What is a mineral vs crystal?

Minerals and crystals are often used interchangeably, but they are not exactly the same thing. While all crystals are minerals, not all minerals are crystals. The main difference lies in their internal structure.

Table of Contents

What is a Mineral?

A mineral is a naturally occurring inorganic solid with a defined chemical composition and crystal structure. Minerals are formed through geological processes and have a crystalline structure. Over 4,000 minerals have been discovered so far. Some examples include quartz, calcite, halite, and pyrite.

For a substance to qualify as a mineral, it must meet the following criteria:

It must be solid and inorganic (not coming from living things or organic compounds).
It must have a defined chemical composition and crystal structure.
It must have formed through geological processes and occur naturally.

Minerals are identified and classified based on their chemical composition and crystal structure. The chemical composition refers to the elements and compounds that make up a mineral. The crystal structure refers to the orderly arrangement of atoms in a mineral.

What is a Crystal?

A crystal is a solid material whose constituents atoms, molecules, or ions are arranged in an orderly, repeating pattern extending in all three spatial dimensions. Crystals are formed through a process called crystallization, which involves ions, atoms or molecules gathering together in a solution and taking on an organized crystalline arrangement.

A crystal’s internal structure consists of a regular, geometric arrangement of molecules held together by chemical bonds. This gives crystals smooth flat faces and defined angles. The shapes of crystals depend on the type of mineral and the conditions under which the crystal formed.

Some defining characteristics of crystalline solids include:

They have a precise melting point.
They have smooth, flat faces with defined angles.

They have a distinct crystal shape such as cubic, tetragonal or hexagonal.
Their atoms are arranged in a regular, repeating 3D pattern.

Some common examples of crystals include diamonds, rubies, emeralds, quartz and ice.

Key Differences Between Minerals and Crystals

While all crystals are minerals, not all minerals are crystals. Let’s look at some key differences:

Minerals	Crystals
Naturally occurring solid with a defined chemical composition	Solid material with a regular, orderly atomic structure
Do not always have a crystal shape or structure	Always have a crystal structure with flat faces and defined angles
Classified by chemical composition and crystal structure	Classified by their symmetry, shape and angles
Examples: Quartz, calcite, gypsum	Examples: Diamonds, salt, snowflakes

The main difference is that crystals have an ordered internal structure that gives them their defined shapes and angles. Not all minerals exhibit crystal structures. Minerals like obsidian and opal don’t have crystal shapes.

How Crystals Form

Crystals form through a process called crystallization. This refers to the formation of a crystalline solid from a solution or molten material. There are two main types of crystallization:

Precipitation: This occurs when the ions/atoms in a solution start to come together in an orderly, repetitive structure to form a crystalline solid.
Freezing: This occurs when atoms/molecules in a liquid start to form an orderly pattern as the material freezes.

In order for crystallization to occur, the solution or melt needs to be supersaturated. This means that is contains more dissolved particles than its equilibrium state.

As the solution cools or reaches lower pressure, the excess dissolved particles start to adhere together in a regular, geometric pattern, forming the crystal’s smooth faces and defined angles. The crystal continues to grow as more particles are deposited onto the orderly structured lattice.

Different crystal shapes are formed based on the type of mineral and the specific conditions like temperature, pressure, and pH levels during crystallization.

Factors Affecting Crystal Formation

There are several factors that affect crystal formation:

Temperature – Higher temperatures increase solubility allowing more particles to dissolve. Lower temperatures cause precipitation of particles to form crystals.
Pressure – Higher pressure increases solubility while lower pressure causes precipitation. Lower pressure is a key factor in crystallization.
pH – Acidic or basic solutions affect the solubility of compounds, impacting crystallization.

Impurities – Impurities interfere with the orderly lattice structure, causing defects in crystals.
Concentration – Supersaturated solutions have high concentrations of dissolved particles, allowing crystals to grow when conditions change.

By controlling these conditions, the shape and structure of crystals can be tailored in a process known as crystal engineering.

Crystal Structure

The crystal structure refers to the orderly, three-dimensional arrangement of atoms, ions or molecules in a crystalline solid. Each mineral has its own unique crystal structure based on its chemical composition.

Some key aspects of crystal structure include:

Unit cell – The basic repeating unit in the atomic lattice of a crystal. Unit cells stack together to form the larger crystal structure.

Lattice points – Points in space occupied by the atoms, ions or molecules within a crystal.
Crystal lattice – The three dimensional network of unit cells that extend in all directions to form the crystal structure.
Crystal planes and faces – Relatively flat surfaces that form the external faces of a crystal.

The unit cell is characterized by six lattice parameters – its dimensions along the X, Y and Z axes and the angles between them. These lattice parameters distinguish the different crystal structures.

Types of Crystal Lattices

There are seven common crystal lattice systems we observe in nature:

Cubic – Atoms/ions arranged in a simple cube (e.g. salt, diamond).

Tetragonal – Square base with unequal axis lengths (e.g. rutile).
Orthorhombic – Three unequal axis lengths at right angles (e.g. aragonite).
Hexagonal – Hexagon-shaped unit cell (e.g. quartz, ice).

Rhombohedral – Like hexagonal but different lattice points (e.g. calcite).
Monoclinic – Three unequal axes with one tilted angle (e.g. gypsum).
Triclinic – Three unequal axes and none at right angles (e.g. kyanite).

The structures can be further classified into 32 different crystal classes based on their symmetry. Understanding the crystal structure helps predict a mineral’s properties like shape, hardness, cleavage, fracture etc.

Common Crystal Shapes

Due to their regular internal structure, crystals exhibit defined external shapes with flat faces and smooth surfaces. Looking at a crystal’s shape helps identify key properties and its crystal structure. Some common crystal shapes include:

Cubic – Cube-like shape with six square faces (e.g. halite).

Octahedral – Eight faced shape with triangular faces (e.g. diamond).
Dodecahedral – Twelve faced shape with pentagon faces (e.g. garnet).
Monoclinic – Unequal sides with two longer prism faces (e.g. topaz).

Prismatic – Two equal and parallel faces with polygonal ends (e.g. tourmaline).
Rhombohedral – Parallelogram faces at equal angles (e.g. corundum).
Tetragonal – Rectangular base and pyramidal top (e.g. rutile).

Tabular – Flattened crystal with flat top and bottom (e.g. mica).

These idealized shapes are a result of a crystal’s internal structure. Real-world crystals often exhibit imperfect shapes with rounded edges or distorted faces due to impurities and irregular growth.

Properties of Crystals

The ordered atomic structure of crystals leads to several unique physical and chemical properties:

Regular geometry – Smooth, flat crystal faces and defined angles based on internal structure.
Precise melting point – Crystals melt abruptly at specific temperatures.
Faceted surfaces – External planes relate to the lattice planes between atoms or ions.

Anisotropy – Properties vary based on crystal orientation.
X-ray diffraction – X-rays produce specific diffraction patterns based on the atomic structure.
Piezoelectricity – Ability to generate electric charge under mechanical stress.

Cleavage and fracture – Break along planar surfaces based on the crystal structure.
Optical properties – Light reflection, refraction and polarization effects.

These properties allow crystals to be identified and characterized using microscopic, optical and X-ray analysis techniques.

Atomic Defects in Crystals

While crystals exhibit an ordered atomic structure, this arrangement is rarely perfect. Atomic defects are interruptions to the periodic crystal lattice. Point defects are defects that occur at a single lattice site inside a crystal:

Vacancy Defects – Missing atoms or ions in the lattice.
Interstitial Defects – Extra atoms squeezed between lattice sites.

Substitution Defects – Lattice atoms replaced by different atoms.

Other defects like dislocations, grain boundaries, stacking faults and surface defects can disrupt larger regions of the crystal lattice. These imperfections lead to real-world crystals with irregularities and flawed geometries.

Crystallization in Nature

Crystallization processes play an important role in nature. Some examples include:

Snowflakes crystallizing in cold clouds.
Gems forming when minerals crystallize from hot fluids.
Sea salt crystals precipitating from saline water.

Ice crystals freezing from water.
Kidney stones arising from crystallization of calcium compounds in body liquids.

Crystallization allows atoms to minimize energy and achieve stable packing arrangements. The wonderful diversity of crystal shapes and structures arise from the different atoms, bonding and conditions involved in crystallization.

Conclusion

In summary, all crystals are minerals but not all minerals are crystals. Crystals exhibit the defined external shape, structure and symmetries that arise from their precise, orderly atomic structure and bonding. This distinctive atomic arrangement gives crystals unique physical, optical and electromagnetic properties.

While ideal crystals have perfect geometries, real-world crystals contain defects in their lattice structure leading to irregularities in their shape and properties. Nevertheless, crystalline materials find widespread applications in technology, science and industry due to their robust mechanical properties and anisotropic characteristics.

So next time you come across a crystal, take a closer look at its geometric shape and facets, for they reveal the intricate atomic order within!