Minerals and Mineral Groups

Lesson Objectives

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

Vocabulary

• chemical compound
• crystal
• inorganic
• mineral
• silicates

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! Figure 2.1 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 also 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.

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, streak, and cleavage.

Crystalline Solid

Minerals are crystalline solids. A crystal is a solid in which the atoms are arranged in a regular, repeating pattern (Figure 2.2  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.

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 a diamond created in a laboratory by placing carbon under high pressures 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 up 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 in the Figure 2.2 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 containing covalently bonded carbon is diamond, but a softer mineral that also contains calcium and oxygen along with carbon is calcite (Figure  below).

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

Minerals are divided 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. Silicates 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 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.

Silica tetrahedrons combine together in six different ways to create different types of silicates (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.

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. Carbonates include other elements, such as calcium, iron, and copper. Calcite (CaCO₃) is the most common carbonate mineral (Figure  below).

Azurite and malachite, shown in the Figure  below, are carbonates that contain copper instead of calcium.

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 (seeFigure  below).

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.

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).

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). Some gigantic 11-meter gypsum crystals have been found. That is about as long as a school bus!

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

• The Definition of a Mineral: http://www.minsocam.org/msa/ima/ima98(04).pdf.
• Mineral Identification: http://geology.csupomona.edu/alert/mineral/minerals.htm.
• Dana Classification of Minerals: http://webmineral.com/danaclass.shtml.
• A Lot of Different Minerals: http://hyperphysics.phy-astr.gsu.edu/hbase/geophys/mineral.html#c1.
• Mineral Groups: http://mineral.galleries.com/minerals/silicate/class.htm.
• Giant Crystal Cave, National Geographic: http://news.nationalgeographic.com/news/2007/04/photogalleries/giant-crystals-cave/photo3.html.

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?