High School Earth Science/Formation of Minerals

Minerals are all around you. They are used to make your house, your computer, even the buttons on your jeans. But, where do minerals come from? There are many types of minerals, and they do not all form in the same way. Some minerals form when salt water on Earth's surface evaporates. Others form from water mixtures that are seeping through rocks far below your feet. Still others form when mixtures of really hot molten rock cool.

Lesson Objectives

Describe how melted rock produces minerals.
Explain how minerals form from solutions.

Formation from Magma and Lava

You are on vacation at the beach. You take your flip-flops off to go swimming because it is one of the hottest days of the summer. The sand is so hot it hurts your feet, so you have to run to the water. Imagine if it were hot enough for the sand to melt. Some minerals start out in liquids that are that hot.

There are places inside Earth where rock will melt. Melted rock inside the Earth is also called molten rock, or magma. Magma is a molten mixture of substances that can be hotter than 1,000°C. Magma moves up through Earth's crust, but it does not always reach the surface. When magma erupts onto Earth’s surface, it is known as lava. As lava flows from volcanoes it starts to cool, as Figure 3.19 shows. Minerals form when magma and lava cool.

Figure 3.19: Lava is melted rock that erupts onto Earth's surface.

Rocks from Magma

Magma cools slowly as it rises towards Earth’s surface. It can take thousands to millions of years to become solid when it is trapped inside Earth. As the magma cools, solid rocks form. Rocks are mixtures of minerals. Granite, shown in Figure 3.20, is a common rock that forms when magma cools. Granite contains the minerals quartz, plagioclase feldspar, and potassium feldspar. The different colored speckles in the granite are the crystals of the different minerals. The mineral crystals are large enough to see because the magma cools slowly, which gives the crystals time to grow.

Figure 3.20: Granite is a type of rock that forms from magma. It contains the minerals quartz (clear), plagioclase feldspar (shiny white), potassium feldspar (pink), and other minerals.

The magma mixture changes over time as different minerals crystallize out of the magma. A very small amount of water is mixed in with the magma. The last part of the magma to solidify contains more water than the magma that first formed rocks. It also contains rare chemical elements. The minerals formed from this type of magma are often valuable because they have concentrations of rare chemical elements. When magma cools very slowly, very large crystals can grow. These mineral deposits are good sources of crystals that are used to make jewelry. For example, magma can form large topaz crystals.

Minerals from Lava

Lava is on the Earth's surface so it cools quickly compared to magma in Earth. As a result, rocks form quickly and mineral crystals are very small. Rhyolite is one type of rock that is formed when lava cools. It contains similar minerals to granite. However, as you can see in Figure 3.21, the mineral crystals are much smaller than the crystals in the granite shown in Figure 3.20. Sometimes, lava cools so fast that crystals cannot form at all, forming a black glass called obsidian. Because obsidian is not crystalline, it is not a mineral.

Figure 3.21: Rhyolite rocks contain minerals that are similar to granite, but the crystal size is much smaller.

Formation from Solutions

Minerals also form when minerals are mixed in water. Most water on Earth, like the water in the oceans, contains minerals. The minerals are mixed evenly throughout the water to make a solution. The mineral particles in water are so small that they will not come out when you filter the water. But, there are ways to get the minerals in water to form solid mineral deposits. Minerals are smaller than sand particles and germs.

Minerals from Salt Water

Tap water and bottled water contain small amounts of dissolved minerals. For minerals to crystallize, the water needs to contain a large amount of dissolved minerals. Seawater and the water in some lakes, such as Mono Lake in California or Utah's Great Salt Lake, are salty enough for minerals to "precipitate out" as solids.

When water evaporates, it leaves behind a solid "precipitate" of minerals, which do not evaporate, as the Figure 3.22 shows. After the water evaporates, the amount of mineral left is the same as was in the water.

Figure 3.22: When the water in glass A evaporates, the dissolved mineral particles are left behind.

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 and sink to the bottom. Salt (halite) easily precipitates out of water, as does calcite, as the Figure 3.23 shows.

Figure 3.23: The limestone towers are made mostly of calcite deposited in the salty and alkaline water of Mono Lake, in California. These rocks formed under water when calcium-rich spring water at the bottom of the lake bubbled up into the alkaline lake, forming these calcite "tufa" towers. If the lake level drops, the tufa towers appear in interesting formations.

Minerals from Hot Underground Water

Cooling magma is not the only source for underground mineral formations. When magma heats nearby underground water, the heated water moves through cracks below Earth's surface.

Hot water can hold more dissolved particles than cold water. The hot, salty solution reacts with the rocks around it and picks up more dissolved particles. As it flows through open spaces in rocks, it deposits solid minerals. The mineral deposits that form when a mineral fills cracks in rocks are called veins. Figure 3.24 shows white quartz veins. When the minerals are deposited in open spaces, large crystals can form. These special rocks are called geodes. Figure 3.25 shows a geode that was formed when amethyst crystals grew in an open space in a rock.

Figure 3.24: Quartz veins formed in this rock.

Figure 3.25: An amethyst geode that formed when large crystals grew in open spaces inside the rock.

Lesson Summary

Mineral crystals that form when magma cools are usually larger than crystals that form when lava cools.
Minerals are deposited from salty water solutions on Earth's surface and underground.

Review Questions

How does magma differ from lava?
What are two differences between granite and rhyolite?
What happens to the mineral particles in salt water when the water evaporates?
Explain how mineral veins form.

Vocabulary

lava: Molten rock that has reached the Earth's surface.
magma: Molten rock deep inside the Earth.
rocks: Mixtures of minerals.

Points to Consider

When most minerals form, they combine with other minerals to form rocks. How can these minerals be used?
The same mineral can be formed by different processes. How can the way a mineral forms affect how the mineral is used?

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