Ron is posting a sequence of pictures of rocks on his desk (“Deskcrops”) and today’s struck a chord. I first went to Gore Mt. on a mineralogy trip in 1997 on one of those typical northern New England / New York spring days. We had sun, rain, snow, and cloudy conditions during at least one point on the trip (several of them while actually in the upper mine!). I was wearing my green rain jacket, which to this day still has garnet shards in one of the upper pockets due to all of the small garnets I picked up while in the mine 🙂
My return to Gore Mt occurred as part of a trip at Vassar in 2007 under slightly more favorable weather conditions (sunny, but cool). I have no idea whether I took pictures during the first trip–if I did, I can’t find the prints / slides–but during the second trip I ran around trying to look all the different garnet textures present. So, I’ll post a few pictures from the trip & try to explain what’s going on. (Apologies for some of the picture quality–this was the first semester with my new camera & I was still figuring some things out.)
As Ron mentioned, the garnets have several different types of rims surrounding them. In some cases, you can see differences for the same garnet:
The variety of rims are most likely due to variances in the bulk composition of the rock as you move around the outcrop. In rocks that are more hornblende-rich, all of the plagioclase would have been used to form garnet leaving hornblende rims around the garnet. In regions where the rock was more plagioclase-rich, garnet growth would have consumed a large percentage of the present hornblende and left a plagioclase rim around the garnet.
Besides the variation in rims (both in composition and width), the other thing that fascinated me on my second trip was the variance in garnet sizes.
I believe the size variation is due to the presence / relative absence of fluids in the system during garnet growth. In situations where diffusion is relatively quick (high temperature and/or high amounts of fluid present), large garnet porphyroblasts are more likely. When the diffusion is slower (lower temperatures and/or low amounts of fluid present), the various components that are needed to grow garnet can not diffuse as far and thus add onto the closest garnet they can find resulting in the growth of lots of little garnets. So, some of the rock had a larger amount of fluid available (large garnets), while other regions were drier. The assumptions I’m making is that all of the garnets grew at about the same time in the rock (which is probably fair since there aren’t any major faults) and that we had lots of nucleation sites (probably due to rapid overstepping of the garnet-in reaction). In the larger garnet region, many of the nucleation sites simply didn’t get enough components to keep growth and ended up dissolving back away or merging into another larger garnet.
At Gore Mt, the garnets are simply everywhere, which is why the mine has been used a prime source of non-jewelry grade garnets. The garnets have been used for a variety of purposes including sandpaper (garnet has a hardness between 7.5 & 8.5 depending on composition, so its harder than quartz sandpaper) and other abrasives. Maybe my favorite application is that you can buy slabs for kitchen counters:
Gore Mt is open to the public, but you will be taken to the lower mine on the general tour. Geology groups should contact the mine directly to arrange to access the upper mine (which has better garnets & exposure in my opinion).
Final parting shot is of the floor of the upper mine, but both look like this. When the sunlight hits the garnets, the floor of the mine looks blood red. Very, very cool.