Geology 4 (Minerals)

The instructor introduces the mineral lecture, stating it will cover fundamental concepts like mineral properties and chemical composition. He advises students to review previous lectures on atomic structure and bonding before watching this lecture.

The instructor shows mineral samples like labradorite, elbaite, gold, and natrolite, remarking on their beauty. He explains that properties like luster, cleavage, hardness, streak, and specific gravity help identify and diagnose minerals.

Although minerals exhibit diverse colors, the instructor cautions that color is an unreliable property for mineral ID because the same mineral can display different colors based on impurities. He gives the example of quartz, which comes in many colored varieties.

The instructor discusses using hardness and cleavage planes to distinguish minerals. He introduces Mohs scale rating mineral hardness and explains cleavage planes in minerals like calcite, halite, and fluorite.

Crushing a mineral into powder for streak testing or X-ray diffraction reveals its true color and composition. The instructor shows how hematite and galena have different streak colors from their whole mineral form.

Beyond basic properties, some minerals display special properties like magnetism, hydrochloric acid reactivity, elasticity, and water content. The instructor notes these can aid identification.

The instructor shows the elemental composition of Earth, noting it is mostly oxygen, silicon, and aluminum. This limits the minerals that can form to those involving these elements.

Silicates like quartz comprise most minerals due to the abundance of silicon and oxygen. Their structure involves tetrahedrons of a central silicon ion surrounded by 4 oxygens. These link together to form crystal structures.

Silicate tetrahedrons have a negative charge. Positively charged ions like potassium balance these charges, allowing tetrahedrons to link into sheets and 3D networks forming larger crystal lattices.

Quartz's 3D silicate framework makes it hard with no cleavage. In contrast, talc's sheet structure causes softness and cleavage. This illustrates how structure determines mineral properties.

Minerals are divided into mafic (dark colored, iron/magnesium-rich) and felsic (light colored, iron/magnesium poor). Mafic minerals like olivine and augite are denser than felsic ones like quartz and feldspar.

Quartz and feldspars dominate Earth's continental crust. Quartz is purely silicon-oxygen. Feldspars contain potassium, sodium, or calcium in addition to silicon-oxygen.

Ions of similar size and charge can substitute into mineral crystal lattices, creating solid solutions like the feldspar sanadine. In metals, solid solutions are called alloys like steel.

As the most abundant mineral, quartz is hard, fractures conchoidally, lacks cleavage, and has a vitreous luster. Its crystal shape is hexagonal. Varieties like amethyst are gemstones.

Feldspars like orthoclase and plagioclase show two perfect 90 degree cleavage directions. Their compositions readily substitute ions, creating abundant solid solutions.

Muscovite micas have perfect one-directional cleavage forming paper-like sheets that were used as glass in medieval Moscow, leading to its name.

Clays like kaolinite appear amorphous but electron microscopy reveals they are composed of microscopic silicate sheets and crystals.

Olivine and pyroxenes like enstatite and augite are common mafic silicates, often occurring as solid solutions between magnesium and iron end-members.

The minerals hornblende (amphibole) and biotite (mica) contain water, which influences their melting behavior in volcanoes.

Non-silicates like carbonates (calcite, dolomite) and oxides (magnetite) comprise a smaller fraction of the crust but are locally abundant and significant.

Polymorphs like graphite/diamond and calcite/aragonite have the same chemistry but different crystal structures due to varied temperature/pressure conditions.