Sunday, October 14, 2007

The Barton Garnet Mine and Fall in the Adirondacks

This Saturday I was able to tag along on a mineralogy field trip to Gore Mountain, in the Adirondack Highlands, home of the Barton Garnet Mine. I was especially excited about this, even as far back as my mineralogy course (12 years ago?) I remember seeing samples of these garnets. First a little background.

The Barton Garnet Mine is an open pit garnet mine that operated for more than 100 years (1878-1983). The mining level is now below the water table, so to avoid the costs of pumping water out of the pit the operational mine has moved about 4 miles away. What is left at the original mine is now open to the public. The biggest part of the tourist attraction is the first pit (shown below), where you can search for gem-quality chunks of garnet. The garnets themselves can be enormous (10cm or so), but small chips of them, especially some of the darker red chips, are gem quality, can be cut, and are therefore potentially valuable. That is the main draw it seems, you can keep whatever you find (well, at $1 a pound, but that is almost nothing for chips of garnet).

The picture above is from the main pit, where you can collect the chips. You can see it was a superb fall afternoon. The mine is at about 2600 feet, and with the wind made it pretty chilly, but that just kept other folks away.

The garnets are entirely hosted by a hornblende-rich garnet amphibolite unit that is itself in fault contact with a meta-syenite to the south, and grades into an olivine meta-gabbro to the north. The garnets can be enormous, up to 35cm in diamter, even though the modal percent of garnet is not all that unusual for the Adirondacks (5-20%). I think the most impressive thing about these garnets is how the are often mantled with thick haloes of hornblende (shown below). The larger the garnet, the larger the hornblende halo. The garnets are largely pyrope (Mg-garnet 37-43%) and almandine (Fe-garnet 40-49%)

Although it is pretty and would make an exceptional counter top and/or stone fireplace, the primary reason this garnet is mined is as an industrial abrasive. Garnet is hard (8 or so on Mohs scale, although according to our guide this particular garnet can top out over 9), and because garnet does not have any cleavage (which means it is a self-sharpening abrasive, every time it breaks it is a conchoidal fracture, which creates a new sharp edge), garnet is a very good abrasive. Most sandpaper is made with garnet, the polishes used on the lens of the Hubble telescope were garnet based, most sandblasting is done with garnet, the list is endless. And (again according to our guide), 95% of the world's industrial garnet comes from this mine. The founder of the mine (Henry Hudson Barton) was actually a jeweler who married into a wood-working family, and used his knowledge of minerals to pioneer the use of garnet as the abrasive in sand paper.

Aside from the garnets it was a great day outside. Fall has arrived in the Adirondacks, and the colors were exceptional. Just some pictures from right near the visitor's center at the mine.
To finish with the pictures of fall, I'm just going to end with this. Fall colors always remind me of this poem, and in particular of cruising down M street on the back of my Dad's bike on my way to school.

October's Party by George Cooper

October gave a party;
The leaves by hundreds came-
The Chestnuts, Oaks, and Maples,
And leaves of every name.
The Sunshine spread a carpet,
And everything was grand,
Miss Weather led the dancing,
Professor Wind the band.

The Chestnuts came in yellow,
The Oaks in crimson dressed;
The lovely Misses Maple
In scarlet looked their best;
All balanced to their partners,
And gaily fluttered by;
The sight was like a rainbow
New fallen from the sky.

Then, in the rustic hollow,
At hide-and-seek they played,
The party closed at sundown,
And everybody stayed.
Professor Wind played louder;
They flew along the ground;
And then the party ended
In jolly "hands around."


Chuck said...

Very cool. Although I seem to recall from looking at these rocks in my undergraduate days that the "amph haloes" were actually just areas of plag depletion- where the aluminium had been sucked into the growing garnet.

That was a long time ago, could be wrong.

Awesome fall colors, metamorphic petrology aside.

Kim said...

I've been to that mine! I've got a garnet the size of a softball from there.

The department where I began my teaching career had a bowling-ball-sized garnet from that mine. It was an unusual sample, because all the hornblende had been stripped away, leaving only the garnet.

One late night, the seniors were a bit punchy from too much thesis-writing...

...and they bowled it.

It broke.

It was very sad.

Thermochronic said...

Chuck - I claim no authority, but I'll look into it. I hadn't been sure if I was going on the trip, and had hoped to read the handouts en route, but instead I drove (and DJ'd), and am therefore catching up. Plag depletion..I like that.

Kim - I bet bowling with enormous garnets would be fun (perhaps knocking down spodumene pins) but man, breaking an enormous garnet would not be. I was pretty excited to see the place, I think it is an excellent stop on Adirondack field trips. The guides and mine operators are actually really nice and knowledgeable, and they seem excited for groups of geologists to see things, even abandoned parts of the mine.

BrianR said...

the Adirondacks are great...I went on a week-long canoe trip up in Saranac lake region...beautiful.

nice garnets

Thermochronic said...

I agree, this was my first time really seeing much of the Adirondacks. I have been told there are tons of great hikes, I was really blown away by how many nice little lakes we drive by, combined with the fall colors it was pretty nice. I kind of felt like I was in an LL Bean catalog.

BrianR said...

"I kind of felt like I was in an LL Bean catalog."

you should get some flannel shirts...oh wait

John Van Hoesen said...

Nice pics - the Barton Mine is about an hour and a half away from my house, been over there quite a few times. They are indeed making countertops, I want one!


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