Crystals can come in a wide variety of colours, which is determined by their chemical composition and structure. The colour of a crystal is closely related to which minerals are present and how the atoms are arranged within the crystal lattice. The colour can provide important information about the crystal’s properties, origins and possible uses.
What Causes Colour in Crystals?
The main ways that colour arises in crystals are through transition metal ions, charge transfer between metals and ligands, and structural defects in the crystal lattice. Let’s take a closer look at each of these mechanisms:
Transition metal ions – Many crystals contain transition metals like iron, copper, cobalt, manganese, chromium, etc. These metals have a partially filled d orbital that can absorb specific wavelengths of light. The absorbed wavelengths are subtracted from white light, leaving the complement color to be observed. For example, ruby owes its red color to Cr3+ ions.
Charge transfer – This involves electron transfers between transition metals and their surrounding ligands or atoms. For instance, sapphire gets its blue color from charge transfers between Fe2+ and Ti4+ ions with O2- ligands. The specific energies involved determine the color.
Structural defects – Defects in the repetitive structure of a crystal lattice can also cause colors through mechanisms like absorbance of impurity atoms or gaps in energy levels. Amethyst’s purple is created by lattice defects around Fe4+ impurities.
Common Coloured Crystals
Many brightly colored crystalline minerals occur commonly in nature. Here are some of the most well-known examples:
| Crystal | Colour | Cause of Color |
|---|---|---|
| Amethyst | Purple | Lattice defects around Fe4+ ions |
| Aquamarine | Blue-green | Fe2+ ions |
| Azurite | Deep blue | Cu2+ ions |
| Carnelian | Orange-red | Iron oxide impurities |
| Citrine | Yellow | Fe3+ impurities |
| Emerald | Green | Cr3+ and V3+ ions |
| Garnet | Red, pink, green, orange, purple | Transition metal ions (depends on variety) |
| Jasper | Red, yellow, brown | Iron oxide impurities |
| Lapis lazuli | Deep blue | Sulfur impurities |
| Malachite | Green | Cu2+ ions |
| Opal | Multicolored | Diffraction of light |
| Quartz | Purple, yellow, pink, green, blue, red | Various impurities |
| Ruby | Red | Cr3+ ions |
| Sapphire | Blue | Fe2+-Ti4+ and Fe2+-Fe3+ charge transfer |
| Topaz | Blue, yellow, pink, red | Fe2+, Fe3+ and Cr3+ ions |
| Turquoise | Blue-green | Cu2+ ions |
Colours in Synthetic Crystals
In synthetic crystals grown in laboratories, scientists have even more control over the colors produced. Impurity ions can be added at precise concentrations to yield specific hues. Common ways to synthetically colour crystals include:
- Doping or substitution – Intentionally replacing some host ions with transition metal or rare earth ions.
- Interstitial incorporation – Adding impurity ions at interstitial sites between normal lattice sites.
- Surface treatments – Coating or clustering metallic nanoparticles on the surface.
- Radiation exposure – Bombarding the crystal with electrons, neutrons or ions to create defects.
For example, the blue color of synthetic cubic zirconia is created by doping ZrO2 with oxygen vacancies or ions like Fe, Co and Ni. Synthetic diamonds get their colours from added nitrogen, boron and hydrogen atoms. The variety of colors attainable is far greater than with natural crystals.
Factors that Affect Crystal Color
There are several factors that determine the exact hue of coloured crystals:
- Type of impurity – Specific transition metals and rare earth elements impart characteristic colors.
- Oxidation state – The charge on metal ions affects the energies absorbed.
- Coordination – The atoms surrounding the metals influence the colors.
- Concentration – More impurities mean deeper, more saturated colors.
- Site symmetry – Similar ions can give different colors in different lattice sites.
- Crystal field splitting – The crystal structure changes d orbital splitting and energies.
- Band gap – Insulating vs. conductive crystals have very different colors.
- Defect concentration – More structural defects create more color centers.
- Particle size – Nanoscale crystals allow quantum confinement effects.
Considering all these factors allows scientists to fine-tune the colors of synthesized crystals for various practical applications.
Origins of Crystal Color in Nature
In naturally forming crystals, the colors arise through the geologic processes and environments in which the minerals are created. Some general principles for how natural crystal colors originate are:
- Magmatic crystals – Crystallization from magma or lava often incorporates transition metals.
- Hydrothermal crystals – Heated, mineral-rich waters dissolve impurities.
- Metamorphic crystals – Heat and pressure restructure minerals and lattices.
- Sedimentary minerals – Impurities result from weathering and dissolved substitutional ions.
- Biogenic crystals – Organisms directly control the composition.
- Cosmic irradiation – High energy radiation in space alters crystals.
For instance, amethyst forms when iron impurities are incorporated as granitic rocks containing quartz crystals undergo hydrothermal alteration. Emeralds result when chromium and vanadium substitute for aluminum in beryl crystals in hydrothermal deposits. Understanding natural environments gives clues about potential crystal colors.
Uses of Colored Crystals
The appealing colors of crystals make them highly valued for many practical uses, including:
- Gemstones – Colored crystals like rubies, sapphires and emeralds are prized as jewels and ornamental stones.
- Lasers – Transition metal doping provides specific wavelengths for tunable solid-state lasers.
- Color filters – Narrow absorption bands allow only certain colors to pass.
- Scintillators – Impurities emit specific colors when irradiated, enabling radiation detection.
- Phosphors – Colored crystals are used in LEDs, fluorescent tubes and cathode ray displays.
- Photovoltaics – Coloration indicates what photon energies can be absorbed for solar cells.
- Quantum dots – Size-confined nanocrystals fluoresce pure colors when excited.
Overall, the color of a crystal provides key insights into its composition and properties, which enable advanced applications across many fields.
Conclusion
In summary, crystal color arises from mechanisms like transition metal ions, charge transfer between metals and ligands, and structural defects. Many common crystals exhibit vivid colors like amethyst, emerald and ruby due to these effects. Crystal color can be precisely controlled in synthetic structures through doping, radiation exposure and other techniques. Naturally coloured crystals originate through magmatic, hydrothermal, metamorphic and other geologic processes. Colored crystals are highly valued for applications including gemstones, lasers, color filters, and quantum dots. Their colors reveal critical information about a crystal’s composition and structure on the atomic scale.
