Minerals

• Different or same mineral (Figure: halite and calcite)

• Figure: breaking calcite with a hammer (small rhombs)

• Minerals: naturally-occurring inorganic crystalline solids with specific chemical composition
• What minerals are present on Earth will depend on:
• abundance of elements
• chemistry--valence and size of ion (see later discussion)
• Abundance of Elements (Figure: pie diagrams of whole-Earth and Crust elements)

• Atomic structure
• Fundamental unit of matter - atom
• Atom composed of:
• electron - negative charge of 1; revolves around nucleus
• proton - positive charge of 1
• neutron - no charge
• element - basic component, no further chemical separation possible
• atomic number Z - number of protons

• atomic weight - average weight of an atom of an element (in amu)
• mass number (A) - number of protons & neutrons
• Example:   general: AQ; for carbon-14:  ¹⁴C
• isotope - same # of protons, different # of neutrons
• stable isotopes - ¹²C, ¹³C, ²⁰⁸Pb
• unstable or radioactive isotopes - ¹⁴C, ⁴⁰K, ²³⁸U

• bond - results from electrostatic attraction
• compound  - distinct substance formed by chemical bonding of two or more atoms in definite proportion
• molecule - smallest unit of compound with properties of that compound

• Atomic structure
• electrons travel in orbitals
• orbitals organized into shells
• chemically stable atom - outer shell is filled (p. 47)
• Valence - number of electrons that an element tends to gain or lose
• Periodic Table - elements organized by atomic number and chemical behavior (p. 43)

• ions - charged atoms or molecules
• cations - positive ions (Na⁺ Ca²⁺, Fe³⁺)
• anions - negative ions (Cl^-, Br^-)
• complex ions - polyatomic ions (SO₄^2-, HCO₃^-)

• ionic - electrons not shared
• large difference in ability to attract electrons
• ex: NaCl, CaCl2
• bond strength increases as charge increases
• covalent - electrons shared
• small difference in ability to attract electrons

• Ex:  CH₄, SiO₂, SO₄^2-
• metallic bond - metals; freely mobile electrons
• van der Waals forces - weak bonds
• more than one bond type common - Ex: CaCO₃
• Ca-CO₃ bond ionic
• C-O bonds covalent

• Factors
• Abundance of elements
• Valence
• Size of ion

• Valence
• in part determines which and how many atoms bond (see periodic table)
• strive to achieve electrical neutrality
• Examples
• Na⁺ + Cl^- = NaCl
• Ca²⁺ + 2Cl = CaCl₂
• Si⁴⁺ and 2O^-2 = SiO₂

• Size of ion
• Controls arrangement of atoms
• Measured in terms of ionic radius
• When atoms, ions, or molecules are arranged in a regular, repeating 3-D pattern -> crystal.

• Features of crystals:
• smooth faces when space allows
• same angle between faces
• size depends on time
• Minerals: naturally-occurring inorganic crystalline solids with specific chemical composition
• coordination number - number of anions directly bonded to a cation (or vice-versa)
•  Coordination number vs. ionic radius ratio

• one ion can substitute for another if
• similar in size
• similar in charge
• Ex: Fe²⁺ for Mg²⁺ in Mg₂SiO₄ (type of olivine); sometimes written as (Mg, Fe)₂SiO₄
• Anions can also substitute
• Br^- for Cl^- in NaCl

• substituting ion may make up <1% of composition ? trace element
• coupled substitution - Na⁺, Si⁴⁺ for Ca²⁺, Al³⁺ in anorthite (CaAl₂Si₂O₈)

• because of substitution, composition may for a continuum ? solid solution
• def: single, homogenous crystal phase which may vary in composition without the appearance of an additional phase
• polymorph - same chemical composition, different structure