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Questions and Answers
Indicatrix is a hypothetical concept having practical implications.
Indicatrix is a hypothetical concept having practical implications.
True
Which of these types of interference colors are associated with Quartz?
Which of these types of interference colors are associated with Quartz?
What are the two types of interference colors of Biotite?
What are the two types of interference colors of Biotite?
What controls all the properties of minerals?
What controls all the properties of minerals?
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Calcite has four faces of __ hardness and two faces of __ hardness.
Calcite has four faces of __ hardness and two faces of __ hardness.
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Kyanite has two hardness values. What are they?
Kyanite has two hardness values. What are they?
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What is R.I. related to in mineralogy?
What is R.I. related to in mineralogy?
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The optical properties of minerals are consistent regardless of the orientation of the section cut.
The optical properties of minerals are consistent regardless of the orientation of the section cut.
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If we have minerals with one optic axis, they are classified as what?
If we have minerals with one optic axis, they are classified as what?
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What is the first mineral in which birefringence was observed?
What is the first mineral in which birefringence was observed?
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Study Notes
Mineralogy
- Indicatrix is a theoretical concept used to understand the optical properties of minerals.
- Optical properties of minerals are controlled by their structure.
- Structure controls the refractive index, and refractive index is dependent on the velocity of light.
- The refractive index (RI) determines how light passes through a mineral.
- Quartz has three interference colors: gray, yellow, and blue.
- Biotite has two interference colors: light brown and dark brown.
- Calcite has four faces with a hardness of 3 and two faces with a hardness of 2.
- Kyanite has two hardnesses: 5 and 7.5.
- The variation in optical properties can be explained by the indicatrix concept.
- The indicatrix represents the refractive index in different directions within a mineral.
- The indicatrix concept is used to understand the optical properties of cubic, tetragonal, and biaxial minerals.
Cubic Minerals
- Cubic minerals are isotropic, meaning they have the same RI in all directions.
- Indicatrix for cubic minerals is a sphere.
Tetragonal Minerals
- Tetragonal minerals have one optic axis.
- They have a circular section perpendicular to the c-axis.
- They have an elliptical section parallel to the c-axis.
- The indicatrix for tetragonal minerals is an ellipsoid.
Biaxial Minerals
- Biaxial minerals have two optic axes.
- They have two circular sections perpendicular to the optic axes.
- They have three principle sections.
- The indicatrix is a triaxial ellipsoid.
Uniaxial Minerals
- Uniaxial minerals have one optic axis.
- Tetragonal and hexagonal minerals are uniaxial.
- The principle sections are where the RI has the maximum value.
- The circular section has a constant RI value (w).
- Random sections have an RI between the minimum (w) and maximum (e).
Interference Colors
- Interference colors are produced by the interaction of light waves.
- The colors observed depend on the mineral's birefringence.
- Birefringence is the difference between the RI of the ordinary (O) and extraordinary (E) rays.
Extinction
- Extinction is the position of a mineral where it appears dark under crossed polarizers.
- Extinction is always measured in orthoscopic mode.
Pleochroism
- Pleochroism is the variation in color observed when a mineral is viewed in different directions.
- It is due to different absorption of light by the mineral in different directions.
Conoscopy vs. Orthoscopy
- Conoscopy: Light converges to a point, allowing for observation of interference figures.
- Orthoscopy: Light is parallel, allowing for observation of the mineral's internal structure.
Birefringence
- Birefringence is the difference between the RI of the E-ray and O-ray.
- It explains why crystals appear differently under polarized light.
- The E-ray has a higher RI than the O-ray.
Bxa and Bxo
- Bxa (Acute Bisectrix): the angle bisector between the two acute angles.
- Bxo (Obtuse Bisectrix): the angle bisector between the two obtuse angles.
- They are vibration directions.
Biaxial (+) and Biaxial (-) Minerals
- Biaxial (+) Minerals: The Bxa is closer to α (minimum RI) than γ (maximum RI).
- Biaxial (-) Minerals: The Bxa is closer to γ (maximum RI) than α (minimum RI).
- The optic axis is between α & γ.
Properties of Minerals
- Structure: Governs all mineral properties.
- Refractive Index (RI): Determined by the structure and affects how light passes through a mineral.
- Indicatrix: A 3D representation of RI in different directions within a mineral.
- Interference Colors: Produced due to mineral birefringence.
- Extinction: The mineral position appearing dark under crossed polarizers.
- Pleochroism: Variation in color observed when a mineral is viewed in different directions.
- Conoscopy and Orthoscopy: Two modes of examining minerals under polarized light.
- Birefringence: The difference in RI between the E-ray and O-ray.
- Bxa and Bxo: Vibration directions within a mineral.
- Biaxial (+) and Biaxial (-) : Distinguishes biaxial minerals based on the Bxa position relative to α and γ.
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Description
Explore the fascinating world of mineralogy with this quiz focused on the optical properties of minerals and the concept of the indicatrix. Understand how the structure and refractive index affect light behavior in various minerals like quartz, biotite, and calcite. Test your knowledge on the properties of cubic and other mineral structures.
