3.2MINERALS

WHAT IS A MINERAL?

Naturally occurring

Inorganic

Crystalline structure

Solid

Fixed chemical composition

CHARACTERISTICS OF MINERALS

All minerals contain one or more of the naturally occurring elements (about 90).

CHARACTERISTICS OF MINERALS

Earth scientists have identified more than 4000 minerals, but only about 20 of them are common.

The common minerals are called rock-forming minerals because they form the rocks making up Earth’s crust.

Of the 20 rock-forming minerals, only about half of them are so common they make up 90% of the mass of Earth’s crust.

COMMON ROCK-FORMING MINERALS

Quartz

Orthoclase (Potassium) Feldspar

Dolomite

Olivine

Halite

Muscovite Mica

Biotite Mica

Pyroxene

Calcite

Gypsum

Amphibole

Plagioclase Feldspar

MINERAL GROUPS

Silicate Minerals Minerals containing a combination of the

two most common elements in Earth’s crust, Silicon (Si) and Oxygen (O).

Make up 96% of the Earth’s crust. The basic building block of silicate minerals is the silicon-

oxygen tetrahedron, SiO4.

Non-silicate Minerals Minerals not containing a combination of Si and O. Make up remaining 4% of Earth’s crust.

- Native Elements - Sulfates- Oxides - Sulfides- Carbonates - Halides

Silicon-Oxygen Tetrahedron

CRYSTALLINE STRUCTURE

All minerals in Earth’s crust have a crystalline structure.

A crystal is a solid whose atoms, ions, or molecules are arranged in a regular, repeating pattern. Each type of mineral crystal is characterized

by a specific geometric arrangement of atoms.

Scientists can study a mineral’s crystals by using X-rays. The X-rays pass through a crystal and strike a

photographic plate producing an image showing the geometric arrangement of the atoms.

SILICATE CRYSTALLINE STRUCTURE

Silicon-oxygen tetrahedra combine in different arrangements to form different silicate minerals. This is due to the kinds of bonds forming between the oxygen atoms

of the tetrahedra and other atoms. The oxygen atoms may be shared with neighboring tetrahedra or

form with other elements outside of the tetrahedra.

There are 6 kinds of arrangements tetrahedra form:1. Isolated 4. Double-chain2. Ring 5. Sheet3. Single-chain 6. Framework

SILICATE CRYSTALLINE STRUCTURE

Example: Olivine

Example: Beryl

Example: Pyroxene

Example: Amphibole

Example: Mica

Example: Quartz, Feldspars

NON-SILICATE CRYSTALLINE STRUCTURE

Non-silicate minerals have a diverse chemical composition, thus displaying a vast variety of crystalline structures. Common ones include:

1. Cubes2. Hexagonal prisms3. Irregular masses

The crystal structure of non-silicates determine their characteristics. Example: Native elements, like Gold (Au), have very high densities

because their crystal structures are based on the packing of atoms as close together as possible, known as close-packing.

Gold

PHYSICAL PROPERTIES OF MINERALS

The physical properties of minerals include:

COLOR

LUSTER

STREAK

CLEAVAGE

FRACTURE

HARDNESS

DENSITY

SPECIAL PROPERTIES

CRYSTAL SYSTEM

COLOR

Not a reliable clue for identifying minerals.The same mineral can come in a variety of colors.

For example: Quartz

Rose Quartz

Crystalline Quartz

Milky Quartz

Amethyst Quartz

STREAK

The color of a mineral in powdered form is known as its streak.

Determined by rubbing a mineral against a piece of unglazed porcelain known as a streak plate.

LUSTER

The way the surface of a mineral reflects light is known as its luster.

When you say an object is shiny or dull, that’s luster!If a mineral is shiny like a

metal, it is considered metallic.

If a mineral is dull, it is considered to be non-metallic.

Galena Hematite

LUSTER

Non-metallic lusters include:

Vitreous (glassy)

Silky (fiber-like)

Resinous (plastic-looking)

Quartz

Satin Spar Gypsum

Orange Garnet

LUSTER

Waxy (greasy)

Adamantine (diamond-like)

Earthy (Earth-like)

Pearly (pearl-like)

Jade

Diamond

Limonite

Muscovite Mica

CLEAVAGE

The splitting of minerals along smooth, flat planes is known as cleavage.

Determined using the “sandwich” rule.

Different types include: Basal (1 plane) Prismatic (2 planes) Rhombohedral (3

planes NOT at right angles) Cubic (3

planes at right angles) Octahedral (4 planes) Dodecahedral (6 planes)

Mica – Basal Cleavage (1 plane)

Halite – Cubic Cleavage (3 planes at

right angles)

Calcite – Rhombohedral Cleavage (3 planes not at right

angles)

Fluorite – Octahedral Cleavage (4 planes)

Sphalerite – Dodecahedral

Cleavage (6 planes)

Barite – Prismatic Cleavage (2 planes)

FRACTURE

The breaking of minerals unevenly along curved, irregular, or other-shaped surfaces is known as fracture.

Different types include: Conchoidal (curved) Irregular (uneven) Fibrous (splintery) Hackly (jagged-edged)

Quartz – Conchoidal Fracture

Aerinite – Irregular Fracture

Serpentine – Fibrous Fracture

Iron – Hackly Fracture

HARDNESS

A mineral’s resistance to being scratched is considered its hardness.

To determine hardness, scientists use Moh’s Hardness Scale.

Fredrich Mohs

2.5

3.5

4.5

5.5

6.5

CRYSTAL SYSTEM

A mineral crystal forms in one of six basic systems.

Include:

Isometric (Cubic) Tetragonal Hexagonal Orthorhombic Monoclinic Triclinic

Isometric

Tetragonal

Hexagonal

Orthorhombic

Monoclinic

Triclinic

DENSITY

The ratio of the mass of a substance to the volume of the substance is called density. Usually measured in g/cm3. Most of the

common minerals in Earth’s crust have densities between 2 and 3 g/cm3.

Water has a density of 1.0 g/cm3 and is used a reference point for other substances. The ratio of an object’s density to the

density of water is called its specific gravity.

SPECIAL PROPERTIES

Some minerals have to ability to glow under ultraviolet (UV) light and exhibit fluorescence. Some continue to glow after the UV light is

turned off and this is known as phosphorescence.

Some minerals will effervesce, or “fizz”, when a drop of a weak acid (such as HCl) is placed on it.

Calcite has a special property known as double refraction, when light is bent causing an image to be doubled.

Calcite and Willemite

Chalk

Calcite

SPECIAL PROPERTIES

Magnetite and Pyrrhotite are naturally magnetic minerals attracting iron. Non-silicate minerals containing iron are

more likely to be magnetic than others without iron.

Some minerals exhibit a distinct taste, such as Halite (table salt).

Some minerals are radioactive and can be detected using a Geiger Counter device. Radioactivity results as unstable nuclei decay

over time into stable nuclei by releasing particles and energy.

Magnetite

Halite

Geiger Counter

SPECIAL PROPERTIES

In reflected light, some minerals display a silky band known as chatoyancy. Commonly called the “Cat’s Eye Effect”. Comes from the French word chat meaning

“cat” and oiel meaning “eye”. It is the result of closely packed parallel

fibers within the mineral.

A similar effect called asterism, is the phenomenon in which a six-sided star shape appears when a mineral reflects light.

Chalcedony Quartz

Cabochon Corundum