Minerals form under specific conditions. If geologists can identify the minerals present in a rock, they can use this information to understand how that rock formed. They can then identify other sources of the mineral by finding locations with similar conditions.

Minerals can form from the cooling of molten rock, from solutions, and from metamorphism. Several factors affect mineral formation, including the chemical composition of the material from which the minerals formed, and the temperature and pressure during formation.

Minerals from Molten Rock

Dr. Wise studies minerals that form from molten rock, a liquid mixture of dissolved pieces of rock, gases, and minerals. Molten rock has two different names, depending upon where it is located. Molten rock located below Earth’s surface is called magma. If molten rock erupts at the surface, it is called lava.

Molten rock can solidify, or harden, when it cools. It hardens as atoms and molecules bond with one another to form mineral crystals. Generally, larger mineral crystals can form when molten rock cools slowly, because atoms and molecules have more time to bond. The terms “larger” and “smaller” are relative, however. Larger crystals can be as big as you are, can fit in your hand, or can be seen without a microscope. Smaller crystals may only be visible with a high-powered microscope.

Magma is located underground and insulated from the cooler air and water at Earth’s surface, so it cools and solidifies very slowly. Lava, on the other hand, cools and hardens almost instantly as it comes in contact with water or air, leaving little time for molecules and atoms to bond. Therefore, mineral crystals that formed from magma are typically larger than those formed from lava.

Magma can sometimes produce abnormally large mineral crystals – as large as 11 m (36 ft) long and as heavy as 50,000 kg (55 tons)! Rocks made of these types of mineral crystals are called pegmatites. Mike Wise is particularly interested in pegmatites because the minerals they contain are often useful but rare, including beryl, lepidolite, tantalite, and topaz. The large size of these mineral crystals makes them easier and more profitable to mine. By understanding how a pegmatite forms, Dr. Wise can predict other locations where rare minerals may be found.

The chemical composition of the molten rock can also influence mineral formation. For example, silicates are a group of minerals containing silicon dioxide (SiO₂) molecules that form from molten rock. These minerals are the building blocks of most of the rocks we see on Earth’s surface. Silicates that form from molten rock rich in silicon dioxide molecules form felsic minerals, such as quartz and muscovite mica. If molten rock is low in silicon dioxide content, they form a group of silicates called mafic minerals, which include olivine, pyroxene, and amphibole.

Minerals from Solution

When a substance like salt dissolves in water, it is called a solution. Solutions are homogenous mixtures of more than one substance, and can be found throughout nature. The air we breathe is a solution. It contains a mixture of substances, such as oxygen, carbon dioxide, and nitrogen. Oceans, lakes, and rivers are also solutions. They contain atoms and molecules from the land around them and the air above them.

Under different conditions, solutions can produce a variety of minerals, such as halite, gypsum, and calcite. Geologists are still studying many of the processes which naturally create minerals from these solutions.

Some minerals can form when the liquid in a solution (usually water) evaporates. The molecules left behind undergo crystallization to make minerals, like halite and gypsum. This is what happened on a very large scale in Bolivia.

The world’s largest salt flat, Salar de Uyuni, covers over 9,000 km² (5,600 mi²) in the Potosi region of Boliva. Salar de Uyuni formed as the water from Lake Minchin, a prehistoric salt lake that covered southern Bolivia about 40,000 years ago, dried out. As the water from the lake evaporated, sodium (Na) and chlorine (Cl) atoms combined to form halite (NaCl) crystals. This layer of halite has been measured to be over 10 meters (33 ft) thick at the center of Salar de Uyuni and extends as far as the eye can see!

Minerals can also form by precipitating out of a solution. Think back to the salt water. Salt dissolves in water. However, if you keep adding salt to water, at some point the solution becomes saturated and no more salt will dissolve. If you heat the water, you add energy to the system, and therefore, more salt can dissolve. Precipitation of salt crystals occurs when you allow this heated salt water solution to cool. Salt that was once dissolved in the heated water will precipitate out from the solution and collect on the bottom of the container.

This phenomenon occurs at hot springs like the ones in Yellowstone National Park. As the thermally heated water from the underground hot springs reaches the surface and cools, minerals dissolved in the hot water precipitate from the solution.

As calcium-rich solutions flow into caves, impressive looking stalactites and stalagmites can form. A stalactite is a mineral deposit shaped like an icicle that hangs downwards from a roof or wall of a cave. Stalagmites are mineral deposits that rise upwards from the cave floor. They are usually cone-shaped and form from the dripping stalactites above them.

Stalactites and stalagmites usually form when water, rich in dissolved calcium bicarbonate (Ca(HCO₃)₂), drips from the roof of a cave. When a droplet of this solution is exposed to the air in the cave, it reacts with the air and releases carbon dioxide (CO₂). During this reaction, the droplet deposits a thin film of the mineral calcite (CaCO₃). Over time, these drops will deposit enough calcite to form stalactites and stalagmites.

Minerals from Metamorphism

Minerals can also form through a process called metamorphism. These minerals usually form below Earth’s surface where temperatures and pressure are very high. Over time, these conditions will metamorphose, or change, the mineral into a new mineral.

Talc, garnet, chlorite, and wollastonite are minerals that can form through metamorphism. Wollastonite can be used in plastics, ceramics, paints, and as a substitute for asbestos. It forms when certain minerals within limestone are raised to high temperatures due to either deep burial or being close to hot molten rock.

From Molten Rock
	Over time, magma cools, allowing atoms to bond and form crystals. Large mineral crystals can form. Examples: Quartz, Olivine, Feldspar		As lava begins to cool, atoms bond and form crystals. Minerals with small crystals can form. Examples: Pyroxene, Quartz
From Solution
	As solutions become saturated with molecules or chemically react with their surroundings, molecules can crystallize and deposit out of the solution. Examples: Calcite, Silver		When solutions evaporate, the molecules left behind can crystallize and be deposited as minerals. Examples: Halite, Gypsum
From Metamorphism
		Minerals beneath Earth’s surface under high temperatures and pressures can change into a different type of mineral. Examples: Jade, Talc, Garnet