Crystals are formed through a process called crystallization, which occurs when molecules gather to stabilize as a liquid cools and hardens. This can happen when magma hardens or when water evaporates from a natural mixture. In underground cavities, crystals grow through atoms that connect in regular three-dimensional patterns. As more atoms join and create a uniform and repetitive pattern, the crystal grows. The shape of crystals can vary considerably and reflects the internal arrangement of the atoms.
There are many different types of crystals, and they can be formed in various ways. Igneous crystals like diamonds and rubies are formed when magma cools and hardens slowly. Metamorphic crystals like garnets and kyanite are formed when existing rocks are subjected to high heat and pressure. Sedimentary crystals like halite and gypsum are formed from the precipitation of minerals from water. Some crystals are grown from vapor or solution. Underground cavity-grown crystals form as atoms connect in regular patterns with more joining over time.
Temperature and pressure play a significant role in crystal formation. Higher temperatures cause atoms and molecules to gain energy and move apart, expanding the crystal lattice. High pressures compress the lattice. The response of crystal structures to temperature and pressure changes results from a balance of strong intramolecular bonding, medium intermolecular interactions, and weaker van der Waals contacts. The formation of crystals is a complex process affected by the shifting conditions of the earth.
How Crystals Form
Cooling and Solidification of Magma
Many crystals originate deep underground from the slow cooling of molten magma. Magma contains dissolved compounds like silicates and sulfides that act as building blocks for crystallization. As magma cools over extended periods of time, these dissolved elements transition from a disordered state into an orderly crystal lattice.
For example, diamonds form about 100 miles underground, where temperatures reach 2200°F. In these extreme conditions, carbon atoms in molten rock are compressed into rigid crystal lattices. If magma cools quickly, such as during a volcanic eruption, crystals have less time to form and tend to be very small. Slow cooling allows for larger crystal growth.
Evaporation from Aqueous Solutions
Crystallization can also occur when water evaporates from a saturated mineral solution, leaving the dissolved particles behind. This occurs in places like lake beds, where evaporation concentrates dissolved salts like sodium chloride. Eventually, the salt ions arrange themselves into orderly repeating cubic lattices known as halite crystals.
Similar evaporation crystallization happens during the formation of gypsum crystals in sea water, copper sulfate crystals from aqueous solution, and snowflakes in freezing atmospheric water vapor. In each case, the loss of water provides the right conditions for ordered crystalline structures to emerge.
Growth in Underground Cavities
Larger crystals often grow in protected environments like underground geodes and caves. In these cavities, silica-rich groundwater slowly deposits dissolved minerals on cave walls. The minerals accumulate in repetitive 3D patterns, forming huge selenite gypsum crystals up to 36 feet long in the Naica Mine of Mexico or cylindrical celestite crystals in Ohio’s Put-in-Bay geodes.
The open space allows the crystals to achieve their large sizes. Given unlimited time and space, crystals continue to grow. Most natural crystals do not achieve their maximum possible size due to crowding, impurities, or changing environmental conditions.
Major Crystal Growth Processes
There are several ways that crystals can form, depending on the medium in which their ions, atoms, or molecules are suspended before crystallization occurs:
Vapor Deposition
Some crystals grow through deposition of gaseous material called vapor. This occurs by heating a solid until it vaporizes, then allowing the vapor to cool and condense on a surface, solidifying in crystal form. Corundum, silicon, and other crystals can be formed this way.
Growth from Molten Material
Crystallization from molten liquid is common in metal alloys and igneous rocks. As mentioned earlier, diamond formation results when tremendous heat and pressure liquefy carbon deep underground. Allowing the melt to cool at a suitable rate allows crystalline structures to emerge.
Crystallization from Solution
Many crystals can grow from a supersaturated solution as solvent evaporates or temperature changes. Alum, table salt, and borax are examples of crystals formed through solution crystallization. This process is useful for purifying and growing crystals in labs.
Factors Affecting Crystal Formation
Many variables influence crystal formation and growth, including:
Temperature
Higher temperatures increase molecular motion, allowing crystals to form as molten material cools or vapor condenses when gaseous particles lose energy. Sudden temperature changes can cause defects or cracks in crystal lattices.
Pressure
Increasing pressure compresses the crystal lattice and atoms move closer together. Extreme pressures deep underground squeeze carbon into diamond crystals. Reducing pressure can cause crystals to crack or cleave along planes between atoms.
Impurities
The presence of impurity atoms or molecules can disrupt the orderly repetition of the crystal lattice. This is why industrial crystallization processes ensure very pure starting compounds.
Space to Grow
Unrestricted space allows crystals to achieve maximum sizes, such as in geodes. Crowded or confined spaces limit growth. Crystals may compete, deform, or even fuse together when space is limited.
Types of Crystals
There are several major categories of crystals based on where or how they form:
Igneous Crystals
Igneous crystals are born through the solidification of molten magma. They include common minerals like feldspar, mica, granite, and quartz, along with precious gems like diamonds, emeralds, and rubies. Their beauty originates from the ordering of atoms as hot, chaotic magma cools.
Metamorphic Crystals
Heat and pressure transform existing minerals into new metamorphic crystals, as seen in garnet, kyanite, and other gems. Metamorphic crystals have a banded or stretched look reflecting the extreme conditions of their formation.
Sedimentary Crystals
Evaporation and chemical precipitation produce sedimentary crystals like halite and gypsum. Layered growth patterns emerge as these crystals grow slowly over time from liquids present in sedimentary environments.
Hydrothermal Crystals
Mineral-rich hot water flowing through rock fractures forms brilliant hydrothermal crystals, including variations of quartz like the purple amethyst and clear, faceted rock crystal. Access to mineral nutrients allows these crystals to grow to substantial sizes.
Vapor-Grown Crystals
As noted earlier, vapor deposition can produce igneous and hydrothermal crystals, as well as manufactured crystals like silicon and other electronics materials grown through carefully controlled vaporization and condensation.
Crystal Shapes and Structures
The shapes and formations of crystals provide clues to their composition and growth conditions. Some characteristics include:
- Cubic crystals like halite indicate equal growth in all directions.
- Hexagonal quartz points to strong directional bonds.
- Jagged, asymmetric surfaces suggest growth disturbances.
- Striations or banding reflect changing conditions during formation.
- Doubly terminated points form when crystals grow freely without attachment at either end.
- Inclusions of foreign material imply purity was compromised, disrupting the lattice.
- Color variations arise from impurities or structural defects.
The amazing diversity of crystal shapes reflects variances in their underlying atomic lattices. Scientists study these structures to better understand the forces governing crystal growth.
Table Comparing Different Crystal Shapes and Structures
Crystal Type | Structure | Example Crystals |
---|---|---|
Cubic | Evenly aligned cube shape reflects equal growth in all directions | Halite, Galena, Pyrite |
Hexagonal | Hexagonal prism shape based on strong directional bonding | Quartz, Apatite, Beryl |
Rhombic | Parallelogram shape with oblique angles | Rhodochrosite, Wulfenite, Hemimorphite |
Monoclinic | 3 unequal crystallographic axes | Orthoclase, Augite, Gypsum |
Triclinic | No 90° angles between crystallographic axes | Copper, Aragonite, Axinite |
Doubly Terminated | Comes to a point at both ends | Quartz, Rutile, Stibnite |
Banded | Striations indicate changing conditions during formation | Agate, Onyx, Fluorite |
Druzy | Tiny crystals coating a surface | Quartz, Calcite, Hematite |
Frequently Asked Questions
What is crystallization?
Crystallization is the process by which atoms, molecules, or ions suspended in a liquid, molten, or gaseous state come together to form a highly organized solid crystal lattice as conditions change.
What conditions are needed for crystals to form?
The key conditions needed are: 1) a concentrated source of the crystals’ building blocks (atoms, ions) in a fluid or molten state, 2) proper temperature conditions for the solid crystal lattice to emerge, usually cooling, 3) time for the crystal to grow, 4) space for the crystal to form without restriction, 5) low concentrations of impurities.
Where do crystals form naturally?
Common natural environments for crystal formation include solidifying magma underground, evaporating water bodies depositing minerals, hydrothermal vents, decomposing pegmatite veins, and cavities in volcanic and sedimentary rock.
Why do crystals have geometric shapes?
The crystal’s shape reflects its internal orderly atomic structure. Atoms align along certain geometric planes and axes as the crystal lattice forms. Common shapes like cubes, hexagons, and rhombohedrons emerge.
What makes crystals colorful?
Trace elements and impurities get incorporated into the crystal lattice, absorbing certain wavelengths of light and imparting color. Structural defects in the lattice can also scatter light waves, producing color.
Are all crystals valuable gemstones?
No, only a few varieties like diamonds, emeralds, and rubies are precious gemstones. But even common crystals like quartz and calcite have visual appeal. The main value of crystals is in understanding their geometric growth patterns.
How fast do crystals grow?
Growth rate depends on conditions. Fast growth yields small crystals, like flash-frozen ice crystals. Geologic crystals can take thousands to millions of years to reach large sizes, accumulating atoms slowly over time.
Conclusion
The wondrous world of crystals forms through a remarkable process called crystallization. Ordered atoms, molecules, and ions suspended within an appropriate medium transition from chaos to precise geometric arrangements as conditions change. Examining factors that enable crystal growth gives insight into the basic forces guiding the assembly of matter. From this ordering process emerges the natural beauty of quartz, salt, and thousands of other crystalline creations.
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