Lesson Objectives

  • Describe the characteristics that all minerals share.
  • Identify the groups in which minerals are classified and their characteristics.

Introduction

Minerals are categorized based on their chemical composition. Owing to similarities in composition, minerals within a same group may have similar characteristics.

What is a Mineral?

Minerals are everywhere! The Figure below shows some common household items and the minerals used to make them. The salt you sprinkle on food is the mineral halite. Silver in jewelry is a mineral . Baseball bats and bicycle frames both contain minerals. Although glass is not a mineral, it is produced from the mineral quartz. Scientists have identified more than 4,000 minerals in Earth’s crust. A few are common, but many are uncommon.
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Silver and halite are minerals; the mineral quartz is used to make glass.
Geologists have a very specific definition for minerals. A material is characterized as a mineral if it meets all of the following traits. A mineral is an inorganic, crystalline solid. A mineral is formed through natural processes and has a definite chemical composition. Minerals can be identified by their characteristic physical properties such as crystalline structure, hardness, density, flammability, and color.

Crystalline Solid

Minerals are crystalline solids. A crystal is a solid in which the atoms are arranged in a regular, repeating pattern (Figure below). The pattern of atoms in different samples of the same mineral is the same. Is glass a mineral? Without a crystalline structure, even natural glass is not a mineral.
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Sodium ions (purple balls) bond with chloride ions (green balls) to make table salt (halite). All of the grains of salt that are in a salt shaker have this crystalline structure.

Inorganic Substances

Organic substances are the carbon-based compounds made by living creatures and include proteins, carbohydrates, and oils. Inorganic substances have a structure that is not characteristic of living bodies. Coal is made of plant and animal remains. Is it a mineral? Coal is a classified as a sedimentary rock, but is not a mineral.

Natural Processes

Minerals are made by natural processes, those that occur in or on Earth. A diamond created deep in Earth’s crust is a mineral. Is carbon formed into diamond at high pressures in a laboratory a mineral? No. Do not buy a laboratory-made “diamond” for jewelry without realizing it is not technically a mineral.

Chemical Composition

Nearly all (98.5%) of Earth’s crust is made of only eight elements – oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium – and these are the elements that make up most minerals. All minerals have a specific chemical composition. The mineral silver is made up of only silver atoms and diamond is made only of carbon atoms, but most minerals are made up of chemical compounds. Each mineral has its own chemical formula. Halite, pictured above, is NaCl (sodium chloride). Quartz is always made of two oxygen atoms bonded to a silicon atom, SiO2. If a mineral contains any other elements in its crystal structure, it's not quartz.
A hard mineral with the element carbon all covalently bonded is diamond, but a softer mineral that also contains carbon along with calcium and oxygen is calcite (Figure below).
Structure of calcite.
Structure of calcite.

The structure of calcite shows the relationship of calcium (Ca), carbon (C), and oxygen.
Some minerals have a range of chemical composition. Olivine always has silicon and oxygen as well as iron or magnesium or both, (Mg, Fe)2SiO4.

Physical Properties

The physical properties of minerals include:
  • Color: the color of the mineral.
  • Streak: the color of the mineral’s powder.
  • Luster: the way light reflects off the mineral’s surface.
  • Specific gravity: how heavy the mineral is relative to the same volume of water.
  • Cleavage: the mineral’s tendency to break along flat surfaces.
  • Fracture: the pattern in which a mineral breaks.
  • Hardness: what minerals it can scratch and what minerals can scratch it.
How physical properties are used to identify minerals is described in the lesson on Mineral Formation.

Mineral Groups

Mineralogists divide minerals into groups based on chemical composition. Most minerals fit into one of eight mineral groups.

Silicate Minerals

The roughly 1,000 silicate minerals make up over 90% of Earth's crust. Silicate minerals are by far the largest mineral group. Feldspar and quartz are the two most common silicate minerals. Both are extremely common rock-forming minerals.
The basic building block for all silicate minerals is the silica tetrahedron, which is illustrated in the Figure below. To create the wide variety of silicate minerals, this pyramid-shaped structure is often bound to other elements, such as calcium, iron, and magnesium.
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One silicon atom bonds to four oxygen atoms to form a silica tetrahedron.
Silica tetrahedrons combine together in six different ways to create different types of silicates (Figure below) and (Figure below). Tetrahedrons can stand alone, form connected circles called rings, link into single and double chains, form large flat sheets of pyramids, or join in three dimensions.
Muscovite.
Muscovite.

Muscovite has platy cleavage because of the sheet-like structure of the silica tetrahedra.
Rhodonite.
Rhodonite.

The silica tetrahedra in pink rhodonite are in single chains to create a tabular crystal shape.

Native Elements

Native elements contain atoms of only one type of element. Only a small number of minerals are found in this category. Some of the minerals in this group are rare and valuable. Gold, silver, sulfur, and diamond are examples of native elements.

Carbonates

The basic carbonate structure is one carbon atom bonded to three oxygen atoms (Figure below). Carbonates include other elements, such as calcium, iron, and copper. Calcite (CaCO3) is the most common carbonate mineral (Figure below).
3D ball-stick model of carbonate ion.
3D ball-stick model of carbonate ion.

The carbonate ion is one carbon atom bonded to three oxygen atoms.
A fossil shell with calcite.
A fossil shell with calcite.

Calcite crystals in a bivalve shell.
Azurite and malachite, shown in the Figure below, are carbonates that contain copper instead of calcium.
Two carbonate minerals - blue azurite and green malachite.
Two carbonate minerals - blue azurite and green malachite.

Two carbonate minerals: deep blue azurite and opaque green malachite.

Halides

Halide minerals are salts that form when salt water evaporates. Halite is a halide mineral. But table salt is not the only halide. The chemical elements known as the halogens; fluorine, chlorine, bromine, or iodine bond with various metallic atoms to make halide minerals.
A fluorite is a halide containing calcium with fluorine.
A fluorite is a halide containing calcium with fluorine.

Fluorite is a halide containing calcium and fluorine.

Oxides

Oxides contain one or two metal elements combined with oxygen. Many important metals are found as oxides. Hematite (Fe2O3), with two iron atoms to three oxygen atoms, and magnetite (Fe3O4) (Figure below), with three iron atoms to four oxygen atoms, are both iron oxides.
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Magnetite is the most magnetic mineral. Magnetite attracts or repels other magnets.

Phosphates

Phosphate minerals are similar in atomic structure to the silicate minerals. In the phosphates, phosphorus, arsenic, or vanadium bond to oxygen to form a tetrahedra. There are many different minerals in the phosphate group, but most are rare (Figure below).
Turquoise.
Turquoise.

Turquoise is a phosphate mineral containing copper, aluminum, and phosphorus.

Sulfates

Sulfate minerals contain sulfur atoms bonded to oxygen atoms. Like halides, they form where salt water evaporates. The sulfate group contains many different minerals, but only a few are common.
Gypsum is a common sulfate with a variety of appearances (Figure below) and (Figure below). Some gigantic 11-meter gypsum crystals have been found. That is about as long as a school bus!
Gypsum.
Gypsum.

Gypsum from South Australia.
White gypsum sands at White Sands National Monument.
White gypsum sands at White Sands National Monument.

The white gypsum sands at White Sands National Monument look like snow.

Sulfides

Sulfides are formed when metallic elements combine with sulfur. Unlike sulfates, sulfides do not contain oxygen. Pyrite or iron sulfide, is a common sulfide mineral known as fool’s gold. People may mistake pyrite for gold because the two minerals are shiny, metallic, and yellow in color.

Lesson Summary

  • For a substance to be a mineral, it must be a naturally occurring, inorganic, crystalline solid that has a characteristic chemical composition and crystal structure.
  • The atoms in minerals are arranged in regular, repeating patterns that can be used to identify that mineral.
  • Minerals are divided into groups based on their chemical composition.
  • The chemical feature of each groups is: native elements – only one element; silicates – silica tetrahedron; phosphates – phosphate tetrahedron; carbonates – one carbon atom with three oxygen atoms; halides – a halogen bonded with a metallic atom; oxides – a metal combined with oxygen; sulfates – sulfur and oxygen; sulfides – metal with sulfur, no oxygen.

Review Questions

  1. What is a crystal?
  2. Which elements do all silicate minerals contain?
  3. Obsidian is a glass that forms when lava cools so quickly that the atoms do not have a chance to arrange themselves in crystals. Is obsidian a crystal? Explain your reasoning.
  4. What are the eight major mineral groups?
  5. What is the same about all minerals in the silicate group? What is different about them?
  6. One sample has a chemical composition with a ratio of two iron atoms to three oxygen atoms. Another sample has a chemical composition with a ratio of three iron atoms to four oxygen atoms. They contain the same elements: Are they the same mineral?
  7. How does the native elements mineral group differ from all of the other mineral groups?
  8. On a trip to the natural history museum you find two minerals that are similar in color. You can see from their chemical formulas that one mineral contains the elements zinc, carbon, and oxygen. The other mineral contains the elements zinc, silicon, oxygen, and hydrogen. Your friend tells you that the minerals are in the same mineral group. Do you agree? Explain your reasoning.

Further Reading / Supplemental Links

Vocabulary

silicates Minerals of silicon atoms bonded to oxygen atoms. mineralogist A scientist who studies minerals. mineral A naturally occurring, inorganic, crystalline solid with a characteristic chemical composition. inorganic Not organic; not involving life. crystal A solid in which all the atoms are arranged in a regular, repeating pattern. chemical compound A substance in which the atoms of two or more elements bond together.

Points to Consider

  • Why is obsidian, a natural glass that forms from cooling lava, not a mineral?
  • Why are diamonds made in a laboratory not minerals?
  • Is coal, formed mostly from decayed plants, a mineral? Is it a rock?
  • Artists used to grind up the mineral azurite to make colorful pigments for paints. Is the powdered azurite still crystalline?

Mineral Identification

Lesson Objectives

  • Explain how minerals are identified.
  • Describe how color, luster, and streak are used to identify minerals.
  • Summarize specific gravity.
  • Explain how the hardness of a mineral is measured.
  • Describe the properties of cleavage and fracture.
  • Identify additional properties that can be used to identify some minerals.

Introduction

Minerals can be identified by their physical characteristics. The physical properties of minerals are related to their chemical composition and bonding. Some characteristics, such as a mineral's hardness, are more useful for mineral identification than others. Color is readily observable and certainly obvious, but it is usually less reliable than its other physical properties.