GEOL 333 #5 - Chemical Principles in Minerals II - Bonding, Solid Solution

I) Atomic bonding in minerals - To form chemical compounds, atoms combine (bond) by losing, gaining or sharing electrons (e-'s). Transfer or sharing of e-'s between atoms changes e- configuration of each atom. Two factors control whether atoms will bond: (a) each atom must be chemically stable and (b) resulting compound must be electrically neutral.

Types of bonds include:

Ionic bond - involves transfer of e-'s creating ions of and and - charge (cations and anions), which creates complete outer shells of e-'s; bond is due to attraction of opposite charges, e.g., NaCl (halite) = Na+ and Cl- ions, Na loses one e- to form Na+ and Cl gains that e- to form Cl-; minerals with ionic bonds commonly are very soluble (dissolve abundantly in water)

Covalent bond - involves pairs of atoms that share e-'s to complete outer shell, outer shells of e-'s from different atoms overlap; these bonds are strongest (making minerals with these bonds very hard, e.g., diamond, pure C with 4 valence e-'s. C atoms in diamond share 1 e- with 4 neighbors and form very strong covalent bonds; minerals with covalent bonds commonly are insoluble (dissolve only little in water)

Metallic bond - in pure metals (e.g., Cu, Au, and Ag). Many atoms share e-'s but e-'s are "loosely held", i.e., free to move around, results in excellent electrical conductivity. Pure metals = malleable and relatively soft due to weak bonds (+ same size for all atoms). Alloys (steel or bronze) are mixtures of metals; much stronger because atoms of different size < planes of weakness.

Van der Waals bond (VDW) - very weak bonds due to residual electrical fields (alternating and and - charges) in neutral atoms. Graphite has sheets of carbon; covalent (strong) bonds within sheet and weak VDW bonds between sheets. Graphite very soft, diamond = hardest, due to bonding differences. Talc = VDW (softest).

Hydrogen bond - attraction of H side of water molecule (or OH molecule) to anion; very weak bonds, e.g., clay minerals such as kaolinite (very soft).

Bonding in Minerals - Most minerals have mixed bonds, especially silicates which are ~50% ionic and 50% covalent.

II) Ionic (atomic) Substitution - Minerals have definable chemical composition. Some minerals have fixed chemical composition, e.g., quartz (SiO2) and halite (NaCl), but others have large chemical variation within fixed limits (e.g., olivine, plagioclase feldspar, and calcite). Large chemical variations occur if substituting ions have similar size and similar ionic charge (usually does not differ by more than 1). Minerals with large chemical variation due to ionic substitutions have large solid solution.

Examples - Minerals with Extensive Solid Solution:

Complete solid solution = olivine (Mg, Fe)2SiO4, ranges from Mg-rich end member (forsterite - Fo - Mg2SiO4) to Fe-rich end member (fayalite - Fa - Fe2SiO4). Most common olivine mineral in nature = (Mg1.6Fe0.4)SiO4. Complete solid solution occurs because Mg and Fe have same charge and similar ionic size.

 

 

 

 

Plagioclase feldspar (Na, Ca)(Al, Si)4O8 has complete solid solution from Na-end member (Albite - Ab - NaAlSi3O8) to Ca-end member (Anorthite - An - CaAl2Si2O8). Na has 1+ charge and Ca has 2+, so coupled substitution must occur, involves Si4+ and Al3+. Why?

For every substitution of Ca2+ for Na+, equal amount of Al3+ substitutes for Si4+.

 

 

 

 

Alkali (K/Na) feldspar has only limited solid solution because K+ (1.33 Å) is much larger than Na+ (0.97 Å).

Ca/Mg-carbonates show limited solid solution due to large differences in size of Ca2+ (1.08 Å) vs. Mg2+ (0.66 Å). Calcite (CaCO3) varies up to 10 - 15 mole % substitution of Mg, dolomite (Ca0.5Mg0.5CO3) ranges from ~45 - 55 mole % Ca, and magnesite (MgCO3) does not show solid solution.

For most solid solution series, difficult to determine exact composition from hand sample. Optical microscope can help, but chemical analysis is often necessary.

Why is mineral solid solution important?