Optical Mineralogy Microscopy: Instruments and Techniques

Optical mineralogy microscopy usually utilizes polarized light microscopes and electron microscopes as tools for the study of earth materials. Theoretically, optical mineralogy microscopy is very often a means to achieve an end and is very rarely the end itself, whether it is being used for professional or educational purposes. At its epicenter, optical mineralogy microscopy aims to achieve knowledge in the nature and characteristics of minerals not only to study their origin but also to determine their uses.

What is optical mineralogy?
Optical mineralogy is just one of the subjects included in the study of geosciences, where earth materials such as minerals are examined and analyzed through the measurement of their optical properties. This is to help distinguish different minerals from one another and determine their origin. With a small number of exceptions including organic substances, fluids, melts and glasses, optical mineralogy extends its reach to all the significant elements that make up the earth’s materials, with the aim to reveal their characterization, structure and composition.

With the use of optical mineralogy microscopy, certain characteristics of minerals can be determined. Some minerals, for example, may appear transparent and colorless. Using the appropriate microscopy technique, certain colors that have previously remained invisible may be observed. The minerals’ shapes and outlines may also be seen, most characteristics that are often not revealed when observed with the naked eye.

Techniques used for optical mineralogy microscopy
The most commonly used technique in optical mineralogy microscopy is polarized light microscopy. This technique helps determine a mineral’s refractive index, birefringence, pleochroism and extinction angle – properties that help a geoscientist examine and analyze the nature and characteristics of a mineral sample.

Refractive index
The refractive index of any substance, including minerals, indicates the measurement of how light is affected as it travels through the specimen, expressed in numerical value. Fused quartz glass, for example, will have a refractive index of 1.46 while zircon will have 1.92. The refractive index of a diamond is 2.42. Generally, the denser the substance, the higher its refractive index. The maximum refractive index value is also dependent on the mineral, an important consideration in optical mineralogy microscopy.

Birefringence
The birefringence is the difference between two light rays’ refractive indices, expressed in numerical terms. When light travels through a specimen, it is split into two components, each one having a refractive index of its own, called a refringence. This occurs because a faster beam of light will emerge first when it bounces off a mineral, with an OPD or optical path difference. The pyroxene mineral augite, for example has a maximum refractive index of 1.724 and a minimum refractive index of 1.700. The difference, or birefringence of this mineral is 0.024.

Pleochroism
In polarized light microscopes, often used in optical mineralogy microscopy, a polarizing filter is utilized to observe a mineral. This is often used in conjunction with a rotating specimen stage. This allows passing light to vibrate at right angles. For colored mineral samples such as tourmaline and chlorite, rays of light will appear in different colors. When rotated on the specimen stage, these colors change. This property of the material is called pleochroism and is an important component of optical mineralogy microscopy, allowing observers to determine mineral characteristics.

Extinction angle
When rotating a specimen using an optical mineralogy microscope such as the polarized light microscope, certain sections will appear gray or black when the mineral’s position is changed. This is called the extinction angle. An observer can then take note of this extinction, measure its angle and compare it with that of other structures in the mineral. These angles can help determine to which group of minerals the sample belongs to because certain groups exhibit similar characteristics.

Ideally, optical mineralogy microscopy should allow easy manipulation and access to the specimen, where an observer can rotate materials to examine it in detail or to produce its extinction angles. Alternatively, a fixed stage microscope may also be used, provided there are polarizer and analyzer prisms or filters.

The future of optical mineralogy microscopy
Earth materials will always need to be examined, if only to determine their uses and reveal their evolution through the ages. Aside from polarized light microscopy, other techniques such as electron microprobes and x-ray diffractions may also be used to determine a mineral’s chemical composition and structure.

While extremely valuable as tools of analysis and investigation, these techniques have not been successful at totally eradicating the use of polarized light microscopy. On the contrary, the need to produce a more reliable and detailed study of minerals have made it important that different tools be used in optical mineralogy microscopy, considering that data and sample collection continues to improve over time.