Gold, diamonds, and emeralds are all beautiful, but they are also all minerals. That is, they are natural substances with distinct chemical compositions that exhibit crystals. Minerals can be striking, like these gemstones, or relatively dull and mundane in appearance, such as calcite, barite, and feldspar.
Knowing that minerals can look so different from each other, what do all minerals have in common? What are the types of minerals found on Earth? How can different kinds of minerals that look very similar be distinguished from one another? Which minerals are abundant, and which are rare?
What All Minerals Have In Common
All minerals are naturally-occurring (i.e., not man-made) solids. Although some minerals can certainly be synthesized -- diamonds, for example, are routinely made for industrial applications -- all minerals are found in nature. A substance birthed only in a laboratory is not considered a true mineral.
In addition, all minerals have a crystal structure. This means that they are composed of atoms arranged in a repeating, three-dimensional pattern. (Materials such as glass lack this orderly structure.)
Each mineral has a unique chemical composition that determines its crystal structure, physical properties, and how it reacts with other materials. Sometimes the chemical composition of a given mineral can vary slightly, giving rise to "mineral cousins" that look somewhat different from each other, but these "cousins" are otherwise chemically identical and are thus still the same mineral. For example, both ruby and sapphire are gem-quality versions of the mineral corundum (aluminum oxide); their different colors come from small amounts of chromium (ruby) or titanium (sapphire) that contaminate the corundum crystal structure. In contrast, corundum that isn't beautifully colored is used as an abrasive; for example, the scratchy part of an emery board is made from tiny particles of corundum.
Minerals include only inorganic materials. Therefore, both diamonds and coal are made of the element carbon, but only diamond is a mineral. Coal, the result of heat and pressure applied to the remains of plants, is organic in origin and hence not considered a mineral.
Types of Minerals
There are eight types of minerals, which are grouped based on their chemistry:
- Native elements
- Oxides and hydroxides
- Sulfides and sulfasalts
- Sulfates
- Halides
- Carbonates
- Phosphates
- Silicates
Different anions (negatively charged form of an atom) are characteristic of each of these mineral types. These anions and examples of minerals for all eight mineral types are given below:
Native elements: No anion; consists of just one kind of atom; e.g. Gold (Au)
Oxides and hydroxides: oxygen or hydroxyl ion; e.g., Hematite and brucite
Sulfides and sulfasalts: Sulfide ion; e.g. Pyrite
Sulfates: Sulfate ion; e.g. Barite
Halides: Chloride, Fluoride, Bromide, or Iodide ions; e.g. Halite (NaCl)
Carbonates: Carbonate ion; e.g. Calcite
Phosphates: Phosphate ion; e.g. Apatite
Silicates: Silicate ion; e.g. Quartz
How to Tell Different Minerals Apart
- Hardness: Hardness is how easily a mineral can be scratched, and it is assessed using Moh's hardness scale. The scale goes from 1 (softest) to 10 (hardest). For example, the softest mineral, talc, has a hardness of 1 and can be scratched by a fingernail. Calcite has a hardness of 3 and can be scratched with a copper coin. Diamond, the hardest natural material known, has a hardness of 10. Only a diamond can scratch another diamond.
- Cleavage: How a mineral breaks when it is struck. This breakage pattern, which is defined by a mineral's crystal lattice, has a shape that is characteristic for each mineral. For example, mica is easily identified by its tendency to break into very thin sheets.
- Fracture: Sometimes, a mineral breaks in a less orderly, but still predictable, fashion. This type of characteristic breakage is called fracture. Quartz, for example, exhibits conchoidal fracture (i.e., the edge of a broken quartz specimen looks like broken glass).
- Color: The color of a sample of a mineral. Color is not a good criterion for identifying minerals because small impurities in mineral structure can greatly influence color (consider how sapphires and rubies are chemically identical except for tiny impurities that make them different colors).
- Streak: The color a mineral makes when it is used to "write" on a white ceramic plate. For example, a hematite sample can look red, brown, or black, but when hematite is scraped across a ceramic plate it always leaves a reddish streak.
- Luster: The way the surface of a mineral reflects light. For example, pyrite, or "fool's gold", is very shiny and has a metallic luster.
- Density: How much mass of a mineral is packed into a given volume. Gold is very dense, while mica is much less dense. The density of a mineral is determined by how heavy its atoms are, and how closely packed they are in its crystal structure.
- Habit: Habit simply means "What shape is a single crystal of the mineral?". These crystal shapes are determined by the crystalline arrangement of the mineral at the atomic level. For example, quartz's six-sided crystal habit is easy to recognize.
Common Minerals Versus Rare Minerals
Silicates are by far the most common minerals on Earth (their major components, silicon and oxygen, are in fact the most abundant elements in the Earth's crust). Carbonates and oxide minerals are also very common.
Native elements, such as gold, silver, copper, diamond, and lead, are much rarer. In fact, gold represents just 0.004 ppm (parts per million) of the Earth's crust. The rarity of native elements explains why they are relatively valuable and labor-intensive to obtain.
References
Understanding Earth, 2nd Edition. Frank Press and Raymond Siever. 1994. W.H. Freeman and Co.
Manual of Mineralogy, 20th Edition (after James D. Dana). Cornelius Klein and Cornelius S. Hurlbut, Jr. 1985. John Wiley & Sons, Inc.
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