Atoms and Minerals Magnet and Iron and slide

Atoms and Minerals Magnet and Iron and slide Quartz Si. O 2 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 shells Number of outermost electrons determine types of bonding Outermost (Valence) shell Argon

Some definitions: • Atomic number: number of protons in the nucleus • 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 shells Number of outermost electrons determine types of bonding Shows atomic number (# protons) and atomic mass (# protons + neutrons). Column shows # electrons in outermost shell

Electrons are in shells. Octet Rule: Atoms larger than Hydrogen and Helium need 8 electrons in their outer shell for stability

Neutral Atoms have #protons = # electrons Oxygen has 6 electrons in its valence shell Silicon has 4 electrons in Its outer shell

To satisfy the octet rule atoms can gain or lose electrons In that state they are called IONS They can combine with oppositely charged ions to form neutral molecules Oxygen, normally 6 valence electrons, wants 2 extra Ions 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 (Na. Cl)- An Example of Ionic Table Bonding Salt Cl- Na+ Na+ Halite small Na+ large Cl- Crystalline structure of Internal atomic arrangement is Na. Cl primarily determined by the size of ions involved (a) Small Sodium ions between large Chlorine ions

Covalent bonding – sharing of valence electrons Sharing Electrons in Outermost Shell Cl 2 Chlorine gas

Covalent Bonds in Water H 2 O Water 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. • 12 C 13 C stable 14 C 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 Pb. S 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 (Si. O 2) 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

• 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 Biotite Micas have perfect cleavage

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 charges often 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 has 6 electrons in its valence shell Silicon has 4 electrons in Its outer shell

Remember: atoms can gain or lose electrons They then combine with oppositely charged ions to form neutral molecules Ions Anion (negative) Cation (positive)

Silicate Molecule The Silicon-Oxygen Tetrahedron 2_25 O 2 - Si 4+ O 2 - O 2 The basis of most rock-forming minerals, charge - 4 O 2 -

Silicate Bonding I • Oxygen O atoms may obtain electrons from Si atoms, producing the Si. O 4 -4 Ion. • The negative charge is balanced by positive metal ions. • This occurs in Olivine, (Fe, Mg)2 Si. O 4, a high temperature Fe-Mg silicate. Forms of this mineral are stable 100’s of kilometers below Earth’s surface. • Sort of Ionic Bond

Example OLIVINE Positive ion Fe and Mg Si. O 4 -4 Ion Tetrahedron facing down Tetrahedron facing up Independent tetrahedra

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_26 c An Amphibole Positive ion Cleavages 56 and 124 deg Double chains (c)

Example: Mica Sheet silicates (d)

Clay Minerals (at high magnification) note sheet structure Very small crystals Kaolinite (hand specimen) Clays are also Sheet Silicates, just as Micas are Vietnam Anecdote

Example: Quartz Si. O 2 2_26 e Framework silicates (e) (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

Feldspars that use Calcium (Ca) or Sodium (Na) metals to balance the Si. O 4 - 4 and Al. O 4 -5 charges are called: Plagioclase feldspar Note the Twinning, seems to have ‘stripes’

Summary