[Rockhounds] Garnet & Kyanite

[Matthew Manon]

All,

Garnet + Kyanite is a stable mineral in high-pressure eclogites, and =20
forms at the expense of plagioclase according to the reaction:
3 CaAl2Si2O8 =3D Ca3Al2Si3O12 + 2Al2SiO5 + SiO2
anorthite =3D grossular + kyanite + quartz

In the pure system CaO-Al2O3-SiO2, this reaction takes place at =20
something like 16 kbar (800=FBC).  Generally garnets will have more =20
almandine and pyrope components than grossular, so the reaction can =20
be found at even lower pressures.

Kyanite-bearing Eclogites have been reported in many places, such as =20
Scotland (glenelg, Sanders 1988), the Tauren window (Holland 1979) =20
and the diamondiferous UHP Kokchetav massif (Sobolev & Shatsky 1990)

-Matt

On Nov 6, 2007, at 1:39 PM, Lanny wrote:

> Hi John and Carolyn
>
> There are a number of areas in the metamorphic complex that =20
> Freezeout is in that have kyanite and garnet (dominantly =20
> almandine). On Moses Butte, there is a small zone with kyanite =20
> included in garnet. Andalusite, kyanite, staurolite and garnet all =20
> have been reported on the Goat Mountain - Backdome Peak area east =20
> of Freezeout. Can't relate from experience exactly which ones can =20
> be found in the same chunk of rock, except the garnet and kyanite =20
> together there. It seems that when I'm in that area I've got too =20
> big of a to do list and thus haven't ventured far from the road on =20
> that particular ridge.
>
> Mica pseudomorphs after kyanite and staurolite are reported in =20
> several areas of these metamorphics.
>
> Regards,
>
> Lanny
>
> On Nov 5, 2007, at 5:18 PM, John Siebel wrote:
>
>> Freezeout Mountain, Shoshone County, Idaho sports both kyanite and =20=

>> garnet in mica schist. There is staurolite nearby. Lanny could =20
>> certainly elaborate on this as his field guides got us up there.
>>
>> John
>>
>> ----- Original Message ----- From: "Carolyn Reynard"
>> Kyanite forms at a higher pressure and temperature than garnet, so =20=

>> usually they are not together. There are exceptions I have a =20
>> specimen from Russia that has both. I believe that would indicate =20
>> my Russian specimen is right on the boundary ( isograd) between =20
>> the two. (adjacent metamorphic zones on a map are separated by =20
>> isograds or line of equal grade.)
>>
>> Not sure about the alteration of kyanite to mica, but I do know =20
>> cordierite alters to mica, so there is a good possibility. Kyanite =20=

>> is often found  mica schist.
>>
>> Carolyn Reynard
>>
>> --=20
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> --=20
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