Diaphaneity refers to a mineral’s ability to transmit light. For example, some minerals are transparent. When they are thick, a small amount of distortion may occur, but light passes relatively freely through them. Thus, we can see the clouds in the distance through the clear Iceland spar (calcite) crystal in Figure 3.38. Because it is so clear, Iceland spar has been used in some industrial applications. (Unfortunately, it is not very durable because calcite is very soft.) Very few minerals are as transparent as the calcite seen here. Thin sheets of muscovite and some quartz come close.
Minerals that do not transmit light as well as clear calcite may be translucent. Although it is not possible to see through them as with transparent minerals, if thin enough, translucent minerals transmit light. Both the whitish calcite and the darker colored orpiment in this photo (Figure 3.39) are translucent. Many other minerals exhibit this same property.
For example, quartz comes in many different colors but, unless very finely crystallized, it is generally translucent to some degree. Calcite, gypsum, topaz, and many micas, are often commonly translucent.
Some minerals are neither transparent nor translucent, and are opaque instead. Opaque minerals, do not transmit light unless the mineral is exceptionally thin. Most opaque minerals have metallic lusters and belong to the sulfide, oxide, or native element groups. This photo (Figure 3.40) shows a hexagonal flake of opaque molybdenite (MoS2) on top of translucent quartz. Pyrite and magnetite are two more common examples of opaque minerals.