Understanding Gemological Formulas
Step 1: Introduction to Gemology
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.
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).
However, if the formula reads (Li, Na) Al (PO4) (OH, F), we have a new species, montebrasite, in which hydroxyl (OH) exceeds fluorine (F).
Furthermore, if the formula is (Na, Li) Al (PO4) (OH, F), then sodium (Na) exceeds lithium (Li). Thus, we class the mineral as yet another species, natromontebrasite.* Obviously, the degree of complexity associated with solid solutions can be very great. Any substitution of elements on a crystallographic structural site may (or may not) have an effect on physical properties.
Impurities can also affect properties. A good example is beryl, Be3Al2Si6O18. Beryl often contains elements such as iron (Fe), manganese (Mn), chromium (Cr), vanadium (V), and cesium (Cs). Since these elements are usually present in such small quantities, they’re not written into the gemological formula. However, there are exceptions. For example, when Cr is present in a beryl sample (giving it the rich green color we know as emerald), gemologists know to substitute Cr for aluminum (Al) in the formula.
A detailed knowledge of chemical substitutions and color changes in crystals requires a much greater sophistication in crystal chemical principles than I can expound here.
* Editor’s note: As of 2006, the International Mineralogical Association (IMA) no longer considers natromontebrasite a mineral species. It is now considered a mixture of amblygonite, lacroixite, and wardite.