Second, the oil immersion method to determine the refractive index
There are several methods for measuring the refractive index, and the oil immersion method is considered to be a convenient and effective method for measuring the refractive index. The so-called immersion method is to immerse the debris of the substance to be tested in the immersion oil with the known refractive index, compare the refractive index of the substance to be immersed, and replace it with a series of immersion oils until the refractive index of the mineral and the immersion oil. When equal, the refractive index of the immersion oil is the refractive index of the substance to be tested.
When the refractive index of a crystal is measured by oil immersion, it is also necessary to observe other optical properties of the crystal such as crystal form, color, pleochroism, elongation symbol, extinction type, axial property and optical symbol. The oil can be thoroughly observed by oil immersion, so the oil immersion method has become a complete field for identifying transparent minerals.
(I) Preparation of oil immersion and determination of refractive index value 1. Preparation of immersion oil The oil refractive index is determined by oil immersion method. Two sets of immersion oils with known refractive index are required, and the difference in refractive index of adjacent immersion oil is usually It is between 0.01 and 0.03. One set is low refractive index immersion oil N=1.400～1.700, the interval is mostly 0.003, about 100 bottles; the other set is high refractive index immersion oil N=1.700～2.100, the interval is mostly 0.01.
There are three methods for preparing oil immersion, that is, liquid and liquid are miscible; solid is dissolved in liquid; solid and solid are mixed.
The preparation of the low refractive index immersion oil is mainly the first method, that is, the liquids of different refractive indexes are mixed at a certain ratio. The volume of the immersion oil after mixing is not less than 20 mL, otherwise the correctness of the refractive index will be affected. The liquid used to prepare the oil immersion preferably meets the following requirements: colorless or nearly colorless; the oil should not react with the substance to be studied; the liquids can be mixed with each other in any ratio; the volatility is not strong; Very poisonous. The well-preserved oil should be stored in a special oil-immersed bottle.
To formulate oil immersion, the following formula can be used:

VN = N ₁ N ₁ +V ₂ N ₂
V ₁ +V ₂ = V

Where V—the volume to be immersed in oil;
N—the refractive index value of the oil to be immersed;
V ₁ , V ₂ ——— the volume required for two kinds of oil-impregnated raw materials;
N ₁ , N ₂ --- Refractive index of two kinds of oil-impregnated raw materials.
To prepare a set of low refractive index immersion oil (N=1.400～1.700), only 5-6 kinds of oils are needed, as shown in the table below.

The preparation of high refractive index immersion oil generally has the following types:
(1) sulfur is dissolved in diiodomethane, N = 1.74 to 1.78
(2) various iodides are dissolved in diiodomethane, N = 1.74 to 1.86;
(3) sulfur plus phosphorus dissolved in diiodomethane, N = 1.74 ~ 2.02;
(4) arsenic trioxide , sulfur, arsenic disulfide and diiodomethane mixed, N=1.74~2.00;
(5) Preparation of N> 2.00 immersion oil, generally only mixed with low-melting solid, such as sulfur, selenium, can be equipped with an N = 1.99 ~ 2.92; iodized thallium bromide, thallium, can be equipped with an N = 2.20 ~ 2.90. [next]
The preparation of sulfur and phosphorus in diiodomethane high refractive index immersion oil is now described. After weighing by phosphorus:sulfur:diiodomethane=8:1:1 mass ratio, the sulfur powder is first dissolved in diiodomethane, and after being dissolved, a layer of distilled water is added, and then it is cut into small pieces. The yellow phosphorus was quickly added one by one, and while stirring, it was observed that as the amount of yellow phosphorus dissolved increased, the consistency of the yellow colloidal liquid gradually decreased, the color gradually became lighter, and finally became a transparent and uniform pale yellow phosphorus liquid. At this time, N≈2.04, if the phosphorus solution is opaque, it can be slightly heated to 40 to 50 °C (overheating is harmful). In order to prevent spontaneous combustion of oil-soaked, bottle of immersion oil to be injected into distilled water, and placed in a metal basin for safekeeping.
2. Determination of the refractive index of the immersion oil The prepared immersion oil is measured by a refractometer, and two kinds of refractometers are now introduced.
(1) Abbe refractometer Abbe refractometer can measure the refractive index of liquid or solid, in which the refractive index of liquid is mainly measured. The measurement range is N=1.300 to 1.700, and a new Abbe refractometer is capable of measuring the refractive index value of 1.86. The simple steps for measuring the refractive index value using the Abbe refractometer manufactured by Shanghai Optical Instrument Factory are as follows:
1 Dip the oil onto the lower prism in the Abbe refractometer double prism box, close the double prism and lock the prism box handle.
2 Adjust the mirror to make the field of view bright, rotate the prism to turn the handwheel, and the light and dark boundary line appears to the lens barrel.
3 Turn the achromatic prism handwheel to eliminate the dispersion, and a clear black and white boundary appears in the lens barrel. Rotate the prism and turn the handwheel to move the black and white line to the center of the crosshair.
4 In the reading lens barrel, read the index value of the oil immersion through the reading line, which can be directly read to the third digit after the decimal point, and the fourth digit after the decimal point can be estimated. After the test, the oil on the prism must be wiped clean.
For newly purchased or long-term unused refractometers, the refractive index scale value should be checked and corrected using the standard glass block attached to the Abbe refractometer (with the refractive index value engraved on it). The calibration method is as follows:
1. Flatten the Abbe refractometer, open the circular hole on the prism box, make the prism face up, drop a drop of oil (the refractive index is greater than the refractive index of the standard glass block), and put the standard glass block And exclude bubbles.
2 Rotate the prism to turn the handwheel so that the reading in the reading mirror is at the refractive index of the standard glass block.
3 Rotate the achromatic prism to make a clear black and white boundary appear in the telescope's field of view. If the black and white boundary passes through the center of the crosshair, the Abbe refractometer is normal; if the boundary does not pass through the center of the crosshair, twist the correction screw (located in front of the telescope tube) and move the black and white boundary to the center of the crosshair.
Abbe refractometers are easy to operate and can be read directly with an accuracy of 0.0004 ±, so they are widely used. With achromatic prism system, the average dispersion of the liquid can be obtained. The dispersion curve can be made with the monochromator. There is a hot water device, and the temperature change curve can be made to obtain the temperature coefficient.
(2) Reflective single-circle angle measuring prism refractometer This refractometer is also called single-turn goniometer. This instrument has high precision and a large measuring range, which can measure not only liquid but also solid. The disadvantage is that the instrument is expensive and the adjustment, correction and calculation are cumbersome, so it is not as common as the Abbe refractometer.
(2) Preparation of oil-impregnated flakes
The preparation of the oil-impregnated sheet is carried out as follows:
(1) The mineral to be tested (crystalline or amorphous) is broken into uniform fine particles, and the diameter is generally 0.03 to 0.05 mm. Be careful not to grind when crushing to maintain the original state of the material debris and prevent many fine powders from sticking to the particles.
(2) Take a small amount of mineral debris (10-20 pieces) in the center of the carrier glass, and spread the debris evenly with a knife.
(3) Cover the mineral debris with a cover glass. The cover glass should not be too large as long as the size of the cover glass is generally 1∕4.
(4) Using a dropper in the oil-immersed bottle, take a small amount of oil so that the end of the dropper is close to the edge of the cover glass, relying on capillary action, so that the oil immersion slowly fills the gap between the cover glass and the glass, and Until the bubble exists, the mineral to be tested is surrounded by oil immersion. The immersion oil should not be too much, otherwise the substance to be tested will float and affect the observation.
After the sheet is made, it can be identified. At this time, the stage is preferably laid flat to prevent the measured mineral from slipping.
In actual work, it is necessary to continuously change the oil immersion. If the mineral debris is small, the oil can be sucked out by the filter paper, and then the new oil is replaced. In order to ensure sufficient accuracy, the oil immersion oil should be dripped in the same way, and the chip is used. After rinsing the new immersion oil for 2 to 3 times, the new immersion oil is added to the measurement; if the amount of debris is large, the immersion oil sheet can be washed off and re-sliced.
(III) Method for comparing the refractive index values ​​of minerals and oil immersion in clastic oil-impregnated sheets 1. Becker line method The principle of comparing the refractive index of minerals and oil immersed oil is the same as the principle of comparing the refractive indices of different minerals in flakes. The refractive index of the mineral is determined according to the movement law of the Baker line. Lifting the lens barrel, the Baker line moves toward the mineral, indicating that the mineral has a refractive index greater than that of the immersion oil; conversely, the oil immersion has a refractive index greater than that of the mineral. When viewed with monochromatic light, when the edge of the mineral and the Becker line disappear, the refractive index of the mineral is equal to or nearly equal to the refractive index of the oil immersion. [next]
2. Dispersion method Under the illumination of white light, when the refractive index difference between mineral and immersion oil is very small (below 0.010), two bands of blue and orange are often present at the edge of contact between mineral and immersion oil. This is due to the difference in refractive index dispersion between the oil immersion and the mineral. The refractive index dispersion of the oil immersion is generally greater than the refractive index dispersion of the mineral. The dispersion curve of the oil immersion is steep, and the dispersion curve of the mineral is relatively flat. The intersection of the two dispersion curves is different, and the resulting dispersion is different. The movement of the lens barrel and the ribbon is different.
(1) When the refractive index of the mineral is greater than that of the oil immersion (Fig. 1), for blue light, the refractive index of the mineral is equal to that of the immersion oil; for yellow light, the refractive index of the mineral is greater than that of the immersion oil. At this point, the barrel is lifted, the orange-yellow ribbon moves toward the mineral, and the light blue ribbon barely moves (Fig. 1b, c).

Figure 1 shows the principle of ribbon formation and the law of ribbon movement when the lens barrel is raised when the refractive index of the mineral is greater than that of the oil immersion
(The direction of the arrow represents the direction in which the ribbon moves, and the length of the arrow represents the speed at which the ribbon moves)

(2) When the refractive index of the mineral is less than that of the oil immersion (Fig. 2), for blue light, the refractive index of the mineral is smaller than that of the immersion oil for orange light, and the refractive index of the mineral is equal to that of the immersion oil. At this time, the lens barrel is lifted, and the light blue belt moves toward the oil immersion. The orange band is almost motionless (Figures b, c).

Figure 2: When the refractive index of mineral is less than oil immersion, the principle of ribbon formation and the law of ribbon movement when lifting the lens barrel [next]

(3) When the refractive index of the mineral is equal to the refractive index of the immersion oil (the refractive index is equal to the yellow light), the refractive index of the yellow mineral is equal to that of the immersion oil; for blue light, the refractive index of the mineral is smaller than that of the immersion oil; For orange light, the mineral has a refractive index greater than that of the oil. At this point, the barrel is lifted, the orange-yellow ribbon moves toward the mineral, the light blue ribbon moves toward the immersion oil, and the speed of movement of the two is approximately equal to the width of the ribbon (Fig. 3b, c).

Figure 3: The formation principle and movement law of the ribbon when the refractive index of the mineral is equal to the oil immersion

3. The oblique mineralization method is generally thick in the middle, thin in the edge, and approximately lenticular. When the mineral crumb is higher than the immersion oil, the light transmitted through the mineral debris is concentrated, and vice versa. Light dispersion, as shown in Figure 4. If a spacer is placed on the optical path of the microscope, part of the light is blocked (sometimes a partial polarizer can be pushed in instead of the spacer), and as a result, the mineral particles are dark and bright, because the inverted image is seen in the microscope. The bright and dark edges are opposite to the actual position.

Figure 4 [next]

As seen in Figure 4, when the mineral crumb has a higher refractive index than the oil immersion, the dark side is on the same side as the separator. When the mineral debris has a lower refractive index than the oil immersion, the dark side and the separator are on the opposite side. When white light is used, when the refractive index of the mineral debris is close to that of the oil immersion, the dispersion phenomenon is also observed. The mineral debris appears blue near the dark side of the field of view and orange on the other side.
The advantage of the oblique illumination method is that it can simultaneously compare the mineral debris with different refractive indices in all the fields of view and the oil immersion. The disadvantage is that when the mineral debris is close to the oil immersion oil, the oblique method is not as good as the above method. accurate.
(4) The step of measuring the refractive index value of the mineral by the oil immersion method. The mineral oil-impregnated sheet is placed under the orthogonal polarizer. If all the mineral particles are fully extinction, the mineral is a homogeneous body; if the mineral debris has an interference color The rotating object is dark and dark, and the mineral is heterogeneous.
1. Method for determining the refractive index of homogeneous minerals The homogeneous body has only one refractive index, and the refractive index of the mineral can be measured by comparing the relative sizes of the oil immersion oil and the mineral refractive index by the Beck line and the dispersion band. By changing the oil continuously until the mineral and the immersion oil have the same refractive index, or between the two bottles of immersion oil in the complete immersion oil.
The oil change interval is determined according to the height of the protrusion in the oil immersion and the apparent degree of the contour. If the protrusion is high and the contour is obvious, the oil change can be separated by a few bottles, otherwise the oil needs to be changed in order. Each observation must be recorded as follows (for example):
The first time: the oil immersion index = 1.652, the mineral refractive index < immersion oil, the contour is obvious, the protrusion is higher.
The second time: the oil immersion index = 1.571, the mineral refractive index < immersion oil, the profile is not obvious, and the protrusion is low.
The third time: the oil immersion index = 1.568', the mineral refractive index < immersion oil, the outline is not obvious, and the protrusion is very low.
The fourth time: the oil immersion index = 1.565, the mineral refractive index > oil immersion, the outline is extremely inconspicuous, and the protrusion is low.
From the above records, the refractive index of the chips is between 1.568 and 1.565, which should be:

1.568+1.565
——————— = 1.5665
2

2. Determination method of refractive index of heterogeneous minerals Select a particle with the lowest interference color and determine the axial and optical properties under a cone microscope. If it is a two-axis crystal, the size of 2V can be estimated.
(1) Method for determining the primary refractive index of a monoaxial crystal In the oil-impregnated sheet, the particle with the highest interference color of the same mineral is selected, and under the cone mirror, the transient interferogram is the main axis of the parallel axis of the one-axis crystal. section. The two main refractive indices of Ne and No can be measured on this cut surface. First, let Ne parallel PP, push up the upper polarizer, compare the relative size of Ne and immersion oil refractive index; turn the stage to make NO parallel PP, compare the relative size of NO and immersion oil refractive index. The Ne and NO values ​​can be measured by constantly changing the oil. Minerals that are parallel to the optical axis are generally difficult to find. When changing oil, it is best to use oil to change the oil.
(2) Method for measuring the refractive index of biaxial crystals The biaxial crystals have three main refractive indices of Ng, Nm and Np, and the measurement methods are as follows:
To determine the primary refractive index of the biaxial crystal, it is necessary to find two kinds of directional slices perpendicular to one optical axis and parallel optical axis. A vertical optical axis slice is fully extinct or grayish black under an orthogonal mirror. Under the cone mirror, an optical axis interferogram is perpendicular to the two-axis crystal. The slice of the light rate is a circular section with a radius equal to NM, parallel to pp in any direction, and its refractive index is NM, which is determined in the same manner as the homogen. If you can't find the mineral particles of the vertical optical axis, you can find the oblique optical axis of the vertical optical axis. This particle is characterized by a low interference color. The interferogram is characterized by a black arm that is always parallel when the optical axis plane is parallel to pp or AA, and the black arm passes through the center of the field of view and bisects the field of view. At this time, the direction of the vertical black arm is NM. Nm is parallel to pp, and the relative magnitudes of Nm and immersion oil refractive index are compared under a single polarizer. By constantly changing the oil, the size of Nm can be measured.
Select a particle with a parallel optical axis and measure the Ng and Np values. This particle has the highest interference color and a transient interferogram under the cone mirror. Determine the direction of Ng and Np under orthogonal mirror, make NG parallel lower polarizer, push up the upper polarizer, compare the relative size of Ng and immersion oil refractive index, rotate the stage 90 ̊, make Np parallel lower polarizer, compare The relative size of the refractive index of Np and immersion oil. By continuously changing the oil, the refractive index values ​​of Ng and Np can be measured.
Nm values ​​can also be determined on slices of vertical Bxa or vertical Bxo. In addition to Nm, Ng or Np values ​​can also be measured.
When the three main refractive indices of the biaxial crystal are measured, since the oriented slice is difficult to find, the oil is generally used for oil change. In order to avoid particle orientation changes during oil change, in the preparation of the sheet, 5% of the transparent glue can be used to adhere the mineral particles to the carrier glass.

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