| GO 340 Gemstones & GemologyEmporia State University
|
|
| GO 340 Gemstones & GemologyEmporia State University
|
|
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.
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. 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 left image is diamond 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 equal in all directions. The center and right images below illustrate acicular and fibrous habits. | ||
Natural diamond showing an octahedral form or equant habit. Image taken from The Image Gallery |
Rutile included in quartz resembles blonde hair and has a prismatic form or acicular, needle-like habit. |
Hawk's-eye rough and polished. |
| Therefore, 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. |
| 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). |
| 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. These are introduced below. |
Crystal SystemsCrystal 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. 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. |
Visit...
|
This page originates from the Earth Science department for the use and benefit of students enrolled at Emporia State University. For more information contact the course instructor, S. W. Aber, e-mail: saber@emporia.edu Thanks for visiting! Webpage created: November 15, 2000; last update: January 13, 2009.
The material for this section came primarily from:
Return to the Syllabus or move on to the next lecture.