# Relationship between isogyre curvature and 2v angle

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The most simple device to increase the visual angle is a magnifying glass (loupe) . Bottom: Relation between 2V and isogyre curvature for crystal sections. 2V may be estimated from the curvature of the single isogyre of a centered optic curvature at the 45° position is a function of 2V, ranging from a right-angle bend the relation between isogyre curvature and 2V: Common olivine: slightly less. OPTICAL PROPERTIES OF MINERALS Nature of light (EM Radiation) To . to the interface is the angle of refraction= r The relation between the two angles is Sin . Maximum curvature of isogyre can also be used to estimate 2V angle.

Just as in uniaxial minerals, the crystal would be extinct with the analyzer inserted when either of the privileged directions are parallel to the polarizer. Note that this would not be the maximum interference color for this crystal. In this orientation the interference color exhibited by positions off of extinction would again reflect the thickness of the crystal and the birefringence. If a face like D were lying parallel to the stage, it would show the circular section and one of the optic axes would be orientated perpendicular to the stage.

For this orientation the crystal would show no change in relief on rotation of the stage with the analyzer not inserted, and would remain extinct through a o rotation with crossed polarizers. Faces like E and F, if lying parallel to the stage, would have privileged directions corresponding to random vibration directions. Biaxial Interference Figures Four primary types of biaxial interference are seen.

Only two of these are commonly used, but it is essential to discuss all four so that you can recognize each. The dark isogyres mark the positions where light vibrating parallel to the polarizer has passed through the crystal. At the points of maximum curvature of the isogyres are the two melatopes that mark the positions where rays that traveled along the optic axis emerge from the field of view. Note that the distance between the two melatopes is proportional to the angle 2V between the optic axes.

Also seen are isochromes, which show increasing interference colors in all directions away from the melatopes. The number of isochromes and maximum order of the interference colors seen will increase with increasing thickness and absolute birefringence of the crystal. Shown in the figure is the trace of the optic axial plane which includes the two optic axes.

As the stage is rotated 45o from this initial position, the isogyres will close to produce a cross. In this position the crystal would be extinct in orthoscope mode. The melatopes will be rotated so that both lie along the N-S cross hair.

Rotation by an additional 45o will result in the isogyres then separating again to show the interference figure in the third diagram. Another 45o rotation will again cause the isogyres to close into a cross, this time with the OAP lying parallel to the polarizing direction of the microscope.

The crystal would again be extinct in orthoscope mode. Another 45o rotation would return the view to the first diagram in the series. This is similar to the BXA figure, except one of the isogyres and melatopes will be outside of the field of view unless the 2V angle is very small.

During rotation of the stage, the melatope will remain at the cross-hair intersection and the isogyres will close to form an off-centered cross and then separate to show the curved isogyre in the adjacent quadrant of the field of view. OA figures are easiest to find among randomly oriented grains, because a grain that shows such a figure will show no change in relief on a o rotation analyzer outand will remain extinct through a o rotation analyzer inserted.

Still, every 90o the broad cross will form as the OAP becomes parallel to one of the cross hairs. Optic Normal Figure O. In this figure, when one of the two privileged directions lines up with the polarizer, a broad cross covering almost the entire field of view will be observed. This cross, however will quickly disappear after just a slight rotation of the stage this is why it is often called a flash figure.

If one sees an optic normal figure, then the interference colors observed in orthoscope mode will reflect the absolute or maximum birefringence of the mineral, as discussed above. Determination of Optic Sign Biaxial interference figures are most useful for the determination of optic sign and estimation of the 2V angle, both of which are useful diagnostic properties of biaxial minerals.

There should be an area near the melatopes that shows a 1o gray interference color. Observe this area as you insert the nm or 1o red compensator. These additional properties include color, color changes, cleavage, absorption spectra, transparency, luster, pleochroism, luminescence, fluorescence, and other optical phenomena. Some of these additional phenomena are adularescence, asterism, aventurescence, chatoyancy, iridescence, labradorescence, opalescence, play-of- color, and various inclusions.

Please refer to my "Geological and Mineralogical Glossary" page for the definitions of these terms. Most minerals are usually white or colorless in a pure state. Many impurities can color these minerals and make their color variable. Growth imperfections interfere with light passing through the crystal making it appear darker or nearly black.

Idiochromatic minerals are "self colored" due to their composition. The color is a constant and predictable component of the mineral. Allochromatic minerals are "other colored" due to trace impurities in their composition or defects in their structure.

In this case, the color is a variable and unpredictable property of the mineral.

Pseudochromatic minerals are "false colored" due to tricks in light diffraction. The color is variable but a unique property of the mineral, such as the colors produced in precious opal and the shiller reflections in Sunstone and Labradorite.

Color is the most important characteristic of gemstones, though in the case of most Diamonds it is the absence of color which is most important.

What is responsible for the variations in color? Color is produced by the way a gemstone absorbs light. Light is an electromagnetic vibration at certain wavelengths, but the human eye can only perceive certain wavelengths. The field of the visible color spectrum includes red, orange, yellow, green, blue, and violet.

There are several different reasons why the various gemstone varieties absorb light differently. Some gemstones are said to be idiochromatic or self-colored.

They absorb certain wavelengths of light due to characteristics of their chemical structure. Most gemstones are allochromatic. They are colored by impurities or trace elements in their crystal structure.

If all the different wavelengths of light pass through a gemstone, it will appear colorless. On the other hand, if the gem material absorbs all the light, it will be appear black.

If a stone absorbs all wavelengths except those in the red part of the spectrum, the gem will appear red. The relationship between a chemical impurity and a gemstone color is not a simple one. Sometimes a similar color can result from different trace elements. Also, a single trace element can produce different colors in different gem varieties.

### OPTICAL PROPERTIES OF MINERALS | Bhem Cabalquinto - guiadeayuntamientos.info

This is because there is a complex relationship between the gem's crystal structure and the trace elements.

Another way in which gemstones acquire color is through human intervention in the form of gem treatments. Heat treatment is often used to change the chemical state of an impurity to deepen or lighten color, reduce a certain hue, or improve clarity. All gemstone treatments must be disclosed by the vendor prior to the sale of the gemstone.

The colors of some minerals and gemstones can be altered by time or exposure to sunlight or bright display lights. Some may fade, while others may oxidize. Some porous gems, such as Agate, Lapis Lazuli, Pearls, and Turquoise may be treated to stabilize their color. Gemstones that have had their color altered through the various treatments may also fade, change color, or become spotty over time. Some may fade, while others can darken.

Temporary color changes may also occur. Not all of a given gemstone or mineral will be sensitive to light. Also, certain color varieties or individual colors of a mineral or gem will be light sensitive.