Crystal Habits: Understanding Crystal Structures
Crystal habits, also known as crystal forms or crystal shapes, refer to the characteristic external shapes of crystals resulting from their internal arrangement of atoms or molecules. These habits are influenced by factors such as the crystal's chemical composition, growth conditions, and external environment.
Acicular
Crystals with needle-like shapes. They are elongated and thin, resembling needles or fibers.
Examples
- Natrolite India, Russia, United States
- Rutile Brazil, Australia, Madagascar
- Epidote Austria, Pakistan, United States
- Aragonite Mexico, Spain, Italy
Bladed
Crystals with flat, elongated shapes resembling blades or knives. They are often thin and flat, with parallel faces.
Examples
- Kyanite Brazil, Nepal, United States
- Gypsum Spain, United States, Italy
- Wollastonite Italy, Finland, United States
- Barite Morocco, United States, China
Botryoidal
Crystals with a rounded, globular shape resembling a bunch of grapes. They form concentric layers radiating from a central point and often have a smooth, rounded surface. Examples include malachite and hematite.
Examples
- Malachite Democratic Republic of the Congo, Australia, Zambia
- Hematite Brazil, Australia, United States
- Smithsonite Namibia, Mexico, United States
- Chrysocolla Democratic Republic of the Congo, United States, Peru
Composite
Crystals composed of multiple intergrown individuals or crystal fragments. Composite crystals can have a variety of arrangements, including parallel growth, cross-shaped intersections, or irregular intergrowths. They often exhibit a combination of different crystal habits within the same specimen.
Examples
- Garnet Schist United States, India
- Staurolite United States, Brazil
- Tourmaline in Quartz Brazil, Afghanistan
- Feldspar in Quartz Brazil, Madagascar
Cubic
Crystals with a cube-like shape, characterized by square faces. They have equal dimensions along each axis and can form perfect cubes or variations like octahedrons.
Examples
- Pyrite Spain, Peru, United States
- Halite United States, Germany, Poland
- Galena Peru, United States, China
- Fluorite China, Mexico, Spain
Dendritic
Crystals with branching, tree-like patterns. These formations often resemble ferns or trees and are formed by the growth of crystal branches in multiple directions.
Examples
- Dendritic Agate United States, Brazil, Madagascar
- Dendritic Copper United States, Chile
- Stibnite China, Romania
- Pyrolusite South Africa, Gabon
Fibrous
Crystals composed of fine, parallel fibers. These fibers may be flexible or rigid and can form dense mats or bundles. Fibrous crystals often have a silky or fibrous luster and are commonly found in asbestos minerals like chrysotile.
Examples
- Asbestos various locations worldwide
- Chrysotile Canada, Russia
- Nephrite China, New Zealand
- Riebeckite South Africa
Hopper
Crystals with a skeletal or hopper-like appearance, characterized by the presence of hollow spaces or depressions within the crystal structure. This occurs when the crystal grows faster along its edges or faces than in its interior, leaving behind empty spaces.
Examples
- Halite United States, Germany, Poland
- Gold South Africa, Russia, Australia
- Galena Peru, United States, China
- Fluorite China, Mexico, Spain
Massive
Crystals that lack well-defined external shapes due to rapid growth or confinement within a space. They form irregular masses or aggregates of crystal grains.
Examples
- Chalcedony Brazil, United States, Madagascar
- Obsidian United States, Mexico, Iceland
- Serpentine United States, Italy, Afghanistan
- Jadeite Myanmar, Guatemala, United States
Octahedral
Crystals with eight faces, forming the shape of a double pyramid. Each face is a triangle, and the crystal resembles two pyramids joined at their bases.
Examples
- Diamond South Africa, Russia, Botswana
- Fluorite China, Mexico, Spain
- Magnetite Sweden, Russia, United States
- Pyrite Spain, Peru, United States
Prismatic
Crystals with elongated, prism-like shapes. They have flat, parallel faces that are often arranged in a columnar fashion.
Examples
- Quartz Brazil, United States
- Tourmaline Brazil, Afghanistan
- Beryl/Aquamarine Brazil, Afghanistan
- Apatite Brazil, Mexico
Prismatic with Terminations
In addition to the elongated prism shape, crystals may also have distinct terminations at their ends. These terminations can be pointed (e.g., pyramidal), flat, or stepped, adding complexity to the crystal's overall appearance.
Examples
- Amethyst Brazil, Uruguay
- Apophyllite India
- Axinite Mexico, France
- Topaz Brazil, Pakistan
Radiating
Crystals that grow outward from a central point, creating a radial or star-like pattern. These formations can be composed of thin crystals radiating outward or aggregates of acicular crystals arranged in a radial fashion.
Examples
- Stibnite China, Romania
- Barite Roses United States
- Malachite Democratic Republic of the Congo, Australia
- Vanadinite Morocco, United States
Rhombohedral
Crystals with a shape resembling a distorted cube, characterized by rhombus-shaped faces. They often exhibit threefold symmetry.
Examples
- Calcite Mexico, China, United States
- Dolomite Italy, Switzerland, United States
- Siderite China, Peru, Germany
- Rhodochrosite Argentina, Peru, South Africa
Skeletal
Similar to hopper crystals, skeletal crystals have a porous, lattice-like structure with voids and cavities. However, in skeletal crystals, the voids are often interconnected, giving the crystal a more pronounced skeletal appearance. This can occur in various mineral species under specific growth conditions.
Examples
- Quartz Brazil, United States
- Calcite Mexico, China, United States
- Garnet India, Brazil, United States
- Bismuth China, Peru, Germany
Spherulitic
Crystals that grow radially outward from a central nucleus, forming spherical or globular aggregates. Spherulitic structures can consist of fine fibers, needles, or radiating crystals and are commonly found in volcanic rocks and certain minerals.
Examples
- Spherulitic Quartz Brazil, United States
- Rhyolitic Obsidian United States, Mexico, Iceland
- Orbicular Jasper United States, Madagascar
- Agate Brazil, Uruguay, Mexico
Stalactitic
Crystals that form in elongated, cylindrical shapes resembling icicles or stalactites. Stalactitic formations often occur in caves or other environments where mineral-rich water drips or flows over surfaces, depositing minerals layer by layer.
Examples
- Calcite Mexico, China, United States
- Aragonite Mexico, Spain, Italy
- Malachite Democratic Republic of the Congo, Australia
- Chalcedony Brazil, United States, Madagascar
Tabular
Crystals with flat, plate-like shapes. They are often formed when crystal growth is restricted in one direction.
Examples
- Gypsum Mexico, Spain, United States
- Halite United States, Germany, Poland
- Barite Morocco, United States, China
Trigonal
Crystals with threefold rotational symmetry, resulting in triangular or hexagonal shapes. Trigonal crystals often have a prism-like form with triangular faces or a combination of triangular and hexagonal faces.
Examples
- Quartz Brazil, United States
- Tourmaline Brazil, Afghanistan
- Beryl/Aquamarine Brazil, Afghanistan
- Cassiterite China, Bolivia, Malaysia
Twinned
Crystals that exhibit twinning, a phenomenon where two or more crystals grow together in a specific orientation. Twinning can produce intricate intergrowths and repeated patterns, adding complexity to the crystal's shape. Common types of twinning include penetration twins, contact twins, and polysynthetic twins.
Examples
- Calcite Mexico, China, United States
- Feldspar Brazil, Madagascar, United States
- Spinel Burma, Sri Lanka, Tanzania
- Aragonite Mexico, Spain, Italy