Crystals - Alan Holden 1958
Royal InstitutionKey Moments
Crystals grow by orderly atomic patterns, shaping salt to granite.
Key Insights
Crystals are ubiquitous and defined by a regular, repeating arrangement of atoms or molecules.
Crystal growth happens from solutions, melts, or gases, and tends to preserve the material's characteristic faces.
The idea of building blocks (atoms/molecules) stacked in a regular lattice explains consistent angles and grain formation in solids.
Cleavage and directional splitting reveal internal crystal planes and the underlying atomic structure.
Different minerals in rocks crystallize separately because they favor different ordering patterns, producing distinct grains (e.g., in granite).
Order in crystals contrasts with the disorder of glasses and gases; crystallization is a highly organized, dynamic process.
CRYSTALS ARE EVERYWHERE
Crystals aren’t merely decorative; they surround us in everyday materials and natural forms. You may have encountered a crystal in a museum, in a chemical bottle, or in a clay pit in Brazil as a strikingly large specimen, and a fragment from a cliff in Arkansas proves crystals come in many sizes. Beyond these striking examples, crystals are everywhere at small scales: salt crystals flavor many foods, snowflakes are single ice crystals, and alum crystals can be grown at home. The common thread is a regular, repeating pattern of atoms or molecules; this pattern gives each crystal its characteristic shape. When you grow alum in a mason jar, you watch a long process of deposition, where the crystal increases in size while preserving well-defined faces. This behavior demonstrates a fundamental point: the crystal’s shape is dictated by its internal order, not by how big it becomes. Under a microscope, you can observe the growth in action as atoms or molecules settle onto the crystal’s surface, adding to a stable, orderly lattice. This orderly deposition explains why crystals retain symmetrical shapes even as they enlarge and why the same material tends to form specific angles between corresponding faces. In short, crystals reveal a hidden, universal order that governs the material world around us.
GROWTH MODES: SOLUTIONS, MELTS, AND GASES
Crystals grow in several natural and laboratory settings, and the growth mode strongly influences their appearance. In solutions, as with alum, crystals form from dissolved material that slowly settles out, layer by layer, on the crystal’s surfaces. When observed under a microscope, crystals keep their shapes as they grow, each face developing in a way that reflects the material’s internal symmetry. Growth also occurs from melts; by melting a substance and letting it cool, you can watch the crystal fronts push into the surrounding liquid or melt near the advancing front, creating flat faces as they solidify. If the melt solidifies into a rounded blob and then cools, the solidification tends to restore the flat faces, illustrating how the material’s natural order reasserts itself during crystallization. When two crystals meet during growth, they don’t push each other aside; they stop growing where the supply of material is exhausted, creating a boundary that may not align with their natural faces. The result is a dense mosaic of crystals—like a granite rock—composed of many grains that grew separately before coalescing into a solid mass.
ROCKS, METALS, AND GRAIN STRUCTURE
Most solids are aggregates of many crystals that grew separately before becoming joined in a single piece. Granite provides a vivid example: a mass of distinct grains where quartz, feldspar, and mica each form its own crystal. These grains do not mix into a single crystalline lattice; instead, each mineral crystallizes in its preferred arrangement, producing visually different patches within the same rock. The lesson is that different molecular orders favor different crystal structures, so even when the rock appears uniform to the eye, it is actually a mosaic of crystalline domains. In metals, the concept persists: polished metal objects—such as a cast brass door handle—reveal crystal boundaries upon careful etching that wears away the metal surface. The overall crystal lattice remains, but the surfaces you see often display grain boundaries where crystals meet. Across nature and industry, the same principle applies: the solid world is largely crystalline, with hidden but organized internal architectures that give form to the materials we handle daily.
BUILDING BLOCKS AND LATTICES
A central idea in crystallography is that crystals are built from small, repeating building blocks—atoms or groups of atoms—arranged in a regular, three-dimensional lattice. The notion is reinforced by observations that crystals of the same material exhibit the same angles between corresponding faces, regardless of their size or shape. Consider a cube as a simple building block for alum; a stack of such blocks in all directions would produce the crystal’s characteristic faces. Yet not all crystals can be constructed from perfect cubes, as the actual atomic arrangements vary. Some blocks are elongated or irregular, which produces different lattice geometries, such as the more complex arrangement for garnet. The concept of a repeating unit—whether a cube or a more intricate motif—helps explain why crystals display consistent angles and why their properties emerge from a fundamental, orderly arrangement of building blocks rather than from randomness.
EVIDENCE OF ORDER: CLEAVAGE AND PLANES
Crystallography gains strong support from evidence of cleavage, the tendency of crystals to split along well-defined planes. Mica is famous for cleaving into thin sheets along its layers, as if the atoms form sheets stacked together with strong bonds within the sheets but weaker bonds between them. Other crystals reveal similar directional planes. For example, nickel sulfate hexahydrate cleaves parallel to a specific face, producing flat, uniform slices when the blade consistently strikes along that direction. Sodium nitrate can cleave in three directions, creating a set of well-defined faces. Such directional cleavage implies an underlying orderly arrangement of atoms into planes or blocks that can separate cleanly along certain axes. Early scientists who studied calcite and other minerals recognized that the faces and angles of crystals are not arbitrary; they reflect a stable, repeating internal order—an insight that helped establish the two fundamental ideas of crystallography: discrete building blocks and a regular lattice.
ORDER, SEEDING, AND THE DIFFERENCE IN MATERIALS
A striking demonstration of crystal order is how different materials respond to seeding and crystallization. When a melt such as salol is cooled, it will crystallize only if the right order is presented; seeding a melt with crystals of a compatible substance can cause growth of those crystals while other substances wait. This selectivity explains why granite contains three distinct minerals—quartz, feldspar, and mica—each crystallizing in its own preferred pattern rather than forming a single mixed crystal. The evidence connects closely with the idea that crystallites grow from specific building blocks and that each material tends toward its own order. The process is astonishingly rapid on an atomic scale: countless atoms must arrange themselves into an ordered array, repeatedly, as the crystal grows. A helpful analogy is the Bragg trick with soap bubbles—rafts of bubbles collect in an orderly way at the surface, suggesting how atoms might organize themselves into a crowded, orderly arrangement at vastly greater speeds. The contrast with glass and liquids emphasizes how order emerges from disorder: in a glass, molecules are stuck in a disordered arrangement, while gases are even more chaotic, their only order lying in the individual molecules that constitute them.
Mentioned in This Episode
●Supplements
●People Referenced
Crystal growth at home — quick cheat sheet
Practical takeaways from this episode
Do This
Avoid This
Common Questions
A crystal is a solid whose atoms or molecules are arranged in a regular, repeating pattern. The video shows crystals from salt to snowflakes and alum grown in a jar as examples.
Topics
Mentioned in this video
Crystal of Alum demonstrated in the film; used to illustrate slow, orderly crystal growth from solution
Table salt; described as composed of little crystals and used as an everyday crystal example
A substance used in crystallization experiments; seeds crystals and demonstrates selective ordering
Mineral with familiar cleavage properties; compared to sodium nitrate in early crystallography studies
Mineral grain in granite; used to illustrate distinct crystal grains within a rock
Mineral grain in granite; one of the three minerals that crystallize separately in granite
Mineral grain in granite; cleaves into sheets, illustrating layered atomic structure
Igneous rock composed of distinct mineral grains (quartz, feldspar, mica) that crystallize separately
Crystal used to demonstrate cleavage and flat faces in Nordic directions
Crystal that cleaves in three directions; used to illustrate directional splitting
Brass door handle showing crystal boundaries after polishing and etching
Material melted and allowed to crystallize in a crucible as part of the demonstration
Complex mineral building block used as an example of non-cubic crystallography
Illustrative cubic building block for iron’s crystal structure
Scientist whose concept (soap-bubble analogy) is borrowed to illustrate crystal packing
More from Ri Archives
21 minYoung and the Wave Theory of Light - with Sir Lawrence Bragg
16 minMagnetism #1 - with Sir Lawrence Bragg
25 minPrelude to Power
21 minPlastics and Rubbers - Properties of Matter #4
Found this useful? Build your knowledge library
Get AI-powered summaries of any YouTube video, podcast, or article in seconds. Save them to your personal pods and access them anytime.
Try Summify free