معادن

م.م. صالح عبيد لزام Atoms, Elements, and MineralsPhysical Geology 13/e, Chapter 2

Minerals

A mineral is a naturally occurring, inorganic, crystalline solid, which is physically and chemically distinctive. Form in the geosphere (most minerals), hydrosphere (e.g., halite), biosphere (e.g., calcite), and even the atmosphere (e.g., water ice, as snow) Consistent and recognizable physical and chemical properties

Atoms and Elements

An element is a substance that can not be broken down into others by ordinary chemical reactions An atom is the smallest unit of a substance that retains the properties of that element Composed of 3 types of subatomic particles Protons (positively charged) Neutrons (zero net charge) Electrons (negatively charged) A molecule is the smallest unit of a compound that retains the properties of that substance

Atomic Structure

Protons and neutrons form the nucleus of an atom Represents tiny fraction of the volume at the center of an atom, but nearly all of the mass Electrons orbit the nucleus in discrete shells or energy levels Shells represent nearly all of the volume of an atom, but only a tiny fraction of the mass Numbers of electrons and protons are equal in a neutral atom Ordinary chemical reactions involve only outermost shell (valence) electrons

Isotopes

Atoms of an element with different numbers of neutrons are called isotopes Isotopes may be either stable or unstable Stable isotopes retain all of their protons and neutrons through time Unstable or radioactive isotopes spontaneously lose subatomic particles from their nuclei over time Stable isotopes can be used to track climate change over time

Chemical Bonding

Chemical bonding is controlled by outermost shell (valence) electrons Elements will typically be reactive unless their valence shell is full Atoms or groups of atoms with unequal numbers of protons and electrons, thus having a non-zero charge, are called ions Positive and negative ions are attracted to one another and may stick or chemically bond together

Chemical Bonding

Ionic bonding Involves transfer of valence electrons from one atom to another Covalent bonding Involves sharing of valence electrons among adjacent atoms Metallic bonding Electrons flow freely throughout metals; results in high electrical conductivity
Ionic bonding of NaCl (sodium chloride)


Composition of Earth’s Crust Common elementsNearly 97% of the atoms in Earth’s crust are represented by the 8 most common elementsO, Si, Al, Fe, Ca, Na, K, MgCommon mineral typesMost minerals are silicates (contain Si and O bonded together)Minerals have crystalline structuresRegular 3-D arrangement of atoms Insert Box 2.3 - Fig. 2A here

Silicate Structures

The Silicon-Oxygen tetrahedron Strongly bonded silicate ion Basic structure for silicate minerals Sharing of O atoms in tetrahedra The more shared O atoms per tetrahedron, the more complex the silicate structure Isolated tetrahedra (none shared) Chain silicates (2 shared) Double-chain silicates (alternating 2 and 3 shared) Sheet silicates (3 shared) Framework silicates (4 shared)

Non-silicate Minerals

Carbonates Contain CO3 in their structures (e.g., calcite - CaCO3) Sulfates Contain SO4 in their structures (e.g., gypsum - CaSO4. 2H2O) Sulfides Contain S (but no O) in their structures (e.g., pyrite - FeS2) Oxides Contain O, but not bonded to Si, C or S (e.g., hematite - Fe2O3) Native elements Composed entirely of one element (e.g., diamond - C; gold - Au)

Mineral Properties

Cleavage Breakage along flat planes Fracture Irregular breakage Specific gravity Density relative to that of water Magnetism Attracted to magnet Chemical reaction Calcite fizzes in dilute HCl

Mineral Properties

Color Visible hue of a mineral Streak Color left behind when mineral is scraped on unglazed porcelain Luster Manner in which light reflects off surface of a mineral Hardness Scratch-resistance Crystal form External geometric form
Physical and chemical properties of minerals are closely linked to their atomic structures and compositions

Minerals

A mineral must meet the following criteria:Crystalline solidAtoms are arranged in a consistent and orderly geometric patternForms through natural geological processesHas a specific chemical composition May include some internal compositional variation, such as the solid solution of Ca and Na in plagioclase)Rock-forming mineralsAlthough over 4000 minerals have been identified, only a few hundred are common enough to be generally important to geology (rock-forming minerals)Over 90% of Earth’s crust is composed of minerals from only 5 groups (feldspars, pyroxenes, amphiboles, micas, quartz)

Minerals

Ore minerals Minerals of commercial value Most are non-silicates (primary source of metals) Examples: magnetite and hematite (iron), chalcopyrite (copper), galena (lead), sphalerite (zinc) Must be able to be extracted profitably to be considered current resources Gemstones Prized for their beauty and (often) hardness May be commercially useful Diamond, corundum, garnet, and quartz are used as abrasives

End of Chapter 2