OK, so this is not really geology. YAG is not a naturally occurring mineral, and yttrium is only a REE wannabe. But REE-doped YAG is used in a variety of technological applications that enable geochemical analyses, such as Nd-YAG lasers that form the core of the low-end laser ICPMS systems.
I’m posting because I have heard of YAG for years and years, but never figured out what YAG actually was until tonight, when I got sidetracked in wikipedia.
Garnet is a mineral structure. It is a complex cubic structure that has a formula X3Y2Z3O12. The X site has a distorted 8-fold coordination, the Y site has 6-fold octahedral coordination, and the Z site has tetrahedral coordination. This tetrahedral site is trpically occupied by silica tetrahedrons in the case of most natural garnets, such as almandine, the typical crustal garnet: Fe3Al2(SiO4)3
Figure 1: The garnet crystal structure.Most natural garnet compositions consist of solid solutions between common +2 8-fold and +3 octahedral ions: Mg, Mn, or Ca substituting for Fe, and Fe3+ or Cr substituting for Al. However, coupled substitutions allow for more complicated compositional changes.
The most common coupled substitution is the formation of majorite garnet at high pressure. Pyrope is magnesium aluminum garnet: Mg3Al2(SiO4)3. At high pressure, however, Si can start substituting into the octahedral coordination. Despite this pressure increase, magnesium can also still squeeze into the octahedral site, so the substitution is Mg+Si -> Al2. The resulting mineral, Mg3MgSi(SiO4)3, has a formula unit that simplifies to Mg4Si4O12, or MgSiO3. This is the same formula unit as enstatite (magnesium orthopyroxene), and indeed the transition of enstatite to majorite is one of the main transition zone reactions that marks the change from upper mantle to lower mantle mineralogy.
Yttrium aluminum garnet also has a coupled substitution. Starting with almandine (Fe3Al2(SiO4)3, the Fe is replaced with Y, and the charge balance is maintained by replacing the Si with tetrahedrally coordinated Al. This gives a formula of Y3Al2(AlO4)3, or Y3Al5O12. Substitution of various REE (most commonly Nd) for Y then gives the mineral its unique optical properties via mechanisms which I do not understand.
Rare Earth Revelry