FMF - Friends of Minerals Forum, discussion and message board

[severin]

Hi everyone, I am a total newbie to serious mineral collecting, but I have had this particular specimen for many years and was wondering if anyone might be able to provide me with some information about it.
It was dug from the ground by a backhoe and recovered by my father who was a construction inspector during installation of a new water main. The collection location was in the Lebanon, NH area. He then gave it to me as I loved collecting neat minerals.
I grew up in the Keene, NH area and brought this specimen to the "famous" Gilsum Rock swap which was a short 10 min drive from home, and an annual favorite for myself and my father.
I was told that the stone originated from a volcano in NY state, I questioned this reasoning given the distance from NY, and thought it may be more likely that it was formed from a much more local, now extinct volcano near Claremont, NH, Mount Ascutney.
Any other information you may have would be greatly appreciated. Thanks in advance!

Here are some pics......

[gemlover]

Looks a lot like the Garnets and Anorthosite found in the Adirondack Mountains in New York. Gore Mountain has commercial garnet mine on it, or at least it did in 1968 when I was doing geology studies in that area.

John

[GneissWare]

[Tracy]

Looks very much like Gore Mountain material to me. Gore Mountain has ceased its mining operations but you can tour the quarry and hunt for "souvenirs." It used to be a major source of industrial-grade garnets for the region. I have a bunch of pieces from that site, collected in the late '80's.

- Tracy

[John S. White]

Very likely almandine (a garnet species) with amphibole from Hanover, New Hampshire.

[Pete Modreski]

I don't know about the possible NH garnet locality that John suggests (and John generally knows the accurate info on most everything), but I would also vote for saying it's from the well-known New York location; the Barton garnet mine, Gore Mountain, New York, in the Adirondacks; almandine garnet with horblende. The garnet has been mined for many years to be crushed for abrasives, and innumberable geologists and rock collectors have visited it; I wouldn't be surprised to find a large specimen like this anywhere in New England, or anywhere in the country for that matter. It's a really neat locality--the huge (but all full of parallel fractures and no crystal faces) garnet crystals are very abundant. The rock was originally a gabbro (igneous rock), now metamorphosed to an amphibolite; composed of amphibole (hornblende) plus garnet, some plagioclase feldspar, and here and there some crystals of greenish-brown hypersthene. The garnet is basically all too fractured to be of any serious gem use.

[John S. White]

Here, I would have to say, is the definitive answer. The following comments are from Steve Chamberlain, the impresario of the Rochester Mineralogical Symposium, and an acknowledged expert on the minerals of the state of New York.

From Steve:

It is virtually certain that the specimen in question originally came from either Gore Mountain or the adjacent Ruby Mountain in Warren County, NY. Much of each of these mountains has been mined away to produce garnet for use as an abrasive. The garnets here reach 1 meter in diameter in a dark green to black matrix. The matrix is mostly an amphibole, formerly called hornblende, and an iron-rich enstatite, formerly called hypersthene, along with plagioclase feldspar and inclusions of cristobalite.

The Barton mine has been open for field collecting for decades and specimens, some quite large, have been carted away by collectors for various purposes, including decorative use in the landscape, which I suspect may be how this specimen came to be subsurface to be "dug up" in New Hampshire.

The garnet crystals are heavily fractured and are very gemmy in zones to several cm--but too dark to be useful for gemstones of any significant size. The New York State I issue of Rocks and Minerals has an article on the Gemstones of New York State that shows two faceted garnets from this occurrence in the collection of the New York State Museum; however, such stones are rather rare.

There is a nice web page from Smith College on the locality, with emphasis on the composition of the garnets (which are almandine, but barely so), that gives a pretty readable presentation of the origin of this deposit.

See

<http>

Steve Chamberlain

[Pete Modreski]

Thanks for sharing Steve's extra info, John.

And, I can't resist--for those (as Steve) who want to point out the correct mineral species terminology, that's fine, but for me, there will always be a time and place to use the precisely defined chemical end-member species designations for minerals, but also a place to continue to use the common "field names" to describe what minerals are in a rock; so, for me,

Viva hornblende, hypersthene, biotite, lepidolite, and all the rest! (And "garnet" for that matter, too!)

Cheers, Pete