Garnet's chemical structure consists of isolated silica groups (SiO4) that have various metal atoms arranged around them.
The six main garnet minerals belong in two groups: the aluminum-silicate garnets are the pyralspite group (from pyrope, almandine and spessartine) and the calcium-silicate garnets are the ugrandite group (from uvarovite, grossular and andradite). Within each group the compositions blend smoothly between members, so classifying a garnet specimen is often an arbitrary exercise best left to experts. For the rest of us, the best way to tell is to note the color and the rocks in which the garnet is found.
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