum of Natural Science, Houston, TX
QUILPIE BOULDER OPAL
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“Fire opal" is a transparent or
translucent opal ranging in color
from yellow to orange to bright red.
Fluorescent minerals
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Fluorite
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Fluorite -- Rogerley, England
Rogerley Mine, Rogerley Quarry,
Frosterley, Weardale, North Pennines, Co.
Durham, England, UK
Dallas Gem Mine (Benitoite
Mine; Benitoite Gem Mine; Gem
Mine), Dallas Gem Mine area,
San Benito River headwaters
area, New Idria District, Diablo
Range, San Benito Co.,
California, USA
Benitoite is a rare
blue barium
titanium silicate
mineral, found in
hydrothermally
altered serpentinite.
Benitoite fluoresces
under short wave
ultraviolet light,
appearing bright
blue to bluish white
in color. The more
rarely seen clear to
white benitoite
crystals fluoresce
red under longwave UV light.
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Double refraction
COLOR PROVIDES BEAUTY AND
DETAILS ON THE INNER
WORKINGS!
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Plagioclase, Hornblende, Quartz and Biotite in a Gneiss from near Flin Flon, Manitoba
Properties and Features Seen in Thin Section (field of view about 2.5 mm)
Seen with plane polarized light (PP)
Opaque, Isotropic and Anisotropic
Minerals
Color and Pleochroism
Relief
Bubbles
Cleavage
Seen with cross-polarized light (XP)
Birefringence and Interference colors
Twinning
Exsolution
Pleochroic Halos
http://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin/plagioclase.htm
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Example of Interference Colors: Calcite and Diopside
This view shows several large diopside grains (high relief with cleavage) surrounded by calcite. In
the PP view, both are nearly clear, although thecalcite has been stained red to help identification.
In the XP view, the diopside shows maximum interference color of 1st order yellow. The calcite,
on the other hand, shows interference colors of such high order that they appear white.
The twin lamellae in the calcite show hints of high-order pastels -- this is typcial of carbonate
minerals and others with very high birefringence.
The field of view is about 2.5 mm.
http://www.und.nodak.edu/instruct/mineral/320petrology/opticalmin
Opaque Minerals, Isotropic Minerals, Anisotropic Minerals, Birefringence and Interference Colors
Opaque minerals do not transmit light in thin sections. So, they appear black in both PP and XP light at all times. Common opaque minerals aregraphite, oxides
such as magnetite or ilmenite, and sulfides such as pyrite.
Isotropic minerals are minerals that have the same properties in all directions. This means light passes through them in the same way, with the same velocity, no
matter what direction the light is travelling. There are few common isotropic minerals; the most likely ones to see in thin section aregarnet and spinel.
Anisotropic minerals have different properties indifferent directions. So, light travels through them in different ways and with different velocities, depending on the
direction of travel through a grain.
Isotropic and anisotropic minerals are, most of the time, easily distinguished because isotropic minerals do not transmit light (are always black) when viewed under
XP light. A complication arises because anisotropic minerals will appear isotropic if grains are oriented in a specific way -- if they are oriented so they are viewed
"down an optic axis." Additionally, as the microscope stage is rotated, anisotropic minerals in any orientation go "extinct" (turn black) every 90o. The odds of a grain
lying in just the "wrong" orientation so as to cause confusion are small, but to overcome this, you should always look at a number of different grains of the same
mineral, and rotate the stage for each, to determining if it is isotropic or anisotropic.
Interference colors and birefringence: Anisotropic minerals, unless viewed down an optic axis, cause polarized light to be split into two rays as it travels through
a grain. The rays may not travel at the same velocity or follow the exact same path. A value, termed birefringence, describes the difference in velocity of the two
rays. When the rays emerge from the grain, they combine to produce interference colors. Interference colors are only seen in XP light! Note that the colors may
be any hue in the specturm. As birefringence increases, the colors repeat (see figure below), but get more and more pastel (washed out). To describe interference
colors we must specify both a hue and an order (e.g., 2nd order red; see chart below). Minerals with low birefringence show only white, gray and black interference
colors. Minerals with very high birefringence -- such as calcite -- show such weak colors that they may appear "pearl" white.
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