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Optical properties

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Optical Propertities

TopazThe 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.

Microcline Feldspar variety AmazoniteColor 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.  

Pyrite has metallic lusterLuster 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. 

DescloiziteStreak  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.  

Index of refractionThe 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.

Dispersion by a glass prism

CalciteBirefringence 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.

Ilvaite (black), Covellite (bright blue), Sphalerite (gray)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).

OpalOpalescence 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.

 

 

Quartz variety TigereyeChatoyancy 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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