**About these garnets
These garnets are Pyrope-almandine solid state solutions
(Mg3Al2Si3O13 to Fe3Al2Si3O12) with chromium (Cr3+)impurities. Calcium is
also likely to subsitute in for the aluminun, since it has nearly the same
size (calcium is .01 A larger.) The first
metal site is octahedral, the second tetragonal dodecaheral. The chromium
is likely subsittuting in the octaheral site due to its outer electrons.  
RAMAN. These crystals are associated with diamonds. These particular
samples were
anthill garnets in Arizona and pectite garnets in Namibia. Garnets are
cubic and can be polished in any orientation. Samples were polished from
1500 microns to 150 microns. Some of the thinner samples exhibited a color
change while on a hotplate about 80 degrees C.

**What have observed

Have taken visible spectra and found absorption peaks at about 400 nm and
570 nm. When the sample is heated, the 400 nm peak shifts slightly and the
570 peak shifts more. Also the band at 570 widens. Overall the graphs
appear to move to the right, from lower to higher wavelength, or
equivlanetly, to lower energies. As observed by eye, the sample starts out
a reddish purple. With heating, the sample becomes slighly orange, then
grey, then finally an intense dark green.


**What this means
By heating thei pyrope, we are allowing it to move to a lower energy
level. HEating it causes the crystal lattice to expand. Before the metal
cations were sharing electrons despite the repulsion between the cation's
electrons and the silcon tetrahera electrons. Now that the crystal lattice
ahs expanded, the electrons will be contiuned to be sahred, only the
repulsion is now less. The ligands between teh differend atoms has
lengthed. We're most concerned with chromium, so let's see what its
electrons are doing.

Cr3+ has 3 outer electrons in its 3d3 orbital. Their spins are
identical. So there is a triplet
excited state where any one of these 3 electrons moves to a higher energy
state while retaining its spin orientation. These three main bands are the
most proabble excited states. The two large bands in the visable (400 and
570nm) are probably two of these. The third is the UV. (how know this?
can calculate?) Charts of Cr3+'s energy levels can be found. However term
diagrams with a perfect octahedral are errenous. But can also find term
diagrams with distorted octahera. (what are radius sizes of all these
cations??)