Natural and faceted Quartz, variety amethyst,
from various USA states. Faceted amethyst
in foreground are 18.7, 44.5, and 33.2
carats (from left to right). Photo by
S.W. Aber, 4/2009, taken at Smithsonian
National Museum of Natural History

GO 340 Gemstones & Gemology
ES 567 Gemstones of the World
Dr. Susan Ward Aber, Geologist & Gemologist
Emporia State University
Emporia, Kansas USA

http://academic.emporia.edu/abersusa/go340/crystal.htm

Crystal Structure

An introduction to crystal structure helps in understanding the physical and optical properties of gems that are needed in identification. Mineral gemstones are inorganic crystal treasures found in the natural world. Other gemstones are organic remains of a past life and include gemstones such as amber and jet. Inorganic gems are crystalline, while organic gems are amorphous or without crystalline form. This lecture will focus on explaining the crystalline nature of gemstones.
Image right is natural pyromorphite, crocoite, and amethyst crystals. The crocoite is a lead chromate nd referred to as red lead. This specimen is from Dundas, Tasmania, Australia. Pyromorphite is a lead chlorophosphate from a mine in Kellogg, Idaho, USA. The highest pedestal is amethyst crystal aggregate, likely from Brazil.

Photo by S.W. Aber, 4/2009, taken at
Smithsonian National Museum of Natural History
Minerals are naturally occurring, macroscopically homogeneous solids with definite chemical compositions and characteristic atomic structures. These inorganic substances grow in a symmetrical form and the external shape is a reflection of the orderly internal arrangement of atoms. Well-developed minerals showing crystal symmetry are termed euhedral; poorly developed minerals are still crystalline, but called an anhedral crystal.

Natural diamond showing an octahedral form or equant habit. Image taken from The Image Gallery.
Thus, all minerals have a characteristic crystalline structure, which means they grow in a very regular, symmetrical arrangement. Symmetry refers to a sameness or repetition of pattern and is what distinguishes the orderly crystalline or atomic structure. If the resulting external shape is not defined by a form, which is a perfect reflection of the internal arrangement of atoms, it is defined by the mineral's habit. These terms can be descriptive such as octahedral or prismatic forms and equant or acicular/fibrous habits.
The diamond image is displaying an octahedral form, which could also be called an equant habit. The octahedron is an eight-sided polygon... and equant means the shape is roughly equal in all directions. The two minerals to the right, are illustrating acicular and fibrous habits and a prismatic form!

Crystalline structure is in part responsible for properties such as hardness, cleavage, fracture, specific gravity, and external shape. The specific properties attributed to minerals and gems can be influenced by both crystal structure and composition. Chemically, the size of elements and ions determines the stacking arrangement of the polyhedra shapes of crystals. Structurally, the arrangements are controlled by symmetrical restrictions, yet differing temperatures and pressures can create differing stable arrangements of elements and result in different minerals. When different crystal structures result from arranging the same elements or chemical composition differently, then they are referred to as polymorphs (i.e., many forms, one composition). In contrast, when many different chemical compositions can arrange themselves in a like-symmetry, this is referred to as isomorphism (i.e., one form, many compositions). Finally, crystals may be less stable when exposed to surface conditions and false forms may result. Each of these peculiarities will be expanded on below.


Upper, right image is rutile, with a blonde hair look, included in quartz. The rutile has a prismatic form or acicular, needle-like habit. Lower, right image is Hawk's-eye rough and polished.

structure though. Pseudomorphism means false form or shape. It is when a mineral crystal's chemical composition and/or crystal structure is changed, but the external form is preserved. An example is in the alteration of azurite crystals to malachite, and the replacement of wood by chalcedony to produce petrified wood. A common misconception is that tiger's-eye and hawk's-eye (blue tiger's-eye) are quartz pseudomorphs after the asbestos mineral crocidolite. In fact, the quartz did not replace the asbestos fibers but grew in an intertwined manner as shown by the hawk's-eye rough in image above. Heaney and Fisher (2003) have a new interpretation of the origin of tiger's-eye in that quartz crystal growth is synchronous with the crocidolite through a crack-seal vein-filling process (p. 323).

Image left is azurite-malachite, which are common pseudomorphs; this image does not portray the exact crystal though.

Polymorphism means many forms or shapes. It is when minerals have the same chemical composition but different crystal structures resulting in different minerals. Diamond and graphite are both composed of carbon but obviously different minerals with vastly different physical and optical properties. Graphite is the soft pencil lead and used as a lubricant, while diamond is a hard gemstone and used as an abrasive for cutting in its industrial applications.
Isomorphism means one or the same form or shape. Isomorphic minerals have the same geometric structural arrangement, but with different atoms or ions in the sites that results in different minerals. Minerals with the same anion belong to isostructural groups, such as garnet and spinel groups. These groups have the same structural configuration but a wide diversity in chemical composition. A detailed explanation of isomorphism using the garnet family of minerals is given by Robert James, Your Gemologist, at http://www.yourgemologist.com/IsomorphousReplacement/isomorphous.html.

In spite of all the variations on crystal symmetry and structure there are only a limited number of combinations possible. All of the over 4,000 known minerals, each falls into only one of six different crystal systems.

Image right: These minerals are all in the garnet group; they are different chemical compositions but the same structure or examples of isomorphism. Image taken at Smithsonian; photo by S.W. Aber 2009.

Crystal Systems

Crystal system is a classification scheme developed to explain the orderly atomic arrangement of the chemical components that make up minerals. All minerals can be divided into one of six classifications. These systems are defined by the imaginary crystallographic axes and the angles between the axes.

Crystal Systems from left to right: cubic or isometric, tetragonal, orthorhombic, hexagonal, monoclinic, triclinic. All images above taken from Crystal Systems, http://en.wikipedia.org/wiki/Crystal_system, Daniel Mayer (2007).

The isometric system has three equal crystallographic axes, all at right angles to one another. Typical crystal shapes include a cube, octahedron, and dodecahedron. The tetragonal system has three axes meeting at right angles, but two are the same length, the third is longer or shorter than the other two. These crystals are four-sided prisms and pyramids. The orthorhombic system has three axes meeting at right angles and none of the lengths are equal. The typical shapes are rhombic prisms and pyramids. The hexagonal system has four crystallographic axes, three in one plane, all the same length, and intersecting each other at 120 degrees. The fourth axis is perpendicular to the plane of the other three and a different length than the other three. Typical crystal shapes are hexagonal prisms and pyramids. The monoclinic system has three axes of different lengths, two at right angles to each other and the third is inclined. Typical forms are pinacoids (paired faces) and prisms. The triclinic system has three axes all of different lengths and all inclined to each other. This system is the least symetrical and shapes are usually pinacoids.


Recommended Readings