Unit 13

A mineral is a naturally occurring inorganic solid that has a characteristic chemical composition, an orderly internal structure, and a characteristic set of physical properties.

No. Granite is not a mineral because it does not have a consistent chemical composition, and obsidian volcanic glass is not a mineral because its structure is amorphous, not orderly.

Yes. Ice is a mineral even though it is not solid at room temperature.

Geologists have identified over 4000 different minerals, but fewer than 20 are common and are known as rock-forming minerals.

The two most common elements in Earth’s crust are oxygen and silicon. Minerals containing both are called silicates and make up 96% of Earth’s crust.

Feldspars are the most common silicate minerals. The type of feldspar that forms depends on which metal combines with the silicon and oxygen atoms.

Quartz is made primarily of silica, which is silicon dioxide with the chemical formula SiO2. Quartz is the second most abundant mineral in Earth’s crust.

Quartz is the most common mineral in desert sand, and amethyst is a purple variety of quartz. Amethyst gets its colour from iron impurities exposed to ionizing radiation.

Yes. Pure elements such as gold, silver, and copper can be considered minerals.

Carbonates: calcite or dolomite. Halides: halite or fluorite. Oxides: corundum or hematite. Sulfates: gypsum or anhydrite. Sulfides: galena or pyrite.

A crystal is a solid whose atoms, ions, or molecules are arranged in a regular, repeating pattern. We study crystal structure using X-rays.

The basic building block of silicate minerals is a silicate tetrahedron with four oxygen atoms arranged in a pyramid with one silicon atom in the centre.

Silicate tetrahedra can be linked in different ways, and the different linkages give the minerals different properties. For example, mica minerals like muscovite contain tetrahedra linked in sheets.

Non-silicate minerals have more varied chemical compositions, so they form a wider variety of crystal structures. Examples include cubes and hexagonal prisms.

The density of a mineral depends on the kinds of atoms it has and its crystalline structure, which determines how closely the atoms are packed. Most common minerals have densities between 2 and 3 grams per cubic centimetre.

Cleavage is the tendency of a mineral to split along specific planes of weakness in the crystal structure to form smooth, flat surfaces. Sheet silicates like muscovite have cleavage in one direction.

Fracture is when a mineral breaks unevenly into pieces. A rough surface has an uneven or irregular fracture, a broken surface that looks like a piece of broken wood has a splintery or fibrous fracture, and curved surfaces are conchoidal fractures.

Hardness is the measure of the ability of a mineral to resist scratching. Hardness does not mean resistance to cleavage or fracture.

The Mohs Hardness Scale lists 10 minerals in order of increasing hardness. Gypsum and calcite are low, and diamond is high.

You scratch the mineral against minerals on the Mohs hardness scale to see which one scratches the other. This helps determine the mineral’s hardness range.

Hardness is determined by the strength of the bonds between the atoms that make up the mineral’s structure. Diamond is hard because each carbon atom is firmly bonded to four others, while graphite is soft because its carbon atoms are arranged in layers.

Colour is generally not reliable because many minerals are similar in colour and trace impurities can change colour. Rubies and sapphires are both corundum.

In a streak test, a mineral is rubbed against an unglazed ceramic tile to observe its powdered form. Pyrite leaves a black streak, not a gold one.

Lustre describes how light reflects from a material. It can be metallic, glassy, waxy, pearly, brilliant, or dull/earthy.

Fluorescent minerals absorb ultraviolet light and re-emit energy in the visible spectrum. Phosphorescent minerals continue to glow after the ultraviolet light is turned off.

Chatoyancy is a silky cat’s-eye effect caused by closely packed parallel fibers. Asterism is the appearance of a star-shaped reflection, usually six-sided.

Double refraction creates a double image of any object viewed through the crystal because light rays are split into two parts. Calcite or Icelandic spar exhibits double refraction.

Some minerals that contain iron have tiny magnetic dipoles and are magnetic. Magnetite is an example of a magnetic mineral.

Minerals are radioactive if they contain unstable isotopes. Uraninite (pitchblende) is radioactive and can contain uranium and radium.

You can test for radioactivity with a Geiger counter.

My favourite mineral is quartz because it is common, and I like the glassy and non metallic look to it. It is made of silicon dioxide, or silica. Quartz is fairly hard, with a hardness of 7 on the Mohs scale, and it often breaks with conchoidal fracture. It can come in many colours, including clear, purple, and pink, depending on impurities.