Eclogite and omphacite use for ceremonial artifact

[stoneaxe27]

Is anyone familiar with eclogite? I believe it is found only in southern Appalachian mountains in a few spots in N. Carolina, S. Carolina, East. Tenn. It is composed of omphacite and garnet. Omphacite is jadeite less than 80% pure. Many of the meso american "jade" items are actually omphacite or eclogite. I believe ceremonial items, bannerstones, pipes and birdtones were made out of eclogite and traded to the Southeast, Florida and up to the midwest. I hope someone can provide some information as there is none in the literature. Eclogite is the densest metamorphic rock, about 3.5 specific gravity, feels like hematite in the hand, and fairly hard, 7-7.5 MOHs.

[painshill]

Hi Thomas and welcome to the forum.
I can't help with artefact use, but I have some geological information, US localities and pictures (of native rock/mineral) if that's any use to you. I guess you know that omphacite is a mineral, whereas eclogite is a rock. If this helps respond here and I'll post what I have tomorrow... it's getting late where I am (UK).
Roger

[stoneaxe27]

I WOULD APPRECIATE ANY INFO YOU CAN PROVIDE.

[JoeM]

You may have all ready seen this........

http://gsa.confex.com/gsa/2002AM/finalprogram/abstract_41135.htm

[painshill]

Hi again Thomas
Here’s what I can offer.
OMPHACITE
Omphacite is a mineral and, as such, has a defined composition. It’s a silicate in the pyroxene group which is typically pale green to almost colourless (with varietal forms from dark green to almost black) and a hardness between 5-6 on the Mohs scale. From a strictly mineralogical point of view, the classification has been reserved for clinopyroxene with a co-crystallised composition of jadeite (25%-75%), augite (25%-75%), and aegirine (0%-25%). The crystal structure lies between augite and jadeite. It frequently occurs with other minerals – notably glaucophane, which is bluish-black.


Omphacite (Italy)


Omphacite with Glaucophane (France)

The compositional distinction is no more than a convenience for practical reasons. The term “jade” is normally used to refer to rocks composed mainly of jadeite, and is also used to refer to rocks consisting entirely of omphacite. Omphacite also has various synonyms and varietal forms, including omphacite jade, chrome-omphacite, titanian omphacite, inky-black jade, diopsidjadeite, jadeite-diopside, mayaite and tuxtlite. It is, as you say, a major mineral component of the rock eclogite.
Major omphacite occurrences are marked on the map below (from mindat).



For the USA, it occurs in Arizona (including the Navajo Indian Reservation), California (widely), Colorado, Maryland, North Carolina, Vermont and Washington (including the Colville Indian Reservation).

For the southern Americas, it occurs notably in Colombia on the northern tip of the Guajira peninsula, in Brazil, Cuba, Guatemala and Mexico.

ECLOGITE
Eclogite is a rock, not a mineral and as such it has no precisely defined composition. It would be more correct to refer to “eclogites” as a suite of rocks, generally defined as metamorphic rocks with an original bulk chemical composition aligning with basalt. Primary minerals are green or black pyroxene (omphacite/jadeite and diopside) plus red garnet (pyrope), with small amounts of various other minerals such as kyanite, quartz, rutile or sphene. Eclogite is characterised in particular by the fact that plagioclase is never present. The name derives from the French “éclogite” (selection) because these minerals are not normally found together.
[continues...]

[painshill]

The red pyrope content ranges from about 30% to 55% and this, together with the accessory minerals, can have a dramatic influence on the colour. Here’s some typical and atypical samples:


Eclogite (Brazil)


Eclogite (South Carolina)


Eclogite with Glaucophane (California)


Banded Eclogite (Slovenia)


Eclogite with high Omphacite (Norway)


Eclogite with high Pyrope

Properties such as hardness are therefore not really relevant, since this will vary with composition, particularly since eclogites are generally coarse-grained. They are invariably dense rocks (unusually so) because they are formed by ultra-high pressure metamorphism of igneous rocks… typically basalt or gabbro.

The kind of pressure needed is normally only generated in subduction zones when rocks are forced down into Earth’s mantle or the lower crust of continents. Typically, this means depths of 20-100 miles or deeper and pressures high enough to occasionally create diamond inclusions.

Although eclogites are presumed to be widely present in Earth’s mantle, they are relatively rare rocks as far as we are concerned because they are not normally exposed. It needs an unusual tectonic event of some kind to bring them to the surface… a continental collision for example. Even then, they frequently undergo “decompression melting” as they come to the surface, such that they become other types of rock (in the territory of tonalite-trondhjemite-granodiorite or sometimes andesite). So, as well as being rare, these rocks are only found in certain zones around the world.

Eclogites are found in three main forms, depending on temperature and depth of formation, occurring as bands, lenses or xenoliths (fragments or nodules embedded in a host rock of another type). Xenolithic forms normally arise in volcanic breccia (a breccia is a mixed rock), kimberlite “pipes” (magma extruded from deep underground) and basalts.

The Earth’s present subduction-related plate boundaries are marked in red on the map below, so these are likely areas where eclogites will occur. In addition there are pre-historic locations where tectonic events, upsurges and “superswells” have occurred.



The principal locations in the USA are in Arizona (as xenoliths), the Franciscan Complex of California, in the kimberlites and basalts of Hawaii (as xenoliths), the Stockdale kimberlite of Kansas (as xenoliths), the Blue Ridge belt near Bakersville in North Carolina, the central and southern Appalachian Piedmont near Newberry in South Carolina, and east of Knoxville in Tennessee. Much of the Appalachian material is “retrograded”. That is, it was once eclogite, but is on its way to becoming something else as a result of decompression melting.

For the southern Americas, eclogites occur in the Sao Francisco craton of Brazil, the Guajira Peninsula of Colombia, Cuba, the Dominican Republic, the Raspas formation of Ecuador, the Motagua fault zone of Guatemala and the Cordillera de la Costa in Venezuela, among others.

As regards artefacts, I’m sure you’re right that it was a highly valued material in Southern Central Americas and also in parts of the Caribbean. Interesting that one of the synonyms for omphacite is “Mayaite”. I was offered a beautiful polished eclogite biface collected from Haiti in the 1950’s for $400 recently :laugh: , but I declined it when the vendor was honest enough to admit that he didn’t know if it was any older than that :unsure: . The Taino culture of Cuba also used this material widely. There are some artefact pics towards the end of the article at this link:

http://www.ugr.es/~agcasco/igcp546/C...tGeol/jade.htm
[[[ :angry: These infernal links often don't work... try pasting this into your browser instead]]]

Occasional eclogite artefacts turn up in Europe too (the main deposits are in Norway), but it’s a rare material for tools. Here’s a polished axe from France (around 3,300 – 9,000 years old):



That’s pretty much all I can tell you. Hope it helps.
Roger

[stoneaxe27]

Roger,
Thank you very much ,that is the most pertinent and helpful summary I have seen. Great photos and maps.

[JoeM]

Heck Yes, Roger! Thanks for that reply!
Great specimen pics and explanations!
Very, very helpful.
Joe

[Hoss]

Nice job Roger. You really go all out on you descriptions. Your posts are always a learning experience for me. Thanks!