- Homogeneous: compositionally uniform. Thus a coal ball with fossils (right) is not a mineral.
But the quartz in an indivdiual sand grain is.
- Naturally occurring: A silicon wafer is not a mineral
But a quartz crystal is.
- Solid: Tar, however natural, is not a mineral
- Inorganic: lacking complex compounds of carbon and oxygen. Peat is not a mineral.
But graphite is.
- Crystalline: Atoms having a regular microscopic arrangement. Obsidian, a volcanic glass, is not a mineral.
But pyroxene is.
- With a specific chemical composition: Possible to describe composition using a chemical formula. Halite (table salt) is a mineral with the formula NaCl.
The geometric regularity with which anions and cations arrange themselves in a lattice is reflected on a macroscopic scale in the geometry of the crystal faces of the growing crystal grain. The ions' packing arrangement determines the crystal's macroscopic cubical shape.
When do crystals form?
- Solidification from melt (freezing): When a liquid is cooled below its melting point.
- Precipitation from a solution: When more of a substance is present in a solution than the solution can hold. (A natural example)
- Solid-state diffusion: (pressure-cooking) can cause ions to reorganize themselves into new minerals without melting having occurred when a solid is heated or compressed.
Cation substitution: Remember:
- Anions of a given atomic mass (like chloride) gain electrons to fill their outer shell.
- Cations of similar mass (like sodium) get rid of their outer shell.
EG: the mineral olivine may have iron (Fe2SiO4) or magnesium (Mg2SiO4), or a combination ((Fe2Mg2)SiO4). Another common cation substitutions is aluminum (Al3+) and silicon (Si4+). More
Polymorphs: Depending on the circumstances, the same ions might form crystals with different geometries. We have already seen the common example of diamond and graphite. Such groups are termed polymorphs of carbon. The former forms a three-dimensional lattice, the latter a series of two dimensional sheets. Calcite and aragonite are polymorphs of calcium carbonate (CaCO3).
Categories of rock-forming minerals:
- Native Elements: Minerals consisting of a single element E.g. diamond, gold, sulfur.
- Silicates: Minerals built around tetrahedron-shaped silicate ion (SiO44-) (Marshak calls this the silicon-oxygen tetrahedron).
Silicon and carbon each have four electrons in their outer orbital. Thus, you might expect silicon to behave chemically similarly to carbon. Because the silicon atom is bigger, however, it tends to to bond covalently to four oxygen atoms. Recall that oxygen needs two electrons to fill its outer orbital. After bonding with the silicon, each oxygen still lacks one electron. The result is the complex silica ion, SiO44-. Physically, this ion is a tetrahedron with Si at the center and four O in each corner.
The silicate tetrahedron acts sort of like carbon in organic molecules, coming together to form complex chains and structures. Because of its geometry and high capacity for electron sharing, the silica ion can participate in a variety of complex minerals. Arrangements of the silica ion include:
- Single tetrahedra: E.g. olivine.
- Single chains of tetrahedra: E.g. Pyroxenes including spodumene. Here, the tetrahedra each share two corners with their neighbors.
- Double chains of tetrahedra: E.g. Amphiboles including tremolite.
- Sheets of tetrahedra: E.g. Micas including muscovite.
- Hexagonal rings of tetrahedra: E.g. beryl.
- Complex three dimensional arrays of tetrahedra: E.g. quartz.
- Single tetrahedra: E.g. olivine.
- Carbonates: Minerals built around carbonate ion (CO32-). Most commonly as calcite CaCO3, its polymorph aragonite, or dolomite MgCa(CO3)2.
- Phosphates: Minerals built around the phosphate ion (PO33-). Including apatite Ca5(PO4)3
- Oxides: Minerals built around oxygen ion (O2-). Cations are generally metallic. A common example is hematite (Fe2O3).
- Sulfides: Minerals built around sulfur ion (S2-). Cations are generally metallic. A common example is pyrite (FeS2).
- Sulfates: Minerals built around the sulfate ion (SO42-). An abundant example is gypsum (CaSO4 . 2H2O).
- Halides: Minerals built around the chloride (Cl-) and fluoride (F-) anions halite (table salt) (NaCl).
- Crystal form: The regular macroscopic geometry displayed by the growth of an unconfined crystal. E.G. Calcite crystal faces occur at characteristic angles.
- Cleavage: A crystal lattice may incorporate regular planes of weakness that don't coincide with it's crystal growth form. Find these by breaking the crystal. E.G. Calcite cleaves to form rhombohedrons. Note, the angles formed by the cleavage planes are different from those between the crystal growth faces.
- Luster: How light interacts with the mineral to produce a characteristic visual texture. Examples include:
- Color: The color of the mineral, itself.
- Streak: The color of a streak that the mineral leaves across a ceramic streak plate.
- Density: The mass of the mineral per unit volume expressed in comparison to water.
- Effervescence: A good way to spot carbonate minerals (calcite, aragonite, dolomite) is to drop dilute HCl and watch for fizzing.
- Fluorescence: a mineral's tendency to give off visible light when illuminated with ultraviolet light.
- Refraction: of light passing through. E.G. double refraction of calcite.
- Radioactivity: sensitivity of a geiger counter to the mineral.
- Touch: Some minerals display different textures. For example talc (soapstone) is slippery.
- Flavor: E.G. halite (rock salt)
Key concepts and vocabulary:
- Mineral definition and characteristics:
- Naturally occurring
- With a specific chemical composition
- Crystal faces
- Ways crystals form:
- Solidification from melt
- Precipitation from a solution
- Solid-state diffusion
- Cation substitution
- Categories of rock-forming minerals:
- Native Elements
- Major physical properties of minerals:
- Crystal form
- Mohs hardness scale