What is Mineral Birefringence?

Measuring Refractive Index

Light slows down (or bends) whenever it enters a gemstone. Mineralogists calculate a mineral's refractive index, or RI, by dividing the speed of light in a vacuum by the speed of light as it passes through the mineral. (Since the speed of light in a vacuum is always faster than the speed of light through any mineral, an RI is always a number greater than 1). Mineralogists use a device called a refractometer to measure a mineral's RI. Since the RI ranges of minerals have been well established, this is a valuable identification technique.

Which Minerals Show Birefringence?

Minerals can be categorized as anisotropic or isotropic based on whether they exhibit birefringence. This property depends on the symmetry of the mineral's crystal structure and its interaction with light.

Isotropic Minerals

Isotropic minerals with an isometric or cubic crystal system, like garnet and fluorite, have only one RI since these crystals don't polarize light. They aren't doubly refractive, they have the same RI in all directions, and, thus, they have no birefringence. Stated differently, isotropic minerals do not exhibit birefringence because their crystal structure is symmetrical in all directions, and light travels through them at the same speed regardless of direction.

Anisotropic Minerals

Birefringence occurs in anisotropic minerals, where the crystal structure causes light to split into two rays traveling at different speeds and refracted at different angles. These minerals belong to crystal systems that lack complete symmetry.

The crystal systems with birefringence are: tetragonal, orthorhombic, monoclinic, triclinic, and hexagonal. Minerals with these crystal systems are doubly refractive because they have two RIs based on the direction light enters them. Examples of anisotropic minerals are calcite, quartz, olivine, and muscovite.

How do You Calculate Birefringence?

The difference between a mineral's highest and lowest RIs is its birefringence number. The greater that number, the more noticeable the effects of double refraction will be to the naked eye.

Some minerals have ranges for RI values for each of their axes. For example, microcline, a variety of feldspar, has the following RI values:

α axis = 1.514-1.529; β axis = 1.518-1.533; and γ axis = 1.521-1.539.

In cases like these, you calculate the birefringence as a range. You take the maximum difference of the smallest values and the maximum difference of the highest values. Thus:

1.521 - 1.514 = 0.007

1.539 - 1.529 = 0.010

So, the birefringence of microcline is 0.007-0.010.

Birefringent Effects

Pleochroism

Pleochroism (the appearance of different colors when viewed from different crystallographic directions) occurs in anisotropic minerals, such as naturally trichroic zoisite, which are also capable of birefringence. You can see different pleochroic colors depending on your viewing angle.

Fuzziness

Other effects can include a fuzzy, out-of-focus appearance, such as in this piece of adamite.

Double Images

Some minerals are so birefringent that they create a double vision effect. If the stone is faceted, the facets on the opposite side of the viewer will appear to be doubled. Some minerals, such as calcite, will create a double image of whatever lies behind it.

International Gem Society

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