Polarised Light Observation for High-Resolution Imaging of Minerals
Magnified observation of the minerals contained within ore and rocks is required in a wide range of fields, including plant construction in the oil industry, material research in the construction industry, and research and development at universities. In many cases, it is difficult to identify substances contained within minerals using a loupe or the naked-eye, so petrographic microscopes are used.
This section describes the basics of petrographic microscopes, explains typical mineral observation items, and introduces examples of polarised light observation of minerals using our 4K digital microscope.
- Different Mineral Observation Methods for Different Purposes
- What Is a Petrographic Microscope?
- What Are (Parallel and Crossed) Nicol Prisms?
- Items in Polarised Light Observation of Minerals
- 4K Digital Microscope Application Example in Polarised Light Observation of Minerals
- A 4K Digital Microscope That Enables Observation and Analysis of Various Objects Including Minerals
Different Mineral Observation Methods for Different Purposes
Loupes or stereoscopic microscopes are used to observe microscopic mineral aggregates and crystals at low magnifications (10x - 100x). On the other hand, when observing mineral types and structures within ore and rocks a magnification of 50x or higher is required, and the specimen is prepared on a glass slide and observed under a petrographic microscope.
What Is a Petrographic Microscope?
A petrographic microscope is a type of optical microscope that uses polarised light, which transmits light vibrating in a fixed direction, to enable observation using different vibration directions of light depending on the material. A petrographic microscope uses a lens with installed Nicol prisms (polarising plates or polarising filters; explained later) and lighting.
General petrographic microscopes use a polarising plate called the polarising Nicol prism (polariser) to change the irradiated light into polarised light. The status of the polarised light that passed through the specimen is then detected with the analysing Nicol prism (analyser), another polarising plate between the objective lens and the eyepiece. This status is converted into contrast between light and dark or between different colours on the specimen, visualising the optical characteristics of the target.
This principle can be used to identify microscopic minerals contained in rocks and to selectively capture and observe rock structures according to the optical properties of the mineral under polarised light conditions. When observing ore and rocks, the target is cut to a thickness of approximately 0.03 mm (0.0012") and is prepared as a specimen on a glass slide that is placed on the stage, ensuring the light passes through the specimen.
Because petrographic microscopes can magnify microscopic targets to observe their optical properties, these devices are used not only with minerals but in the research of various items such as glass, plastics (films, etc.), polymers, fibres, polymeric materials, and the medicine made from polymeric materials.
What Are (Parallel and Crossed) Nicol Prisms?
Normal light vibrates in various directions. This light can be changed to vibrate in a single direction (polarised) by passing it through a polarising filter (polarising plate) called a Nicol prism. Changing the direction in which light is polarised by rotating the specimen side in relation to the Nicol prism changes the vibration direction of the captured light, allowing for selective detection of a specific target and its characteristics.
The two types of typical polarised light observation methods using Nicol prisms are shown below.
Parallel Nicol prisms
This refers to the observation of a specimen prepared on a glass slide with the Nicol prisms arranged in the same direction, an arrangement that is also called open Nicol. As shown in the figure on the left, the only light that passes is that vibrating in the same direction as the Nicol prisms.
Crossed Nicol prisms
This is also called orthogonal Nicol. It refers to the observation of a specimen prepared on a glass slide and placed between two Nicol prisms arranged so the directions in which they polarise light are orthogonal. As shown in the figure on the right, the light that vibrates in directions at a right angle and parallel to the two Nicol prisms does not pass. Although overlapping these two Nicol prisms results in a black image, light can pass through the crossed Nicol prisms and be observed depending on the vibration direction of the light from the specimen placed between the prisms.
Items in Polarised Light Observation of Minerals
This section explains typical observation items to show how petrographic microscopes are used to observe minerals.
When ore or rocks are observed with the naked eye, it is only possible to determine their general shape, such as column- or board-shaped. On the other hand, if you cut a thin slice of ore, prepare this specimen on a glass slide, and use a petrographic microscope to observe it with parallel Nicol prisms, you can magnify the shape of the mineral in the cross section and observe it selectively.
Multiple linear cracks (stripes) that are parallel or that cross each other at fixed angles are known as cleavage and can be seen with polarised light observation with parallel Nicol prisms.
This index, which indicates the degree to which light refracts, can be examined when light passes through a mineral. In observation with a petrographic microscope, the refractive index can be examined by checking whether parallel Nicol prisms cause the cracks (stripes) on and the contour of the mineral to appear as clear black marks.
When a mineral whose colour can be checked changes colour if the specimen is rotated under polarised light with parallel Nicol prisms, the phenomenon is referred to as pleochroism. With colour changes attributable to pleochroism, the same colour can be seen twice with each specimen rotation of 360°. For example, during polarised light observation of hornblende, rotating the specimen by 90° causes light brown and dark green-brown to alternately appear, indicating pleochroism.
Zonal structures are structures in which the composition of a single crystal differs on its exterior (which grows last) and interior (which grows first) due to its growth. These structures are commonly seen in minerals such as plagioclase and pyroxene contained in igneous rock. Crossed Nicol prisms are used in the polarised light observation of zonal structures.
Each time the specimen is rotated 360° during polarised light observation with crossed Nicol prisms, the image switches between bright and dark four times. The position where the image appears brightest is called the diagonal position. The colour of the mineral that can be observed at the diagonal position is the interference colour.
Each time the specimen is rotated during polarised light observation with crossed Nicol prisms, multiple mineral crystals appear bright or dark. For minerals that appear dark four times during a rotation of 360°, the darkest position is the extinction position. The angle between this position and the vertical direction of the field of view is the extinction angle.
Positive and negative extension
With a lambda filter (sensitive colour test plate/full-wave plate) inserted between crossed Nicol prisms, rotating the specimen causes the cracks (stripes) and thin edges of the crystal to appear yellow and blue. If the Z’ direction of the lambda filter and the extension direction of the mineral roughly match when the rotation is stopped with the target at a position that makes it appear blue, the extension is positive. On the other hand, if the extension direction matches the X’ direction, the extension is negative.
Crystal twinning refers to systematic changes in the atom arrangement direction at the lattice plane in a mineral crystal. With crossed Nicol prisms, non-extinct parts can be viewed as straight light and dark stripes in the crystal of a mineral that has undergone crystal twinning. Rotating the specimen inverts the light and dark parts.
Exsolution texture refers to a structure in which a solid mineral remains in this form but separates (undergoes exsolution) into two or more types of minerals under a gentle temperature decrease. Polarised light observation of these textures is possible with crossed Nicol prisms.
4K Digital Microscope Application Example in Polarised Light Observation of Minerals
In polarised light observation of minerals, it is necessary to clearly perceive changes in the viewing method due to the angle. However, the following problems are present in mineral observation: it is very difficult to determine conditions for the transmitted illumination; proficiency, experience, and a large amount of time are required; and evaluations vary from one observer to another.
KEYENCE’s ultra-high accuracy VHX Series 4K Digital Microscope uses a high-performance optical system, a 4K CMOS image sensor, and a proprietary observation system that provides access to diverse functions with simple operations.
Furthermore, corresponding to observation with parallel or crossed Nicol prisms, high-resolution 4K images can be acquired with simple operations even during polarised light observation of minerals, allowing for efficient and fast mineral identification and structure observation.
This section introduces an example of polarised light observation of minerals using the VHX Series.
Polarised light observation of minerals
The VHX Series 4K Digital Microscope’s proprietary observation system provides fully motorised stage movement including focusing and rotation, allowing for accurate and efficient polarised light observation.
In addition to this system, which provides high functionality with simple operations, a lineup of lenses is available to meet the needs of observers, supporting observation with parallel or crossed Nicol prisms.
A dual-objective zoom lens (VH-ZST) can be used to achieve a wide magnification range (20x to 2000x) without changing the lens. The wide variety of illumination options provided by lens-controlled mixed lighting and various optical adapters enables the optimal environment for mineral identification and structure observation. This high-performance optical system and lighting can be used to capture images with a 4K CMOS image sensor, allowing for observation of minerals with 4K images whose accuracy is far higher than anything possible until now.
A 4K Digital Microscope That Enables Observation and Analysis of Various Objects Including Minerals
The VHX Series 4K digital microscope provides high performance and functionality not only as a petrographic microscope capable of high-accuracy 4K image quality but as a stereoscopic microscope, metallurgical microscope, and measuring microscope as well, covering a wide range of observation modes including brightfield, darkfield, polarised light, and differential interference contrast. The observation and analysis of a wide range of targets can be controlled automatically, strongly supporting research work.
Although advanced observation with 4K images was not possible conventionally, the VHX Series is equipped with many functions that make this a reality. The Multi-lighting function optimises the determination of lighting conditions at the press of a button, and the depth composition function makes it possible to capture fully focused images of the entire field of view, even when this field includes a three-dimensional target at high magnification.
This product also allows for advanced analyses such as highly accurate sub-micrometre level 2D and 3D measurements, acquisition and profile measurement of 3D images, and automatic area measurement/count with intuitive operations.
Additionally, Excel can be installed directly on the VHX Series to automatically create reports using templates. This one device is all you need to seamlessly and efficiently carry out all your observation and analysis work.
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