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Optical mineralogy is the examination of optical properties in minerals and rocks. The study of light’s behavior, properties, and diffusion, known as optics, is applied to the evaluation of rocks and minerals. We rely on the study of optical mineralogy to unravel the genesis of rocks and minerals - optical mineralogy can both help trace back geological origins and determine mineralogical evolution.
Methods and Techniques
In the field of optic mineralogy, the petrographic microscope is most frequently used to analyze minerals. Under the polarizing microscope, mineral samples are proportioned into grain mounts, or thin sections, to be observed. Polarized light microscopy enables three main features: it polarizes the light source before entering the mineral, passes the light with a polarization angle 90 degrees to the source light, and highlights the contrast of birefringent material in the mineral.
Within the study, there are a series of commonly observed properties and techniques; these include but are not limited to the following: refractive index, birefringence, pleochroism, and extinction angle. All of these properties help differentiate and categorize minerals, as well as further define geological history.
The refractive index refers to the speed of light as it spreads through a mineral. In optics, this occurrence translates to a dimensionless number. The refractive index shows the extent to which the path of light bends, or is refracted when going through a mineral.
Birefringence concerns which materials have refractive indexes, in which they rely on the polarization and propagation of light. In other words, minerals that are said to be directionally dependent are known as birefringent. While observing birefringence, it is possible to come across the phenomena of double refraction - this occurs on birefringent minerals when a ray of light is polarized and subsequently divides into two separate rays.
Following refractive index and birefringence, is the optical phenomenon of pleochroism. Pleochroism is when a mineral radiates various colors from different viewing angles - this most clearly takes place with polarized light. In relation to the optical axis, if the absorption of light changes from angle to angle in a mineral, then pleochroism ensues. Like birefringent minerals, pleochroism also occurs for anisotropic, or directionally dependent, minerals and crystals.
The extinction angle is the measure at which all light travels uninterruptedly within the mineral. If this occurs, light is unable to permeate the second polarizer. Optically, the measure stands between the general shape of the crystal or mineral, known as the habit, and when cross-polarized light dims, which is known as the mineral’s extinction. Directionally dependent minerals show extinction for every 90 degrees of rotation - however, metallic, glass, and isotropic minerals do not reveal any light.
Isotropic vs. Anisotropic
Expectedly, isotropic and anisotropic minerals reveal different optical properties. Since isotropic minerals are not directionally dependent, they have a single refractive index and no restriction on the direction of light going through them. In contrast, anisotropic minerals, making up 90 percent of solid substances, perform beam splits with their directionally dependent nature. This separation of light rays is visually detailed by polarizing microscopy.
Minerals vary in appearance and the study of optical mineralogy enables scientific explanation for these visual variations. Quartz and calcite are identifiably transparent, while chlorite is green, and garnets are pink, and optical mineralogy allows us to understand why these minerals are alike and different. Every mineral will have multiple characteristics, whether it be tints, cleavage, or refractive index. Minerals can weather better than others; some will break down easily and become cloudy, whereas others can sustain clarity and newness, and even produce secondary products from their original makeup.
As steps are taken to identify minerals, these varying characteristics must be considered for the purpose of discovery, categorization, and classification. Optical mineralogy differentiates minerals by evaluating chemical composition and structure.
Sources and Further Reading