Summary
Highlights
This module introduces minerals as the building blocks of rocks, explaining that the Earth's crust is composed of various rocks, each made of one or more minerals. It also defines mineralogy as the study of minerals and mineralogist as the person who studies them.
To be considered a mineral, a substance must be naturally occurring (not man-made or manufactured), inorganic (without carbon, never alive), a homogeneous solid, have a definite chemical composition, and possess a crystalline structure.
Mineraloids are naturally occurring, mineral-like substances that lack crystallinity. They possess a chemical composition beyond the generally accepted ranges for specific minerals. Opal is given as a prime example of a mineraloid due to its non-crystalline structure.
Minerals are crucial for everyday life. Examples include quartz for glass, graphite for pencils, fluoride for toothpaste, chalcopyrite for coins, rutile and ilmenite for white paint, talc and muscovite for makeup, and silver and platinum for jewelry. Humans need 16 essential minerals for survival, such as sodium, potassium, calcium, magnesium, copper, and phosphorus. There are over 4,200 known minerals globally, with gold and silver being rare examples.
Mineralogists identify minerals based on their chemical and physical properties. Luster refers to the brightness and manner in which a mineral reflects light. Main types include metallic and non-metallic (glassy). Non-metallic lusters can be adamantine (brilliant like diamond), earthy (dull), silky, greasy (oily), resinous (waxy), vitreous (like broken glass), and pearly.
Cleavage is the ability of a mineral to break along flat planes due to its internal structure. Examples include muscovite (one direction), feldspar (two directions), and halite/calcite (three directions). Minerals with no cleavage, like quartz, break with an irregular fracture, specifically conchoidal fracture, similar to glass.
Specific gravity is a mineral's density, calculated as the ratio of an object's mass to the mass of an equal volume of water. Objects lighter than water float, while those heavier sink. Crystal form (or habit) describes the characteristic external shape of an individual crystal or crystal group, bounded by flat faces related by symmetry.
Tenacity measures a mineral's cohesiveness. Terms include brittleness (breaks or powders easily, e.g., pyrite), malleability (can be hammered into thin sheets, e.g., gold), ductility (can be stretched into wire, e.g., copper), flexibility and elasticity (bends and returns to original shape, e.g., biotite), and sectility (can be sliced with a knife, e.g., gypsum).
Additional identification tests include taste (e.g., halite tastes salty), odor (e.g., clay minerals have an earthy smell), striations (parallel lines on cleavage surfaces), magnetism (attraction to magnets for iron oxide-rich minerals), double refraction (images appear double when viewed through clear minerals like calcite), X-ray fingerprints (unique deflection patterns of X-rays), and chemical tests (e.g., carbonates react with hydrochloric acid to produce CO2 gas).
Mineralogists typically follow a sequence for identifying minerals: luster, hardness (using Mohs scale), color, streak, crystal form or habit, cleavage, specific gravity, and other properties.
Hardness refers to a mineral's scratchability and durability, measured using the Mohs hardness scale (1-10). Talc is the softest (1), and diamond is the hardest (10). Common items like fingernails (2.5), a steel knife (5.5), and a streak plate (6.5-7) are used as references.
The color of a mineral can be variable and may change depending on the surface, making it less reliable for identification. Streak is the color of the mineral's pulverized powder, which is generally constant and distinctive, even if different from the crystal's color.