Understanding Gemological Formulas
by Dr. Joel Arem
Step 1: Introduction to Gemology
Lesson 3
“Quartz Japan Twin Huaron Peru” by Didier Descouens. Licensed under CC By-SA 3.0.
"The chemical composition of a mineral is a primary aspect of its definition as a species. That composition can be expressed in a chemical formula that identifies the elements and proportionate number of atoms in a molecule or chemical compound of that mineral. Gemological formulas are chemical formulas of gems and other minerals of interest in the world of jewelry and gem collecting. For example, quartz has a chemical formula of SiO2, one atom of silicon to two atoms of oxygen.
Substitutions
Many gemological formulas contain elements grouped in parentheses with commas, for example (Fe, Mg). This indicates that either iron (Fe) or magnesium (Mg), or both, may occupy a specific position in the crystal structure. The element listed first within a parenthesis is the one present in a greater amount on the structural site. In some cases this determines the species!
For example, amblygonite is (Li, Na) Al (PO4) (F, OH).
Elements within parentheses in gemological formulas may occupy the same position in the crystal structure of a gemstone. These elements are listed in descending order according to the amounts present at that position. “Amblygonite.” © Rob Lavinsky, http://www.iRocks.com. Used with permission.
However, if the formula reads (Li, Na) Al (PO4) (OH, F), we have a new species, montebrasite, in which hydroxyl (OH) exceeds fluorine (F).
“Montebrasite, Cut” by Enrique Zimbres. Licensed under CC By-SA 3.0.
Furthermore, if the formula is (Na, Li) Al (PO4) (OH, F), then ... "
https://www.gemsociety.org/article/unde ... -gemology/
