Minerals

Introduction

How is a mineral different from a rock? A useful analogy is to think about a rock as a chocolate chip cookie, which is comprised of different ingredients: flour, eggs, white sugar, brown sugar, vanilla, and, of course, chocolate chips. Similarly, most rocks are comprised of multiple "ingredients" and these consist of different types of minerals.

The International Mineralogical Association recognizes approximately 5,500 different kinds of minerals. Minerals make up the rocks of the Earth’s lithosphere and mantle. They contain many of the nutrients in soil necessary for plant growth. They are crushed and processed to extract metals for all purposes and are also used in construction. Minerals that are used in manufacturing include silica for glass, kaolinite for ceramics, gypsum for wallboard (drywall), fluorite for the fluoride in toothpaste, montmorillonite clay for kitty litter, and halite (better known as common table salt and rock salt) to clear ice from roads. Some minerals are considered to be precious or semi-precious gemstones and are used in jewelry (learn more about gems below). Others are used to dye fabric and color paint. Metallic minerals have many applications in manufacturing. For example, iron comes from hematite and magnetite, and from it we make steel. Lead, from the mineral galena, is used in the manufacture of batteries and in the solder found in electronic devices. Titanium, from the mineral ilmenite, is used in airplanes, spacecraft, and even white nail polish. Aluminum comes from bauxite and is known for being both lightweight and strong—many of the parts that make up today's automobiles are made of this metal. Copper comes from a variety of copper-bearing minerals, including chalcopyrite, and is used to make electrical wire, tubing, and pipe.

In short, nearly every aspect of our lives—both biologically and socially—requires minerals. In fact, the average American uses, in one way or another, over 38,000 pounds of minerals every year (learn more).

Scientifically Speaking, What are Minerals?

To a geologist, a mineral is "a naturally occurring homogeneous solid with a definite ... chemical composition and a highly ordered atomic arrangement. It is usually formed by inorganic processes" (Klein and Hurlbut, 1999). What does this all mean?

• Naturally occurring: Not made by people.
• Homogeneous: One uniform compound.
• Solid: Not a gas or liquid.
• Definite chemical composition: Can be expressed by a chemical formula.
• Highly ordered atomic arrangement: There is a geometric relationship between the atoms that comprise the mineral.
• Inorganic processes: Not biological in origin and typically crystallizing from a liquid. (Note that this part of the definition is somewhat problematic, as some organisms use minerals such as calcite or aragonite to create their skeletons; the shells of clams and snails are examples.)

Because the structure and properties of minerals are defined by their underlying atomic composition (as is the case for all matter in the universe), a brief, simplified review of atomic-level chemistry is required.

In order to make use of minerals, we must first be able to identify them. This means understanding how minerals are formed, what they’re made out of, and how their constituent atoms and elements are put together. Earth scientists who identify and research minerals are called mineralogists.

Other Properties of Minerals

Besides color, luster, hardness, and cleavage, other properties of minerals also help aid in their identification. For example, some minerals, especially metals, are much denser than others. Finding the exact density is straightforward, but it does require measuring the volume of the sample. Placing an unknown mineral in water (or other liquid) to find its volume by displacement can be a risky undertaking since several minerals react violently with water, and many more break down with exposure. Some minerals like magnetite are magnetic (affected by magnetic fields), while a few are natural magnets (capable of producing a magnetic field).

Some minerals have rare properties. For example, there are minerals that exhibit luminescence of all types, giving off light due to a particular stimulus. Some minerals are radioactive, usually due to the inclusion of significant amounts of uranium, thorium, or potassium in their structure. Carbonate minerals will effervesce when exposed to hydrochloric acid. Double refraction describes the result of light passing through a material that splits it into two polarized sets of rays, doubling images viewed through that material. For example, a single line on a sheet of paper will appear as two parallel lines when viewed through a clear calcite crystal.

What Distinguishes a Regular Mineral from a Gem?

Minerals are assigned to the category of gemstones based primarily on our interpretation of what has value. Typically, the beauty, durability, and rarity of a mineral qualify it as a gemstone. Beauty refers to the luster, color, transparency, and brilliance of the mineral, though to some degree it is dependent on the skillfulness of the cut. Not all gems are prized for these reasons; for example, scarcity may be artificially inflated, or a mineral may be valued for its unusual color. (A recent article in the New Yorker magazine by Ed Caesar provides an overview of how the global diamond market operates, as well as the hunt for the world's largest diamonds.)

Gemstones can be further categorized as precious or semiprecious stones. Precious stones, including diamond, topaz, and sapphire, are rare and translucent to light. They are more durable because they are hard, making them scratch resistant. On the Mohs scale of hardness, the majority of precious gemstones have values greater than 7. Semi-precious stones are generally softer, with hardness scale values between 5 and 7. The minerals peridot, jade,  garnet, amethyst, citrine, rose quartz, tourmaline, and turquoise are examples of semi-precious stones that can be cut and used in jewelry.

Gems may have common names that differ from their geological ones, and these names may be dependent on mineral color. For example, the mineral beryl is also referred to as emerald, aquamarine, or morganite depending on its color. Corundum can also be called sapphire or ruby, and peridot is another name for olivine.

How Do Mineral Deposits Form?

Economically recoverable mineral deposits are formed by geologic processes that can selectively concentrate desirable elements in a relatively small area. These processes may be physical or chemical, and they fall into four categories:

Magmatic Processes

Magmatic processes separate minor elements of magma from the major elements and concentrate them in a small volume of rock. This may involve either the early crystallization of ore minerals from the magma while most other components remain molten or late crystallization after most other components have crystallized. Magmatic processes responsible for the formation of mineral deposits are usually associated with igneous intrusion (formed during mountain building events, rifting, and volcanic activity), which can range in composition from granite (felsic) to gabbro (mafic). Metamorphism may also cause recrystallization of minerals and concentration of rare elements. Under conditions of extreme high-temperature metamorphism, minerals with the lowest melting temperatures in the crust may melt to form small quantities of pegmatite magmas.

Hydrothermal Processes

Hydrothermal processes involve hydrothermal solutions that dissolve minor elements dispersed through large volumes of rock, transport them to a new location, and precipitate them in a small area at a much higher concentration. Hot water enriched in salts such as sodium chloride (NaCl), potassium chloride (KCl), and calcium chloride (CaCl2) is called a hydrothermal solution, or simply "brine." The brine is as salty or even saltier than seawater, acidic, and may contain minute bits of dissolved minerals such as gold, lead, copper, and zinc. The presence of salt in the water stops the metallic minerals from precipitating out of the brine because the chlorides in the salt preferentially bond with the metals. Additionally, because the brine is hot (from over 600°C (~1100°F) to less than 60°C (140°F)), the minerals are more easily dissolved, just as hot tea dissolves sugar more easily than cold tea does.

Hot water brines can have varying origins. Most bodies of magma contain mineral-enriched, superheated water, which is released into the surrounding rock as the magma cools. Rainwater can become a hydrothermal solution as it filters through rocks and picks up soluble materials along its path. Seawater, which is already enriched in salt, often becomes a hydrothermal solution in the vicinity of volcanic activity on the ocean floor where tectonic plates are pulling apart.

Hydrothermal solutions move away from their source of heating through cracks, faults, and solution channels into the adjacent cooler rocks. Some of these fluids may travel very long distances through permeable sedimentary rock. As the water moves quickly through fractures and openings in the rock (where it experiences changes in pressure or composition and dilution with groundwater), it can cool rapidly. This rapid cooling over short distances allows concentrations of minerals to be deposited. When a hydrothermal solution cools sufficiently, the dissolved salts form a precipitate, leaving behind concentrated mineral deposits in a vein or strata-bound deposit.

Sedimentary Processes

Sedimentary processes gather elements dispersed through large volumes of water and precipitate them in a sedimentary environment, such as in sedimentary layers on the ocean floor or on lakebeds. Sedimentary mineral deposits form by direct precipitation from the water.

Weathering and Erosion

Weathering and erosion break down large volumes of rock by physical and chemical means and gather previously dispersed elements or minerals into highly concentrated deposits. Residual weathering deposits are mineral deposits formed through the concentration of a weathering-resistant mineral, as a result of surrounding minerals being eroded and dissolved. In contrast, mineral deposits formed by the concentration of minerals in moving waters are called placer deposits.

Classification of Minerals

The approximately 5,500 different types of minerals are classified based upon their underlying chemistry. Even though there are thousands of minerals, only 30 or so are common and abundant in Earth's crust. These are referred to as the "rock forming minerals." These are organized here into two broad groups: 1) silicate minerals; and 2) non-silicate minerals, which include native element minerals, halides, oxides, sulfides, sulfates, and carbonates.

These mineral groups are presented on the following pages, beginning with silicate minerals.

References and Further Reading

Caesar, Ed. The woman shaking up the diamond industry. The New Yorker, February 3, 2020 issue.

International Mineralogical Association. List of Minerals. Last updated January 2020.

Klein, C., and C. S. Hurlbut, Jr. 1999. Manual of Mineralogy, 21st Edition, revised. John Wiley & Sons, Inc. New York, 681 pp.

Mason, B., and C. B. Moore. 1982. Principles of Geochemistry, 4th Edition. John Wiley & Sons, Inc., New York.