Wk8: Minerals

Definition of a mineral 

A naturally occurring crystalline solid with definite (but variable) chemical composition and ordered atomic structure; inorganic and formed by biotic/abiotic reactions.

State of minerals 

Minerals are always solid.

Natural vs synthetic minerals 

Minerals must be naturally occurring; synthetic versions are not minerals.

Number of known minerals (2025) 

6,145 valid minerals approved by the International Mineralogical Association.

Most abundant minerals in continental crust 

Feldspars (~51%), quartz (~12%), pyroxenes (~11%), micas/clays (~5%).

Why silicate minerals dominate Earth’s surface 

Crust is rich in oxygen and silicon (O = 46.6%, Si = 27.7%), forming abundant silicate structures.

What determines mineral composition 

Elemental abundance in crust: O, Si, Al, Fe, Ca, Na, K, Mg.

Silicate structural types 

Isolated (olivine), single chain (pyroxene), double chain (amphibole), sheet (mica/clay), framework (quartz/feldspar).

Geochemical reactions and Earth system 

Reactions alter minerals, influence weathering, climate, carbon cycle, and planetary boundaries.

Ocean acidification reaction sequence 

CO₂ dissolves → forms carbonic acid → dissociates to HCO₃⁻ + H⁺ → increases acidity.

Ocean pH change since 1850 

Drop from 8.2 to 8.1 = 26% increase in acidity; fastest rate in 300 million years.

Impacts of ocean acidification 

Shell dissolution, coral decline, disrupted carbon cycle, food‑web impacts.

Pyrite oxidation (acid sulfate soils) 

FeS₂ + O₂ + H₂O → sulfuric acid + iron hydroxide; lowers pH to 2–3.

Where pyrite forms 

Anoxic coastal sediments, black shales, hydrothermal ore deposits.

How pyrite becomes exposed 

Drought, drainage, excavation, mining, road/rail cutting.

Environmental impacts of pyrite oxidation 

Acid mine drainage, heavy metal release, long‑term soil/estuary acidification.

Definition of weathering 

In‑situ breakdown of rocks into residues or altered products via mechanical, chemical, or biological processes.

Mechanical weathering examples 

Abrasion, thermal expansion, pressure release, frost/salt wedging, hydration.

Chemical weathering examples 

Dissolution, oxidation/reduction, hydration/dehydration, leaching, acid reactions.

Mineral stability (slowest → fastest weathering)

Hematite → gibbsite → quartz → clays → muscovite → K‑feldspar → biotite → plagioclase → amphibole → pyroxene → olivine → calcite → halite.