Formation of Minerals
Mix of minerals. Photo: RoseCreekTreasure

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.

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. Minerals form when magma and lava cool.

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 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.

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 the mineral crystals are much smaller than the crystals in the granite. 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.

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 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.  After the water evaporates, the amount of mineral left is the same as was in the water.

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. 

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. 

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