Minerals Geology 115

Mineral Definition: “A naturally-occurring homogenous inorganic solid substance with a definite chemical composition and a characteristic crystalline structure.” What about ice (frozen H2O)?

Mineral Definition: “A naturally-occurring homogenous inorganic solid substance with a definite chemical composition and a characteristic crystalline structure.” Ice does not have a “characteristic crystalline structure” -- thus it is a mineraloid.

Atomic theory John Dalton (1800): “Matter is made out of atoms -- the smallest units with distinguishable chemical properties.”

Molecules Atoms form associations called molecules; atoms in molecules are held together by chemical bonds

Crystals Molecules can attract each other due to intermolecular forces -- much weaker than chemical bonds If the molecules are held together in a regularly spaced lattice, a crystal is formed

Ice’s structure Note large-scale irregularities in the symmetry of the lattice -- ice is a mineraloid

Mineral intermolecular force Molecules in minerals are held together by ionic bonds -- the atoms lose or gain electrons and are thus called ions Positively charged ions are cations; negatively charged ions are anions

Geologists and minerals Minerals are the components of rocks Unlike minerals, rocks do not have a single definite chemical composition, and minerals retain their integrity when incorporated in a rock

Granite = rock

Mineral classification Basic classification is by chemical composition, typically by the anion it contains: Carbonates (CO32–) Halides (e.g., Cl–) Phosphates (PO43–) Sulfates (SO42–) Oxides (O2–) Sulfides (S2–)

Silicates But no mineral class is as prevalent as those made with the silicate anion (SiO44–) -- 95% by volume of the crust Tetrahedral shape

Silicate structure Since silicates are all the same shape, it is the arrangement of the silicates, and the cations that electrically balance the silicates that determine the structure of the mineral Olivine and quartz are examples; in fact, quartz is made only of silicon and oxygen, so is the most common mineral on Earth

Silicate structure Knowing the way the ions stack can tell you some mineral properties Sheet silicates (phyllosilicates) form thin sheets because there are not many ionic bonds between different levels of molecules Mica minerals, like biotite and muscovite, are examples

Silicate structure Tectosilicates (framework silicates) have a much more complex structure but have regular planes of weaker forces -- leads to characteristic 60°/120° cleavage of these minerals Feldspar (=“field stone”) minerals, like plagioclase and orthoclase, are examples

The properties of minerals are determined by their chemical composition and crystal habit; there are over 7000 unique minerals

Crystal habit Crystal shapes are determined by local (nearest neighbor) intermolecular attractions

Cleavage Cleavage, on the other hand, relies on the alignment of weak areas held together by only intermolecular forces through the whole crystal

Color (and luster) Mineral color can be due to its intrinsic chemical composition (e.g., copper ores) or the crystal’s refractive properties or “impurities” such as a low concentration of metal ions not in the mineral’s chemical formula Latter reason explains much of quartz’s color variability

Specific gravity SG is the ratio of the mineral’s density to water’s density; it can be thought of as a “unitless” density. SG measures how efficiently packed the atoms are in a crystal, and gives some indication of composition (e.g., the lead in galena).

Hardness Hardness is an indication of molecule alignment in crystals (similar to cleavage) but also measures the strengths of the intermolecular forces

Strength ≠ Hardness Carbon nanowire is one of the strongest (tensile) materials known, but is no harder than graphite

Sadly, minerals aren’t usually large enough to identify So what techniques are available? Optical mineralogy

Optical mineralogy Relies on the behavior of polarized light transmitted through a very thin cross-section of a rock sample

X-ray crystallography Basic premise: shoot X-rays at a crystal, look at the shadow pattern of atoms, determine structure and, eventually, identity