Crystal Form
Image credit: R.Weller, Cochise College. http://skywalker.cochise.edu/wellerr/mineral/minlist.htm
The Bottom Line on Crystal Form - Here on Top!
Minerals are crystalline solids that grow into distinctive geometric forms. However, there are over 5,000 known minerals, but only some 48 ideal crystal forms. When well-developed and larger in size, the form of a crystal is a beautiful object and highly valued.
Seed (microscopic) crystals grow larger from the chemical bonding of ions and molecules in a liquid solution to a pre-existing solid surface. Because of variation in the chemical and physical environments in which crystals grow, crystals can develop different geometric forms. As a result, we can't just look at a crystal, recognize its form (say, cubic) and use this feature to positively identify that one mineral by name. For example, galena and halite both form cubic crystals.
Although crystal form is not a defining property of a particular mineral, it is such a beautiful expression of nature that it is worth our study to connect some geometric discoveries by ancient Greeks with our modern knowledge of mineral structure.
As a practical matter, in many student kits mineral samples may not show well-developed crystal form (e.g., cheaper samples!), or a sample may be an aggregate of microscopic crystals too small to display the crystal form. These circumstances may make it difficult or impossible to identify crystal form.
Bottom line: Crystal form is an expression of the internal geometry of the crystalline structure, but it may not be well enough developed or large enough for us to recognize in a student sample to aid in mineral identification. Therefore, we won't use crystal form for identifying a mineral, except in combination with other properties.
Errors/Suggestions: Contact Scott Brande (see footer).
Crystal Form
One of the defining characteristics of a mineral solid is its crystalline structure - crystallinity. This feature means a geometrically regular (long distance) arrangement of the ions (charged atoms) and molecules. When individual crystal seeds grow larger without significant interruption, a macroscopic (visible) crystal may exhibit a geometric shape. Consult your textbook or other resources for additional details.
For world-class specimens, check out the Mineral Gallery at the Smithsonian Institution.
Platonic Solids - Distinctive Crystal Shapes
Although there are over 5,000 minerals known on earth, there aren't 5,000 differently shaped crystals. Many mineral crystals take the geometric form of one of the fundamental Platonic solids, three dimensional forms with flat surfaces (faces) all of the same shape, such as triangles, squares, and pentagons. Here are some examples.
Tetrahedron - four faces, each a triangle of equal side length. Click image for rotating animation.
Cube - six faces, each a square. Click image for rotating animation.
Octahedron - eight faces, each a triangle of equal side length. Click image for rotating animation.
Dodecahedron - 12 faces, each a pentagon of equal side length. Click image for rotating animation.
Platonic Solids Animation Credits: Tetrahedron: User Cyp on en.wikipedia (https://commons.wikimedia.org/wiki/File:Tetrahedron.gif ), „Tetrahedron“, https://creativecommons.org/licenses/by-sa/3.0/legalcode; Cube: user:Kjell André (https://commons.wikimedia.org/wiki/File:Hexahedron.gif ), „Hexahedron“, https://creativecommons.org/licenses/by-sa/3.0/legalcode ; Octahedron: Cyp (https://commons.wikimedia.org/wiki/File:Octahedron.gif), „Octahedron“, https://creativecommons.org/licenses/by-sa/3.0/legalcode ; Dodecahedron: User Cyp on en.wikipedia (https://commons.wikimedia.org/wiki/File:Dodecahedron.gif ), „Dodecahedron“, https://creativecommons.org/licenses/by-sa/3.0/legalcode
Visualization of Crystal Form
The platonic solids are ideal forms as each face is the same shape and has the same side length (e.g., square, equilateral triangle). Crystals may grow into forms with different angles and face shapes. Thanks to Mark Holtkamp, who created and posted 3D visualizations of all 48 possible crystal forms. Browse Smorf Crystal Forms for more examples.
Trigonal Prism. The term "prism" means the crystal is elongated in one direction. This trigonal prism has triangular faces on each end of the elongated middle (the prism part). The prism has three sides (faces), one for each side of the triangles at the ends. The prism faces are each rectangular in shape. Both triangular ends may be either an isosceles (two equal length sides) or equilateral triangle (three equal length sides).
Hexagonal prism. Here the elongation connects hexagonal (six) sided ends. The faces on the elongated middle number six, one for each side of the hexagons on the two ends. The shape of each prism side (face) is a rectangle.
Rhombic dipyramid. A "dipyramid" means two pyramids, one at each end of the prism. Each pyramid five-sided (four sides and a base). Each pyramid has four triangular faces and a four sided (rhombic) base (not 90 degree angles at the corners). The two pyramids are joined base-to-base so the pyramids point in opposite directions.
Rhombic dodecahedron. "Dodecahedron" means 12 (2+10) faces, each a rhomb (four sides not at right angles). Can you count the 12 sides as the model rotates?
"Real World" (Physical) Crystals
Physical crystals, the ones we handle, grow in nature within a tremendous variety of chemical and physical environments. Differences in these environments may be caused by variation in the chemical composition of the fluid solution from which crystals precipitate (for example, including major and/or trace elements), interference of growth by adjacent solids (for example, including other crystals or the wall rock), and many other factors.
As a result, the same mineral may grow into what appear to be different geometric forms. However, each mineral species exhibits the same underlying fundamental characteristics. In the science of crystallography, the crystal is a physical expression of the geometric latticework of its ions and molecules held in solid form by chemical bonds.
Here are some examples of nature's variation in one of quartz - one of the most important of minerals in modern civilization.
Quartz
Sometimes called "cathedral" quartz, this form is named for its similarity in form to the tower of a cathedral because the central large crystal is surrounded by smaller crystals.
Image credit: Amir Akhavan
Quartz
In the Adirondack Mountains of New York and other regions, quartz crystals precipitated in cavities of limestone or other somewhat soluble rock. These crystals are unusually clear ("water clear") and doubly terminated (points on both ends), making this form a prized possession when discovered and dug from the weathered cavities in the earth.
Image credit: Amir Akhavan
Crystals and Crystalline Aggregates
Mineral crystals form as ions and molecules floating around in a liquid solution become chemically bound to a pre-existing solid surface. Variation in the chemical and physical environment (temperature, concentrations of ions and molecules, etc.) control the growth rate of crystals. Single large crystals form, for example, when variations are minimal during growth and there are no interfering objects. Masses of microscopic crystals may form, for example, when precipitation rates and concentrations of solutes are high.
Single crystal form may be well developed in some mineral samples, while in others, a sample may simply look like a fine-grained lump of rock because the crystals are all microscopic in size.
What Is A Single Crystal?
A single crystal is an inorganic, naturally formed solid with
a specified chemical composition, and
a repeating and ordered geometric arrangement of ions held in place by chemical bonds
A single crystal
Image credit: Scott Brande
A model of a single crystal composed of two different elements.
Image credit: Public domain
Single Crystal Growth
Watch this video of a single crystal growing in volume (the visible change) by the addition of ions from solution (the invisible part). Note that the layers of different faces growing outward eventually meet and intersect at a straight line boundary.
Click the image to play video in separate tab.
Image credit: The Royal Institution, from YouTube: https://youtu.be/4sMPJOuskUU . Accessed [11 July 2019]
Examples of Single Crystals
A mineral sample may be composed of a single larger (macroscopic) crystal. A single crystal is ideal for study.
large enough for observation
exhibits well-developed geometric form(s) [some minerals exhibit multiple forms]
What Is A Crystalline Aggregate?
A crystalline aggregate is a solid composed of a bunch of individual crystals. Here's an example with 11 individual crystals.
each crystalline region shows a differently oriented geometric arrangement
red lines mark boundaries between individual crystals
A mineral sample may be composed of a crystalline aggregate. Such samples are more difficult to study for a variety of reasons.
crystals may be too small to be easily observed individually
multiple crystals meet in different orientations, preventing formation of a single, macroscopic geometric form
Notice that within each crystalline domain the circles are all arranged parallel and oriented in the same geometric pattern. Notice also that the geometric orientation changes across a boundary.
Image credit: Scott Brande
Examples of Crystalline Aggregates
Here are some examples of mineral samples composed of crystalline aggregates. Individual crystals cannot be observed, so properties such as cleavage and fracture cannot be determined. However, other properties, such as hardness, streak and reaction to acid may be determined.
Instructions: How To Recognize Distinctive Crystal Form
Caution - Crystal form is more easily determined when crystals are relatively large, and occur singly or in an aggregate with only a few crystals. When crystals are tiny and tightly clustered, it may be very difficult to observe crystal form without magnification.
Material(s) for test
a well-developed mineral crystal, macroscopic in size (for example, greater than about 5 mm or about 1/4 inch)
Procedure for the test
Observe the crystal
Determine the two dimensional shape of a single face
Determine the number of faces for a complete crystal
Compare the face shape and number of faces to a reference diagram or chart
Possible test results and interpretation
For example (as above), cube, tetrahedron, trigonal prism, rhombic dipyramid, octahedron, dodecahedron, hexagonal prism, etc.