EARTHSCI LESSON 4

Main Idea

The diversity of minerals on Earth are based on the similarities and differences of their chemistry.

Geosphere

One of the four terrestrial planets; refers to solid Earth.

Geosphere Composition

Composed of rock and regolith, aggregates of minerals.

Mineral

A naturally-occurring, inorganic solid with a definite chemical composition and an ordered internal structure.

Mineral Characteristic: Naturally-occurring

Minerals form naturally.

Mineral Characteristic: Inorganic

Minerals formed through inorganic processes; exclude those derived from living organisms.

Mineral Characteristic: Solid

Liquids and gases—even natural ones like petroleum—are not minerals.

Mineral Characteristic: Definite Chemical Composition

Minerals have an exact formula with elements and compounds in specific ratios.

Mineral Characteristic: Ordered Internal Structure

Atoms are arranged in a regular, repetitive crystal structure.

Volcanic Glass

Not a mineral because it is amorphous and has no ordered structure.

Mineraloids

Mineral-like substances that do not meet all criteria of a mineral. Examples: amber, obsidian, opal, pearl.

Silicates

Minerals containing silicon-oxygen tetrahedrons (SiO₄²⁻). Major rock-forming group. Examples: olivine, quartz.

Carbonates

Minerals containing CO₃²⁻ combined with elements like calcium or magnesium. Examples: calcite, dolomite.

Sulfates

Minerals containing SO₄²⁻. Examples: gypsum, barite.

Halides

Compounds with halogen ions (Cl, F, I, Br). Examples: halite, fluorite.

Oxides

Compounds of metallic ions and oxygen. Examples: magnetite, hematite.

Sulfides

Compounds of metallic ions and sulfur. Example: galena.

Native Elements

Elements found uncombined with others. Examples: gold, silver, diamond.

Crystal Structure

Dependent on chemical composition. Minerals with similar composition often share structure.

Six Crystal Systems

Triclinic, Monoclinic, Orthorhombic, Tetragonal, Hexagonal, Isometric.

Physical Properties of Minerals

Characteristics used to identify minerals: crystal form, habit, cleavage, fracture, luster, color, streak, hardness, density, magnetism, taste, feel, reaction to acid.

James Dwight Dana

American geologist who created Dana’s System of Mineralogy, still in use today.

Crystal Form

Definite structure crystallized from composition.

Crystal Habit

Outward appearance of mineral’s crystal form. Types: granular, tabular, dendritic, acicular, reniform, drusy, encrusting.

Cleavage

Tendency to break along planes of weakness. Described by number, direction, quality (excellent, good, poor, absent).

Fracture

Break without cleavage. Can be irregular or conchoidal (smooth, curved surfaces).

Luster

Appearance of light reflected from surface. Metallic or nonmetallic. Nonmetallic types: vitreous, resinous, pearly, silky, greasy, earthy, dull.

Color

Most obvious property but unreliable; affected by impurities. Types: idiochromatic (inherent color), allochromatic (due to impurities), pseudochromatic (optical effects).

Streak

Color of a mineral in powdered form; tested on a streak plate.

Hardness

Strength of chemical bonds; tested by scratching.

Mohs’ Scale of Hardness

Relative scale created by Friedrich Mohs (1812). Talc = 1, Gypsum = 2, Calcite = 3, Fluorite = 4, Apatite = 5, Orthoclase = 6, Quartz = 7, Topaz = 8, Corundum = 9, Diamond = 10.

Mohs’ Mnemonic

“The Geologist Can Find An Ordinary Quartz Tourists Called Diamonds.”

Density / Specific Gravity

Weight of a mineral relative to water. Most minerals ~2.7; gold = 19.

Other Mineral Properties

Magnetism (magnetite), Taste (halite = salty), Effervescence (calcite reacts with acid), Feel (talc = greasy), Solubility, Melting Point.