optics

Optical Properties of Minerals

The specimens on the left show some of the glorious variety of color in minerals. Color in minerals results from the way light interacts with certain atoms, or groups of atoms, present in the mineral.

It can come about in three ways. First, the color-active atoms may be a principal component of the mineral, such as copper in the dioptase specimen (A). Second, the color may be due to trace amounts of an atom or a mistake in the crystal structure. For example, iron in quartz produces amethyst (B). Finally, the color effect may be produced by the inclusion of small particles of a different mineral of substance in the mineral, as with hematite inclusions in quartz, resulting in jasper (C).

The yellow of sulfur (42) and the golden metallic sheen of pyrite (44) are examples of color that can be used as a diagnostic property. In many minerals, however, the color can vary over an immense range, mostly because of impurities. Even in such instances, the color range of a mineral can be a useful diagnostic property.

Optical properties occur because of interference in the light rays as they strike and interact with the mineral, causing a play of colors called iridescence. Labradorite (D) and the opal (E) are particularly striking examples of this property.

The reflection of light in a starlike pattern, from the polished piece of corundum, is known as asterism (F), and hence this specimen is called a star sapphire. This effect is well displayed in the Star of India in the Morgan Gem Hall.

In the top photo you can see some very important optical properties. First is the phenomenon known as streak, which is the color of the powder made from a mineral (you can see the color of the streak made on the white piece of paper under the mineral). Intriguingly, the color of the powder does not always match the color of a mineral. This contrast is well-demonstrated by the white powder and the lavender crystal of specimen number 46, flourite, at the upper right.

The next property is known as luster, the surface appearance of a mineral. Moving from the resinous sulfur (42) to the silky asbestos (G), to the waxy chalcedony (H), it is easy to see that the mineral kingdom displays a wide range of lusters.

Bear in mind that these different lusters are caused not only by the kind of mineral, but also by the mineral's texture and by the way the mineral grew.

Text from Natural History Museum, New York, NY