Light in Minerals

Image credit: Scott Brande

The Bottom Line on Non-Metallic Minerals - Here On Top!

Bottom Line: If a mineral transmits any light, it is non-metallic. But if fragment of a sample remains dark in front of a bright light source, we may not conclude the mineral is metallic. Details and explanation below.

Errors/Suggestions: Contact Scott Brande (see footer).

Degrees of Transparency

When lights falls on a mineral, it may interact with the chemical compound in different ways. What's important to us for mineral identification are three terms: opaque (opacity), translucent (translucency) and transparent (transparency).

• opaque minerals block light from penetrating through the surface (and therefore through the mineral); one can only see the surface of the crystal

• translucent minerals permit some light to enter the interior of the mineral, but the light does not pass through unimpeded; writing or an object cannot be distinguished through a crystal

• transparent minerals pass light largely unimpeded; writing or an object can be seen through the crystal in bright enough light

The term "diaphanous" (diaphaneity) refers to the degree of transparency according to these different interactions of light with a mineral.

Opacity

Opaque samples block the transmission of all light through the mineral. A single mineral crystal may appear opaque for various reasons, including

• a weathered surface that scatters and blocks light

• a thicker sample that cumulatively absorbs light as it passes through the mineral

• a darker color due to elements in the compound (such as metals) that selectively absorb light

• a metallic element that reflects light

Translucency

Translucent samples transmit only some of the light through the mineral. A single mineral crystal may appear cloudy for various reasons, including

• inclusions of clay or other external material incorporated into the crystal as it grows

• flaws in the crystal lattice that scatter light

• the sample is composed of many crystals, oriented in different directions

Transparency

Transparent samples transmit light through the mineral as in a piece of clear glass. However, we recognize that minerals may be colored as well as transparent, as long as the mineral appears clear enough for writing or an object to be seen through the crystal.

Transparency in Calcite Crystals

Transparency in Gypsum Crystals

Bright Light Through Thin Fragments

Recognizing Non-metallic Minerals by Light Transparency

One fact that distinguishes metallic minerals from non-metallic minerals is that metallic minerals reflect all light at the surface. However, because light may be partially or totally blocked from passing through a mineral for some of the other reasons (above), we can't be sure that a mineral is metallic when a sample is opaque. However, if a sample does pass any light, we can be sure that is non-metallic.

Thin Fragments Test for the Passage of Light

Even the thinnest of fragments of metallic minerals reflect all light at the surface. Consider that even a strong back light will not shine through a thin piece of aluminum foil.

If a small, thin fragment of a mineral DOES transmit ANY light from behind it, we must conclude the sample is non-metallic.

In the images below, small fragments of crushed minerals (a few millimeters in size) are placed on a white translucent plastic bottle cap. The bottle cap is placed over the lens of a bright flashlight. Paired images of reflected and transmitted light demonstrate whether or not light passes through the fragments. If light is transmitted through the fragment, the sample is non-metallic.

Note: If the sample remains dark in transmitted light (opaque), we CAN'T conclude that the mineral is metallic because of the many other factors that might block the passage of light - see the section on Opacity above.