Mineral properties Geology 101, Fall 2010

The properties of minerals are determined by their chemical composition

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

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