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Earth Science · Unit 5 · Grades 6–8

Minerals and Crystals

Every rock is built from minerals, and every mineral is built from atoms stacked in a pattern.

The salt on your fries, the graphite in your pencil, and the sand in a playground are all minerals or made from them. Minerals are the building blocks of rocks, and many of them grow into beautiful crystals with flat faces and sharp edges.

In this unit you will learn what counts as a mineral, see how crystals grow, visit a cave full of crystals longer than a school bus, and learn the tests geologists use to tell one mineral from another. Then you can try those tests yourself in the Mineral ID Lab.

Play Mineral ID Lab Take the quiz

Quartz and amethyst points, with cubes of pyrite and halite.

What is a mineral?

Geologists use five tests to decide whether something is a mineral. It has to pass all five.

Naturally occurringIt forms in nature, not in a factory. Steel and glass are not minerals.
InorganicIt is not made by living things. Coal (from plants) and pearls (from oysters) are not minerals.
SolidIt is solid at room temperature. Liquid water is not a mineral, but ice in a glacier is.
Definite chemical makeupIt is made of the same elements in about the same amounts wherever it is found. Quartz is always silicon and oxygen, SiO₂.
Ordered crystal structureIts atoms are lined up in a pattern that repeats over and over.

Mineral or rock?

A rock is made of one or more minerals mixed together, like a cookie is made of flour, sugar, and chocolate chips. Granite is a rock made mostly of feldspar, quartz, and mica. To see how rocks change from one kind into another, visit The Rock Cycle.

Atoms and crystals

A crystal is a solid whose atoms are arranged in a repeating pattern. Because the pattern repeats, the outside of a crystal often has flat faces and a regular shape. In halite (rock salt), sodium and chloride atoms take turns in a grid of tiny cubes, so a halite crystal grows into a cube, too.

Each mineral has its own pattern of atoms, so each one tends to grow in its own shape. Drawings are simplified, not to scale.

Where crystals grow

From cooling magmaAs melted rock cools, atoms slow down and lock together into crystals of quartz, feldspar, mica, and other minerals.
From evaporating waterWhen salty water dries up, the salt left behind forms crystals. Thick beds of halite and gypsum formed this way where ancient seas dried out.
From hot water solutionsHot water deep underground carries dissolved minerals. As it cools or changes, the minerals come out of the water and grow crystals in cracks and caves.

Time makes size

The more time a crystal has to grow, the bigger it can get. Magma that cools slowly deep underground makes rocks like granite with crystals you can see. Lava that cools fast on the surface makes basalt with crystals too small to see, or even obsidian, a glass with almost no crystals at all.

The Cave of the Crystals

In 2000, miners digging about 300 meters (1,000 feet) under Naica mountain in Chihuahua, Mexico, broke into a cave filled with giant beams of see-through selenite, a clear form of gypsum. The largest crystals are about 11 meters (36 feet) long, longer than a school bus, and the biggest weigh up to about 55 tons (National Geographic).

How did they get so big? For a very long time, the cave was filled with hot, mineral-rich water that stayed at nearly the same temperature, about 55 to 58 °C (about 130 to 136 °F). At that temperature, gypsum grew very, very slowly, and nothing disturbed it. Scientists think the biggest crystals took hundreds of thousands of years to grow.

The cave is so hot and humid that people could only stay inside for a short time, even with special cooling suits. It is a perfect example of the rule above: lots of time and steady conditions make giant crystals.

How to identify a mineral

There are thousands of minerals, but you can tell common ones apart with a few simple tests. No single test is enough. Geologists use several together, like clues in a mystery.

ColorEasy to see, but often misleading. Quartz can be clear, white, pink, purple, or smoky, because tiny amounts of other elements change its color.
StreakThe color of a mineral’s powder when you rub it on a rough white tile called a streak plate. Streak is more reliable than color. Hematite can look silver, black, or red, but its streak is always red-brown. Pyrite looks gold, but its streak is greenish black.
LusterHow the surface reflects light. Metallic minerals, like pyrite and galena, shine like metal. Nonmetallic lusters include glassy (quartz), pearly (some gypsum), and dull or earthy.
CleavageSome minerals split along flat surfaces where the bonds between atoms are weak. Mica peels into thin sheets. Halite breaks into little cubes.
FractureMinerals without cleavage break unevenly. Quartz breaks with curved, shell-shaped surfaces, called conchoidal fracture.
DensityHow heavy a mineral feels for its size. Galena and magnetite feel surprisingly heavy compared with a same-sized piece of quartz.

Special properties

Fizzes in acidA drop of weak acid on calcite makes bubbles of carbon dioxide gas. (In class, only use acid with your teacher and safety goggles.)
MagneticMagnetite pulls on a magnet. It is the only common mineral that is strongly magnetic.
TasteHalite tastes salty, because it is salt. But never lick an unknown sample: some minerals are toxic.
FluorescenceSome minerals, including some fluorite and calcite, glow in bright colors under ultraviolet (UV) light.

Hardness and the Mohs scale

Hardness is how well a mineral resists being scratched. In 1812, the German geologist Friedrich Mohs ranked ten minerals from softest (talc, 1) to hardest (diamond, 10). A mineral scratches anything softer than itself and is scratched by anything harder.

You don’t need the ten minerals to test hardness. Everyday tools work too: a fingernail is about 2.5, a copper penny or wire about 3.5, glass about 5.5, and a steel nail about 6.5.

The Mohs scale only tells which mineral is harder, not how much harder. Diamond is far harder than corundum, even though they are only one step apart.

Try it

If your fingernail scratches a mineral, it is softer than 2.5. If your fingernail can’t, but copper can, it is between 2.5 and 3.5. If it scratches glass, it is harder than 5.5.

Common minerals

FeldsparThe most common mineral group in Earth’s crust: the USGS estimates feldspars make up about 60% of it. Pink or white, hardness 6, with two good cleavages.
QuartzSilicon and oxygen. Hardness 7, glassy, no cleavage. Most beach sand is mostly quartz, because it is tough and resists weathering.
MicaPeels into thin, bendable, see-through sheets. Muscovite mica is pale and biotite mica is dark.
CalciteHardness 3. It fizzes in weak acid and makes up limestone and marble.
HaliteRock salt. Hardness about 2.5, breaks into cubes, dissolves in water.
GypsumHardness 2, soft enough to scratch with a fingernail. Used to make drywall and plaster.
Graphite and diamondBoth are pure carbon. In diamond, each carbon atom is locked to four neighbors in a strong 3-D framework, so it is the hardest natural mineral (10). In graphite, the carbon atoms form flat sheets that slide over each other, so it is soft (1 to 2) and leaves a mark on paper.

Minerals in everyday life

The U.S. Geological Survey (USGS) tracks nearly 100 mineral commodities that people mine and use. You probably used several before breakfast.

Quartz sandMelted to make glass and fiber-optic cables.
GraphiteThe “lead” in pencils, and part of electric-car batteries.
GypsumThe inside of drywall in most houses.
HaliteTable salt, and salt for icy roads.
CalciteUsed to make cement.
TalcUsed in some deodorants and to make paper smooth.
Your phoneThe USGS says more than half of the parts in a mobile device come from mined materials, including copper for wires and rare earth elements for the tiny magnets in speakers.

Game: Mineral ID Lab

Six mystery samples are on your lab bench. Run tests like a mineralogist: streak, scratch, acid, and magnet. When you think you know what it is, name it. In this lab no one tastes anything.

This lab needs JavaScript. With JavaScript turned off, the six minerals and their test results are listed below.

The six minerals and their test results:

Quartz: Clear to milky white, glassy luster, six-sided crystal with a pointed tip. Streak: No streak shows. The sample is harder than the streak plate and scratches it. Hardness: Scratches glass and a steel nail. Nothing in the kit scratches it. Acid: Nothing happens. Magnet: No pull.
Calcite: Clear to white, glassy luster, breaks into slanted box shapes. Streak: White. Hardness: Copper scratches it, but a fingernail does not. Acid: It fizzes! Bubbles of carbon dioxide gas form. Magnet: No pull.
Gypsum: White to clear, pearly to glassy luster, soft-looking flat crystals. Streak: White. Hardness: Your fingernail scratches it. Acid: Nothing happens. Magnet: No pull.
Hematite: Silvery gray to black, metallic luster. Streak: Red-brown, like dried blood or rust. Hardness: Scratches copper. About as hard as glass. Acid: Nothing happens. Magnet: No pull (or only a very weak one).
Pyrite: Brassy gold color, metallic luster, cube-shaped crystals. Streak: Greenish black. Hardness: Scratches glass. Acid: Nothing happens. Magnet: No pull.
Magnetite: Black, metallic luster, heavy for its size. Streak: Black. Hardness: Scratches copper. About as hard as glass. Acid: Nothing happens. Magnet: A magnet snaps onto it!

Quick quiz

Nine questions. Choose one answer for each, then press Check my answers.

1. Which of these is NOT a mineral? Quartz, a natural crystal of silicon and oxygen Coal, made from buried plant remains Halite, natural rock salt

2. What is the main difference between a mineral and a rock? A rock is made of one or more minerals Rocks are always softer than any mineral Minerals only form in deep caves

3. Two crystals of the same mineral formed from magma. One is huge and one is tiny. What most likely made the difference? The tiny one is made of completely different atoms The huge one cooled over a much longer time The huge one formed near the surface

4. A sample looks silvery black but leaves a red-brown streak. What is it most likely? Hematite Magnetite Graphite

5. Your fingernail will not scratch a mineral, but a copper wire will. About how hard is it? Harder than glass, over 5.5 Softer than talc, under 1 Between 2.5 and 3.5

6. Mica peels into thin, flat sheets. Quartz breaks into curved, shell-like surfaces. What do these show? Streak color and metallic luster Cleavage and fracture Density and magnetism

7. A drop of weak acid makes a white mineral fizz. Which mineral is it most likely to be? Gypsum Milky quartz Calcite

8. Diamond and graphite are both made of pure carbon. Why is one so hard and the other so soft? Their atoms are arranged differently Graphite in pencils is always mixed with clay Diamond is much hotter inside

9. Which mineral group makes up the biggest part of Earth’s crust? Calcite crystals Native gold Feldspar

Answer key: 1. Coal, made from buried plant remains. 2. A rock is made of one or more minerals. 3. The huge one cooled over a much longer time. 4. Hematite. 5. Between 2.5 and 3.5. 6. Cleavage and fracture. 7. Calcite. 8. Their atoms are arranged differently. 9. Feldspar.

Talk about it

Look around your classroom or kitchen. How many things can you find that are made from minerals? Which ones surprised you?
Why is streak a better clue than color? Can you think of a time when the way something looks fooled you?
The Naica crystals took hundreds of thousands of years to grow, and the mine that found them changed the cave. Should people protect places like this, even if it means nobody can visit?
Diamond and graphite are made of the same atoms but act completely differently. What does that tell you about why structure matters?

Sources

U.S. National Park Service, Minerals (the five characteristics required of all minerals) and Mohs hardness scale (the ten index minerals; fingernail 2.5, copper penny 3.5, knife or glass plate 5.5, steel nail 6.5).
U.S. Geological Survey, What is the difference between a rock and a mineral?
U.S. Geological Survey, Mineral resource of the month: feldspar (feldspars are estimated to make up 60 percent of Earth’s crust).
U.S. Geological Survey, Minerals and uses activity (quartz sand, graphite, gypsum, halite, calcite, and talc uses) and Mineral commodities (nearly 100 commodities; more than half of mobile-device parts).
Smithsonian National Museum of Natural History, What is a mineral?
British Geological Survey, Rocks and minerals.
Mineralogical Society of America, Mineral ID key (using luster, hardness, cleavage, and streak together).
Mindat, Hematite (red-brown streak), Pyrite (greenish-black streak), and Magnetite (strongly magnetic).
National Geographic, Giant crystal cave’s mystery solved (discovered in 2000, about 300 meters deep; crystals up to about 11 meters long and about 55 tons grew in water near 58 °C).
Gázquez, Calaforra, Forti, and Badino, The caves of Naica: a decade of research, Boletín Geológico y Minero (selenite crystals up to 11 meters; growth near 55 °C).

Credits

Written by Mary, Jugami’s Earth science teacher. The crystal and Mohs scale drawings are original, hand-coded SVG made for Jugami. They are simplified sketches, not drawn to scale.