I may have missed the GSA meeting, but…

I just saw a post by Life in a Plane Light which really makes me wish that I had been able to attend the Fall GSA meeting this year. As a metamorphic petrologist I love garnet. Sure, it is a pretty mineral, but the usefulness of this mineral in learning about the history of metamorphic rocks is what makes it truly fascinating. Fortunately, while I may not have been able to attend the session on Garnet and Its Use in Unraveling Metamorphic and Tectonic Processes , the abstracts are available on line to read at will. I've just done well more than my 1000 words of reading from the geologic literature reading these abstracts, and wondering why I hadn't thought of looking at the program sooner. Just because one can't attend a meeting doesn't mean one can't benefit from the information that is shared there.

Why Garnet?

A friend of mine recently asked me “Why garnet?”, and it occurred to me that others might also like to hear one petrologist’s thoughts as to why it is such a well-studied metamorphic mineral.

1. They are pretty!

2. They are very common in a wide rang

e of metamorphic rock types.

3. They are stable across a reasonably broad range of pressures and temperatures of relevance for metamorphism.

4. The often form “porphyroblasts” (metamorphic crystals that are noticeably larger than those which surround them).

5. They are easy to identify in hand-samples—their nice “garnet-red” colour often contrasts with the other minerals in the sample (though some of the less common varieties come in other colours, including green and yellow).

6. They are easy to identify in thin-section (a slice of rock only ~3 microns thick, which means that light transmits through most of the minerals so that one can look at it in an optical microscope): they have a high “relief” (they look like they are taller than the things next to them, even though they aren’t) and they are isotropic (they are solid black when the polarizing filters are crossed, no matter how the stage is turned, making them stand out against the changing bright blues, pinks, and yellows that the other minerals become when the polarizing filters are crossed).

7. They have a rather broad range of possible chemical compositions, with iron, magnesium, manganese, and calcium all able to slot into the same position in the crystal structure (this is part of what gives it a broad range of stable temperatures and pressures) and aluminum and silica can also do a certain amount of swapping one for the other. There are a handful of other, less common elements which can also substitute for others in its crystal structure.

8. They have very slow diffusion, which means that once they reach a certain size the center of the grains no longer get involved in chemical reactions. As a result it is normal for the composition of garnets to be “zoned”, with the center containing more Mn than the rims, and the rims containing more Mg than the core (each of the other major elements also typically change their concentration from core to rim).

We metamorphic petrologists talk about the garnet cores being “armored” by the rims. The rims are, in theory, in equilibrium (or trying to achieve equilibrium) with the matrix minerals at any given time—this means that the minerals present will be participating in the chemical reactions that are causing the growth of some minerals and the dissolution of others. For many minerals the normal grain size is small enough that the reactions involve the entire grains, but garnets often grow large enough that only the outermost shell is involved in the reactions, with the inner portion “freezing” in whatever composition was stable when it was the outer portion.

So, just as an Everlasting Gobstopper (do they still make those candies?) changes colors as you suck on it, so garnets show a range of compositions from core to rim. Part of the changes in garnet composition are due to rare ingredients having been used up making garnet (plus or minus any other zoned minerals present). So Mn, which tends to prefer garnet to any other mineral in metamorphic rocks, starts out “high” in garnet, but there is usually so little of it available in any given metamorphic rock it is soon used up and the garnets have gradually less and less Mn as they grow, until eventually the outer portions have no measurable Mn at all. The other reasons garnets change their composition is due to changes in pressure or temperature. Different recipes of garnet are stable at different pressures and temperatures. So if the conditions change different types of garnet grow on the outside of the pre-existing garnet. These features all combine to make it a very well-studied mineral because of all of the inform

ation one can extract about the history of the rock.

A large garnet in the wild (southwest coast of Tasmania, photo taken by Andrew McNiel):

Garnets (2mm) in thin-section from Collingwood River, Tasmania (also shown are biotite (brown), muscovite (pale but wavy lines), quartz (colorless and without lines + dots in the garnet):

Same garnets as above, but in crossed-polarized light:

I couldn’t help myself

This morning one of my friends posted as her facebook status update the comment "Tell me something I don't know..." "Without mucus, your stomach would digest itself." "Ok, tell me something ELSE I don't know. Something less... disgusting...".

Since she isn’t a geologist, I couldn’t resist typing up the following paragraph to share with her, as something she (and, likely, most of her other on-line friends) probably didn’t already know.

The presence of even a small amount Mn lowers the temperature at which garnet first starts to crystallize in a metapelitic rock; Mn is preferentially incorporated into garnet as compared to the other minerals. It substitutes into the same position in the garnet crystal structure as Fe, Ca and Mg (all of which are usually far more common). As a result the earliest garnet grown in a metamorphic rock is usually the highest in Mn-concentration, and as the crystal grows and depletes the reservoir of Mn its composition changes, gradually incorporating less and less Mn and more and more Fe into its crystal structure. The analysis of a typical crystal of garnet in such rocks will usually show a bell-shaped curve for Mn—decreasing in quantity towards the edges of the grain, while Fe increases. (Ca and Mg are also usually zoned, but they tend to respond more to changes in pressure to dictate which has the greater concentration.)