Minerals

AtomsIonsElements and CompoundsElements in the Earth’s CrustMinerals

Atomic Components

An atom is the smallest subdivision of matter. An atom consists of three types of particles. The nucleus, or center, of an atom contains neutral particles, called neutrons, and positively charged particles, called protons. Around the nucleus is a thinly populated region that contains electrons, particles with a negative electrical charge.

IONS

In an atom, the number of protons and the number of electrons are equal; therefore, the atom is neutral or has no electrical charge. If an atom gains or loses electrons, the charge of the atom becomes negative or positive, respectively. Charged atoms are called ions. If you have ever shuffled your feet on a carpet and then touched a door knob, you are familiar with an electrical charge.

Elements and Compounds An element is a substance which cannot decompose into other

substances by ordinary chemical means. One atom of an element will have all the characteristics of that element. Although you may not realize it, you are already familiar with several elements. For example, the element carbon is in diamonds, used for jewelry, and in graphite, used in pencil lead. We also use the elements gold and silver for jewelry. We need the element oxygen in the air we breathe. We worry about the carbon dioxide emitted by our cars.

Carbon dioxide is a compound, or a combination of two or more elements. In the case of carbon dioxide, two atoms of oxygen combine with one atom of carbon. Another common compound, table salt, is made of the elements sodium and chlorine.

Major Elements in the Earth’s Crust

Element (Symbol) Weight percent_______________________________

Oxygen (O) 46.6Silicon (Si) 27.7Aluminum (Al) 8.1Iron (Fe) 5.0Calcium (Ca) 3.6Sodium (Na) 2.8Potassium (K) 2.6Magnesium (Mg) 2.1Total 98.5__________________________

Major Elements in the Earth’s Crust

Eight elements combine to make most minerals and rocks found in the Earth's crust. The elements are oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium. The letters in the parentheses are abbreviations. You may already be somewhat familiar with some forms of these elements. Silicon combines with two atoms of oxygen (in the nomenclature of chemist, SiO2,or silica) to make the mineral quartz, which is common on many mainland beaches. Silica is also used for window glass. Aluminum is used for soda cans. Iron is used for the frame of automobiles and reacts with oxygen in the air to produce rust. Magnesium is used in flares. Calcium is a major component in our teeth and bones. Sodium is in table salt.

Potassium is used in fertilizer.

Mineral Definition

A mineral, by definition, must satisfy five conditions:

• It must be naturally occurring.

• It must be inorganic.

• It must be a solid element or compound.

• It must have a definite composition.

• It must have a regular internal crystal structure.

Mineral Commentary

This definition excludes the thousands of compounds invented by humans in laboratories because these compounds are not naturally occurring. Compounds that are found in only plants or animals are also excluded. Liquids are excluded because they are not crystalline, their atoms are free to move. Minerals can be a single element, like diamond, which is made of carbon of compounds of two or more elements, like quartz, which contains one silicon and two oxygen atoms. Definite composition indicates that a chemical analysis of a given mineral will always produce the same ratio of elements. For example, quartz will always have one silicon for every two oxygen atoms. Therefore, minerals can

be expressed by chemical formulas, such as SiO2 for quartz.

COMMON IONS IN MINERALS

Common ions in minerals. Charges and relative sizes are shown.

IONS CAN SUBSTITUE IN MINERALS

In some minerals, elements of similar size and charge substitute for each other. The amount of iron and magnesium in olivine varies because these elements can substitute for each other in the structure of the mineral. The formula for olivine, (Mg,Fe)2SiO4, indicates that for every two magnesium and/or iron atoms, there are one silicon atom and four oxygen atoms. Substitution of sodium for calcium occurs in the mineral plagioclase.

MINERALS HAVE A REPEATING STRUCTURE

Regular internal crystalline structure indicates that the atoms are arranged in a regular repeating pattern. This diagram shows the structure of the mineral halite. Atoms of chlorine and sodium are arranged in a three dimensional repeating pattern.

Minerals in Igneous Rocks

Orthoclase Orthoclase

Minerals in Igneous Rocks

Quartz Muscovite: Biotite

Minerals in Igneous Rocks

Pyroxene Augite

Minerals in Igneous Rocks

Horneblende