Mineralogy Notes

Minerals: Building Blocks of Rocks

• Minerals are the fundamental components of rocks and the solid Earth.
• Defined by five key characteristics:
  • Specific chemical composition
  • Specific atomic arrangement (crystal form)
  • Solid state
  • Inorganic origin
  • Naturally occurring
• Example: Six-sided, pyramidal Quartz Crystals.

Mineral Formation: Crystallization from Magma

• Minerals can form during the crystallization of a magmatic melt.
  • Basalt: Composed of minerals that crystallize from magma derived from a partial melt of the asthenosphere.
  • Andesite: Composed of minerals that crystallize from magma derived from a partial melt of basaltic ocean crust.
• Crystal size is related to cooling rate.
  • Large plagioclase crystals indicate slow cooling underground.

Identifying Minerals in Porphyritic Rock

• Porphyritic rocks contain minerals that crystallized at different times.
• Determining the order of crystallization and relative melting temperatures:
  • Minerals that crystallize first have the highest melting temperatures.
  • Minerals that crystallize last have the lowest melting temperatures.
• Example: Porphyritic andesite with small black minerals, larger white minerals, and smaller minerals in the gray matrix.

Mineral Formation: Precipitation from Aqueous Solution

• Minerals can precipitate directly from an aqueous solution.
• Example: Salt (NaCl) deposits in the Great Salt Lake, Utah.

Crystal Form and Atomic Arrangement

• The arrangement of constituent atoms determines the crystal form of a mineral.
• Requires an unrestricted environment for proper crystal growth.
• Example: Quartz crystal.

Quartz Crystallization in Granite

• Quartz in granite may not form well-defined pyramidal crystals.
• The order of crystallization indicates relative melting temperatures.
  • Quartz as the last mineral to crystallize suggests a lower melting temperature compared to other minerals in granite.

Crystal Systems

• Seven major crystal systems exist, each defined by the arrangement of atoms during mineral growth.
• Famous gems represent various crystal systems.
• Question: Which crystal system does olivine belong to?

Cleavage Planes and Chemical Bond Strength

• Chemical bond strength and atomic arrangement determine the presence of weakness planes (cleavage planes) in minerals.
• Example: Halite (salt) crystal with three cleavage planes at 90° to one another.
• Cleavage planes form between ionic bonds (e.g., between sodium (Na) and chloride (Cl) atoms in halite).

Atomic Structure

• Atoms are the smallest particles defining the chemical properties of matter.
• Composed of protons (+), neutrons (neutral), and electrons (-).
• Protons and neutrons reside in the nucleus, while electrons surround the nucleus in defined energy levels.

Important Chemistry Facts

• The number of protons defines the element's chemical properties.
  • Example: Oxygen always has 8 protons.
• The number of protons plus neutrons defines the mass number (electrons have negligible mass).
• Electrons are arranged in orbits (energy levels) around the nucleus with specific capacities:
  • 1st level: 2 electrons
  • 2nd level: 8 electrons
  • 3rd level: 8 electrons

Chemical Stability and Orbital Valencies

• Atoms achieve chemical stability when charge is balanced and all orbital valencies are filled.
• Orbital capacities:
  • 1st orbital: 2 electrons
  • 2nd orbital: 8 electrons
  • 3rd orbital: 8 electrons

Periodic Table Arrangement

• Elements are arranged in rows (periods) based on increasing atomic number (proton number).
• Atomic mass generally increases along with the atomic number.

Ionic State and Mineral Arrangement

• The arrangement of atoms in a mineral (ionic state) is related to the charge and size (ionic radius) of the atom.

Ionic vs. Atomic Radii

• The ionic radii of Cl-1 is larger than Na+1, while their atomic radii relationship is opposite (Na atomic radii is larger than Cl).
• Na has 2 orbitals in its ionic state (loss of outermost electron), whereas Cl has 3 orbitals in its ionic state, leading to a larger ionic radius.
• Atoms that are not charged have progressively smaller radii within a given periodic table row due to greater nuclear attraction as proton number increases.

Periodic Table Groups

• Elements are arranged in columns (groups) based on the number of electron valencies.
• Transition metals (middle of the periodic table) may have multiple valency states.
• Noble gases: What is significant about them?

Ionic Charges and Bonding

• Atoms can give up, receive, or share electrons to fill their valencies.
• Atoms that tend to give up electrons (+ charged cations) bond with atoms that tend to receive electrons (- charged anions).
• Atoms can also bond by sharing electrons (covalent bonds) to achieve a stable configuration.

Ionic Bonds

• Ionic bonds form through the physical transfer of electrons between a donor cation (+ charge) and a receiver anion (- charge).

Bonding Properties and Valency

• Elements in the same column have similar bonding properties.
• Example: Sodium (Na) and lithium (Li) both transfer one electron when forming bonds.
• Sodium (Na) transfers one electron to chloride (Cl) to form NaCl.

Silicon and Oxygen Valencies

• Silicon (Si+4) and oxygen (O-2) are major components of minerals in the Earth’s crust and upper mantle.

Covalent Bonds

• Electrons are shared between constituent atoms to achieve charge balance and fill valencies.
• Covalent bonds are generally very strong.

Common Ionic Charges

• N and P, as well as metallic elements, can have more than one valency state.
• Transition metals (middle of the periodic table) can also have more than one valency state.

Periodic Table of the Elements

• Overview of the periodic table.

Metallic Bonding

• Valence electrons are shared between nuclei.
• Forms an "electron cloud" or "electron sea model."
• Negatively charged electrons move freely among positively charged nuclei.
• Bond between electrons and nucleus is relatively weak, allowing electrons to wander.
• No set bond configuration.

Physical Properties of Metals

• Conductors of heat and electricity
• High melting points
• Malleable
• Ductile
• Luster
• Opaque

Intermolecular Bonding (Van der Waals Bonds)

• Van der Waals bonds are a type of intermolecular bond.
• Intermolecular attractions occur between neighboring molecules.
• Uneven charge distribution in some minerals leads to Van der Waals bonds.
• Temporary grouping of electrons on one side of an atom's nucleus creates slight positive and negative charges (Bond Dipole moment).
• The positive side attracts electrons of neighboring atoms, and the negatively charged side attracts the nuclei of neighboring atoms.

Intermolecular Bonding (Hydrogen Bonds in Water)

• Van der Waals bonds are much weaker than ionic, covalent, and metallic bonds.
• Can be broken by small amounts of heat.
• Example: Intermolecular bonds in ice break when the temperature is raised to 0°C.
• Covalent bonds within individual water molecules remain unaffected.
• Intermolecular bonds in water form due to the bipolar distribution of hydrogen and oxygen atoms.

Van der Waals Bonds - Graphite Example

• Graphite has flat planes of carbon atoms held together by covalent bonds.
• No covalent bonds exist between the planes.
• Weak intermolecular interactions (Van der Waals forces) are present between the planes.
• The planes can easily slip and slide over each other.
• Explains why graphite is soft and a good lubricant.
• Used in pencils due to the weak bonds allowing graphite to shatter easily.

Mineral Hardness

• Relative hardness of a mineral is controlled by its composition and bond strength between constituent atoms.
• Harder minerals can abrade softer minerals.
• Question: What mineral is found on a dentist's drill?

Graphite vs. Diamond: Composition and Atomic Arrangement

• Graphite and diamond have the same composition (carbon), but different atomic arrangements.
• Bonds between carbon sheets in graphite are weak Van der Waals bonds, making it very soft.

Diamond: Structure and Hardness

• Diamond is composed of carbon atoms arranged in a more compacted structure than graphite.
• Strong covalent bonds exist between the carbon atoms.
• Diamond is the hardest naturally occurring mineral.
• Forms very deep within the earth under very high pressures in volcanic pipes.

Streak

• Streak is most diagnostic for metallic minerals.
• Hematite ($Fe<em>2O</em>3$) leaves a distinct reddish-brown streak on a porcelain plate.

Dominant Elements in Earth Rocks

• Silicon (Si^{+4}) and oxygen (O^{-2}) are the dominant elements comprising Earth rocks.
• Other common elements are cations (positive ionic charges): $K^{+1}$, $Na^{+1}$, $Ca^{+2}$, $Fe^{+2, +3}$, $Al^{+3}$, $Mg^{+2}$

Silicate Tetrahedron

• The silicate tetrahedron is a complex anion with a charge of -4 ($SiO_4^{-4}$).
• Achieves charge balance by ionic bonding with available cations or covalent bonds between shared oxygen atoms of adjacent tetrahedra.

Olivine Structure

• ($Mg, Fe)<em>2SiO</em>4$): Olivine structure satisfies its tetrahedral valencies with ionic and metallic bonds with magnesium ($Mg^{+2}$) and iron ($Fe^{+2}$) atoms.
• First silicate mineral to crystallize from a magmatic melt.
• Olivine has no cleavage planes and will break along fractures.
• A major mineral comprising the upper mantle.
• Forms phenocrysts in some basalts.

Olivine Ratio of Si:O

• ($Mg,Fe)<em>2SiO</em>4$: Ratio of Si:O in olivine mineral structure is 1:4.
• One Si atom to 4 unshared O atoms.

Pyroxene Structure

• ($Mg,Fe,Ca,Na)(Mg,Fe,Al)Si<em>2O</em>6$: Pyroxene is a single-chain silicate that shares two oxygen atoms.
• Has two cleavage planes (at right angles).
• A main mineral comprising basaltic ocean crust.

Pyroxene Ratio of Si:O

• Ratio of Si:O in pyroxene (single-chain silicate) mineral structure is 1:3.
• One Si atom to 2 unshared and 2 shared O atoms.

Amphibole Structure

• ($Na,Ca)<em>2(Mg,Al,Fe)</em>5(Si,Al)<em>8O</em>{22}(OH)_2$): Amphibole (double-chain silicate) structure shares either 2 or 3 of its oxygen atoms.
• Forms two cleavage planes (124° and 56°).
• A common mineral in subduction zone rocks (andesite, dacite, diorite, and granodiorite).
• Water is present in the crystal structure. Where does this water originate?

Amphibole Ratio of Si:O

• Ratio of Si:O in amphibole mineral structure is 1:2.75.
• 50% of tetrahedra: One Si atom to 2 unshared and 2 shared O atoms.
• 50% of tetrahedra: One Si atom to 1 unshared and 3 shared O atoms.

Mica Structure (Muscovite and Biotite)

• Muscovite and biotite micas are sheet silicates with one cleavage plane at 180°.
• Cleavage plane forms based on the relative bond strength between constituent atoms within the crystal lattice.
• Biotite contains Fe and Mg, giving it a dark appearance.
• Both micas have dissolved water present.
• Muscovite: $K<em>2Al</em>4(Si<em>6Al</em>2O<em>{20})(OH,F)</em>2$
• Biotite: $K<em>2(Mg,Fe)</em>6Si<em>3O</em>{10}(OH)_2$

Sheet Silicate Ratio of Si:O

• Ratio of Si:O in the sheet silicate mineral structure is 1:2.5.
• One Si atom to 1 unshared and 3 shared O atoms.
• Formulas:
  • Muscovite: $K<em>2Al</em>4(Si<em>6Al</em>2O<em>{20})(OH,F)</em>2$
  • Biotite: $K<em>2(Mg,Fe)</em>6Si<em>3O</em>{10}(OH)_2$

Framework Silicates (Quartz)

• 3-dimensional framework silicates share electrons between four of the oxygen atoms in each tetrahedron.
• Covalent bonds exist between all tetrahedral oxygen atoms.
• Quartz has no cleavage planes and will break by conchoidal fracture.
• A common mineral in granite or rhyolite.

Quartz Ratio of Si:O

• Ratio of Si:O in the quartz 3-dimensional framework silicate mineral structure is 1:2.
• One Si atom to 0 unshared and 4 shared O atoms.
• Chemical formula: $SiO_2$

Feldspar Structure (Potassium and Plagioclase)

• Feldspar structure is similar to quartz except that an aluminum (Al+3) exchanges with a silicon atom (Si+4) within the tetrahedron.
• Requires an additional electron to satisfy the extra valency.
• Bond can be between calcium (Ca+2), sodium (Na+1), and potassium (K+1), depending on the magma composition and crystallization temperature.
  • Potassium Feldspar: KAlSi3O8
  • Plagioclase Feldspar: (Ca,Na)AlSi3O8
• Where does the cleavage plane develop in this mineral group?

Non-Silicate Minerals

• There will be two questions on the first exam related to the non-silicate minerals discussed in the following Power Point slides.

Hematite (Oxide Mineral Group)

• $Fe<em>2O</em>3$: Hematite is a mineral in the oxide mineral group and a by-product of chemical weathering processes.
• An important source of the world’s iron ore used to manufacture steel.
• Iron ore is a major component in steel production.

Calcite (Carbonate Mineral Group)

• $CaCO_3$: Calcite is the main mineral composing the sedimentary rock limestone and the metamorphosed rock marble.
• An important component (cement) of concrete.

Galena (Sulfide Mineral Group)

• PbS: Galena is an important mineral in the sulfide group.
• An important source of the world’s refined lead.
• 80% of refined lead is used in the production of batteries.

Gypsum (Sulfate Mineral Group)

• (CaSO4 – 2H2O): Gypsum is an important mineral in the sulfate mineral group.
• Forms as an evaporite deposit in mineral-rich marine or lacustrine (lake) environments.
• Used primarily for drywall and plaster building materials.

Halides

• Halite (NaCl, common salt) forms an important mineral group which sustains our lives.
• Often forms as an evaporite deposit in desert lakes (playas) or enclosed shallow seas.

Native Elements

• Only a few minerals occur as pure elements in the earth’s crust.
• Include gold (Au), silver (Ag), copper (Cu), platinum (Pt), diamond (C), graphite (C), and sulfur (S).