The word garnet does not denote a single mineral but rather a group of minerals which have similar crystalline structures and physical properties. The garnets generally fall into two chemically similar groups - the pyralspites and ugrandites.
The pyralspite garnets form the first group. These are magnesium (Mg), iron (Fe), and manganese (Mn) garnets which all have aluminum (Al) as the second element in their formula. There are three common pyralspite mineral species:
The name pyrope derives from the Greek words for "fire" and "eye" due to its deep red color. Good quality pyropes can be cut into ruby-like gems.
Pyropes are primarily associated with ultramafic igneous rocks (mantle peridotites) and ultrahigh-pressure metamorphic rocks (eclogites) formed during the deep subduction of basaltic seafloor.
Pyrope can be difficult to distinguish from almandine garnets since they form a solid-solution series between each other. Pure pyrope and pure almandine are the exception, typically pyrope has some Fe and almandine has some Mg mixed into the minerals.
The name almandine (sometimes called almandite) comes from Alabanda, a location in ancient Asia Minor (modern Turkey). It's also a deep red, sometimes looking purplish, and, as mentioned above, sometimes difficult to distinguish from pyrope.
Almandine is the most common type of garnet and typically the garnet found in garnet schists. The deposits at Gore Mountain, NY (the subject of a future post) are mostly almandine and mined for use as an industrial abrasive.
Spessartine (sometimes called spessartite) is named for Spessart in Bavaria, Germany. Spessartines come in a variety of colors but the most striking are orange.
Spessartines form a solid-solution series with almandine and can thus have Fe mixed into them (they're the reddish spessartines).
Spessartine garnets are most often found in granite pegmatites or Mn-rich metamorphic rocks.
The ugrandite garnets form the second group. These are chromium (Cr), aluminum (Al), and iron (Fe) garnets which all have calcium (Ca) as the first element in their formula. There are three common ugrandite mineral species:
Andradite was named for the Brazilian mineralogist José Bonifácio de Andrade e Silva (1763-1838).
There are three color varieties of andradite - black (melanite), green (demantoid), and yellow-green (topazolite). Demantoid (the green is from chromium) is very rare and an expensive gemstone.
Andradite also forms in contact-metamorphosed limestones.
Since garnets were values since ancient times as attractive gemstones there are a host of confusing names (some technically incorrect today) referring to all of the different varieties of these six basic garnet species.