Opaque Minerals, Isotropic Minerals, Anisotropic Minerals, Birefringence and Interference Colors

Opaque minerals do not transmit light in thin sections. So, they appear black in both PP and XP light at all times. Common opaque minerals are graphite, oxides such as magnetite or ilmenite, and sulfides such as pyrite.

Isotropic minerals are minerals that have the same properties in all directions. This means light passes through them in the same way, with the same velocity, no matter what direction the light is travelling. There are few common isotropic minerals; the most likely ones to see in thin section are garnet and spinel.

Anisotropic minerals have different properties indifferent directions. So, light travels through them in different ways and with different velocities, depending on the direction of travel through a grain.

Isotropic and anisotropic minerals are, most of the time, easily distinguished because isotropic minerals do not transmit light (are always black) when viewed under XP light. A complication arises because anisotropic minerals will appear isotropic if grains are oriented in a specific way -- if they are oriented so they are viewed "down an optic axis." Additionally, as the microscope stage is rotated, anisotropic minerals in any orientation go "extinct" (turn black) every 90o. The odds of a grain lying in just the "wrong" orientation so as to cause confusion are small, but to overcome this, you should always look at a number of different grains of the same mineral, and rotate the stage for each, to determining if it is isotropic or anisotropic.

Interference colors and birefringence: Anisotropic minerals, unless viewed down an optic axis, cause polarized light to be split into two rays as it travels through a grain. The rays may not travel at the same velocity or follow the exact same path. A value, termed birefringence, describes the difference in velocity of the two rays. When the rays emerge from the grain, they combine to produce interference colors. Interference colors are only seen in XP light! Note that the colors may be any hue in the specturm. As birefringence increases, the colors repeat (see figure below), but get more and more pastel (washed out). To describe interference colors we must specify both a hue and an order (e.g., 2nd order red; see chart below). Minerals with low birefringence show only white, gray and black interference colors. Minerals with very high birefringence -- such as calcite -- show such weak colors that they may appear "pearl" white.

Note: Interference colors are combinations of differen wavelengths and so are difficult to reproduce on a web page.
The colors shown below may not match exactly what is seen through a microscope.

Using interference colors to identify a mineral: Birefringence and interference colors vary for a given mineral depending on the direction the light follows through a grain. They also change as the microscope stage is rotated. If you are looking down an optic axis, the mineral appears to have no birefringence. So, you should look at a number of grains to determine a mineral's maximum birefringence. The "maximum" (highest order) interference colors are diagnostic for a given mineral. In the chart above, the colors under the mineral names are typical interfence colors for the given mineral. However, interference colors depend on many things (including grain orientation, thin section thickness, and variations in mineral composition) and so the colors indicated shown can only be considered general guides. In general, the exact color is not as important as the "order." With a ltitle bit of practice, determining the order of interference colors, at least for 1st, 2nd and 3rd order, is not too difficult.

Anomalous interference colors: Some minerals show interference colors that are anomalous. This means the colors do not show on charts such as the one above. Typically, these minerals have very low birefringence. Sometimes, however, anomalous colors are caused by other things.

 

Example of Interference Colors: Biotite, Cordierite and Garnet

The above view shows a large grain of cordierite on the left and a large grain of garnet on the right. A minor amount of brown (PP) biotite is also present. In the XP view, the cordierite shows first order gray and white interference colors. The garnet is isotropic and so appears black in the XP view. The biotite doesn't show easily identified interference colors because the brown color of the grain masks them. Still, with a little imagination you can see biotite may have second order colors.

The field of view is about 2.5 mm.


Example of Interference Colors: Biotite, Quartz and Graphite in a Mica Schist

Here we see mostly brown biotite flakes. The biotite show various shades of brown and tan in PP light (it is pleochroic). Biotite has moderately high birefringence. In the XP view, the biotite interference colors range up to 2nd order red (with just a hint of 2nd order blue).

The quartz is clear in the PP view, and shows maximum interference colors of 1st order gray in the XP view.

Note the opaque graphite -- it is black in both views.

The field of view is about 2.5 mm.


Example of Interference Colors: Calcite and Diopside

This view shows several large diopside grains (high relief with cleavage) surrounded by calcite. In the PP view, both are nearly clear, although the calcite has been stained red to help identification. In the XP view, the diopside shows maximum interference color of 1st order yellow. The calcite, on the other hand, shows interference colors of such high order that they appear white. The twin lamellae in the calcite show hints of high-order pastels -- this is typcial of carbonate minerals and others with very high birefringence.

The field of view is about 2.5 mm.