1.2.5: Crystalline and Non-Crystalline Mineral Materials

Later chapters will discuss crystals and crystallinity in more detail. For now, it is sufficient to know that crystalline means “having an orderly and repetitive atomic structure.” The definition of a mineral given above includes all crystalline materials made by geological processes. Because minerals are crystalline, they must be solids. However, the International Mineralogical Association (IMA) has granted a special exception to mercury.

1.20 Cinnabar with native mercury

Mercury, although liquid under Earth-surface conditions, is considered a mineral because it is a naturally occurring native element like copper, gold, silver, and several others (that are solids except at high temperature). Figure 1.20 shows red, partially cubic crystals of the mineral cinnabar (mercury sulfide) from a famous mineral locale near Almaden, south of Madrid in Spain. Also seen are shiny droplets of silver-colored liquid mercury. Both cinnabar and mercury are minerals. Note that water, the most common natural liquid at Earth’s surface is not a mineral, but ice – crystalline H₂O – meets every requirement of the definition and is.

1.21 Obsidian and other volcanic debris in Iceland

Some natural geological substances appear a lot like minerals but are not crystalline. Instead, they are amorphous solids, meaning they contain a random arrangement of atoms. With a few exceptions (e.g., the copper carbonate georgeite), such materials are not accepted as minerals by the IMA and are generally called mineraloids. Examples of mineraloids include obsidian and several other varieties of natural volcanic glass. This photo shows black obsidian in Iceland. The material surrounding the obsidian is made of scoria and other volcanic debris. Obsidian and other volcanic glasses form when lava cools so quickly that atoms do not have time to arrange themselves in an orderly and repetitive way.

1.22 Opal

Opal contains silicon and oxygen. It has about the same composition as quartz but contains up to 10% water. On an atomic scale, opal is not crystalline; it comprises spheres of silica (SiO₂), 150-300 nanometers in diameter, arranged in a random pattern. The spheres cause light refraction and give some opal a beautiful appearance with rainbow-like colors. If the colors change when the sample is viewed at different angles, we call the property a play of colors. The sample in Figure 1.22, from Ethiopia, displays a beautiful example of play of colors. Because of its play of colors, and because it is very hard, opal is often a much-prized gemstone. The most common kind of opal is like the opal shown, with a rainbow of pastel hues. Black opal is darker colored in blue, gray and green. Fire opals contain brilliant sparks of red, yellow and orange. Like mercury, the IMA has granted opal the right to be called a mineral even if it is not crystalline.

Agate and chalcedony, two varieties of microcrystalline silica (SiO₂) related to opal, have the same composition as quartz, but strictly speaking are not minerals because they are not crystalline. They also, typically, consist of more than one mineral. Some mineralogists, however consider agate and chalcedony to simply be textural varieties of quartz.