Minerals of Earth’s Crust Earth Science Chapter 5

Section 1: What is a Mineral? Introduction This section defines what a mineral is and review the two main groups of minerals, the silicates and nonsilicates. It describes the six common crystalline structures of silicates. It also describes the six major classes of nonsilicates

1. Characteristics of Minerals Minerals are the basic materials of Earth’s crust. A mineral is a natural, usually inorganic solid that has a characteristic chemical composition, an orderly internal structure, and a characteristic set of physical properties. To determine whether a substance is a mineral or a nonmineral, scientists ask four basic questions. If the answer to all four questions is yes, the substance is a mineral Is the substance inorganic? An inorganic substance is one that is not made up of living things or the remains of living things Does the substance occur naturally? Minerals form and exist in nature, thus a manufactured substance is not a mineral

Characteristics of Minerals (cont.) Is the substance a solid in crystalline form? Atoms must be arranged in a regularly repeating crystalline structure. Does the substance have a consistent chemical composition? The ratio of elements must be the same throughout the substance

2. Kinds of Minerals Earth scientists have identified more than 3,000 minerals, but fewer than 20 are common. The common minerals are called rock-forming minerals because they form the rocks that make up the Earth’s crust. Of the 20 common rock-forming minerals, 10 are so common that they make up 90% of the mass of Earth’s crust These minerals are: Quartz - Calcite Orthoclase - Dolomite Plagioclase - Halite Muscovite - Gypsum Biotite - Ferromagnesian minerals All minerals can be classified as either silicate or nonsilicate minerals based on their chemical composition

Kinds of Minerals (cont.) Silicate Minerals A mineral that contains a combination of silicon (Si) and oxygen (O) is a silicate mineral The mineral quartz has only silicon and oxygen, but other minerals have one or more additional elements. Feldspars are the most common type of silicate minerals. Silicate minerals make up 96% of Earth’s crust. Feldspar and quarts alone make up more than 50% of the crust.

Kinds of Minerals (cont.) Nonsilicate Minerals Approximately 4% of Earth’s crust is made up of minerals that do not contain compounds of silicon and oxygen, or nonsilicate minerals There are six major groups of nonsilicate minerals based on their chemical composition: Carbonates Compounds that contain a carbonate group (CO3) Halides Compounds that consist of chlorine or fluorine combined with sodium, potassium, or calcium Ex. NaCl or CaF2

Kinds of Minerals (cont.) Native elements Elements uncombined with other elements Ex. Silver (Ag) or copper (Cu) Oxides Compounds that contain oxygen and an element other than silicon Ex. Corundum (Al2O3) or hematite (Fe2O3) Sulfates Compounds that contain a sulfate group (SO4) Ex. Gypsum (CaSO4·2H2O) Sulfides Compounds that consist of one or more elements combined with sulfur Ex. Galena (PbS) or Pyrite (FeS2)

3. Crystalline Structure All minerals have a crystalline structure Each type of mineral crystal is characterized by a specific geometric arrangement of atoms A crystal is a solid whose atoms, ions, or molecules are arranged in a regular, repeating pattern. Sometimes the growth of crystals is hindered, as a result, minerals are commonly made up of masses of crystals that are small If a crystal forms unrestricted the mineral will form as a single, large crystal that has one of the six basic shapes Knowing the crystal shape is helpful in identifying minerals

4. Crystalline Structure of Silicate Minerals Even though there are many kinds of silicate minerals, their crystalline structure is made up of the same basic building blocks. Each building block has four oxygen atoms arranged in a pyramid with one silicon atom in the center. This four-sided structure is called a silicon-oxygen tetrahedron The silicon-oxygen tetrahedra combine in different arrangements to form different silicate minerals The various arrangements are the result of the kinds of bonds that form between the oxygen atoms of the tetrahedra and other atoms

Crystalline Structure of Silicate Minerals (cont.) There are six kinds of arrangements that tetrahedra form. Isolated Tetrahedral Silicates and Ring Silicates Isolated tetrahedra - only atoms other than silicon and oxygen atoms link silicon-oxygen tetrahedra. Ex. Olivine forms when oxygen atoms of tetrahedra bond to magnesium (Mg) and iron (Fe) Ring silicates – form when shared oxygen atoms join the tetrahedra to form three-, four-, or six-sided rings. Ionic bonds hold the rings together, and the rings align to create channels that can contain a variety of ions, molecules, and neutral atoms.

Crystalline Structure of Silicate Minerals (cont.) Single-Chain Silicates and Double- Chain Silicates Single-chain silicates – each tetrahedron is bonded to two others by shared oxygen atoms. Most single-chain silicate minerals are called pyroxenes. Double-chain silicates – two single chains of tetrahedra bond to each other Double-chain silicates are called amphiboles

Crystalline Structure of Silicate Minerals (cont.) Sheet Silicates and Framework Silicates Sheet silicates – each tetrahedron shares three oxygen atoms with other tetrahedra. The fourth oxygen atom bonds with an atom of aluminum (Al) or magnesium (Mg), which joins one sheet to another. Ex. Muscovite and biotite are sheet silicates Framework silicates – each tetrahedron is bonded to four neighboring tetrahedra to form a three-dimensional network Framework silicates, other than quartz, contain some tetrahedra in which atoms of aluminum or other metals substitute for some of the silicon atoms Ex. The feldspars and quartz

5. The Crystalline Structure of Nonsilicate Minerals Because nonsilicate minerals have diverse chemical compositions, nonsilicate minerals display a vast variety of crystalline structures Common crystal structures for nonsilicate minerals include cubes, hexagonal prisms, and irregular masses. They may form tetrahedra that are similar to those in silicates, however the ions at the center are not silicon. The classes of nonsilicate minerals can be divided into smaller groups based on the structural similarities of the minerals’ crystals. The structure determines the minerals’ characteristics One type of crystal structure is called closest packing. Each metal atom is surrounded by 8 to 12 other metal atoms that are as close to each other as the charges of the atomic nuclei will allow