Polarized Light Observation for High-Resolution Imaging of Minerals

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.

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 polarize 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.

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 polarized 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 color can be checked changes color if the specimen is rotated under polarized light with parallel Nicol prisms, the phenomenon is referred to as pleochroism. With color changes attributable to pleochroism, the same color can be seen twice with each specimen rotation of 360°. For example, during polarized 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 polarized light observation of zonal structures.

Each time the specimen is rotated 360° during polarized 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 color of the mineral that can be observed at the diagonal position is the interference color.

Each time the specimen is rotated during polarized 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.

With a lambda filter (sensitive color 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. Polarized light observation of these textures is possible with crossed Nicol prisms.

The VHX Series 4K Digital Microscope’s proprietary observation system provides fully motorized stage movement including focusing and rotation, allowing for accurate and efficient polarized 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.

Polarized light observation of minerals (parallel Nicol prisms/crossed Nicol prisms) using the VHX Series 4K digital microscope