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Optical
Propertities
 The
optical properties described on this page include
color, luster,
streak,
index of refraction,
dispersion, and a particular kind of
luminescence called
fluorescence. All of these
properties are also dependent on the interaction of light, mineral structure and
composition, and human vision. Since the late 19th century, it has been
accepted that light is a form of electromagnetic energy. Visible light
represents a range of the electromagnetic spectrum of wavelengths from about 350
to 750 nm (nm = nanometer, a nanometer is one billionth of a meter) that can be
seen by the human eye. Optics can be understood best if light is regarded
as a wave phenomena and waves are described by their velocity, wavelength, and
frequency, which are related by the equation
f = v/l
where f is the
frequency or number of wave crests per second measured in cycles per second or
hertz (Hz), v is velocity in meters per second, and
l
is the wavelength or distance from one wave crest to the next wave crest (light
can also be described by particle theory, but we can ignore that aspect of the
behavior of light for the moment). With a few exceptions, the frequency of
light is constant in wide range of transparent materials. If the frequency
is constant, the wavelength of light, therefore, changes as the velocity
changes, in order for this equation to hold true.
Monochromatic light is
essentially one wavelength of light and is perceived as whatever wavelength is
present. Polychromatic light consists of more than one wavelength.
When polychromatic light light strikes the eye, the combination of wavelengths
is still perceived as one color, however, the wavelength of that color light
might not be present. All colors can be generated with combinations of two
or more wavelengths except for the wavelengths of 420, 500, and 660 nm that
correspond to violet, green, and red light. When the entire visible
spectrum is present, the human eye perceives it as white light.
 Color
is one of the most obvious physical properties of a mineral. For some
minerals it is diagnostic (idiochromatic color), for most minerals color
is not. Most minerals can be a wide range of colors due to impurities
caused by chemical substitution and the effects of radiation (allochromatic
color). This is the case of the blue microcline crystal shown on the left.
What ever the reason for mineral color, color depends on the selective
absorption or reflection of certain wavelengths of light by the mineral during
light transmission or reflection. A white mineral looks white because it
reflects essentially all of the visible spectrum. A transparent mineral
transmits essentially all of the visible light spectrum. A black mineral
absorbs all wavelengths of light. A colored mineral, selectively absorbs
certain wavelengths of light and transmits or reflects the remaining light that
our eyes see. The color of a mineral is, therefore, dependent on the
wavelength or color of the incident light and the optical properties of the
mineral.
 Luster
When white light (natural light composed of the entire spectrum of wavelenghth
of visible light) shines on a mineral, the light may be transmitted, scattered,
reflected, refracted, or absorbed from the surface. Luster is the term
given to the character of the light that is scattered and reflected from the
mineral surface. There are three main catagories of luster. These
are metallic, submetallic and nonmetallic luster. Minerals with metallic luster
reflect light in the same way that metals do.
Submetallic luster is intermediate between metallic and nonmetallic luster.
Nonmetallic luster is further divided into adamantine (brilliant or diamondlike
luster), vitreous (glassy luster), resinous (similar to the luster of resin),
greasy, pearly (slightly irridescent due to light dispersed through the
microlayers of the mineral), silk (shimmery), waxy (slight reflection), and
earthy or dull.
 Streak
The color of metallic minerals, that is minerals having metallic
luster, is a fairly constant property, whereas the color of nonmetallic minerals
is generally not due to the coloring effect of minor impurities (allochromatic
color). The streak or the color of the powdered mineral is typically constant
and is, therefore, more diagnostic for nonmetallic minerals. The term streak
refers to the way the powdered mineral is obtained. The mineral of
interest is drawn across a clean ceramic plate. This produces a streak of
powdered mineral as long as the mineral is softer than the plate which has a
hardness of 6.5.
The streak of many metallic minerals is different
than the color of the massive or unpowdered mineral and is a great aid to
identification. For instance, both hematite and magnetite can be black or
very dark gray, but hematite always has a red-brown streak similar to the color
of dried blood, hence its name. Magnetite always has a black streak.
Arsenopyrite is silver in color but has a black streak. Pyrrhotite is
bronze in color with a black streak. Descloizite (pictured above left), is
silver gray, but it has an orange or red-brown streak.
Index of Refraction
The velocity of light
depends on the nature of the material that it travels through and the wavelength
of the light. The maximum possible velocity of light is 3.0 x 1010
cm/second in a vacuum. When light enters any other substance, the velocity
of the light is reduced due to complex electromagnetic interactions with the
electron cloud surrounding each of the ions or atoms in the substance. In
general, the greater the number of atoms and ions per volume of substance, the
greater will be the reduction in the velocity of light in the medium. The
atomic number of the elements that make up the mineral are also important.
The higher the atomic number, the greater the number of electrons in the
electron cloud around each of the ion.
When a light ray strikes the interface
between the air and a mineral at any angle other than the perpendicular (qv),
the portion of the light that enters the mineral is refracted at some angle (qm),
or bent. The refractive index of a mineral (nm) can be thought
of a measure of how effective a material is in bending light according to the
equation nm / nv = Vv/Vm where
nv is the index of refraction in a vacuum, Vv is the
velocity of light in a vacuum, and V is the velocity of light in the mineral.
 The index of
refraction can also be expressed as the ratio of the angle at which light hits
the mineral (called the angle of incidence) by the angle at which light is
refracted or bent as it enters the mineral (called the angle of refraction).
Snell's Law, named for the Dutch scientist, Willebrod Snell that defined the
relationship, states that nm / nv = (sine qv)
/ (sine qm) where nm and nv are the index of
refraction of the material and index of refraction of the vacuum (or 1)
respectively, qv is the angle of incidence, and qm is the
angle of refraction. The index of refraction of a mineral is very roughly
proportional to the density of the mineral and inversely proportional to the
velocity of light in the mineral. Typically, the higher the index of
refraction, the denser the mineral and the slower the velocity of light in the
mineral. The index of refraction for minerals typically falls between 1.4
to 2.0 with a few exceptional mineral exceeding 2.4. The higher the index
of refraction, the more brilliant and sparkling the mineral will appear.
The extremely high index of refraction of diamond is one of the many reasons why
this mineral is so valuable as a gemstone.
The index of refraction is dependent on mineral symmetry and it is a vectorial
property. Isotropic materials (isometric minerals and amorphous materials)
have essentially the same structure in all directions and so they have only one
index of refraction. Uniaxial minerals (hexagonal and tetragonal minerals)
are defined by two indices of refraction; one index defines the velocity of
light traveling parallel to the unique c-axis, the other index describes the
index at right angles to the c-axis. Biaxial minerals (orthorhombic,
monoclinic and triclinic minerals) have two planes of equal refractive indices
and are called biaxial. The interested reader should refer to
Introduction to Optical Mineralogy by William D. Nesse for more information.
Optical
Classification |
Mineral system |
# of indices of refraction |
Symbol of index |
| Isotopic |
Isometric minerals
Amorphous materials |
One |
n |
| Unixial |
Tetragonal
Hexagonal |
Two, one unique and
parallel to c-axis |
nw (omega) and ne (epsilon) |
| Biaxial |
Orthorhombic
Monoclinic
Triclinic |
Three |
na (alpha), nb (beta), and ng
(gamma) |
A table showing the the indices of
refraction of a number of isotropic, uniaxial, and biaxial minerals appears
on a separate page.
Dispersion The index of
refraction of an isometric mineral is not the same for all wavelengths of light.
When a ray of white light travels from air into a triangular glass prism, the
light not only bends but it also is separated into its component colors or
wavelengths, the colors of the spectrum. The violet light is bent slightly
more than the red, for instance, because it travels more slowly through the
glass. As the light emerges from the prism the colors separate even more.
This phenomenon, called dispersion, can be observed in some minerals such as
diamond. Jewelers refer to this characteristic in gemstones as fire.
The next time you look at a diamond, observe the rainbow of colors caused by the
splitting of white light into its component wavelengths.
 Birefringence
and
Double Refraction A
single light ray is split into two rays as it is transmitted into uniaxial and
biaxialB
minerals. The two rays travel at different velocities through the mineral, which
is why uniaxial minerals have two indices of refraction (nw and ne). The
difference between the two indices of refraction is called
birefringence. One of the two rays (w) called the
ordinary ray, behaves as if it were traveling
through an isotropic material (such as an isometric mineral), while the
extraordinary ray (e) does not (it vibrates at right angles to the vibration
direction of ray w). In most uniaxial and biaxial minerals, the difference
in the velocities, and hence the two indices of refraction, of the two rays is
small; for instance, the birefringence of quartz is only 0.009. On the
other hand, the birefringence of calcite is so large (d = 0.172), that it
produces an optical phenomenon called double refraction
- the visible doubling of an image viewed through the mineral -
caused by the splitting of one light ray into two.
B
The behavior of light in biaxial minerals is extremely complex and will
not be discussed any further except to say that the two light rays produced
through the splitting of the one incident ray both behave as extraordinary rays.
The interested reader is encouraged to read
Neese or another textbook on optical mineralogy for more information.
 Pleochroism
The splitting of light into two light rays may result in an effect called
pleochroism in some colored transparent uniaxial and biaxial minerals.
Pleochroism is produced because the two rays of light are differencially
absorbed as they pass through the colored anisotropic mineral. Because the
absorption is different for the two light rays, the wavelengths and colors of
the two light rays are different when they emerge from the crystal. The
color of ruby and sapphire (uniaxial corundum) is caused in part by a blending
of the two colors. Biaxial minerals, or those that crystallize in the
orthorhombic, monoclinic, and triclinic crystal systems, can have three distinct
colors. Tiny crystals of biaxial ivaite, the black mineral in the
photograph to the left, is strongly pleochroic (yellow, dark brown, and dark
green).
Iridescence The flash
of colors produced when light strikes the surface of an precious opal or some
compositions of plagioclase feldspar is called iridescence. This effect is
caused by the interference of light with a mineral interior as the angle of the
incident light changes. An internal iridescence can be caused by
light rays diffracted and reflected from closely spaced exsolution lamellae
(explained below), repeated parallel twin lamellae, tiny parallel mineral
inclusions, or closely spaced parallel fracture and cleavage planes. The
regular spacing of silica spheres in precious opal, alignment mineral inclusions
in chrysoberyl, and plagioclase exsolution lamellae may act as diffraction
gratings for white light and split it into its component wavelengths. The
human eye sees as these component wavelengths as flashes of color.
The iridescence shown by some members of the alkali
and plagioclase feldspars is the result of exsolution lamellae. Exsolution
lamellae are thin plates of alkali or plagioclase feldspar with different
compositions that "unmix" from an initially homogeneous solid solution.
The initial bulk composition of the mineral does not change as a result of
exsolution. Iridescence results because the fine-grained planar exsolution
lamellae of alkali and plagioclase feldspars act as a diffraction grating for
white light. (Look here for more
information about exsolution in the
feldspars).
 Opalescence
is the shimmery reflection from the interior of precious opal. This
phenomenom is caused by the diffraction of light by regularly spaced planes
defined by the closely packed similarly-sized silica spheres. Precious opal has
this quality. Common opal, a mineraloid, lacking long range order does not
display opalescence as beautifully.
 Chatoyancy
is the shimmer of reflected light in a mineral with a structure consisting
of closely packed parallel fibers or parallel mineral inclusions. Tiger
eye quartz, cat's eye chrysoberyl, some serpentine group minerals, and some
satin spar gypsum display chatoyancy.
Asterism is a six-winged star
shape produced by reflections from included microscopic rutile needles that are
oriented in three directions at 120o to each other in hexagonal ruby
and sapphire crystals. Quartz can also display asterism,
Luminescence is defined as the
emission of light by a mineral due to causes other than heating to
elevated temperatures. Minerals that emit light in the visible spectrum as
a result of exposure to ultraviolet, X-rays, or cathode rays are luminescent.
Fluorescence, or luminescense excited through exposure to
ultraviolet light, is of particular interest in mineralogy. The term
fluoresceence was coined by Sir George Stokes, who discovered the
scientific basis of this phenomenon while experimenting with fluorite irradiated
by natural sunlight in 1982. Manganese (Mn), one of the
transition
elements or metals, is a commonly the source of electrons that are
subject to excitation in minerals. When these electrons fall back to their
initial or ground states, some of the exciting energy is lost as heat, resulting
in a shift in the wavelength of the energy to lower energy (longer wavelength)
visible light.
Many minerals fluoresce, but none do so more
brilliantly than those from Franklin Furnace and Sterling Hill, New Jersey.
This locality is unusual in that it contains a lnumber of minerals (franklinite
and zincite) that are rare, if not unique to this one site. The Texas
Memorial Museum has several beautiful specimens from Franklin Furnace in the
research collection.
In the pairs of pictures below, the photographs on
the left were taken with visible light, and those on the right were taken with
ultraviolet light.
This rock is a metamorphosed impure limestone from
Franklin Furnace, containing franklinite, zincite, willemite, and calcite.
Franklinite is a black spinel group mineral, similar in appearance, structure,
and physical properties to magnetite. Both franklinite and magnetite are
magnetic, but franklinite is considerably less so than magnetite. Zincite,
an oxide mineral, is dark red only because of the substitution of manganese for
zinc; pure zincite would be white or colorless. Neither franklinite nor
zincite is fluorescent under ultraviolet light, but willemite (pale green in
natural light) is neon green in ultraviolet light, and calcite (opaque white in
natural light) is bright red in ultraviolet light. Scroll down to see
magnified views of the same specimen.
These magnified views of the Franklin Furnace
specimen clearly show the black franklinite and red zincite grains in the
natural light photograph. Franklinite and zincite do not fluoresce, and so
look black in the photograph taken with ultraviolet light. The neon
green of willemite and the bright red of the calcite are clearly visible in the
photograph taken with ultraviolet light. Willemite looks yellow or tan in
the photograph to the right, but is actually a pale green.
This rock, which is also from Franklin
Furnace, is composed primarily of green willemite, with smaller, discontinuous
streaks and lenses of black franklinite and red zincite. Zincite is most
obvious in the thick red lens on the right side of the specimen just above the
"G25" label in the visible light photograph. The contrast between the
fluorescent willemite and nonfluorescent franklinite and zincite is striking
under ultraviolet light.
This rock from Franklin Furnace is
primarily composed of white calcite, pink rhodonite, and black franklinite.
The far left portion of the specimen in the photograph to the right was
partially shielded from the ultraviolet light, causing the outlines of the
specimen in the two photographs to appear to be different.
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