Chapter 5: How Minerals Form: Processes and Conditions

Minerals do not all form the same way. Even though their structures follow clear rules, their origins can be very different depending on temperature, pressure, chemistry and the geological environment. This chapter explains the four major pathways of mineral formation and how each one contributes to Earth’s mineral diversity.

1. Crystallization from Molten Rock (Igneous Minerals)

When molten rock cools, atoms begin to arrange themselves into repeating patterns. These patterns become crystals.

The type of mineral that forms depends on:

• Temperature
• Rate of cooling
• Chemical composition of the magma

Higher-temperature minerals like olivine (silicate) and pyroxene (silicate) crystallize first. As the melt cools further, minerals like feldspar (aluminosilicate), quartz (silicate) and mica (aluminosilicate) appear. This sequence is known as Bowen’s Reaction Series, a model that explains the order in which common igneous minerals crystallize.

Slow cooling deep underground allows large, well-formed crystals to grow. Quick cooling at the surface results in small or microscopic grains.

2. Crystallization from Water-Rich Fluids (Hydrothermal Minerals)

Hot fluids inside Earth carry dissolved elements. When these fluids enter cracks and cool down, the dissolved material begins to crystallize.

This process forms classic minerals such as:

• Quartz (silicate)
• Tourmaline (borosilicate)
• Topaz (fluoro-silicate)
• Fluorite (halide)
• Beryl (aluminosilicate)
• Calcite (carbonate)
• Ore minerals like galena (lead sulfide), sphalerite (zinc sulfide) and chalcopyrite (copper-iron sulfide)

Key concepts:

Hydrothermal fluids: Hot, mineral-rich water from magma or deep circulation

Veins: Minerals deposited in fractures when fluids cool or react with rocks

Geodes: Hollow cavities where crystals grow inward from the walls

The chemistry of the fluid controls what minerals form.

3. Changes Caused by Heat and Pressure (Metamorphic Minerals)

When existing rocks are buried, compressed or heated (without melting), the minerals inside them become unstable. They reorganize or change into new minerals that are stable under the new conditions. This process is called metamorphism.

Examples of new minerals that develop during metamorphism include:

• Garnet (silicate)
• Kyanite (aluminosilicate)
• Staurolite (silicate)
• Chlorite (silicate)
• Epidote (silicate)
• Serpentine (silicate)
• Talc (silicate)

Two important terms:

Recrystallization: Minerals grow larger or change shape without changing composition

Neocrystallization: Entirely new minerals form from chemical reactions during metamorphism

4. Precipitation at Low Temperatures (Surface or Near-Surface Minerals)

Some minerals form right at Earth’s surface through chemical reactions with water and air.

Types of low-temperature formation:

• Evaporation of water leaves behind salts like halite (halide) or gypsum (sulfate)
• Chemical weathering produces minerals such as clays (aluminosilicates), iron oxides and carbonates
• Biogenic activity creates minerals through shells, corals and microbial processes, including calcite (carbonate), aragonite (carbonate) and some phosphates

These minerals help shape soils, sediments and landscapes.

This chapter examined the main processes that form minerals within Earth and at its surface. Despite their varied origins, minerals share systematic chemical and structural relationships. In the next chapter, we will explore how these relationships form the basis of mineral classification and the major groups that define Earth’s mineral kingdom.