Wells

The Formation of Minerals

Prof. John J. Renton

Outline Lecture Notes** (JA)

• A Mineral occurs naturally and is solid, inorganic, with a reasonable well-defined chemical composition and crystal structure.
• Shouldn't use a mineral name unless all 5 above conditions apply (eg, not petroleum, synthetic rubies.
• Atomic nucleus contains protons and neutrons. Atomic number = protons (H = 1, C = 6, Fe = 26, U = 92). Atomic mass = protons + neutrons (Al = 27, Pb = 207).
• Isotopes have same atomic number, but different mass, all react chemically in same way. One isotope always dominates (>99%) in nature. Atomic mass is fractional because it takes isotope abundance into account.
• One (-ve) electron for every (+ve) proton - atoms are neutral. Electrons orbit in shells (energy levels) from K to Q, depending on number. Outer layer determines chemical properties.
• Atomic bonds can be ionic or covalent or, less frequently, van der Waals (electrostatic), metallic (outer electrons can move freely -> electrical, thermal conductivity), or hydrogen.
• Octet rule says atoms nonreactive when outer shell is full (normally 8 electrons). Noble (inert) gases - He, Ne, Ar, etc - all have full outer shells and only react when forced to.
• Na has 1 outer (valence) electron, Cl has 7, so both can achieve 8 by donating/accepting an electron, turning them into charged ions which attract and form an ionic bond. Not very strong - easily broken by water -> solution.
• C has 4 valence electrons and can bond with other atoms by sharing electrons to give 8 in total, forming a strong covalent bond (diamond is pure C and hardest known substance, cannot be attacked chemically, only burned).
• Most minerals are mixture of covalent and ionic bonds. Calcite (limestone) is 80% ionic, 20% covalent and water soluble, though less so than NaCl (100% ionic). Over a million years can dissolve -> Mammoth Caves.
• Quartz (sand) is 80% covalent, 20% ionic and only attacked by HF - mot resistant of common minerals.
• Chemical reactivity is function of bonding type - more covalent = resistant, more ionic = less resistant. Chemical weathering is the same. Granite very long-lasting because of covalency within its constituent minerals.
• Chemistry is very, very important [to geology].

Further information

**more detailed lecture notes available to group members on request.