Polarization microscope Single polarization, orthogonal polarization and cone light observation are the core observation methods of mineralogy and petrography. They are step-by-step and complementary, and are used to obtain optical information of different dimensions of crystals.
Single polarization is a lower polarization accessory (polarizing lens) using a microscope. After the light passes through the lower polarizer, it becomes polarized light that vibrates in a single direction, then passes through the mineral sheet (usually about 30um thick) and finally enters the objective and eyepiece. Preliminary identification of the mineral, observation of its morphology, cleavage, color changes, and estimation of the relative size of its refractive index.
What is mainly observed under single polarization is the properties related to the absorption and refraction of light waves by minerals, and the symmetry of amorphous crystals.
2.1 Understanding: Observe the presence or absence of understanding, the number of groups (such as one group, two groups), and their perfection. The two groups of cleavage will form a "cleavage angle", which is an important indicator for the identification of minerals (such as pyroxene ~ 87°/93°, angular amphibole ~ 56°/124°).
2.2 Color: The color of the mineral in the sheet is the result of its selective absorption of light waves of a specific wavelength. Note: The color in the sheet is the complementary color remaining after the white light is absorbed, which is different from the color of the hand specimen.
2.3 Polychromatism: The phenomenon that the color of heterogeneous minerals changes with the direction of the crystal. When you rotate the stage, if the color or shade of the mineral changes, it indicates that it is polychromatic. This is due to the different ability to absorb light waves in different vibration directions.
Principle: When light enters another medium (mineral) from one medium (e.g. gum, refractive index N ≈ 1.54), refraction and reflection occur at the interface, the significance of which depends on the difference in refractive index between the two (| N minerals - N gum |).
Protrusion: the black thick line (Baker line) at the edge of the mineral and the brightness at the point of contact of the mineral with the gum. According to the height of the protrusions, it can be divided into negative and low protrusions, positive and low protrusions, positive and middle protrusions, high protrusions, positive and high protrusions, etc., which are used to estimate the refractive index range of the minerals.
Roughness: The feeling that the mineral surface looks rough and uneven. The greater the difference between the refractive index and the gum, the more significant the roughness.
On the basis of single polarization, push in the upper polarizer (polarizer, analyzer). The key is that the direction of vibration of the upper and lower polarizers is perpendicular to each other (usually a positive north-south direction, a positive east-west direction).
In orthogonal polarization, the field of view should be all black, called extinction. Because the polarized light from the lower polarizer cannot pass through the upper polarizer perpendicular to its vibration direction.
When heterogeneous minerals are placed on the stage, the light entering the minerals is birefringent and decomposes into polarized light in two vibrational directions perpendicular to each other. An optical path difference (R) occurs between these two polarizations. When they shoot out minerals, the vibration direction of their synthetic light waves changes, so that a part of the light can pass through the upper polarizer to brighten the minerals.
Differentiate between homogeneous (all extinction) and heterogeneous (four brightness and four darkness); determine the extinction type (parallel, symmetrical, oblique extinction); determine the birefringence size and ductility symbol of the mineral.
Properties related to mineral birefringence and interference are mainly observed under orthogonal polarization.
One revolution of the stage (360°) results in four dimming (extinction) and four brightening of the heterogeneous minerals.
When the mineral is in an extinction state, the long and short radii of its photometric body elliptic section (representing the vibration direction of the light) are exactly parallel to the vibration direction of the upper and lower polarizers. This position is the extinction level.
When the mineral is not in the extinction position, it displays a color, which is the interference color.
The interference color is not the color of the mineral itself, but the result of interference by two beams of polarized light produced by birefringence.
The height of the interference color depends on the optical path difference (R), and R = d * (N1 - N2), where d is the thickness of the sheet (fixed at 0.03 mm) and (N1 - N2) is the birefringence.
Therefore, the interference color is an intuitive reflection of the birefringence. According to the Michael Levy Interferometric Chromatogram, the birefringence size of the mineral can be estimated by the interferometric color level.
For strip-like and plate-like minerals, the positive ductility (parallel) or negative ductility (vertical) can be judged according to the angle between the extension direction and the extinction direction under the highest interference color, which is an important symbol for identifying chain-like/layered silicate minerals (such as pyroxene and hornblende).
on the basis of orthogonal polarization. Push the upper bias mirror into the bias mirror (remain orthogonal).
Switch to a high-power objective (such as 40x), because the high-power objective has a short working distance and a large numerical aperture, which can concentrate a wider range of light.
Add a condenser (Bertrand mirror, Abbe condenser) under the stage so that the light is not incident in parallel, but in a conical beam into the mineral sheet.
It is not the image of the mineral itself that is observed under the cone light, but the interference map formed by the light from all directions of the mineral on the focal plane behind the objective lens. This image reflects the orientation and type of the mineral photobody in three-dimensional space. Determining the axiality (uniaxial/biaxial) and optical symbols (positive/negative) of a mineral is the highest order means of optical identification of a crystal.
Homogeneous: The section in any direction is completely extinct with no interference pattern.
Uniaxial crystalline minerals: Unique interference maps are produced, such as black crosses and concentric ring-shaped interference color circles.
Vertical optical axis tangent: typical black cross and concentric ring.
Skewed optical axis tangent: Incomplete black cross and elliptical rings.
Parallel optical axis cross-section: a coarse blurred shadow.
Biaxial crystalline minerals: Interferograms are more complex and common are double leaf curves (like two eyes).
Vertical acute aliquot tangent: a typical hyperbolic black band (extinction shadow).
Vertical cross-section of an optical axis: a simple black cross that splits into hyperbolas when rotating the stage.
This is the core purpose of cone light observation. By observing the interferogram and inserting a test plate (such as a gypsum test plate or a mica test plate) and observing whether the interference color coil in the interferogram increases or decreases, it can be determined whether the mineral is a positive/negative axis crystal or a positive/negative biaxial crystal.
Single polarized light, like looking at the "looks" and "skin tone" of minerals in ordinary light.
Orthogonal polarization, such as a "stress test" on a mineral, to see how it reacts (produces interference color) when it is subjected to forces in different directions (vibration of light).
The cone light is a "CT three-dimensional scan" of the mineral, directly looking at the optical structural skeleton (photometric body) inside it.
morphology, cleavage, color, multicolor, protrusions, roughness
Lower Polarizer + Upper Polarizer + Condenser + High Magnification Objective
Determine axiality (mono/biaxial crystals) and optical symbols (positive/negative)
Tell us the material, the feature you need to see and the magnification range. We will recommend a configuration and quote it.
