Mineral and Rock Notes for Exam Preparation
Mineral and Rock Fundamentals
Geologic Definition of a Mineral
Naturally occurring: Must be found in nature, not man-made.
Generally inorganic: Typically does not contain carbon-hydrogen bonds; however, some minerals of biological origin are considered inorganic.
Solid substance: Must maintain a definite shape and volume.
Orderly crystalline structure: Atoms are arranged in a specific, repeating three-dimensional pattern.
Definite chemical composition (that allows for some variation): Has a specific chemical formula, though some minor substitutions of similar elements may occur.
Definition of a Rock
A solid mass composed of minerals or mineral-like matter that occurs naturally as part of our planet.
Minerals within a rock are joined in such a way that their individual properties are typically retained, forming an aggregate.
Most rocks are aggregates of various minerals.
Some rocks can consist of only one mineral, while others are made up of non-minerals (e.g., volcanic glass).
Atoms: Building Blocks of Minerals
Atomic Number: The number of protons in the nucleus of an atom.
It determines an atom's chemical nature (i.e., its identity as an element).
Electrons fly around the nucleus in orbits or shells.
An electron has approximately 1/2000 the mass of a proton.
Element: A group of the same kind of atoms.
Approximately 90 natural elements exist, with several others synthesized in laboratories.
Elements are organized in the periodic table, where those with similar properties align.
Most elements combine with others to form chemical compounds.
How Do Minerals Form?
1. Precipitation of Mineral Matter
Ions dissolved in an aqueous solution reach saturation and begin to form crystalline solids.
A drop in temperature or loss of water through evaporation brings the solution closer to saturation.
Once saturation is reached, ions start to bond, creating crystalline solids.
Example: Evaporite deposits, such as salts, form from the evaporation of water bodies.
Minerals can also precipitate from slowly moving groundwater, filling fractures and voids.
Example: Geodes, which are hollow, spherical rocks lined with inward-growing crystals.
2. Crystallization of Molten Rock
This process is analogous to water freezing.
When magma (molten rock) is hot, atoms are mobile and move freely.
As magma cools, atoms slow down and begin to chemically combine.
This generates a mosaic of intergrown crystals.
3. Deposition as a Result of Biological Processes
Marine organisms extract calcium or silica from seawater.
They then secrete external skeletons or shells composed of either calcium carbonate (CaCO3) or silica (SiO2).
Examples:
Corals and mollusks primarily use calcium (Ca).
Diatoms and radiolarians primarily use silicon (Si).
Mineral Structures and Compositions
All mineral samples are crystals or crystalline solids.
A crystal or crystalline solid is any natural solid with orderly, repeating internal structures.
Mineral Structures: The specific atomic arrangement that forms the basic building blocks of a mineral crystal, referred to as unit cells.
Key aspects of mineral structures and compositions:
Orderly Packing: Atoms and ions are packed in an orderly fashion through various bonds (ionic, covalent, metallic).
3D Packing: Atoms pack in three dimensions to minimize voids; the shape and symmetry of this packing determine the overall crystal shape.
Polymorphs: Materials with the same chemical composition but different internal structures (different types of packing) are called polymorphs.
Example: Polymorphs of Carbon
Diamond: Carbon atoms are covalently bonded into a compact, three-dimensional framework, which explains its extreme hardness.
Graphite: Carbon atoms are bonded into sheets that are joined together by very weak electrical forces, making it soft and flaky.
Mineral Groups
Nearly 4000 minerals have been named.
Rock-forming minerals:
Only a few dozen make up this category.
These are common minerals that constitute most of the rocks in Earth's crust.
They are primarily composed of the eight most abundant elements in the continental crust (which collectively account for more than 98\% by weight).
Economic minerals:
Less abundant than rock-forming minerals.
Minerals used extensively in the manufacture of products (e.g., metals, building materials).
These two groups are not always mutually exclusive; for instance, Calcite (CaCO_3) is both a rock-forming mineral and an economic mineral.
The major elements in Earth’s crust (outermost layer) make up 98.5\% of its weight, including Oxygen at 46.6\% and Silicon at 27.7\% of the total.
Silicate vs. Nonsilicate Minerals
Silicate Minerals
Most common type of minerals (over 800 known silicates).
Account for more than 90\% of Earth's crust.
Silicon (Si) and oxygen (O) are the basic building blocks.
Silicate Structures:
All silicates contain oxygen and silicon, the two most abundant elements in Earth's crust.
The fundamental building block is the silicon–oxygen tetrahedron, which consists of four oxygen ions surrounding a much smaller silicon ion (SiO_4^{4-}).
Single tetrahedra link together to form various complex structures (e.g., single chains, double chains, sheets, three-dimensional networks).
Most silicates form from molten rock cooling and crystallizing.
Examples of Silicate Minerals (See Page 15 for detailed table):
Light Silicate Minerals (e.g., Quartz (SiO2), Muscovite (KAl2(AlSi3O{10})(OH)2), Potassium Feldspar (KAlSi3O8), Plagioclase Feldspars (Ca,NaAlSi3O_8))
Common characteristics: generally lighter in color and lower specific gravity.
Feldspars (Potassium and Plagioclase) account for about 51\% of the crust.
Quartz makes up about 12\% of the crust.
Dark Silicate Minerals (e.g., Olivine ((Mg,Fe)2SiO4), Pyroxenes ((Mg,Fe)SiO3), Amphiboles (Ca2(Fe,Mg)5Si8O{22}(OH)2), Biotite (K(Mg,Fe)3AlSi3O{10}(OH)2))
Common characteristics: generally darker in color and higher specific gravity due to high iron and magnesium content.
Olivine (single tetrahedra) has no cleavage.
Pyroxene group (single chains) has two planes of cleavage at right angles.
Amphibole group (double chains) has two planes of cleavage at 60^\circ and 120^\circ angles.
Micas (sheets) possess one plane of cleavage.
Nonsilicate Minerals
Not as common as silicates but are economically very important.
Make up approximately 8\% of Earth's crust.
Divided into groups based on the common negatively charged ion or complex ion they share.
Common Nonsilicate Mineral Groups (See Page 17 for detailed table):
Carbonates (CO_3^{2-}):
Calcite (CaCO_3): Used in Portland cement, lime.
Dolomite (CaMg(CO3)2): Used in Portland cement, lime.
Halides (Cl^- , F^- , Br^-):
Halite (NaCl): Common salt.
Fluorite (CaF_2): Used in steelmaking.
Sylvite (KCl): Used as fertilizer.
Oxides (O^{2-}):
Hematite (Fe2O3): Ore of iron, pigment.
Magnetite (Fe3O4): Ore of iron.
Corundum (Al2O3): Gemstone, abrasive.
Ice (H_2O): Solid form of water.
Sulfides (S^{2-}):
Galena (PbS): Ore of lead.
Sphalerite (ZnS): Ore of zinc.
Pyrite (FeS_2): Sulfuric acid production.
Chalcopyrite (CuFeS_2): Ore of copper.
Cinnabar (HgS): Ore of mercury.
Sulfates (SO_4^{2-}):
Gypsum (CaSO4 ullet 2H2O): Plaster.
Anhydrite (CaSO_4): Plaster.
Barite (BaSO_4): Drilling mud.
Native elements (single elements):
Gold (Au): Trade, jewelry.
Copper (Cu): Electrical conductor.
Diamond (C): Gemstone, abrasive.
Graphite (C): Pencil lead.
Sulfur (S): Sulfadrugs, chemicals.
Silver (Ag): Jewelry, photography.
Physical Properties for Mineral Identification
The definite crystalline structure and chemical composition of minerals give them unique physical and chemical properties.
These properties are primary diagnostic tools, determinable by observation or simple tests on hand samples.
Key Physical Properties:
Crystal form: The external expression of a mineral's orderly internal arrangement of atoms.
Luster: The appearance of a mineral in reflected light (e.g., metallic, glassy, pearly, dull, earthy).
Color: Often the most conspicuous property, but can be highly variable due to impurities, making it less reliable for some minerals (e.g., quartz).
Streak: The color of a mineral's powder when rubbed across a streak plate; more consistent than mineral color.
Hardness: A mineral's resistance to scratching or abrasion; typically measured on Mohs scale of hardness.
Cleavage/Fracture: How a mineral breaks.
Cleavage: The tendency of a mineral to break along planes of weak bonding, producing smooth, flat surfaces.
Fracture: Irregular breakage when there are no planes of weak bonding.
Taste: Dissolvable minerals may have a distinct taste (e.g., Halite is salty).
Smell: Some minerals have a characteristic odor (e.g., some sulfides when scratched).
Feel: The tactile sensation (e.g., Talc feels soapy, Graphite feels greasy).
Magnetism: Whether a mineral is attracted to a magnet (e.g., Magnetite).
Double Refraction: The ability of a mineral to split a single light ray into two, producing a double image (e.g., Calcite).
Reaction to hydrochloric acid: Some minerals (e.g., carbonates like Calcite) effervesce (fizz) when dilute hydrochloric acid is applied.
Specific gravity: The ratio of a mineral's weight to the weight of an equal volume of water; a measure of density.