7.1 Mineral Formation and Properties

Minerals are the building blocks of Earth's crust, and they form through several distinct geological processes. Understanding how minerals form and what properties define them gives you a foundation for interpreting Earth's geological history and recognizing why certain minerals show up in certain environments.

Mineral Formation Processes

Crystallization from Magma

When magma cools and solidifies, atoms arrange themselves into ordered crystal structures. The cooling rate and chemical composition of the magma are the two main factors that control what minerals form and how large their crystals grow.

• Slow cooling (deep underground) gives atoms more time to organize, producing larger crystals. Granite is a classic example, with visible crystals of quartz, feldspar, and mica.
• Rapid cooling (at or near the surface) produces tiny crystals or even a glassy texture. Obsidian forms this way, cooling so fast that crystals don't have time to develop at all.
• Not all minerals crystallize at the same temperature. Bowen's reaction series describes the predictable order in which minerals crystallize from cooling magma. High-temperature minerals like olivine crystallize first, while quartz crystallizes last at lower temperatures.

Precipitation from Aqueous Solutions

Minerals can dissolve in water and later precipitate out when conditions change. Shifts in temperature, pressure, or water chemistry trigger this process.

• Surface evaporation: When bodies of water evaporate, dissolved minerals concentrate and precipitate. This is how evaporite deposits like halite (rock salt) and gypsum form, often in arid lake beds or shallow seas.
• Groundwater environments: As mineral-laden groundwater drips through caves, dissolved calcium carbonate precipitates to form structures like stalactites (hanging from the ceiling) and stalagmites (building up from the floor).
• Hydrothermal systems: Hot, mineral-rich fluids circulating through rock fractures cool and deposit minerals in veins. Many economically important deposits of quartz, gold, and silver form this way, often near volcanic activity or hot springs.
• Chemical weathering of surface rocks can also dissolve minerals, which then reprecipitate elsewhere under new conditions.

Metamorphism

Metamorphism transforms existing rocks and minerals through changes in temperature, pressure, and/or chemical environment, all without melting the rock. The minerals recrystallize, grow larger, or change composition while remaining solid.

• Regional metamorphism affects large areas where tectonic forces create intense pressure and heat, such as during mountain building. This produces foliated rocks like schist and gneiss, where minerals align in parallel layers.
• Contact metamorphism happens locally when magma intrudes into surrounding rock. The heat "bakes" the adjacent rock, changing its texture and mineralogy. Hornfels is a common product.
• Hydrothermal metamorphism occurs when hot, chemically active fluids interact with rock, altering mineral compositions through a process called metasomatism. Ions are exchanged between the fluid and the rock, creating new mineral assemblages.

Mineral Physical Properties

Hardness, Cleavage, and Fracture

Hardness measures a mineral's resistance to scratching. The Mohs hardness scale ranks minerals from 1 (talc, very soft) to 10 (diamond, hardest natural substance). A harder mineral will always scratch a softer one, so you can test unknown minerals against known reference points. For quick field tests: a fingernail is about 2.5, a copper penny about 3.5, and a steel nail about 6.5.

Cleavage is the tendency of a mineral to break along smooth, flat surfaces that correspond to planes of weakness in its crystal structure. The number and quality of cleavage planes help identify minerals:

• Mica has perfect cleavage in one direction, so it peels into thin sheets.
• Halite has perfect cleavage in three directions at 90°, breaking into cubes.
• Feldspar has two cleavage planes at nearly 90°.

Fracture describes how a mineral breaks when it doesn't have well-defined cleavage. Quartz, for example, displays conchoidal fracture, producing smooth, curved surfaces like broken glass. Pyrite tends to show uneven, irregular fracture.

Luster, Specific Gravity, and Other Properties

Luster describes how a mineral's surface appears when it reflects light. The first distinction is between metallic (looks like polished metal, as in pyrite) and non-metallic. Non-metallic luster has several subtypes:

• Vitreous (glassy, like quartz)
• Resinous (waxy glow, like sulfur)
• Pearly (like talc)
• Silky (fibrous sheen, like asbestos)
• Dull/earthy (like kaolinite)
• Adamantine (brilliant sparkle, like diamond)

Specific gravity is the ratio of a mineral's density to the density of water. It's especially useful for telling apart minerals that look similar. Gold (specific gravity ~19.3) feels noticeably heavier than pyrite (~5.0), which is one reason pyrite earned the nickname "fool's gold."

Other diagnostic properties worth knowing:

• Color can be helpful but is often unreliable because trace impurities change a mineral's color (quartz comes in many colors).
• Streak (the color of the powdered mineral on an unglazed porcelain plate) is more reliable than color. Hematite, for instance, always leaves a reddish-brown streak regardless of whether the specimen looks silver or black.
• Magnetism: Magnetite is naturally magnetic and will attract iron filings.
• Acid reaction: Calcite fizzes (effervesces) when dilute hydrochloric acid is applied, because the acid reacts with calcium carbonate to release $CO_2$ gas.

Mineral Chemical Properties

Composition and Classification

Minerals are classified into groups based on their dominant chemical anion or anion group. Here are the major categories:

Silicates are by far the most abundant group, making up over 90% of Earth's crust.

A mineral's chemical composition directly shapes its physical properties. Isomorphous substitution occurs when ions of similar size and charge swap places within a crystal structure without changing the overall structure. For example, $Mg^{2+}$ and $Fe^{2+}$ commonly substitute for each other in olivine, which is why olivine's color ranges from green (Mg-rich) to brown (Fe-rich).

Crystal Structure and Its Influence

Crystal structure refers to the ordered, repeating three-dimensional arrangement of atoms within a mineral. There are seven crystal systems that describe the symmetry and geometry of crystal lattices: cubic, tetragonal, hexagonal, trigonal, orthorhombic, monoclinic, and triclinic.

The type of chemical bonding between atoms has a direct effect on physical properties. Diamond, for instance, has carbon atoms linked by strong covalent bonds in all directions, giving it a hardness of 10. Graphite is also pure carbon, but its atoms are bonded in flat sheets with weak forces between them, making it one of the softest minerals (hardness 1-2). This is a perfect example of polymorphism: the same chemical composition producing completely different minerals because of different crystal structures.

Formation conditions control which crystal structure develops. High-pressure metamorphism deep in Earth's mantle can force carbon atoms into diamond's compact structure, while at surface pressures, carbon naturally forms graphite. The same principle applies to other polymorphs, such as the silica minerals quartz (low pressure) and coesite (high pressure).