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Archive for March, 2008

Sign of Elongation

Thursday, March 13th, 2008

        Length fast and Length slow are the terms most commonly used in optical mineralogy. When the fast ray vibrates more or less parallel to the length of an elongate mineral, then the elongation is called length fast. On the other hand, if the slow ray vibrates more or less parallel to the length […]

Change of Relief in Anisotropic Minerals

Thursday, March 13th, 2008

        A change of relief is usually exhibited by most minerals that display moderate to strong birefringence as the stage of petrographic polarizing microscope is rotated in plane light. This characteristic of some minerals can be observed both in thin section and in grain mount when they are evaluated with the aid of the […]

Pleochroism of Anisotropic Minerals

Thursday, March 13th, 2008

When evaluated in plane light or with the upper polar removed from its slot in a petrographic polarizing microscope, many colored anisotropic minerals are found exhibiting a change of color as the stage is rotated. This property where there is a change of color as the stage of the polarized microscope is rotated is called […]

Determining the Thickness of the Mineral in Thin Section

Thursday, March 13th, 2008

    In optical mineralogy, the most commonly used material in determining the thickness of a mineral in thin section is quartz. This is because quartz is an abundant and can be easily identified from other minerals. The interference color for different quartz grains in thin section are commonly found ranging from black to a maximum […]

Extinction

Thursday, March 13th, 2008

Anisotropic minerals usually go dark or extinct when evaluated between crossed nicols once every 90-degree rotation of the stage of the petrographic polarizing light microscopes. This is of course with the orientation wherein the optic axis is vertical as prescribed in optical mineralogy rules. When one vibration direction of the mineral is oriented parallel to […]

Categories of Extinction

Thursday, March 13th, 2008

Extinction, as described in optical mineralogy, has four different categories namely parallel extinction, inclined extinction, symmetrical extinction, and no extinction angle. In parallel extinction, the extinction angle is 0-degree or when the cleavage or length is aligned with one of the other of the crosshair. Inclined extinction on the other hand is the angle measured […]

The Orders of Interference Colors

Thursday, March 13th, 2008

Most mineralogists that focus in optical mineralogy are familiar with the color chart where the interference colors are found following a repeating sequence. The sequence is usually found with the change from red to blue occurring at retardations of approximately 550, 1100, and 1650nm. These found boundaries are used to separate the sequence of colors […]

Optics of Uniaxial Minerals

Thursday, March 13th, 2008

Some of the optical properties of uniaxial minerals can be illustrated using the cleavage rhomb of calcite. This has been profoundly studied in optical mineralogy.  It has been studied that there are two images that may appear, each is composed of plane polarized light vibrating at right angles to the other if the cleavage rhomb […]

Determining the Thickness of a Mineral in Grain Mount

Thursday, March 13th, 2008

In optical mineralogy, the same technique used in determining thickness in thin section is employed in grains of mineral in a grain mount. But then this method is usually not precise because it cannot be assumed that all grains may have the same thickness. The same is thru minerals with cleavage that may not be […]

Anomalous Interference Colors

Thursday, March 13th, 2008

Anomalous interference colors as discussed in optical mineralogy are not shown in the interference color chart and there are some minerals that display this characteristic when viewed under petrographic polarizing light microscopes. When birefringence and retardation of mineral when viewed with the aid of polarized microscope are significantly different for different wavelengths of light, this […]

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