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Mineral Formation
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Printed: January 25, 2015
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C HAPTER
Chapter 1. Mineral Formation
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Mineral Formation
Lesson Objectives
• Describe how melted rock produces minerals.
• Describe how hot rock produces different minerals.
• Explain how minerals form from solutions.
Vocabulary
•
•
•
•
lava
magma
rock
vein
Introduction
Minerals form under an enormous range of geologic conditions. There are probably more ways to form minerals
than there are types of minerals themselves. Minerals can form from volcanic gases, sediment formation, oxidation,
crystallization from magma, or deposition from a saline fluid, to list a few. Some of these methods of mineral
formation will be discussed below.
Formation from Hot Material
A rock is a collection of minerals. 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 1,000o C. Magma cools slowly inside Earth, which gives mineral crystals time to grow large enough to
be seen clearly ( Figure 1.1).
When magma erupts onto Earth’s surface, it is called lava. Lava cools much more rapidly than magma when it is
below the surface. In a cooling lava, mineral crystals do not have time to form and are very small. The chemical
composition will be the same as if the magma cooled slowly.
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 chapter “Rocks.”
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FIGURE 1.1
Granite is rock that forms from slowly
cooled magma, containing the minerals
quartz (clear), plagioclase feldspar (shiny
white), potassium feldspar (pink), and biotite (black).
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 1.2.
FIGURE 1.2
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, which sink. Halite easily precipitates out of water,
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Chapter 1. Mineral Formation
as does calcite. Some lakes, such as Mono Lake in California ( Figure 1.3) or The Great Salt Lake in Utah, contain
many mineral precipitates.
FIGURE 1.3
Tufa towers form when calcium-rich spring
water at the bottom of Mono Lake bubbles
up into the alkaline lake. The tufa towers
appear when lake level drops.
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 1.4).
FIGURE 1.4
Quartz veins formed in this rock.
When minerals are deposited in open spaces, large crystals form ( Figure 1.5).
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FIGURE 1.5
Amethyst formed when large crystals
grew in open spaces inside the rock.
These special rocks are called geodes.
Lesson Summary
• Mineral crystals that form when magma cools slowly are larger than crystals that form when lava cools rapidly.
• Minerals form when rocks are heated enough that atoms of different elements can move around and join into
different molecules.
• Minerals are deposited from salty water solutions on Earth’s surface and underground.
Review Questions
1.
2.
3.
4.
5.
What is the difference between magma and lava?
Under what circumstances do large crystals form from a cooling magma?
Under what circumstances do small crystals form from a cooling magma?
What happens to the mineral particles in salt water when the water evaporates?
Explain how mineral veins form.
Further Reading / Supplemental Links
• Gems and Where They’re Found: http://socrates.berkeley.edu/~eps2/wisc/Lect3.html .
• How to Grow Your Own Crystals: http://www.sdnhm.org/kids/minerals/grow-crystal.html .
Points to Consider
• Is a mineral a static thing or does it change? If it changes, on what time frame?
• When most minerals form, they combine with other minerals to form rocks. How can these minerals be used?
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Chapter 1. Mineral Formation
• The same mineral can be formed by different processes. How can the way a mineral forms affect how the
mineral is used?
References
1. Kevin Walsh, modified by CK-12 Foundation. http://www.flickr.com/photos/86624586@N00/10179314/ .
CC BY 2.0
2. Rebecca Calhoun. CK-12 Foundation . CC BY-NC 3.0
3. Flickr:Zengame. http://flickr.com/photos/zengame/48497087/ . CC BY 2.0
4. Image copyright Phon Promwisate, 2014. http://www.shutterstock.com . Used under license from Shutterstock.com
5. User:Pseudopanax/Wikipedia. http://commons.wikimedia.org/wiki/File:Opened_dark_purple_amethyst_cathe
dral_in_Crystal_Mountain_Museum.jpg . Public Domain
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