2.3 Mineral formation and classification

Mineral Formation Processes

Minerals form through several distinct processes, and each one produces different types of rocks. Understanding how minerals form helps you connect the dots between igneous, sedimentary, and metamorphic rocks later in the course.

Crystallization from magma

When magma cools, the atoms within it slow down and begin bonding into organized crystal structures. The result is igneous rock.

• Cooling rate matters. Slow cooling (deep underground) gives crystals more time to grow, producing coarse-grained rocks like granite. Fast cooling (at the surface) creates fine-grained rocks like basalt.
• Magma composition and pressure also control which minerals crystallize first. Minerals with higher melting points, like olivine, tend to form early, while quartz crystallizes later as the remaining melt cools.

Precipitation from aqueous solutions

Minerals can dissolve in water and later come out of solution when conditions change. Think of it like dissolving salt in warm water, then watching crystals appear as the water evaporates.

• Evaporation concentrates dissolved ions until minerals precipitate. This is how evaporite minerals like halite (rock salt) and gypsum form in shallow seas and desert lakes.
• Cooling or chemical reactions in water can also trigger precipitation. Limestone, for example, often forms when calcium carbonate precipitates in warm, shallow ocean water, sometimes aided by organisms like corals.
• Hydrothermal systems (hot, mineral-rich water circulating through rock) are another major setting for precipitation, producing ore deposits of metals like copper and gold.

Metamorphic reactions

Deep in the crust, existing minerals can recrystallize or chemically react under intense heat and pressure. The rock never fully melts; instead, its minerals rearrange into new, more stable forms.

• Limestone recrystallizes into marble. Sandstone becomes quartzite. Shale transforms into schist.
• These reactions can change both the mineral composition and the texture of the rock, often producing aligned or layered crystal structures.

Mineral Classification and Properties

Geologists classify minerals into groups based on their chemical composition, specifically the dominant anion or anion group in their structure. Silicates dominate the Earth's crust, but the other groups are important for understanding ore deposits, sedimentary environments, and more.

Silicates

Silicates are built around the silicon-oxygen tetrahedron ($SiO_4$), a structure where one silicon atom is bonded to four oxygen atoms. These tetrahedra can link together in chains, sheets, or 3D frameworks, producing a huge variety of minerals.

• This is by far the most abundant mineral group, making up roughly 90% of the Earth's crust.
• Examples: quartz, feldspar, mica, olivine, garnet, pyroxene.

Carbonates

Carbonates contain the carbonate ion ($CO_3^{2-}$) bonded with metal cations like calcium, magnesium, or iron.

• They often form through biological activity (shells, coral reefs) or by precipitating from water.
• Carbonates react visibly with dilute hydrochloric acid, which is a handy identification test.
• Examples: calcite, dolomite, aragonite, siderite.

Oxides

Oxides consist of oxygen bonded with one or more metal cations.

• They form across many environments, from cooling magma to surface weathering.
• Many important ore minerals are oxides. Hematite and magnetite, for instance, are major sources of iron.
• Examples: hematite, magnetite, corundum, rutile.

Sulfides

Sulfides contain sulfur bonded with metal cations and typically form in hydrothermal or magmatic settings.

• Many economically valuable ore minerals are sulfides. Galena is the primary ore of lead, and chalcopyrite is a major copper ore.
• Examples: pyrite, galena, sphalerite, chalcopyrite.

Sulfates

Sulfates contain the sulfate ion ($SO_4^{2-}$) bonded with metal cations.

• They commonly form through evaporation of saline water or through oxidation of sulfide minerals at the surface.
• Examples: gypsum, anhydrite, barite.

Native elements

Native elements occur as pure, uncombined elements or natural alloys.

• They can form through magmatic concentration, hydrothermal deposition, or weathering processes.
• Examples: gold, silver, copper, sulfur.

Solid solution in minerals

Solid solution occurs when one element substitutes for another in a mineral's crystal structure without breaking the overall structure. This is possible when the swapped elements have similar sizes and charges. The result is a continuous range of compositions within a single mineral group.

Here are the three classic solid solution series you should know:

• Plagioclase feldspar series: Ranges from sodium-rich albite ($NaAlSi_3O_8$) to calcium-rich anorthite ($CaAl_2Si_2O_8$). Any mixture of Na and Ca is possible. As calcium content increases, the mineral gets denser, darker, and has a higher melting temperature.
• Olivine series: Ranges from magnesium-rich forsterite ($Mg_2SiO_4$) to iron-rich fayalite ($Fe_2SiO_4$). More iron means a darker color, higher density, and lower melting point.
• Pyroxene series: Ranges from magnesium-rich enstatite ($MgSiO_3$) to iron-rich ferrosilite ($FeSiO_3$). The same pattern holds: more iron shifts color, hardness, and stability.

The key takeaway is that minerals aren't always fixed recipes. Solid solution means a single mineral name can cover a spectrum of compositions.

Factors affecting mineral stability

A mineral that's stable deep in the Earth may break down at the surface, and vice versa. Three main factors control which minerals survive in a given environment.

• Temperature
  • High temperatures can cause minerals to melt, recrystallize, or react to form new minerals.
  • Low temperatures favor precipitation and crystallization from solutions.
• Pressure
  • High pressure pushes minerals toward denser, more compact crystal structures. The classic example: graphite (low pressure) transforms into diamond (very high pressure), even though both are pure carbon.
  • Low pressure allows less dense or water-bearing minerals like mica and hydrated clays to remain stable.
• Chemical environment
  • Which elements are available, and in what concentrations, determines what can form.
  • Changes in pH, oxygen levels, or ion concentration trigger reactions. For example, pyrite ($FeS_2$) exposed to air and water oxidizes to form iron oxide minerals like hematite.
  • Surface weathering environments promote the breakdown of silicates and the formation of clay minerals and oxides.
• Stability fields
  • Each mineral has a specific range of temperature, pressure, and chemical conditions where it's stable. Outside that range, it transforms or reacts.
  • Geologists map these ranges on phase diagrams, which show boundaries between stability fields. These diagrams help predict which minerals form in different settings, such as specific metamorphic facies or stages of magma cooling.