GO 340 Gemstones & Gemology

Emporia State University

www.emporia.edu/earthsci/amber/go340/chemical.htm

Crystal Chemistry

An introduction to crystal chemistry helps in understanding what gems are made of and how this affects the gemstone environments and associations. Most gemstones are minerals, that is inorganic crystal treasures found in natural world.

Composition...

A mineral is a homogeneous solid, that grows in a symmetrical form, as a result of the regular geometrical arrangement of atoms, ions, and molecules. A mineral's make-up or composition is expressed in a shorthand manner with a chemical formula derived from quantitative chemical analysis, which shows the amount of each type of element present. Chemical testing can be destructive. Today the composition of gems is often determined with an electron microprobe or visible, ultraviolet, and infrared spectroscopy, and these specialized instruments can detect elements even in trace amounts. The chemical make-up aids in identification and classification, while distinguishing natural from synthetic materials.

Atoms and Ions...

Atoms are the units that all matter is composed of, the smallest subdivisions that retain the characteristics of elements. Fast-moving electrons are in energy levels or orbital shells around the atomic nucleus. The orbital shells farthest from the nucleus are incompletely filled and electron movement between energy levels accounts for optical properties such as color, fluorescence, and phosphorescence. Electrons in the outermost shell are the valence electrons and when in the most stable configuration, these shells are filled. The outer shells are filled by gaining or losing electrons, creating a positive or negative charge and cation or anion.

Classification...

Minerals are arranged into groups based on dominant anion (nonmetal) or anionic group. Minerals within the same chemical group have similar chemical properties, origins and occurrences, and physical properties.

Chemical Classifications
Chemical Class Anion or Anionic Group An Example
Silicates Silicon and Oxygen Tourmaline, (Mg,Fe)2 SiO4
Oxides Oxygen Corundum, Al2O3
Carbonates Carbon and Oxygen Rhodochrosite, MnCO3
Native Elements One element, such as Carbon Diamond, C
Sulfides Sulfur Sphalerite, ZnS
Halides Halogen ions, such as Fluorine Fluorite, CaF2
Phosphates Phosphorus and Oxygen Apatite, Ca5(PO4)3 (F,Cl,OH)
Sulfates Sulfur and Oxygen Gypsum, CaSO4 2H2O

Chemical Bonding...

The forces that bind atoms, ions, or ionic groups together in crystalline solids are electrical, with their type and intensity responsible for the physical and chemical properties of minerals. The stronger the bond the harder the crystal and higher the melting point. The high hardness of diamond is because of the strong electrical bonding forces linking the carbon atoms. These electrical forces holding inorganic minerals together are chemical bonds, such as: ionic, covalent, van der Waals, metallic, hydrogen, or some combination.

Ionic Substitution, Solid Solution, Exsolution...

The solution or melt in which the mineral crystallizes can contain many elements not primary to the chemical composition. Such additional elements can be present in the crystal structure in minute amounts substituting for a major element within the mineral. This ionic substitution can cause color, such as chromium present in emerald (variety of beryl) creating green and iron present in aquamarine (also a variety of beryl) creating blue. When ionic substitution is extensive, it is termed solid solution. Substitution is common if the ionic radius differs by less than 15% assuming the overall neutral charge of the mineral is maintained. An example is with the olivine group of minerals, where forsterite is a magnesium silicate, and fayalite is an iron silicate. The iron and magnesium substitute for one another because they have like charges and similar ionic radii size. "With no iron, forsterite is colorless, but with increasing iron the mineral darkens, going from light-toned olive green to dark green to black in fayalite" (Hurlbut and Kammerling, 1991, p. 30). The gem variety of olivine, peridot, has 10% magnesium of forsterite replaced by iron.

Exsolution is responsible for adularescence and asterism in gemstones. When minerals crystallize at high temperatures, high internal thermal energy allows for less stringent space requirements and ionic substitution is extensive (Hurlbut and Kammerling, 1991, p. 30). When the mineral cools, the poorly fitting ions are forced to migrate through the crystal structure and a type of unmixing occurs. For example, a potassium-rich feldspar, called orthoclase, can tolerate sodium replacement of potassium at high temperatures, but forces these ions to migrate forming small localized areas of a sodium-rich feldspar, called albite. These pockets of albite intertwined with orthoclase result in an optical phenomenon called adularescence, which is an overall shimmery blue-white glow and localized flashes of color. This exsolution interaction gives the schiller or adularescence phenomenon to moonstone.

An example of asterism is found in corundum and referred to as star ruby and star sapphire. The aluminum and oxygen of corundum can accomodate titanium substituting for aluminum in the crystal structure. Upon slow cooling the titanium reacts with the oxygen producing needle-like crystals of the mineral rutile. The hexagonal crystal structure of corundum constrains the rutile crystals to orient 60 degrees to one another and if enough are present when the stone is cut en cabochon (a smooth convex top) perpendicular to the long c-axis direction, the star or asterism will result (Hurlbut and Kammerling, 1991, p. 30). Some corundum with titanium can be heat treated, slowly cooled and enhance the asterism, while some corundum is heated and cooled rapidly to reduce the star effect and improve the transparency of the gem.


Milky white adularescence
of moonstone.


Star ruby showing asterism.
Image taken from the Mineral and Gemstone Kingdom.

Required reading! Go to:

  • What is a crystal? This lecture from Jill Banfield's Gems and Gem Materials course at UC Berkeley is found at http://socrates.berkeley.edu/~eps2/wisc/Lect4.html

    Optional and fun information on chemical elements:

  • WebElements Periodic Table at http://www.webelements.com/index.html from the University of Sheffield.

    The material for this section came primarily from:

    Return to the Syllabus or move on to the next lecture.

    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: 1999; last update: January 14, 2008.

    Copyright 1999-2008 Susan Ward Aber. All rights reserved.