First, the optical properties of transparent minerals under a single polarizer. Only one polarizer is a single polarizer. The morphology, cleavage, color and pleochroism of mineral crystals can be observed under a single polarizer; mineral protrusions, matte surfaces and Baker lines are studied.
1. Morphology of mineral crystals Each mineral has a certain crystal habit and thus forms a certain shape. For example, garnets are often rhombohedral dodecahedrons, mica is pseudo-hexagonal, and hornblende is columnar. This particular form helps us identify minerals. At the same time, the morphology and degree of crystallization of minerals are often related to the formation conditions, so studying the morphology of minerals can also help us understand the genesis of minerals.
The mineral form seen in the flakes is not the three-dimensional form of the mineral crystals, but the form of the mineral surface. The shape and size of the cut surface in the same direction of the same crystal can be quite different. Figure 1 below is a cubic crystal. The shape of the facet can be square, triangle, hexagon, rectangle and other shapes due to the different direction of the facet. Therefore, the study of the shape of minerals under the microscope can not only be based on the shape of the individual sections, it must be comprehensively observed, carefully analyzed and compared, it is best to observe with the hand specimens in order to make an accurate conclusion.
Figure 1 Relationship between mineral crystal morphology and the direction of the section
Figure 2 Crystallization degree and cleavage [next]
Generally, the degree of crystallinity of the crystal can be determined according to the degree of development of the crystal face under the microscope. Can be roughly divided into 3 categories:
(1) The automorphic crystal form is intact, and the crystal is surrounded by a well-developed crystal plane. The self-formed crystals in the flakes are regular polygons, and the intersection of the crystal and the flakes is all straight (Fig. 2a). The self-formed crystals represent mineral crystals with early crystallization or strong crystallization ability, such as hornblende.
(2) Semi-automorphic crystal: The crystal form is relatively complete. The crystal is surrounded by partially developed crystal faces and incompletely developed crystal faces, and has irregular polygons in the flakes. The intersection of the crystal and the sheet is straight, and the part is irregular (Fig. 2b). The crystallization time of the semi-automorphic crystal is often later than that of the self-shaped crystal, such as biotite.
(3) His crystal form is incomplete, and the crystals are all composed of incomplete crystal planes. It is irregularly rounded in the flakes. Crystals sheet irregular line of intersection of the whole curve (FIG. 2c), such as a quartz crystal. His crystal form has a late crystallization time and cannot be developed into a complete crystal form due to space constraints.
2. Cleavage and determination of its angle The cleavage has been introduced in the minerals section. Many minerals have cleavage, but the direction, perfection degree, number of groups and cleavage angle of different mineral cleavage are different, so cleavage is an important basis for identifying minerals.
When the sheet is ground, a slit is formed in the direction of the cleavage plane due to the action of mechanical force. In the process of sticking the ore, the slit is filled with gum. Since the refractive index of the mineral is different from the refractive index of the gum, the refraction occurs when the light passes through, and the slits are displayed. Therefore, the cleavage of the mineral appears as a parallel slit in the sheet, which is called a cleavage slit. According to the degree of perfection of cleavage, the performance of cleavage seams is also different, generally can be divided into 3 levels:
(1) Extremely complete cleavage The cleavage seam is fine and straight and runs through the entire mineral grain, such as biotite (Fig. 2b).
(2) Complete cleavage The cleavage seam is thin, thick, and not completely coherent, such as amphibole (Fig. 2a).
(3) Incomplete cleavage The cleavage seam is intermittent, and sometimes only the general direction of cleavage, such as the cleavage of olivine, can be seen. The degree of clarity of the cleavage seam is not only related to the degree of perfection of the cleavage, but also by the relative size of the refractive index of the mineral and the gum. The larger the difference between the two, the clearer the cleavage seam; otherwise, the cleavage seam is not clear. Therefore, some minerals have cleavage, but because the refractive index is close to the gum, there is no obvious cleavage or cleavage in the flakes, as is the case with feldspar minerals.
In addition to the nature of the cleavage, the width of the cleavage slit is also related to the direction of the tangential surface. When slicing the vertical cleavage plane, the cleavage slit is the narrowest and represents the true width of the cleavage. At this time, the lens barrel is lifted, and the cleavage slit does not move to both sides, as shown in Fig. 3. When the slice direction is oblique to the cleavage plane, the cleavage slit is necessarily larger than the true width, as shown in Fig. 3. At this time, raise the lens barrel. The cleavage seam is moving to both sides. When the angle between the slice direction and the cleavage plane (a) gradually increases, the cleavage slit gradually becomes wider and becomes more and more blurred. When the angle a increases to a certain extent, the cleavage slit is invisible. This angle is called the visible critical angle of the cleavage slit. In addition, the cleavage slit is not seen when the slicing direction is parallel to the cleavage plane. Therefore, there are cleavage minerals, and it is not always possible to see the cleavage seam in the sheet, which is mainly controlled by the direction of the section.
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