4.4 Mineral Formation

Is carbon a girl's best friend?

Yes! (At least if you think that diamond is a girl's best friend, anyway.) When people think of carbon they think of black dust left over from a fire, but the diamond is just carbon that was squeezed very hard at extremely high pressure. Formed at lower pressure, the carbon mineral is graphite, the mineral that is pencil "lead." Graphite would make a very different sort of ring.

Mineral Formation

Minerals form in a variety of ways:

• crystallization from magma
• precipitation from ions in solution
• biological activity
• a change to a more stable state as in metamorphism
• precipitation from vapor

Formation from Magma

Imagine a rock that becomes so hot it melts. Many minerals start out in liquids that are hot enough to melt rocks. Magma is melted rock inside Earth, a molten mixture of substances that can be hotter than 1,000°C. Magma cools slowly inside Earth, which gives mineral crystals time to grow large enough to be seen clearly (Figure below).

When magma erupts onto Earth's surface, it is called lava. Lava cools much more rapidly than magma. Crystals do not have time to form and are very small. The chemical composition between minerals that form rapidly or slowly is often the same, only their size differs.

Existing rocks may be heated enough so that the molecules are released from their structure and can move around. The molecules may match up with different molecules to form new minerals as the rock cools. This occurs during metamorphism, which will be discussed in the "Metamorphic Rocks" concept.

Formation from Solutions

Water on Earth, such as the water in the oceans, contains chemical elements mixed into a solution. Various processes can cause these elements to combine to form solid mineral deposits.

Minerals from Salt Water

When water evaporates, it leaves behind a solid precipitate of minerals, as shown in Figure below.

Water can only hold a certain amount of dissolved minerals and salts. When the amount is too great to stay dissolved in the water, the particles come together to form mineral solids, which sink. Halite easily precipitates out of water, as does calcite. Some lakes, such as Mono Lake in California (Figure below) or The Great Salt Lake in Utah, contain many mineral precipitates.

Minerals from Hot Underground Water

Magma heats nearby underground water, which reacts with the rocks around it to pick up dissolved particles. As the water flows through open spaces in the rock and cools, it deposits solid minerals. The mineral deposits that form when a mineral fills cracks in rocks are called veins (Figure below).

When minerals are deposited in open spaces, large crystals form (Figure below).

Minerals Under Pressure

In the last several years, many incredible discoveries have been made exploring how minerals behave under high pressure, like rocks experience inside the Earth. If a mineral is placed in a special machine and then squeezed, eventually it may convert into a different mineral. Ice is a classic example of a material that undergoes solid-solid "phase transitions" as pressure and/or temperature is changed. A "phase diagram" is a graph which plots the stability of phases of a compound as a function of pressure and temperature.

A phase diagram for water (ice) is included in the Figure below. The phase diagram is split up into 3 main areas for the solid crystalline phase (ice), the liquid phase (water), and the gas phase (water vapor). Notice that increasing pressure lowers the freezing point and raises the boiling point of water. What does that do to the stability conditions of the liquid phase?

Summary

• Minerals form as magma cools.
• Minerals form when they precipitate from hot fluids that have cooled down.
• Minerals form when the concentration of ions gets too great in a fluid.

Review

1. How do minerals form in veins?
2. How do minerals form from cool water?
3. When do large crystals form from magma? When do small crystals form from magma?