The optical properties of minerals refer to their behavior in the presence of light and how they interact with light when observed using various optical techniques. These properties include transparency/opacity, color, luster, refractive index (RI), pleochroism, birefringence, dispersion, extinction, and crystallography.
- Color: The color of a mineral can be a useful diagnostic tool. However, it should be noted that color can vary greatly depending on impurities, and so it is not always a reliable indicator of a mineral’s identity.
- Luster: Luster refers to the way that a mineral reflects light. Minerals can be metallic, glassy, pearly, or dull, and each type of luster can be used to help identify a mineral.
- Transparency: Some minerals are transparent, while others are opaque. Minerals that are transparent can be further categorized as either colorless, colored, or pleochroic (displaying different colors when viewed from different angles).
- Refractive index: The refractive index of a mineral is a measure of how much light is bent as it passes through the mineral. This property can be used to identify a mineral by measuring the angle at which light is refracted.
- Birefringence: Birefringence refers to the property of a mineral that causes light to split into two rays as it passes through the mineral. This property is particularly useful for identifying minerals in thin sections under a microscope.
- Dispersion: Dispersion refers to the way that different colors of light are refracted at different angles by a mineral. This property is particularly useful for identifying gems such as diamonds.
- Pleochroism: Pleochroism refers to the property of a mineral that causes it to display different colors when viewed from different angles.
- Fluorescence: Some minerals exhibit fluorescence, meaning that they emit light when exposed to ultraviolet light. This property can be used to help identify minerals in certain settings.
Overall, optical properties are an important diagnostic tool for identifying minerals. By understanding these properties and how they relate to each other, mineralogists can determine the identity of a mineral with a high degree of accuracy.
- Optical Microscopy
- Why use the microscope ?
- Minerals and propogation of light
- Thin section
- Properties of Light
- Plane Polarized Light (PPL):
- XPL (Crossed Polarizers):
- Passage of Light
- Refractive Index
- Opaque Mineral
- Transparent mineral
- Becke Line
- Metamict Texture
- Colour in PPL
- Isotropic Minerals
- Between crossed polars
- Anisotropic minerals
- 1-Light passes through the lower polarizer
- Color & Pleochroism
- Index of Refraction (R.I. or n)
- 2 – Insert the upper polarizer
- 3 – Now insert a thin section of a rock
- 4 – Note the rotating stage
- Estimating birefringence
- Twinning and Extinction Angle
- Appearance of crystals in microscope
Optical microscopy, also known as light microscopy, is a widely used technique in the field of mineralogy for the identification and characterization of minerals. It involves the use of a microscope that utilizes visible light to magnify and analyze mineral samples. Here are some key points about optical microscopy in mineralogy:
- Principle: Optical microscopy is based on the interaction of light with minerals. When light passes through a mineral sample, it can be absorbed, transmitted, or reflected depending on the mineral’s optical properties, such as color, transparency, and refractive index. By observing how light interacts with a mineral under a microscope, valuable information about its physical and optical properties can be obtained.
- Equipment: Optical microscopy requires a specialized microscope equipped with various components, including a light source, lenses, a stage for holding the mineral sample, and eyepieces or a camera for viewing and capturing images. Polarizing microscopes, which use polarized light, are commonly used in mineralogy for studying the optical properties of minerals.
- Sample Preparation: Mineral samples for optical microscopy are typically thin sections or polished thin mounts, which are prepared by cutting a thin slice of a mineral specimen and mounting it on a glass slide. Thin sections are commonly used for studying the mineralogy of rocks, while polished thin mounts are used for analyzing individual mineral grains.
- Techniques: Optical microscopy techniques used in mineralogy include transmitted light microscopy, which involves passing light through a thin section or thin mount to observe the mineral’s internal features, and polarized light microscopy, which involves the use of polarized light to study the mineral’s optical properties, such as birefringence, extinction, and pleochroism. Other techniques, such as reflected light microscopy and fluorescence microscopy, may also be used for specific purposes in mineral identification and characterization.
- Mineral Identification: Optical microscopy is a powerful tool for mineral identification based on their physical and optical properties. By observing a mineral’s color, transparency, crystal shape, cleavage, and other features under a microscope, and by using techniques such as polarization and interference, mineralogists can identify minerals and differentiate between different mineral species.
- Limitations: Optical microscopy has some limitations in mineralogy. It may not be suitable for identifying minerals with similar physical and optical properties, or minerals that are very small or opaque. In such cases, other techniques such as X-ray diffraction, electron microscopy, or spectroscopy may be required for more accurate mineral identification and characterization.
Optical microscopy is a fundamental and widely used technique in mineralogy, providing valuable information about the physical and optical properties of minerals, which is essential for their identification and characterization.
Why use the microscope ?
Microscopes are used in mineralogy for a variety of reasons:
- Mineral Identification: Microscopes are used to observe the physical and optical properties of minerals, such as color, transparency, crystal shape, cleavage, and other features, which are essential for their identification. By examining mineral samples under a microscope, mineralogists can gather critical information that helps them identify different mineral species and distinguish between similar minerals.
- Mineral Characterization: Microscopy allows for the detailed characterization of minerals, including their crystal structure, texture, and inclusions. This information provides insights into the formation and history of minerals, which can be important for understanding their properties and applications.
- Mineralogical Research: Microscopy is used in mineralogical research to study the optical, chemical, and physical properties of minerals, as well as their relationships with other minerals and rocks. Microscopic analysis can provide valuable data for understanding mineral occurrences, mineralogical processes, and geological history.
- Mineral Processing: Microscopy is used in the field of mineral processing to analyze and optimize the beneficiation of ores and minerals. By examining mineral samples under a microscope, mineral processing experts can assess the mineral liberation, mineral associations, and mineralogical characteristics of ores, which can help in developing effective mineral processing strategies.
- Geological Mapping: Microscopy can be used in geological mapping and mineral exploration to identify and map minerals in rocks and ores. This information can be used to understand the distribution, composition, and economic potential of mineral deposits in a given area.
- Education and Teaching: Microscopes are widely used in educational settings to teach students about mineralogy and geology. By using microscopes, students can observe and identify minerals, and learn about their properties, occurrences, and uses.
In summary, microscopes are essential tools in mineralogy for mineral identification, characterization, research, mineral processing, geological mapping, and education. They allow for detailed observation and analysis of minerals, providing valuable insights into their properties, occurrences, and applications.
Minerals and propogation of light
The propagation of light through minerals is a fascinating topic in mineralogy and is closely related to the optical properties of minerals. When light passes through a mineral, it may undergo various interactions, such as absorption, reflection, refraction, and polarization, which can provide important information about the mineral’s composition, structure, and properties. Here are some key points related to the propagation of light in minerals:
- Transparency and Opacity: Minerals can be transparent, translucent, or opaque to light, depending on their chemical composition and internal structure. Transparent minerals allow light to pass through with little or no scattering, while translucent minerals scatter light to some extent, and opaque minerals do not allow light to pass through at all.
- Absorption: Some minerals have selective absorption of certain wavelengths of light due to the presence of specific chemical elements or compounds. This results in the mineral appearing colored when viewed under a microscope or with the naked eye. The absorption spectrum of a mineral can provide information about its chemical composition.
- Refraction: Refraction is the bending of light as it passes from one medium to another with a different refractive index. Minerals with different crystal structures and chemical compositions can exhibit different refractive indices, which can be determined using a refractometer. The refractive index is an important optical property used in mineral identification.
- Polarization: Light passing through certain minerals can become polarized, meaning the light waves oscillate in a particular direction. This property can be observed using a polarizing microscope, which allows for the examination of minerals in cross-polarized light. Polarized light microscopy is a powerful technique used in mineral identification and characterization.
- Pleochroism: Some minerals exhibit pleochroism, which means they show different colors when viewed from different angles under polarized light. This property is caused by the preferential absorption of light in different directions due to the mineral’s crystal structure and can be used as a diagnostic tool in mineral identification.
- Birefringence: Birefringence, also known as double refraction, is the property of certain minerals to split light into two rays with different refractive indices. This can be observed using a polarizing microscope, and the amount of birefringence can provide information about the mineral’s crystal structure and composition.
- Optical Sign: The optical sign of a mineral refers to the direction in which the mineral’s refractive indices are oriented with respect to its crystallographic axes. The optical sign can be determined using a polarizing microscope and is an important characteristic used in mineral identification.
The study of how light interacts with minerals and how it propagates through them is crucial in mineralogy, as it provides important information about the mineral’s composition, structure, and properties. Optical properties of minerals, such as absorption, refraction, polarization, pleochroism, birefringence, and optical sign, are used in mineral identification, characterization, and research. Microscopic techniques, such as polarizing microscopy, are widely used to study the propagation of light through minerals and reveal important details about their optical properties.
A thin section refers to a thin slice of a rock or mineral that is mounted on a glass slide and ground down to a thickness of typically 30 micrometers (0.03 mm) using specialized equipment. Thin sections are used in petrology, a branch of geology that studies rocks and minerals under a microscope to determine their mineral composition, texture, and other important characteristics.
Thin sections are created by cutting a small piece of rock or mineral into a thin slab, which is then affixed to a glass slide using an adhesive. The slab is then ground down to the desired thickness using a series of abrasive materials, such as silicon carbide powder, to achieve a smooth and even surface. The resulting thin section is then polished to improve transparency and clarity, and may be stained with dyes or chemicals to enhance certain features or properties.
Thin sections are commonly examined under a polarizing microscope, also known as a petrographic microscope, which is equipped with polarizers and analyzers that allow for the study of the rock or mineral’s optical properties, such as birefringence, pleochroism, and extinction angles. By analyzing the minerals and their arrangement in the thin section, geologists can identify the rock type, determine the mineral composition, and interpret the rock’s history, such as its formation and deformation processes.
Thin sections are widely used in various fields of geology, including igneous petrology, sedimentary petrology, metamorphic petrology, economic geology, and environmental geology. They are essential tools for studying rocks and minerals at a microscopic level and provide valuable insights into their origin, evolution, and properties. Thin sections are also commonly used in education and research, as they allow for detailed examination and analysis of rocks and minerals, contributing to our understanding of Earth’s geology and its history.
Properties of Light
- Wave-like nature: Light exhibits wave-like properties, such as wavelength, frequency, and amplitude. It can be described as an electromagnetic wave that travels through a medium or vacuum.
- Particle-like nature: Light also behaves as a stream of particles called photons, which carry energy and momentum.
- Speed: Light travels at a constant speed of about 299,792 kilometers per second (km/s) in a vacuum, which is the fastest known speed in the universe.
- Electromagnetic spectrum: Light exists in a range of wavelengths and frequencies, which together form the electromagnetic spectrum. This spectrum includes different types of light, such as visible light, ultraviolet (UV) light, infrared (IR) light, X-rays, and gamma rays, each with its own unique properties and uses.
Plane Polarized Light (PPL):
- Polarization: Light waves can be polarized, which means that their oscillations occur in a single plane, as opposed to in all directions. Polarized light has a specific orientation of its electric field vector.
- Polarizers: PPL is created by passing unpolarized light through a polarizer, which is a filter that transmits only the light waves oscillating in a specific plane while blocking those oscillating in other planes.
- Properties: PPL has properties such as direction, intensity, and color that can be used to study and analyze various materials, such as minerals and crystals, under a polarizing microscope.
XPL (Crossed Polarizers):
- Technique: XPL is a technique used in polarized light microscopy, where two polarizers are crossed, meaning their polarization planes are perpendicular to each other.
- Interference: When a thin section of a mineral or a crystal is placed between crossed polarizers, it can create interference patterns known as interference colors or birefringence, which provide information about the mineral’s optical properties, such as refractive index and crystal structure.
- Identifying minerals: XPL is commonly used in mineralogy to identify and characterize minerals based on their unique interference patterns and birefringence colors, which can help in determining the mineral’s composition, crystal structure, and other properties.
Passage of Light
Reflection is a process in which light, or other forms of electromagnetic radiation, bounces off a surface and returns back into the same medium from which it originated, without changing its frequency or wavelength. This phenomenon occurs when light encounters a boundary between two media with different refractive indices or optical densities.
Key points about reflection:
- Angle of incidence and angle of reflection: The angle at which light strikes a surface is called the angle of incidence, and the angle at which it is reflected is called the angle of reflection. According to the law of reflection, the angle of incidence is equal to the angle of reflection, and the incident ray, the reflected ray, and the normal (a line perpendicular to the surface) all lie in the same plane.
- Specular vs. diffuse reflection: Reflection can be either specular or diffuse. Specular reflection occurs when light reflects off a smooth surface, such as a mirror, and the reflected rays maintain their original direction and form a clear reflection. Diffuse reflection occurs when light reflects off a rough or irregular surface, such as paper or a matte surface, and the reflected rays scatter in different directions, resulting in a less clear reflection.
- Applications of reflection: Reflection is used in many everyday applications, such as mirrors, reflective surfaces on vehicles and road signs for visibility, optical devices like telescopes and microscopes, and in photography and art for creating visual effects.
- Law of reflection: The law of reflection states that the angle of incidence is equal to the angle of reflection, and the incident ray, the reflected ray, and the normal all lie in the same plane. This law is fundamental in understanding the behavior of light when it encounters a reflective surface.
In summary, reflection is the process in which light or other forms of electromagnetic radiation bounce off a surface and return back into the same medium from which it originated, without changing its frequency or wavelength. It involves the angle of incidence and angle of reflection, can be specular or diffuse, has many practical applications, and follows the law of reflection.
The velocity of light depends on the medium through which it passes.Light is an electromagnetic wave which interacts with electrons.The distribution of electrons are different for each material and sometimes for different directions through a material.When light passes from one medium to another there is a difference in velocity. Light rays apparently bend at the contact
Angle of incidence ≠ Angle of Refraction.
The amount of refraction is related to the difference in velocity of light in each medium.Refractive index (R.I.) for air is defined as 1
The absolute refractive index for a mineral (n) is the refraction relative to that in air.
- depends on the atomic/crystal structure
- is different for each mineral
- is constant for a mineral
- is a diagnostic property of the mineral
- between 1.3 and 2.0
There may be one, two or three values of R.I. depending on the atomic structure of the mineral.
Opaque minerals are minerals that do not transmit light and do not allow light to pass through them. They appear opaque or dull when viewed under a microscope or with the naked eye, as they do not have the ability to transmit light through their structure.
Opaque minerals are typically composed of materials that are not transparent or translucent to light due to their physical and chemical properties. They may contain various impurities, minerals, or elements that absorb or scatter light, preventing it from passing through.
Some examples of opaque minerals include native metals like gold, silver, and copper, as well as sulfides like pyrite, galena, and chalcopyrite. These minerals are commonly found in ore deposits and are often associated with metallic ore deposits. Other opaque minerals include certain oxides, carbonates, and sulfates, which can have metallic or non-metallic compositions.
Transparent minerals are minerals that allow light to pass through them, making them appear clear or translucent when viewed under a microscope or with the naked eye. These minerals have a crystalline structure that allows light to pass through their lattice, allowing them to transmit light without scattering or absorbing it.
Transparent minerals can be found in a wide range of colors and can exhibit various optical properties such as pleochroism (change in color with orientation), birefringence (double refraction), and interference colors when viewed under a polarized light microscope. These properties can be used to identify and differentiate transparent minerals.
Some examples of transparent minerals include quartz, calcite, feldspar, garnet, tourmaline, and topaz. These minerals are commonly found in rocks and minerals from various geological settings and have diverse applications in industry, jewelry, and scientific research.
Becke line is an optical phenomenon observed when a mineral or other transparent material is immersed in a liquid with a different refractive index. It is a useful technique used in optical mineralogy for determining the relative refractive index of a mineral compared to the surrounding medium, which can provide information about the mineral’s optical properties.
When a mineral is placed on a glass slide and immersed in a liquid with a refractive index higher or lower than that of the mineral, a bright or dark border appears along the edge of the mineral, respectively. This border is called the Becke line. The direction in which the Becke line moves when the focus is changed can provide information about the relative refractive index of the mineral compared to the surrounding medium.
The Becke line phenomenon occurs due to the difference in refractive indices between the mineral and the surrounding medium. When the refractive index of the medium is higher than that of the mineral, the Becke line moves towards the mineral, and when the refractive index of the medium is lower than that of the mineral, the Becke line moves away from the mineral. The position and movement of the Becke line can be observed and analyzed under a polarized light microscope, and it can be used as a tool for identifying minerals and determining their optical properties.
The Becke line is a valuable tool in optical mineralogy for studying the optical properties of minerals, including their refractive indices, birefringence, and other optical characteristics. It is widely used in the identification and characterization of minerals in geology, petrology, and materials science.
Relief, in the context of optical mineralogy, refers to the difference in brightness or darkness of a mineral compared to the surrounding medium when viewed under a polarized light microscope. It is one of the optical properties of minerals that can be observed and used to identify minerals and determine their characteristics.
Relief is typically observed as a difference in brightness or darkness of a mineral compared to the surrounding medium, which is usually a glass slide or a mounting medium. This difference in brightness or darkness is caused by the difference in refractive indices between the mineral and the surrounding medium. When the mineral has a higher refractive index than the medium, it appears brighter, and when it has a lower refractive index, it appears darker.
Relief can be used as a diagnostic feature for identifying minerals, as different minerals have different refractive indices, and thus exhibit different degrees of relief. For example, minerals with high relief, appearing brighter against the surrounding medium, may indicate minerals with high refractive indices, such as quartz or garnet. Minerals with low relief, appearing darker against the surrounding medium, may indicate minerals with lower refractive indices, such as calcite or plagioclase feldspar.
Relief is typically observed and evaluated under crossed polarizers, which are commonly used in polarized light microscopy. By observing the relief of a mineral, combined with other optical properties such as color, birefringence, and pleochroism, minerals can be identified and characterized, providing valuable information for geological and materials science studies.
Cleavage, in the context of mineralogy, refers to the tendency of minerals to break along specific planes of weakness, resulting in smooth, flat surfaces. It is a property that is determined by the crystal structure of a mineral, and it can be observed and measured in thin section under a polarized light microscope.
Cleavage is a result of the arrangement of atoms or ions in a mineral’s crystal lattice. Minerals with a crystalline structure often have planes of weakness along which the bonds between atoms or ions are weaker, allowing the mineral to break along these planes when subjected to stress. The resulting surfaces are typically smooth and flat, and they can have distinct geometric patterns, depending on the crystal lattice of the mineral.
Cleavage is an important property used in mineral identification, as different minerals exhibit different types and quality of cleavage. Some minerals may have perfect cleavage, where the mineral breaks easily and smoothly along specific planes, resulting in flat surfaces with shiny or reflective appearances. Other minerals may have imperfect or no cleavage, resulting in irregular or rough surfaces when broken.
Cleavage can be described based on the number and orientation of cleavage planes. Common terms used to describe cleavage include basal (occurring parallel to the base of the crystal), prismatic (occurring parallel to elongated crystal faces), cubic (occurring perpendicular to cubic faces), and rhombohedral (occurring at angles other than 90 degrees).
Fracture is a property of minerals that describes how they break when subjected to stress, but do not exhibit cleavage, which is the tendency of minerals to break along specific planes of weakness. Unlike cleavage, which results in smooth, flat surfaces, fracture results in irregular, uneven, or rough surfaces when a mineral is broken.
Fracture can occur in minerals that lack a well-defined crystal structure or do not have prominent cleavage planes. It can also occur in minerals that have undergone deformation or have been subjected to external forces that have disrupted their crystal lattice. Fracture can be caused by a variety of factors, such as impact, pressure, or bending.
There are several types of fracture that can be observed in minerals, including:
- Conchoidal fracture: This type of fracture results in smooth, curved surfaces that resemble the inside of a seashell. It is commonly observed in minerals that are brittle and break with a glassy or vitreous appearance.
- Irregular fracture: This type of fracture results in rough, uneven surfaces with no distinct pattern. It is commonly observed in minerals that do not have well-defined cleavage planes and break randomly.
- Splintery fracture: This type of fracture results in long, splinter-like or fibrous surfaces. It is commonly observed in minerals that are fibrous in nature, such as asbestos minerals.
- Hackly fracture: This type of fracture results in jagged, sharp-edged surfaces with a haphazard pattern. It is commonly observed in minerals that are ductile and break with a tearing or ripping appearance.
Fracture can be an important property used in mineral identification, as it can provide additional information about the physical properties and behavior of minerals when subjected to stress. It can also be used to distinguish minerals with similar physical properties but different fracture characteristics.
Metamict texture refers to a specific type of texture observed in certain minerals that have been altered by high levels of radiation, typically from radioactive elements. This radiation-induced alteration causes the mineral’s crystal lattice to become amorphous, disordered, or completely destroyed, resulting in a characteristic metamict texture.
Metamict texture is commonly observed in minerals such as zircon (ZrSiO4) and thorite (ThSiO4) that contain radioactive elements like uranium (U) and thorium (Th). These minerals may undergo a process called metamictization, in which the radiation damages the crystal structure, leading to amorphization or complete destruction of the original crystalline structure.
Metamict minerals may exhibit certain characteristic features, including:
- Loss of crystalline shape: Metamict minerals may lose their typical crystal shapes and appear as shapeless masses or irregular grains under a microscope.
- Amorphous or disordered structure: Metamict minerals may lack the ordered arrangement of atoms that is characteristic of crystalline minerals, appearing amorphous or disordered.
- High relief: Metamict minerals may exhibit high relief, meaning they appear bright against a dark background under crossed-polarized light due to their amorphous or disordered nature.
- Loss of birefringence: Metamict minerals may lose their birefringence, which is the ability to split light into two different refractive indices, due to their amorphous or disordered structure.
Metamict texture can be an important diagnostic feature used in identifying and characterizing minerals that have been affected by high levels of radiation. It can also provide insights into the geological history and processes that these minerals have undergone, such as their exposure to radioactive elements, which can have implications for their potential use in geochronology, radiometric dating, and other scientific applications.
Colour in PPL
Color observed in plane-polarized light (PPL) is an important property used in the identification and characterization of minerals under a microscope. The interaction of light with minerals can result in various colors when viewed in PPL, and these colors can provide valuable information about the mineral’s composition, crystal structure, and optical properties.
In PPL, minerals can exhibit different colors depending on their optical properties, such as:
- Isotropic minerals: Isotropic minerals are minerals that do not exhibit birefringence and have the same refractive index in all directions. These minerals will appear black or gray in PPL because they do not split light into two different refractive indices.
- Anisotropic minerals: Anisotropic minerals are minerals that exhibit birefringence and have different refractive indices in different directions. These minerals can exhibit a wide range of colors in PPL, including shades of gray, white, yellow, orange, red, green, blue, and violet, depending on the mineral’s crystal structure and composition.
- Pleochroic minerals: Pleochroism is the property of some minerals to exhibit different colors when viewed along different crystallographic directions. In PPL, pleochroic minerals may show different colors when the microscope stage is rotated, providing valuable diagnostic information for identifying the mineral.
- Absorption and transmission properties: Minerals may exhibit selective absorption and transmission of certain wavelengths of light due to their chemical composition and crystal structure, resulting in specific colors being observed in PPL.
The colors observed in PPL can be used in combination with other optical properties, such as relief, cleavage, fracture, and crystal shape, to help identify and characterize minerals. It is important to consult mineral identification references and use proper mineral identification techniques and tools to accurately interpret the colors observed in PPL and make reliable mineral identifications.
Isotropic minerals are minerals that do not exhibit birefringence, which means they have the same refractive index in all directions. As a result, they do not show any interference colors or polarization effects when viewed under a polarizing microscope in plane-polarized light (PPL) or crossed-polarized light (XPL). Instead, isotropic minerals typically appear as black or gray when viewed in PPL, with no changes in color or brightness as the microscope stage is rotated.
Examples of isotropic minerals include:
- Garnet: Garnet is a common mineral group that can occur in a variety of colors, such as red, orange, yellow, green, brown, and black. It is isotropic and does not exhibit birefringence.
- Magnetite: Magnetite is a black mineral that is strongly magnetic and commonly occurs in igneous and metamorphic rocks. It is isotropic and does not show any interference colors in PPL or XPL.
- Pyrite: Pyrite, also known as “fool’s gold,” is a metallic yellow mineral that is commonly found in sedimentary, metamorphic, and igneous rocks. It is isotropic and does not exhibit birefringence.
- Halite: Halite, also known as rock salt, is a colorless or white mineral that is commonly found in sedimentary rocks. It is isotropic and does not show any interference colors in PPL or XPL.
- Sphalerite: Sphalerite is a common zinc mineral that can occur in various colors, such as brown, black, yellow, green, and red. It is isotropic and does not exhibit birefringence.
Isotropic minerals are important to identify and recognize in mineral identification using optical microscopy, as their lack of birefringence and characteristic black or gray appearance in PPL can help distinguish them from anisotropic minerals that show interference colors and polarization effects.
Between crossed polars
Isotropic minerals always look black regardless of orientation of crystal or rotation of stage
The indicatrix is a geometric representation used in mineralogy and optics to describe the optical properties of anisotropic minerals. It is a three-dimensional ellipsoid that represents the variation in refractive indices of a mineral with respect to different crystallographic directions.
Anisotropic minerals have different refractive indices along different crystallographic directions due to their internal crystal structure. The indicatrix helps to describe the relationship between the crystallographic axes of a mineral and the refractive indices associated with those axes.
The indicatrix can be visualized in three dimensions, with its axes representing the principal refractive indices of the mineral. These axes are typically labeled as n_x, n_y, and n_z, with n_x and n_y representing the two perpendicular refractive indices in the plane of the indicatrix, and n_z representing the refractive index along the optical (c-axis) direction.
The shape of the indicatrix can provide information about the optical properties of the mineral. If the indicatrix is a sphere, the mineral is isotropic, meaning it has the same refractive index in all directions. If the indicatrix is an ellipsoid, the mineral is anisotropic, meaning it has different refractive indices along different crystallographic directions.
The indicatrix is a useful tool in studying the optical properties of minerals, and it can be used to determine important optical properties such as birefringence, optic sign, and optic angle, which are critical in mineral identification and characterization.
Anisotropic minerals are minerals that exhibit different physical or optical properties along different crystallographic directions. This is due to their internal crystal structure, which results in variations in properties such as refractive index, birefringence, color, and other optical properties, depending on the direction of observation. Anisotropic minerals are also known as doubly refracting minerals because they split a single incident light ray into two rays with different refractive indices.
Anisotropic minerals can exhibit a wide range of optical properties, including pleochroism (different colors when viewed from different directions), interference colors (colors observed in polarized light), extinction (the complete disappearance of a mineral grain when rotated), and other properties that can be observed using various optical techniques such as polarized light microscopy.
Examples of anisotropic minerals include calcite, quartz, feldspar, mica, amphibole, pyroxene, and many others. These minerals are commonly found in a wide range of rock types and have important industrial, economic, and geological significance. The study of anisotropic minerals and their optical properties is a fundamental part of mineralogy and petrology, and it plays a crucial role in mineral identification, characterization, and understanding the physical and optical properties of rocks and minerals in various geological settings.
Uniaxial – light entering in all but one special direction is resolved into 2 plane polarized components that vibrate perpendicular to one another and travel with different speeds
Biaxial – light entering in all but two special directions is resolved into 2 plane polarized components…
Along the special directions (“optic axes”), the mineral thinks that it is isotropic – i.e., no splitting occurs
Uniaxial and biaxial minerals can be further subdivided into optically positive and optically negative, depending on orientation of fast and slow rays relative to xtl axes
1-Light passes through the lower polarizer
Color & Pleochroism
Color and pleochroism are important optical properties of minerals that can be observed using polarized light microscopy.
Color refers to the appearance of minerals when viewed under normal or white light. Minerals can exhibit a wide range of colors due to their chemical composition and the presence of various impurities or structural defects. Color can be used as a diagnostic property in mineral identification, although it is not always reliable as some minerals can exhibit similar colors.
Pleochroism, on the other hand, is the phenomenon where minerals exhibit different colors when viewed from different crystallographic directions under polarized light. This property is due to the anisotropic nature of minerals, which causes them to absorb light differently along different crystallographic axes. Pleochroism is often observed in minerals that have a significant difference in absorption of light along different crystallographic directions.
Pleochroism is typically observed using a polarizing microscope, where the mineral is placed between crossed polarizers, and the stage is rotated to different orientations to observe changes in color. By rotating the stage, the mineral may exhibit different colors, ranging from no color (extinction) to one or more distinct colors. The number of colors and the intensity of pleochroism can provide important clues for mineral identification, as different minerals have unique pleochroic properties.
Index of Refraction (R.I. or n)
The index of refraction (R.I. or n) is an optical property of minerals that describes how much a mineral bends or refracts light as it passes through it. It is defined as the ratio of the speed of light in a vacuum to the speed of light in the mineral.
The index of refraction is a valuable tool in mineral identification as it can help differentiate minerals with similar physical properties. Different minerals have different indices of refraction due to variations in their chemical composition, crystal structure, and density.
The index of refraction is typically determined using a refractometer, which is a specialized instrument used in mineralogy and gemology. The refractometer measures the angle at which light is bent as it passes through a transparent mineral sample, and the index of refraction is calculated based on this angle.
The index of refraction can be used in conjunction with other optical properties, such as pleochroism, extinction angle, and birefringence, to help identify minerals in thin sections or polished mineral samples. It is an important parameter in the study of minerals and their optical properties, and it can provide valuable information about the composition and structure of minerals.
Relief is an optical property of minerals that refers to the degree to which a mineral appears to stand out or contrast against the surrounding medium when viewed under a microscope in transmitted light. It is related to the difference in refractive indices between the mineral and the surrounding medium, typically a mounting medium or the mineral’s host rock.
Minerals with higher relief appear to stand out more prominently against the surrounding medium, while minerals with lower relief appear more similar in brightness or color to the surrounding medium. Relief is typically observed in thin sections of minerals using transmitted light microscopy, where the mineral is viewed between crossed polars or in plane-polarized light.
Relief can be useful in mineral identification as it can provide clues about the refractive index of a mineral, which can help narrow down the list of possible minerals based on their known refractive indices. Relief can vary depending on the mineral’s chemical composition, crystal structure, and other factors. For example, minerals with higher refractive indices, such as quartz, may exhibit higher relief, while minerals with lower refractive indices, such as feldspars, may exhibit lower relief.
Relief can also be used to determine the relative abundance of different minerals in a rock, as minerals with higher relief may appear more abundant compared to minerals with lower relief. In some cases, relief can provide information about the alteration or weathering of minerals, as altered minerals may exhibit different relief compared to unaltered minerals.
2 – Insert the upper polarizer
3 – Now insert a thin section of a rock
Conclusion has to be that minerals somehow reorient the planes in which light is vibrating; some light passes through the upper polarizer
4 – Note the rotating stage
Most mineral grains change color as the stage is rotated; these grains go black 4 times in 360° rotation – exactly every 90o
Birefringence is an optical property of minerals that refers to the difference in refractive indices between the two mutually perpendicular vibration directions of light passing through a mineral. It is typically observed in minerals under polarized light microscopy, where the mineral is viewed between crossed polars or in conoscopic view.
Estimating birefringence in minerals can be done through several methods, including:
- Visual estimation: Birefringence can be estimated visually by observing the interference colors that a mineral exhibits when viewed between crossed polars. Interference colors are a result of the phase difference between the two orthogonal light waves passing through the mineral, which is determined by the mineral’s birefringence. Using a standard reference chart or Michel-Lévy chart, the birefringence can be estimated based on the observed interference colors.
- Retardation measurement: Birefringence can be estimated by measuring the retardation of a mineral using a retardation plate or a quarter-wave plate. The retardation is the difference in optical path length between the two orthogonal light waves passing through the mineral, which is directly related to the birefringence. By measuring the retardation and applying appropriate calibration, the birefringence can be estimated.
- Birefringence dispersion: Some minerals exhibit birefringence dispersion, where the birefringence changes with wavelength of light. By measuring the birefringence at different wavelengths, such as using a conoscopic prism or a spectroscope, the birefringence dispersion can be determined, which can provide information about the mineral’s composition and optical properties.
It’s important to note that estimating birefringence is a qualitative method and may not provide precise quantitative values. The accuracy of the estimation depends on factors such as the quality of the microscope, the mineral’s thickness, and the observer’s experience and skill in interpreting interference colors or measuring retardation. Therefore, it’s often necessary to confirm birefringence estimates with other methods, such as using advanced techniques like refractometry or spectroscopy, for more accurate and precise results.
Extinction is a term used in optical mineralogy to describe the phenomenon where a mineral goes from being brightly illuminated to dark or nearly dark under crossed polars in a polarizing microscope. It is a useful property for identifying minerals and understanding their crystallographic orientation.
There are two main types of extinction:
- Parallel extinction: In this type of extinction, the mineral goes extinct (becomes dark) when its crystallographic axis is parallel to the polarizer and analyzer in a crossed polars configuration. This means that light passing through the mineral is blocked by the analyzer, and the mineral appears dark. Minerals with parallel extinction are typically isotropic or have their crystallographic axes aligned with the polarization directions of the microscope.
- Inclined extinction: In this type of extinction, the mineral goes extinct (becomes dark) at an inclined angle to the polarizer and analyzer in a crossed polars configuration. This means that the mineral is not fully aligned with the polarization directions of the microscope, and as the stage is rotated, the mineral goes from bright to dark or vice versa. Minerals with inclined extinction are typically anisotropic, meaning they have different refractive indices in different crystallographic directions.
Extinction can provide important information about the crystallographic orientation and symmetry of minerals, which can be used for mineral identification and characterization. For example, minerals with parallel extinction are typically isotropic, meaning they have the same optical properties in all crystallographic directions, while minerals with inclined extinction are typically anisotropic, meaning they have different optical properties in different crystallographic directions. The angle of extinction can also provide information about the mineral’s crystal symmetry and crystallographic orientation, which can aid in mineral identification and interpretation of the mineral’s crystal structure.
Twinning and Extinction Angle
Twinning is a phenomenon where two or more individual crystals of a mineral grow together in a symmetrical manner, resulting in a twinned crystal with characteristic intergrown patterns. Extinction angle is a term used in optical mineralogy to describe the angle between the direction of maximum extinction of a twinned mineral and the direction of maximum extinction of the untwinned mineral.
Twinning can affect the extinction behavior of minerals in a polarizing microscope. When a twinned mineral is observed under crossed polars, the extinction behavior may differ from that of an untwinned mineral due to the arrangement of the twinned crystals. Twinning can cause the extinction direction of the twinned mineral to deviate from the extinction direction of the untwinned mineral, resulting in a characteristic extinction pattern.
The extinction angle is the angle between the direction of maximum extinction of the twinned mineral and the direction of maximum extinction of the untwinned mineral. It is measured in degrees and can provide important information about the twinning type and orientation of the twinned crystals. The extinction angle is a key feature used in identifying and characterizing twinned minerals.
There are several types of twinning, including simple twins, multiple twins, and complex twins, and the extinction behavior and extinction angle can vary depending on the type of twinning. The extinction angle can be measured using a polarizing microscope with a conoscopic or conoscope attachment, which allows for precise determination of the angle between the extinction directions of the twinned and untwinned crystals.
Appearance of crystals in microscope
The appearance of crystals under a microscope depends on several factors, including the type of crystal, the lighting conditions, and the observation mode (e.g., transmitted or reflected light, polarized or unpolarized light). Here are some common appearances of crystals in a microscope:
- Euhedral Crystals: Euhedral crystals are well-formed crystals with distinct crystal faces that are characteristic of the mineral species. They typically exhibit sharp edges and smooth faces, and their crystallographic features can be easily observed under a microscope. Euhedral crystals are often seen in igneous and metamorphic rocks.
- Subhedral Crystals: Subhedral crystals are partially developed crystals that have some well-formed crystal faces but also exhibit some irregular or incomplete growth. They may have rounded edges or incomplete faces, and their crystallographic features may be less distinct compared to euhedral crystals.
- Anhedral Crystals: Anhedral crystals are poorly formed crystals that lack well-defined crystal faces and edges. They may appear as irregular grains or aggregates of mineral particles without any discernible crystallographic features. Anhedral crystals are commonly found in sedimentary rocks or in areas of rapid crystallization.
- Polycrystalline Aggregates: Polycrystalline aggregates are composed of multiple crystals that are randomly oriented and intergrown. They may appear as granular or crystalline masses under a microscope, without distinct crystal faces or edges. Polycrystalline aggregates are common in many types of rocks and minerals.
- Twin Crystals: Twin crystals are formed when two or more crystals grow together in a symmetrical manner, resulting in characteristic intergrown patterns. Twinning can create unique appearances under a microscope, such as repeated patterns, parallel or intersecting lines, or symmetrical features.
- Inclusions: Inclusions are small mineral or fluid-filled cavities within crystals that can affect their appearance under a microscope. Inclusions may appear as dark or light spots, irregular shapes, or fine patterns within the crystal, and they can provide important information about the mineral’s formation history and environmental conditions.
The appearance of crystals in a microscope can provide valuable information for mineral identification, crystallography, and understanding the formation and properties of minerals. Proper techniques in sample preparation, lighting conditions, and observation modes can enhance the visibility and characterization of crystal features under a microscope.