The Chemistry of the Garnet Group
Spessartite Garnet
Garnet is a group of minerals. Scientists classify minerals into groups, species, and varieties. A group is the most inclusive of these classifications. It constitutes a collection of minerals that share similarities in their chemical composition, crystalline structure, and other natural properties.
Garnet’s chemical formula includes the silicate molecule (SiO4). As a group, garnets share the same general chemical formula: X3Y2(SiO4)3. The species and varieties of garnet are distinguished by the incorporation of various metal ions such as iron (Fe), aluminum (Al), magnesium (Mg), manganese (Mn), and chromium (Cr). Some garnets also contain calcium (Ca).
Andradite Garnet var. Demantoid
When different atoms are substituted into the general chemical formula X3Y2(SiO4)3, different species of garnet are created. Given all the possible chemical substitutions within the garnet group, a potential for up to 60 garnet species exists, but to date only about 20 of these species are known in nature. Many of these are rarely encountered as mineral specimens, and most of them are not gem quality.
Almandite Garnet
Six garnet species are important in the gem and jewelry industry: pyrope, almandite, spessartite, grossularite, andradite, and uvarovite.
Pyrope, almandite, and spessartite belong to what are known as the aluminum garnets. Sometimes this cluster of garnets is called the pyralspite (pyrope-almandite-spessartite) series. In this series, Y is always aluminum (Al), and X is magnesium (Mg) in pyrope, iron (Fe) in almandite, and manganese (Mn) in spessartite.
Andradite Garnet var. Topazolite
Andradite, grossularite, and uvarovite belong to what are known as the calcium garnets. Sometimes this cluster of garnets is called the ugrandite series (uvarovite-grossularite-andradite). In this series, X is always calcium (Ca), and Y is aluminum (Al) in grossularite, iron (Fe) in andradite, and chromium (Cr) in uvarovite.
Garnet Chemistry:
General Chemical Formula: X3Y2(SiO4)3
Pyralspite Series (Aluminum Garnets):
Pyrope: Mg3Al2(SiO4)3
Almandite: Fe3Al2(SiO4)3
Spessartite: Mn3Al2(SiO4)3
Ugrandite Series: (Calcium Garnets):
Grossularite: Ca3Al2(SiO4)3
Andradite: Ca3Fe2(SiO4)3
Uvarovite: Ca3Cr2(SiO4)3
Uvarovite Garnet
Garnet chemistry is further complicated by the fact that when garnets are found in nature, they are usually mixtures of two or more species. Therefore, the garnet species listed above should be thought of as theoretically pure end members of a very complex group of minerals.
Grossularite Garnet var. Hessonite
Garnet mixtures are possible because chemical elements substitute for one another in the mineral’s crystalline structure. This interchange of elements is very extensive within each of the two series (especially the pyralspites), but it also occurs to a lesser degree between the two series.
Grossularite Garnet var. Tsavorite
Intermediate stones composed primarily of pyrope-almandite, pyrope-spessartite, almandite-spessartite, grossularite-andradite, and uvarovite-andradite mixtures have all been found, with other species elements being incorporated to some degree as well. The chemical formula for the mixed or intermediate species does not correspond to the pure formulae listed above. The physical properties of these transitional garnets also show a continuum of expression as well.
Rhodolite Garnet
Because of their popularity, some of the mixed, or transitional, stones have been given trade names so that they can be readily marketed. Since the use of trade names is somewhat controversial, a few experts advocate a separate species or varietal designation for these stones instead. An example of a commercially important garnet intermediate is rhodolite, a reddish-purple garnet that is a mixture of pyrope and almandite. Other important trade names for transitional species include malaia and color-change garnets. When it comes to garnet nomenclature, another controversial gemstone is hydrogrossular garnet. While most gemologists consider hydrogrossular to be a translucent or opaque variety of grossularite, it is often listed as a garnet species in its own right.
Pink Hydrogrossular Garnet