Posted by: Alan Pitts | May 23, 2010

Unakite

This beautiful rock is unakite, which I found during our Igneous and Metamorphic Petrology field trip to the Blue Ridge province of Virginia.

We found plenty of this at our first stop of the trip at an abandoned quarry outside the town of Vesuvius.

The pink colored mineral is orthoclase feldspar. The blue-grey mineral is quartz, which is blue from trace amounts of Ti in small rutile inclusions in the quartz.  The green mineral is epidote, a metamorphic mineral, and is often described as “pistachio”.  I would tend to argue that pistachio is more of a flavor than a color.  However, after a series of tests back at the lab I can now say with a strong certainty that epidote definitely looks more like pistachio than is tastes.

I’ve been told on several occasions that unakite is the state rock of Virginia, but never have been able to confirm it with any official source.  It is not exclusively found in the state of Virginia and actually is named for the Unaka Mountains in North Carolina and Tennessee.

On the freshly exposed face of this other sample you can see very fine grains with a metallic luster.  Initially we thought it was specular hematite or magnetite.  At this point I’m going with hematite, since it is not magnetic.  As soon as I get my hands on a streak plate, I’ll know for sure.

And just one more that I took later with a different lighting arrangement:

It’s not all about good looks though, it also has an interesting history.  It started out as granitic pluton over a billion years ago associated with the Grenville orogeny and creation of the super-continent Rodinia.  Since the Grenville, the tectonic setting on (what is today) the east coast of the North America has fluctuated between convergent, divergent, and passive margins several times.  This rock was lucky enough to find itself caught up in one or maybe several of these later orogenic events, either the Taconian ( appx. 460 mya) and/or the Alleghenian which formed Pangea (appx 300-250 mya).  These periods of increased heat and pressure altered the mineral composition of the rock, producing the epidote and giving the characteristic color.

I’m not sure if I totally support it as the state rock of Va, but I think it is at least a good candidate for the following reasons;  It has an exciting story that illustrates some of the key events in the geologic history of Virginia and the world.  It has a unique color scheme making it easy to identify.  Also, in the gem world, unakite is considered a semi-precious stone which is highly polishable and makes nice carvings.

Im going to think more on this one…. I wonder if I can come up with some others that would be worthy rock-representatives of the great state of Virginia.

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Responses

  1. Pete Berquist of TNCC thinks charnockite should be the state rock. I like unakite better myself, because as you point out, its story has several chapters.

  2. If there isn’t a state rock already, I agree it should be unakite (unless No. Carolina or Tennessee already has it?).


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