What Are Crystals

A crystal is a solid material in which atoms, ions, or molecules are arranged in a highly ordered, repeating three-dimensional pattern known as a crystal lattice. This periodic internal structure distinguishes crystals from amorphous solids, such as glass, where atomic arrangements lack long-range order.

In geology, crystals form the fundamental building blocks of minerals, which are naturally occurring inorganic substances that compose rocks. Each mineral exhibits a characteristic crystal structure determined by the bonding and spatial arrangement of its constituent atoms, influencing its symmetry, external morphology, and physical properties.

What Are Crystals. Fabulous Quartz with Dumortierite inclusions from Brazil. Photo Timothy Paul.

Crystal Structure, Symmetry, and Form

At the core of every crystal is the unit cell—the smallest repeating unit that defines the crystal’s atomic arrangement. Think of it as a 3D blueprint: when repeated in all directions, it forms the crystal lattice, an infinite grid of atoms.

Symmetry and Crystal Systems

Crystals are classified into seven crystal systems based on their symmetry:

• Cubic (e.g., halite, NaCl)
• Tetragonal
• Orthorhombic
• Hexagonal (e.g., quartz, SiO₂)
• Trigonal
• Monoclinic
• Triclinic

These systems, further divided into 14 Bravais lattices and 230 space groups, determine properties like hardness, cleavage, and optical behavior. For example, halite’s cubic structure arises from alternating sodium and chloride ions, while quartz’s hexagonal symmetry produces six-sided prisms.

From Unit Cell to Macroscopic Crystal

The repetition of unit cells creates the crystal’s macroscopic form. Simple minerals like halite have small, straightforward unit cells, while complex silicates (e.g., micas and clays) feature larger, more intricate arrangements.

External Form and Growth

A crystal’s habit—its external shape—reflects its internal symmetry when grown in open spaces. According to Steno’s Law (1669), the angles between corresponding faces remain constant, regardless of size. However, in confined environments (e.g., within rocks), crystals may develop irregular (anhedral) shapes while retaining their internal order.

Additional Structural Features

• Twinning: Symmetrical intergrowths of two or more crystals.
• Zoning: Compositional bands that record changing growth conditions.
• Defects: Imperfections that affect color and mechanical strength.
• Polymorphism: The same chemical composition can form different structures under varying conditions (e.g., diamond vs. graphite).

Steno’s Law revealed the connection between internal atomic order and external form long before modern tools like X-ray diffraction confirmed it.

Crystal lattice structure examples: cubic, octahedral, and dodecahedral forms with cleavage and fracture patterns in minerals like halite and fluorite.

How Crystals Form

Crystals grow when atoms or ions arrange themselves into a highly ordered, stable structure. This process, called crystallization, is driven by changes in temperature, pressure, composition, or solubility. It happens in two main steps:

Nucleation: The formation of tiny, stable clusters of atoms or ions.

Growth: The addition of more atoms or ions to these clusters, building the crystal.

The size, shape, and texture of crystals depend on factors like cooling rate, diffusion, and the presence of fluids or impurities. Here’s how crystals form in different geological settings:

Igneous Crystallization (From Melts)

When magma or lava cools, it becomes supersaturated, causing minerals to precipitate.

• Slow cooling (deep underground): Produces coarse-grained rocks with large, well-formed crystals, such as quartz and feldspar in granite.
• Rapid cooling (near the surface): Results in fine-grained or glassy textures, like basalt or obsidian.
• Multi-stage cooling: Creates porphyritic textures, where large crystals (phenocrysts) are embedded in a finer matrix.

Example: The large quartz crystals in granite form over millennia as magma cools slowly deep within the Earth.

Influences on crystal shape: Temperature, melt composition, volatile content (e.g., water, CO₂), and diffusion rates. Extreme conditions can produce skeletal, hopper, or dendritic crystals.

Metamorphic Crystallization (Solid-State Transformation)

Heat and pressure cause existing minerals to recrystallize (grow into interlocking mosaics) or neocrystallize (form new minerals).

• Recrystallization: Existing grains grow and reduce defects, as seen in the calcite crystals of marble.
• Neocrystallization: New minerals, like garnet porphyroblasts in schist, form from chemical reactions.
• Phase transformations: Minerals change structure with pressure and temperature, such as kyanite, andalusite, and sillimanite.
• Role of fluids: Metamorphic fluids enhance diffusion, catalyze reactions, and transport chemicals, a process called metasomatism.

Sedimentary and Diagenetic Crystallization (From Solutions)

Crystals form when solubility changes or evaporation concentrates dissolved ions.

• Evaporites: Minerals like halite (table salt) and gypsum precipitate in evaporating basins.
• Diagenetic cements: Calcite or quartz fills pores and fractures in sandstones.

Example: Halite crystals form as seawater evaporates in arid environments.

Influences: Ionic strength, pH, and impurities affect how crystals nucleate and grow.

Hydrothermal Crystallization

Hot, mineral-rich fluids circulate through fractures, depositing crystals as conditions change.

• Common minerals: Quartz, sulfides, tourmaline, and fluorite.
• Features: Crystals in open spaces (vugs or veins) often show growth zoning and fluid inclusions, which record their formation history.

Vapor Deposition/Sublimation

Crystals form directly from gases, such as sulfur around volcanic fumaroles. Growth is controlled by gas supersaturation and diffusion.

Example: Bright yellow sulfur crystals near volcanic vents are created by sublimation.

Key Kinetic Factors

Nucleation: Crystals form more easily on surfaces (heterogeneous nucleation) than in bulk solutions (homogeneous nucleation).

Growth vs. Diffusion: Fast attachment with limited supply creates skeletal or dendritic crystals, while ample supply allows well-faceted growth.

Ostwald Ripening: Over time, larger crystals grow at the expense of smaller ones, influencing the final crystal size and texture.

Properties of Crystals

Internal Structure and Mechanical Properties

A crystal’s lattice, formed by the repetition of a unit cell, defines its symmetry and governs mechanical properties. The type and strength of atomic bonds within the lattice influence:

• Hardness: Strong bonds, like diamond’s covalent carbon network, result in high hardness (10 on the Mohs scale), while weak interlayer bonds in talc produce softness (1 on the Mohs scale).
• Density: Dense minerals like galena (PbS) have closely packed lattices, contrasting with lighter minerals like quartz (SiO₂).
• Cleavage: Weak planes in the lattice allow minerals like mica to split into thin sheets. Quartz, lacking such planes, shows conchoidal fracture, breaking with smooth, curved surfaces.

These properties help geologists identify minerals and infer formation conditions, such as the pressure and temperature under which a crystal formed.

External Morphology (Crystal Habit)

When crystals grow in open spaces, their habit—the external shape—reflects their internal lattice symmetry. Common habits include:

• Cubic: Halite (NaCl) forms perfect cubes.
• Hexagonal: Quartz (SiO₂) grows as six-sided prisms.
• Dodecahedral: Garnet forms 12-sided shapes.
• Tabular: Barite appears as flat, plate-like crystals.

In crowded environments, like most rocks, crystals are anhedral, lacking distinct faces but retaining internal order. The habit aids in field identification and reveals growth conditions, such as available space or chemical environment.

Optical Properties

Crystals interact with light in ways that are diagnostic under a petrographic microscope:

• Birefringence: Calcite (CaCO₃) splits light into two rays, creating a double image, due to its anisotropic lattice.
• Pleochroism: Some crystals, like tourmaline, change color depending on the light’s direction, aiding identification.
• Extinction Angles: The angle at which a crystal appears dark under polarized light is unique to its lattice orientation.

These properties allow geologists to distinguish minerals in thin sections and infer their structural characteristics.

Electrical and Elastic Properties

The lattice symmetry also governs specialized properties:

• Piezoelectricity: Quartz generates an electric charge under pressure, used in electronics like watches.
• Pyroelectricity: Tourmaline produces a charge with temperature changes.
• Anisotropic Elasticity: Crystals expand or deform differently along lattice directions, affecting their durability.

These properties, rooted in symmetry, are critical for technological applications and understanding crystal behavior under stress.

Geological Significance

Crystal properties are essential for:

• Mineral Identification: Hardness, cleavage, and optical traits distinguish minerals like quartz from calcite in the field or lab.
• Environmental Insights: Properties like crystal size or habit reflect formation conditions (e.g., slow cooling produces large crystals in granite).
• Industrial Applications: Piezoelectric quartz powers electronics, while durable gems like garnet are valued in jewelry and abrasives.

By studying these properties, geologists unlock clues about Earth’s history and harness crystals for practical uses.

Distinctions: Crystals, Minerals, Rocks, Gems, and Mineraloids

Understanding the differences between crystals, minerals, rocks, gems, and mineraloids is essential for geologists to accurately identify and classify Earth materials. Each term describes a distinct aspect of geological substances, defined by their structure, origin, or use.

Crystal: A solid with a highly ordered, repeating three-dimensional atomic lattice, whether natural (e.g., quartz) or synthetic (e.g., laboratory-grown sapphire). The defining feature is the periodic arrangement of atoms, which governs properties like symmetry and cleavage.

Mineral: A naturally occurring, inorganic solid with a specific chemical composition and crystalline structure. All minerals are crystals due to their lattice structure, but not all crystals are minerals (e.g., synthetic crystals are not). Examples include quartz (SiO₂) and calcite (CaCO₃).

Rock: An aggregate of one or more minerals and/or mineraloids, bound together. For example, granite is composed of quartz, feldspar, and mica, while limestone consists primarily of calcite.

Gem: A mineral or crystal valued for its beauty, durability, and rarity, often cut and polished for jewelry. Examples include ruby and sapphire (both corundum, Al₂O₃, with trace elements affecting color) and diamond (carbon).

Mineraloid: A naturally occurring, non-crystalline solid lacking a periodic atomic lattice. Examples include obsidian (volcanic glass), opal (amorphous silica), and amber (fossilized resin).