Atoms and Minerals Magnet and Iron and slide

Quartz SiO2 common mineral

Minerals: Building blocks of rocks

• Definition of a mineral:• Naturally occurring• Inorganic solid• Ordered internal molecular structure• Definite chemical composition

• Definition of a rock:• A solid aggregate or mass of minerals

• Atomic structure• Central region called the nucleus

– Consists of protons (positive charges) and neutrons (neutral charges)

• Electrons– Negatively charged particles that orbit around the nucleus– Located in discrete energy levels called shells

Flattened structure of an atom# protons (+) equals # electrons (-)Electrons in shellsNumber of outermost electrons determine types of bonding

ArgonOutermost (Valence) shell

Some definitions:Some definitions:

• Atomic number:Atomic number: number of number of protons in the nucleusprotons in the nucleus

• Atomic Mass: Atomic Mass: total mass of protons and neutrons within an atom’s nucleus

• We can see these on a Periodic Table

Periodic Table of the Elements

# protons (+) equals # electrons (-)Electrons in shellsNumber of outermost electrons determine types of bonding

Shows atomic number (# protons) and atomic mass (# protons + neutrons). Column shows # electrons in outermost shell

Atoms larger than Hydrogen and Helium need 8 electrons in their outer shell for stabilityOctet Rule:

Electrons are in shells.

Neutral Atoms have #protons = # electrons

Oxygen has6 electrons in its valence shell

Silicon has 4 electrons inIts outer shell

To satisfy the octet rule atoms can gain or lose electronsIn that state they are called IONS

They can combine with oppositely charged ions to form neutral molecules

IonsOxygen, normally 6 valence electrons, wants 2 extra

Silicon, normally 4 valence electrons, would like to be rid of, or share, 4

Chemical Bonding 1: Ionic

• Chemical bonding• Formation of a compound by combining two or

more atoms• Ionic bonding

• Atoms gain or lose outermost (valence) electrons to form ions

• Ionic compounds consist of an orderly arrangement of oppositely charged ions

• Usually Columns I (alkali metals e.g. Na) and VII (halogens e.g. Cl)

Halite small Na+ large Cl-

Na+

Na+

Na+

Na+

Na+

Cl-

Cl-Cl-

Crystalline structure ofNaCl

(a)Small Sodium ions between large Chlorine ions

Internal atomic arrangement is primarily determined by the size of

ions involved

Table Salt

Halite (NaCl)- An Example of Ionic Bonding

Covalent bonding – sharing of valence electrons

Cl2 Chlorine gasSharing Electrons in Outermost Shell

Covalent Bonds in Water

H2OWater is polar

Other Bond Types

• Metallic bonding– Valence electrons are free to migrate among

atoms– Weaker and less common than ionic or covalent

bonds• Intermolecular bonding

– Hydrogen bonds- charged regions in water attract

– Van der Waals bonds- electrons momentarily grouped on same side of nucleus

Isotopes• Isotopes and radioactive decay

• Atomic mass is the total mass of neutrons plus protons in an atom

• An isotope is an atom that exhibits variation in its atomic mass, i.e. different numbers of neutrons

• Some isotopes have unstable nuclei that emit particles and energy in a process known as radioactive decay.

• 12C 13C stable 14C radioactive

Structure of minerals

• Polymorphs• Two or more minerals with the same chemical

composition but different crystalline structures• Diamond and graphite (both carbon) are good

examples of polymorphs» The transformation of one polymorph to another

is called a phase change» Example: Graphite in a High Pressure Cell

Makes Diamond• Some polymorphs make good PT indicators

Diamond and graphite – polymorphs of carbon

Physical properties of minerals• Crystal Form

• External expression of the orderly internal arrangement of atoms

The mineral garnet often exhibits good crystal form

Physical properties of minerals

• Luster• Appearance of a mineral in reflected light• Two basic categories

– Metallic– Nonmetallic

• Terms are used to further describe nonmetallic luster are vitreous (glassy), pearly, silky, earthy (like dirt), adamantine (greasy)

Galena PbS displays metallic luster

Valuable ore of Lead

Physical properties of minerals

• Color• Generally an unreliable diagnostic

property to use for mineral identification• Often highly variable for a given mineral

due to slight changes in mineral chemistry• Exotic colorations of some minerals

produce gemstones• But we use it anyway

Quartz (SiO2) exhibits a variety of colors

Physical properties of minerals

• Streak• Color of a mineral in its powdered form• Helpful in distinguishing different

minerals with similar composition• Hardness

• Resistance of a mineral to abrasion or scratching

• All minerals are compared to a standard scale called the Mohs scale of hardness

Streak

• Cleavage• Tendency to break along planes of weak

bonding• Produces flat, shiny surfaces• Described by resulting geometric shapes

– Number of planes– Angles between adjacent planes Micas have perfect cleavage

BiotiteMica

Three directions of perfect cleavage – fluorite, halite, and calcite

Each Cleavage Plane is paired

Physical properties of minerals

• Fracture• Absence of cleavage when a mineral is

broken. Shown: conchoidal fracture in Quartz

• Specific Gravity• Ratio of the weight of a mineral to the

weight of an equal volume of water• Average value is approximately 2.7• Simply hefting a mineral works too.

Physical properties of minerals

• Other properties• Magnetism• Reaction to hydrochloric acid• Malleability• Double refraction• Taste• Smell• Elasticity

Is it calcite or dolomite?

Classification of Minerals

• Nearly 4000 minerals have been identified on Earth (We discuss a few)

• Rock-forming minerals• Common minerals that make up most of

the rocks of Earth’s crust• Only a few dozen members• Composed mainly of the 8 elements that

make up 98% of the continental crust

Commonly formed Ion chargesoften called “oxidation state”

Metals can form more than one Ion. Fe+2 is name Ferrous, Fe+3 is named Ferric

Classification of Minerals

• Silicates• Most important mineral group

– Comprise most of the rock-forming minerals– Very abundant due to large amounts of

silicon and oxygen in Earth’s crust• Basic building block is the silicon-oxygen

tetrahedron molecule– Four oxygen ions surrounding a much

smaller silicon ion

The Component Atoms

Oxygen has6 electrons in its valence shell

Silicon has 4 electrons inIts outer shell

Remember: atoms can gain or lose electronsThey then combine with oppositely charged ions to form

neutral molecules

Ions

Anion (negative)

Cation (positive)

O2 -

O2 -

O2 -

O2 -

Si4+

2_25The Silicon-Oxygen Tetrahedron

The basis of most rock-forming minerals, charge - 4

Silicate Molecule

Silicate Bonding I

• Oxygen O atoms may obtain electrons

from Si atoms, producing the SiO4 -4 Ion.

• The negative charge is balanced by positive metal ions.

• This occurs in Olivine, (Fe,Mg)2SiO4, a high temperature Fe-Mg silicate. Forms of this mineral are stable 100’s of kilometers below Earth’s surface.

• Sort of Ionic Bond

Tetrahedronfacing down

Tetrahedronfacing up

Positive ionExample OLIVINE

Independent tetrahedra

Fe and Mg

SiO4 -4 Ion

Silicate Bonding II

• Alternately, the oxygen atoms may complete their outer electron shells by sharing electrons with two Silicon atoms

in nearby silicon tetrahedra.

• A sort of covalent bond

A Pyroxene

Single chains weakly paired

2_26c

Positiveion

Double chains(c)

An Amphibole

Cleavages 56 and 124 deg

Sheet silicates(d)

Example: Mica

Clay Minerals(at high magnification)

Kaolinite(hand specimen)

note sheet structure

Clays are also Sheet Silicates, just as Micas are

Vietnam Anecdote

Very small crystals

2_26e

Framework silicates(e)

Example: Quartz SiO2

(3-D, also the Feldspars)

Classification of Minerals

• Common Silicate minerals• Feldspar Group

– Most common mineral group– two directions of perfect cleavage at 90 degrees– In Feldspars, some of the Silicon atoms (oxidation

state +4) are replaced by Aluminum (oxidation state +3)

– Ion is not symmetrical

– Pearly Luster

A Potassium Feldspar

Plagioclase feldsparNote the Twinning, seems to have ‘stripes’

Feldspars that use Calcium (Ca) or Sodium (Na) metals to balance the SiO4 - 4 and AlO4 -5 charges are called:

Summary