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The optical microscope is a fundamental analytical tool used by geologists to identify minerals in a rock sample. With regards to petrological microscopy, two major classes of minerals exist; those that transmit light and those that reflect light.
Mineral types that transmit light include the majority of silicates, such as the common rock-forming minerals quartz (SiO2), olivine [(Mg,Fe)SiO4) and feldspars of various compositions, as well as carbonates, such as calcite (CaCO3) and dolomite [(Mg,Ca)CO3]. Opaque minerals that reflect light, rather than transmit it, include many oxide phases such as magnetite (Fe3O4) and sulphide ore minerals like chalcopyrite (CuFeS2) and sphalerite [(Zn,Fe)S].
In transmitted light microscopy a section of rock is polished to a typical thickness of 25μm, mounted on a glass slide using araldite epoxy resin and placed over an aperture through which polarised light is shone. In reflected light microscopy the light is shone down the lens to reflect off a polished block of rock sample mounted in resin. In either instance the brightness can be adjusted to highlight or improve the visibility of certain features.
The shape a mineral grain exhibits can indicate, qualitatively, what it might be. A euhedral crystal, one where the crystal is seen in its typical crystal form without being cut by other minerals that formed before it or deformed into another shape through post-formation processes, will exhibit a somewhat unique shape. Quartz, for example, is known for forming crystals with a hexagonal cross-section. However, such cross-sections are also seen in euhedral olivine crystals. Other optical properties must, therefore, be considered for identification.
Relief (how much a mineral grain “stands out” from its surrounding) can be used to discern between certain minerals. Minerals with higher refractive indices show distinct contours resulting from refraction and internal reflection of light at mineral interfaces. Larger differences in refractive indices between adjacent grains impart a stronger outline and pronunciation of the more refractive phase.
Cleavage planes within certain minerals, such as pyroxenes and amphiboles, can be distinguished from each other based on the angle between their cleavage planes. Pyroxenes show two planes at 90o to each other while amphiboles typically show oriented at 120o. Olivine, on the other hand, shows no cleavage planes but will exhibit conchoidal fractures curving throughout. Micas, including muscovite and biotite, show only one cleavage plane oriented parallel throughout.
The color of minerals under the microscope, generated through selective absorption of certain wavelengths is a useful diagnostic tool. Discerning between micas can be done based on their color with Fe-bearing biotites appearing cream to brown and muscovites appearing white. Biotites are strongly pleochroic, meaning when the microscope stage is rotated the intensity of the color they exhibit changes under different crystal orientations.
Most microscopes are fitted with an analyzer. This can be inserted to set the vibration directions of the light perpendicular to each other. Under “crossed-polars” minerals will exhibit interference colors if they are anisotropic, meaning their crystal atoms or ions are arranged in different ways in different axial directions. If the mineral is isotropic, like garnet which crystallizes in the cubic form, then under crossed polars it appears black. The order of interference color is indicative of the mineral species.
In reflected light microscopy reflectance is the first property that becomes apparent with pyrite (FeS2), typically stated as having a reflectance of ~55%, galena (PbS) ~43% and magnetite (Fe3O4) ~20%. The resin that the polished block is set in is generally around 5% and many silicate and carbonate phases show a similar reflectance to the resin. Reflectance is the proportion of reflected light to incident light.
Pleochroism may also be exhibited along with bireflectance, this latter property being a change in reflectance depending on the orientation of the examined crystal. Bireflectance is exhibited by hematite (Fe2O3), while covellite (CuS) is strongly pleochroic changing between deep blue and blue-white depending on orientation.
Anisotropism, similar to the interference colors seen in transmitted light, is a property of those opaque minerals with atoms arranged in different ways in different directions. Changes in color and brightness will be observed in anisotropic minerals under crossed polars, while isotropic minerals will appear black.
There is some middle-ground in terms of minerals being either opaque or transparent. Many minerals allow some light to penetrate beneath the mineral surface and reflect it off flaws or cracks in the crystal. The occurrence and color of these internal reflections can be diagnostic with sphalerite [(Zn,Fe)S] exhibiting red-brown internal reflection and cassiterite (SnO2) exhibiting yellow internal reflections.
When viewing minerals down the microscope the surrounding phases can affect how the central grain is viewed, so using as many features to identify a mineral is always recommended.
Sources
- Craig JR, Vaughan DJ (1994) Ore microscopy and ore petrography. 2nd Edition. John Wiley and Sons.
- Raith MM, Raase P, Reinhardt J (2012) Guide to thin section microscopy. 2nd Edition.
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