Petrographic microscopes

The petrographic microscope is an essential tool for studying the mineral content and texture of fine-grained rocks. It also provides a rapid and accurate means for identifying minerals through their optical properties.

Polarized Light

The crystals of many rocks and deposits are too small to be distinguished—much less identified—by the naked eye. Individual crystals can be distinguished under an ordinary microscope, but identification in this manner is still difficult. A powerful improvement was discovered in 1828 by Scottish geologist William Nicol when he applied to the microscope his newly invented polarizing prisms cut from calcite crystals and found that different minerals have very distinctive appearances in polarized light. The modern petrographic microscope is a refinement of Nicol's discovery.

The petrographic microscope is similar to a standard biological microscope but has adaptations for use with polarized light. A polarizer (or “polar”) beneath the condenser lenses polarizes the light before it passes through the specimen. A circular, rotatable stage allows the slide and specimen to be turned with respect to the polarized light. A second, removable polarizer called the analyzer, oriented crosswise to the lower polar and above the specimen, can be inserted into the light path in the tube, providing “crossed polars.” There are other accessories used for special purposes.

A polarizing microscope adapted to view reflected rather than transmitted light is commonly used by metallurgists to study the identity, size, and texture of metal crystals produced in industrial processes. Economic geologists use the reflection microscope to identify opaque, metal-bearing minerals and to determine their abundance in samples from an ore body.

Interference Colors

Under the polarizing microscope, crystals show bright, distinctive colors called interference colors. As the stage is turned, these colors move and change or become dark (“extinct”) in ways that can be used to identify the minerals. It is the interaction of light with the crystal structure that causes the distinctive behavior.

Light is an electromagnetic wave. As a ray of light travels along, its electric field strength oscillates back and forth transverse to the path of the ray, somewhat like the vibration that travels along a horizontal rope when it is shaken up and down at one end. The vibration direction and the travel path are perpendicular. The distance traveled by the ray between one maximum of the transverse field and the next is the wavelength of the light. Each color of visible light has its own particular wavelength, ranging from roughly 700 nanometers for red to 400 nanometers for violet (a nanometer is one-billionth of a meter). White light is a mixture of all colors.

Index of Refraction

All light travels at the same speed in a vacuum, but in a transparent medium, it is slowed by interaction with matter. The speed is characteristic of a given medium and is indicated by its index of refraction; the greater its index, the more slowly light moves through it. The speed in air (index 1.0003) is essentially the same as in a vacuum (1.0000); however, in quartz (index 1.54), it is only 65 percent of that value, and in diamond (index 2.42), it is only 40 percent.

The atoms or molecules in crystals are arranged in a strict order, repeated over and over to make the crystal structure (or “lattice”). The structure is responsible for many characteristic features of minerals, such as the natural shapes and faces of crystals and the likelihood of breaking along flat surfaces called cleavages. The pattern of any given mineral is very distinctive, so the ways in which light interacts with the different structures can be used to identify the various minerals.

Minerals are classified as isotropic or anisotropic. In isotropic crystals, light can travel in any direction at the same speed, and the index of refraction has the same value for all orientations. Isotropic minerals all have a molecular structure and spacing that are identical along each of three perpendicular crystal axes. Some common examples are halite (rock salt) and garnet. Glass substances, which are equally disordered in all directions, are also isotropic.

In anisotropic minerals, the molecular structure and spacing are different along one or more crystal axes. In such crystals, the interaction of light vibrations with matter (and therefore, the speed of light and the index of refraction) depends on the direction of travel. The analyst, to identify the minerals, must know the detailed differences among the crystal structure systems and how light interacts with them. For the present purposes, however, a single fact is important: The speed of light traveling along most paths through anisotropic crystals depends on the direction of vibration. Even along the same path, light vibrating in one direction may be faster or slower than light vibrating in another, with both vibration directions being perpendicular to the travel path.

The polarizer under the stage allows the analyst to select the vibration direction. Without it, the rays of light rising vertically through the specimen vibrate parallel to the stage but with random orientation. The polarizer absorbs all these rays except those vibrating in one specific direction, which is usually fixed back-to-front or sideways in the field of view. The analyst turns the specimen on the stage to change the orientation of the crystals to the polarized light.

Retardation

An anisotropic crystal viewed down the microscope tube, in general, has one direction with a maximum index of refraction and another perpendicular to it with a minimum index. A ray vibrating parallel to the first would be the slowest, while the other would be the fastest. A ray oriented in any other direction actually separates into two parts, each part vibrating in one of the two directions but following the same vertical path. The part vibrating in the higher index direction is slower than the other and falls progressively farther behind; it is said to be retarded. The amount of retardation depends on how far the parts travel (the thickness of the crystal) and on the difference in their speeds (and, therefore, on the difference in their indices of refraction, called the birefringence). Because each color has its own particular wavelength, the amount of retardation affects whether the vibrations of the two parts of a given color are in step with each other (in phase) as they exit the crystal or determines how much they are out of step.

The analyzer (the upper polarizer) blocks all light vibrating parallel to the lower polarizer because it is oriented at right angles. Thus, the glass of the slide and any isotropic crystals appear black, as they do not alter the polarization. Similarly, the two separated parts of a ray from an anisotropic crystal, if they happen to emerge in step, recombine in the original polarization, and this light is blocked as well. Although if one part is retarded out of step with the other (so that, recombined, they have a rotating “elliptical polarization”), the analyzer in effect deals with each part individually. It resolves each part once more and allows only those portions parallel to the analyzer vibration direction to pass. The passed portions of each part now vibrate in the same plane but are out of step with each other. Depending on how much they are out of step, the vibrations may reinforce each other and strengthen the color or oppose each other and weaken the color. Colors that are weakened or canceled are subtracted from the original white light, and what remains to be viewed is the complementary color. A sheet of mica placed between crossed polars shows this effect well, even without a microscope.

Michel-Levy Chart

The interference colors that result from this process are one of the most striking features of crystals viewed under crossed polars. Because they result from the subtraction of specific colors from white light—some more and some less—they fall in a sequence that is distinctly different from an ordinary spectrum. Beginning with black when there is no retardation (the passed rays are in step), as retardation increases, the colors go through gray and white to orange and red for the first “order,” then through several cycles from red through blue for higher orders, eventually merging to pinks and greens, and finally to more or less white for very high orders. The sequence is displayed on a Michel-Levy chart (which shows the sequence of colors as a function of birefringence). The chart is named after French geologist Auguste Michel-Levy. The colors that actually appear in a given crystal give important information about the mineral.

Sample Preparation

Samples for microscopic examination are usually prepared either as a powder or as a thin section. The powder is made by crushing a mineral grain and screening it very finely; a small amount of the powder is then placed on a microscope slide with a drop of oil. The thin section is made by sawing a slice off the sample, gluing it to a slide, then further sawing and grinding the slice until it is only 0.03 millimeters thick. In such thin samples, most minerals are transparent or translucent, although metals and many sulfide minerals are still opaque. Special reflection techniques can be used to examine opaque minerals.

Microscopic analysis of crushed mineral grains (powder) is the most efficient way to identify any mineral (and some nonmineral substances) whose crystals are large enough to be distinguished with a microscope. Thin sections are less efficient, but they have other advantages because they preserve the structure of the original sample; they are essential for the study of fine-grained rocks. By calculating the relative abundance of each kind of mineral, examining the shapes of grains and the ways they contact each other, and studying the distribution of grains and larger structures like bedding, the analyst can identify the rock type, estimate its properties, and interpret clues to its history. For example, a thin section of sandstone under the microscope would show the shape of the sand grains, fine details of its bedding, the amount of cement between grains, the amount of empty space, or porosity, and the presence and distribution of any mineral grains besides the quartz sand. This information could be used to estimate its mechanical strength for engineering purposes, its ability to hold water or oil, or its potential as a quarry stone, raw material for glassmaking, or ore of uranium.

Mineral Identification

With either preparation, the first goal is to identify the minerals present by observing their visible properties. Features of shape, such as a characteristic crystal form, habit, cleavage, or fracture (keeping in mind that only a cross-section is visible), give the first clues to identity. For example, garnets often exhibit a polygon-like cross-section of their characteristic crystal form, and mica usually shows its perfect one-directional cleavage. Typical colors may be present (with polars uncrossed), although they are much fainter than in a hand sample. Some minerals, like tourmaline or biotite mica, change color as they are turned in the polarized light; these minerals are called pleochroic.

The relief of a crystal indicates the contrast between its index of refraction and that of its surroundings. A mineral with high relief appears to stand out from its background and have very distinct boundaries, while one with low relief is hard to distinguish from its background. If neighboring minerals or a medium (mounting or immersion) of a known index are present, the analyst can estimate the index of refraction of an unknown mineral from its relief. The analyst can measure the index of minerals in powdered form exactly by comparison with standard index oils (called the immersion method). If the index of the mineral matches the oil closely, the grain boundary almost disappears. Anisotropic crystals require a different oil for each vibration direction. Having measured the indices, the analyst can then consult a table to identify the mineral.

If the indices cannot be measured directly, as in a thin section, the birefringence (the difference between maximum and minimum indices) gives useful information for identification. The interference colors in a crystal depend on its birefringence and its thickness (usually approximately 0.03 millimeters in a thin section). The analyst compares the highest interference colors found in a crystal to a Michel-Levy chart, determines the corresponding birefringence, and consults a table to identify the mineral.

The relationships between the vibration directions and visible features such as crystal faces and lines of cleavage give another clue to identity. At every quarter turn as an anisotropic crystal is turned on the stage, there is a point at which the crystal becomes completely dark, or extinct. Extinction occurs whenever the crystal's vibration directions are parallel to the polarizer or analyzer. The angle between an extinction direction and a crystal face or cleavage can distinguish between many otherwise similar minerals, such as the pyroxenes and amphiboles. Isotropic minerals like garnet are extinct at all positions of the stage.

Interference Figures

Interference figures provide another powerful means of identifying crystals. They are shadows with distinctive shapes that appear with crossed polars and diverging light because polarized light is blocked from certain areas of the crystal image. Special lenses are used to cause the light to diverge and to change the focus of the eyepiece.

The shadow figures, which depend on the nature and orientation of the crystal, take the shapes of Maltese crosses or sweeping curves that move in distinctive ways as the stage is turned. The analyst can use them to determine many details about the crystal structure, the relationships of the vibration directions, and other features useful for identification.

Commercial and Public-Safety Applications

The petrographic microscope is an important tool for identifying many kinds of minerals and other substances that cannot easily be distinguished by ordinary physical and chemical tests. It has been used, for example, to determine the nature of corrosion products on metal surfaces. The corrosion products indicate which chemical reactions might be responsible for the damage and, therefore, how the surfaces might be protected. In another application, the microscope has been used to study the different materials traded commercially or displayed in museums as “jade.” Officially, the name “jade” is applied to rocks composed of either an amphibole called nephrite or a pyroxene called jadeite, but the microscope revealed that much of what has been called jade is really composed of other minerals similar in appearance. The study showed historically significant patterns in the use of different kinds of jade in various cultures. In 2025, modern petrographic microscopes combined automated stage movement, machine learning, and digital image processing. These smart microscopes rapidly identified rock textures, mineral composition, and grain relationships that previously required expert manual interpretation. This is especially important in mining, petroleum geology, and environmental studies.

The petrographic microscope has many applications to areas in which geology touches on the economy or on public safety. Rock that has been sheared and fractured, as by faulting, shows distinctive texture and structure in thin section. Knowledge of these features in the rock of a given region can be important in the prediction of earthquake potential or in the evaluation of stability for engineering projects. Thin sections also show the amount of empty space, or porosity, between the grains in a rock, which is essential for estimating the potential of the rock for bearing oil, for carrying groundwater, or for allowing the passage of pollutants and radioactive waste. In the mining industry, thin sections are used to identify and evaluate the abundance of ore minerals and also to determine their grain size and how they are locked into the rock structure; all these factors determine whether the minerals can be recovered at a profit.

There are many anisotropic substances besides minerals. Whenever there is a distinct alignment of long molecules in a substance, polarized light may interact with it and reveal interference colors. Some biological tissues, structures in cells, plastics, and glasses have such anisotropic structures, and polarized light is useful for studying them. In one application, polarized light is used to study the distribution of stress in engineering structures such as machine parts and architectural members. The structure is modeled with a plastic such as Lucite and viewed through crossed polars. When the model is placed under load, the plastic develops interference colors that are concentrated at points of maximum stress.

Fiber-optic systems for transmission lines and optical switching devices developed for telephone and computer communications depend on the differences in the indices of refraction of their various parts. The polarizing microscope, which shows the differences by interference colors, is a key instrument for designing and testing such systems. Although microscope systems using other kinds of radiation are becoming widely employed, the polarizing light microscope will continue to hold a central importance both in the field of geology and outside it.

Principal Terms

anisotropic crystal: a crystal with an index of refraction that varies according to direction with respect to crystal axes

birefringence: the difference between the maximum and minimum indices of refraction of a crystal

crystal axes: directions in a crystal structure with respect to which its molecular units are organized

index of refraction: the ratio of the speed of light in a vacuum to its speed in a particular transparent medium

interference: the combining of waves or vibrations from different sources so that they either are in step and reinforce each other or are out of step and oppose each other

interference color: a color in a crystal image viewed under crossed polars, caused by subtraction (cancellation) of other colors from white light by interference

interference figure: a shadow shape caused by the blocking of polarized light from certain areas of a crystal image

polarization: a method of filtering light so that only rays vibrating in a specific plane are passed

principal vibration directions: directions in a crystal structure in which light vibrates with maximum or minimum indices of refraction

retardation: the progressive falling behind of part of a ray vibrating in a slower direction compared to a part vibrating in a faster direction

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