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FROM ATOMS TO MINERALS

Elements and Atoms

 

An element is a substance that can not be broken down by ordinary chemical reactions.  There are about 88 naturally occurring elements.

 

An atom the smallest unit of matter that retains the properties of an element.

 

Atoms are actually made of 3 types of subatomic particles:

 

Subatomic Particle

Location

Charge

Mass

       
       
       

 

The protons and neutrons occur in the atom's nucleus.  The nucleus has almost no volume, but contains most of the atom's mass.

 

Electrons exist around the nucleus in discrete energy shells or energy levels.  Electron shells comprise most of the atom’s volume, but electrons have almost no mass.

 

Atoms are electrically neutral (# electrons = # protons).

 

The atomic number of an element is equal to the number of protons in an atom of that element.

 

The atomic mass of an element is equal to the combined mass (in AMU) of all subatomic particles in an atom of that element (this is essentially equal to the number of protons + the number of neutrons).  

 


 

Isotopes are atoms of an element with different numbers of neutrons.  For  example there are several isotopes of oxygen.  Two of these are oxygen-16 (with a mass of 16 AMU) and oxygen-18 (mass of 18 AMU).  Sketch these two isotopes of oxygen showing protons, electrons, and neutrons).

 

Oxygen-16

Oxygen-18

 

 

 

 

 

 

Isotopes may be either stable or unstable.

 

Stable isotopes retain all of their protons and neutrons through time. We will see that these isotopes are useful for tracking climate change over time. 

 

Unstable or radioactive isotopes decay to form new atoms by losing subatomic particles.  Radioactive isotopes are useful for measuring the ages of rocks.

 


 

There are 3 Types of Chemical Bonding that Lead to Compounds

 

1) Ionic bonding- Occurs between metal and non-metallic atoms by electron transfer from the metal to the non-metal (NaCl).

 

2) Covalent bonding- Occurs between two non-metallic atoms by electron sharing (the carbonate CO32- ion).

 

3) Metallic bonding- Occurs between metals; the electrons flow freely amongst atoms (Fe-Ni alloy).

 

Most compounds are not purely ionic or purely covalent: they have characteristics of both bonding types.

 


 

What are Minerals?

 

 

Naturally-occurring, solid, inorganic chemical compounds that are the building blocks of rocks.

 

About 4000 minerals are known, buy only a dozen are common in Earth's crust.

 

Minerals are crystalline in nature, meaning that the atoms have very specific arrangements.  This well-defined atomic structure is often reflected by the minerals geometric shape at the macroscopic scale.  However, a mineral can be crystalline at the atomic scale without exhibiting crystal-like shapes at the macroscopic scale.

 

Most minerals form in the geosphere, but some minerals form in the hydrosphere (halite) or even the biosphere (calcite).

 


 

Mineral Chemistry

 

Minerals are chemical compounds and therefore they have chemical formulas.

 

Quartz: SiO2

Alkali Feldspar: (K,Na)AlSi3O8

Biotite: K(Fe,Mg)3AlSi3O10(OH)2

Halite: NaCl

 

Like all chemical compounds, minerals are electrically neutral (the total number of protons = the total number of electrons.

 

Many minerals have compositions that are fixed (i.e., quartz and halite), but some minerals may exhibit a limited range of compositions.  For example, feldspar compositions can vary between K-rich, Na-rich, and Ca-rich varieties as shown below.

 

 

This "solid solution" happens because similarly-sized and charged ions can substitute for each other.  Some crystals even exhibit "chemical zoning" in which the composition varies systematically within the crystal (usually from crystal rim to crystal core).

 


 

How Do We Distinguish One Mineral from Another?


Different minerals have:

 

-Different chemical compositions (in each mineral the elements are bonded together in specific ratios).

 

-Different atomic structures (graphite and diamond, both made of pure carbon, but are classified as different minerals because they have different crystal structures).

 

-Different physical characteristics (we can use physical tests to distinguish minerals).

 


 

Minerals can be Identified Based on their Physical Properties:

 

 

Color-

 

Streak-

 

Luster-

 

Hardness-

 

Crystal form/habit-

 

Cleavage-

 

Fracture-

 

Specific gravity-

 

Magnetism-

 

Reactivity to acid-


 

The Occurrence of Elements and Minerals in Earth’s Crust

 

The 8 most common elements in Earth's crust are: O, Si, Al, Fe, Ca, Na, K, and Mg.  These elements account for 98% of the atoms in Earth's crust.

 


 

These 8 elements combine chemically with one another to make 6 main minerals which account for 90% of the minerals in Earth's crust.  These minerals, the so-called the rock forming minerals, are: feldspar, quartz, pyroxene, amphibole, and mica.

 

The Rock-Forming Minerals

Feldspar

Alkali Feldspar: (K,Na)AlSi3O8

Feldspar

Plagioclase Feldspar: NaAlSi3O8 - CaAl2Si2O8

Quartz: SiO2

 

Amphibole

Hornblende: Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2

Mica

Muscovite: KAl2(AlSi3O10)(F,OH)2

Mica

Biotite: K(Fe,Mg)3AlSi3O10(F, OH)2

Olivine

Fayalite: Fe2SiO4

Forsterite: Mg2Si2O4

Pyroxene

Enstatite: Mg2Si2O6

Ferrosilite: Fe2Si2O6

These photos are from R. Welleror of Cochise College, © 2008.

 

Mineral Structures

 

Minerals have crystalline structures at the atomic level.  The regular, repeating, three-dimensional arrangement of atoms (or ions) is called a lattice structure.

 

The lattice structure of halite, NaCl, looks like:

 

Here, the blue spheres represent Na1+ ions and the red spheres represent Cl1- ions.

 

 

The most common rock-forming minerals contain Si and O bonded together (i.e., SiO2, MgSiO3).  These minerals are called “silicates”.

 

The basic structure for silicate minerals is the silica tetrahedron.  The tetrahedron has 1 silicon at the center, and 4 oxygen atoms at the corners.  The Si and O are covalently bonded to form a negative polyatomic ion (SiO44-).

The extra negative change (-4) means that positive metal ions are needed for charge balance (i.e., MgSiO3).  The metal ions reside between the tetrahedra.

 

Silica tetrahedra can join together by sharing corner oxygen atoms to make the following more complex silicate structures:

 

SILICATE STRUCTURES

Arrangement of Tetrahedra

Drawing

Minerals

Isolated tetrahedra- No oxygen atoms are shared.

Olivine

Chain silicates- Two oxygen atoms on each tetrahedron are shared.

Pyroxene

Double-chain silicates- Alternating 2 and 3 oxygen atoms on each tetrahedron are shared.

Amphibole

Sheet silicates- 3 oxygen atoms on each tetrahedron are shared.

Micas

Framework silicates (not pictured)- All 4 oxygen atoms on each tetrahedron are shared.

Ugh...Can somebody sketch this for me?

Quartz

 


 

Non-Silicate Minerals

 

There are many minerals that are not based on the silica tetrahedron.  These non-silicates are less abundant that silicates but are of great importance.  Some noteworthy non-silicate minerals are:

 

Native elements- Composed entirely of one element (i.e., diamond is pure carbon and gold is pure)

 

Carbonates- Contain CO32- in their structures (i.e., calcite CaCO3)

The non-silicate mineral calcite (Image from the US Department of the Interior).

 

Sulfates- Contain SO42- in their structures (i.e., gypsum CaSO4· 2H2O)

 

Sulfides- Contain S2- (but no O) in their structures (i.e., galena PbS)

 

Oxides- Contain O2-bonded to metal atoms (i.e., hematite Fe2O3)

 

Many non-silicate minerals are “ore minerals”, meaning that they are of commercial value in sufficient quantities.

 


 

Minerals as Gemstones

 

The term "gemstone" is used rather loosely in reference to minerals (and sometimes rocks) that are prized for their beauty and high hardness.

 

Gemstones are commonly used for faceting and/or mounting in jewelry.

 

The world of gemology often uses special names for minerals.  For example, gemstone quality olivine is known as peridot.

Some common gemstones (Public Domain Image by Arpingstone).