Wednesday, March 7, 2012

BROOKITE: TITANIUM DIOXIDE


SPECIMEN FROM MAGNET COVE, ARKANSAS ~17.5 MM IN LENGTH.  NOTE TERMINATED QUARTZ CRYSTALS ADJACENT TO BROOKITE CRYSTALS.  GROUNDMASS IS MAINLY STAINED MICRO QUARTZ CRYSTALS.  SPECIMEN HAS BEEN TREATED WITH ACETIC ACID.

A long time ago, as I am fond of stating, I had the opportunity to spend a short amount of time in Arkansas participating in a geology student field trip.  Now, some of these student trips stick out in my mind like “yesterday” while others are buried in the deep recesses.  I believe rainy and cool weather, coupled with a scarcity of adult beverage locations (for relaxation in the evenings), somehow helped rank this trip as “not very interesting”.  That is unfortunate since, as I think back, there was much to learn, and I messed up by not offering my full attention.  Ah, perhaps one can forgive some youthful discretion.

What I do remember is trying to collect through the brush, mud and bugs at a place called Magnet Cove, and hunting for quartz crystals at Mt Ida.  I still have some of the latter that have been following me around for almost half a century, as well as a couple of other interesting specimens from Magnet Cove.

Earlier this year as I was putting together the blog (March 1) on pseudobrookite from Utah, something rattled my inner brain and I vaguely remembered the mineral brookite from Arkansas.  Did I have a sample stored somewhere?  Perhaps so, but where?  I have moved and changed houses so many times that “stuff” is still packed/buried in boxes and totes.  Sometimes I am able to locate pieces of “stuff”, and sometimes I am convinced that particular piece must not have survived one of the moves 20 years ago.  And, to my deepest regret, I gave away untold boxes of specimens since during one sojourn the moving truck was full and rocks cost scarce money to ship.  Oh well, life goes on.

I did find the brookite specimen, originally held on to because of the “pretty” quartz crystals.  It turns out this was a smart move since Magnet Cove is a premier area in the U.S. for collecting brookite crystals, and although not rare, it is not an overly common mineral.

I later found out that Magnet Cove is of tremendous geologic interest since it (all five square miles) has produced in excess of 100 different minerals, many of them rather uncommon and at least five new to science (such as kimzeyite, a zirconium-rich garnet).  Magnet Cove is an alkaline (low silica, high potassium and sodium minerals) intrusion into the surrounding Paleozoic sedimentary rocks.  The name comes from an abundance of magnetite found in the surface.  The magma associated with the intrusion did not reach the surface and includes some rare and unusual rock types that seem associated with a melt that was originally a CO2-rich basaltic liquid in the earth's upper mantle (Mike Howard, Rockhounding Arkansas).  The intrusion seems related to others scattered across this part of Arkansas including the famous Crater of Diamonds.

And that brings me back to brookite, a mineral that is well-known and collectable from Magnet Cove.  Brookite is a titanium dioxide, TiO2, crystallizing in the orthorhombic system.  MinDat notes that brookite is one of five titanium dioxide minerals (rutile, anatase, akaogiite, unnamed) that occur in nature---all belong to different crystal systems!  The Magnet Cove specimens usually are pyramidal in shape and often sit on top of, under, or next to terminated quartz crystals.  This combination forms an attractive display and all one needs to do is search the web for collectable specimens.

Later in life I was able to return to Arkansas on a couple of non-collecting geology field trips and became fascinated with the folded and faulted Ouachita Mountains (although never to Magnet Cove or Mt. Ida).  These mountains, continuous with, and related to, the Appalachians to the east and other Texas mountains to the west, were the result of continental collision in the late Paleozoic.  This collision compressed and uplifted the area and created very high mountains.  Today, after much erosion, the mountains are much lower but the folds are well-outlined by resistant layers of chert and novaculite (see Pick & Pack, October 2010).  Living in Colorado I feel somewhat close to the Ouachitas since the Ancestral Rocky Mountains in our state are most likely related to this compression zone.  Remember, these mountains shed off the debris that we know as the Fountain Formation.

There are a number of really good articles about Magnet Cove minerals, including;

The Arkansas Issue of Rocks and Minerals, 1989, July-August issue on collecting minerals from Arkansas, including Magnet Cove.

Howard, J. M., 1999, Brookite, Rutile Paramorphs after Brookite, and Rutile Twins from Magnet Cove, Arkansas: Rocks & Minerals, v. 74, no. 2, p. 92-102.

Howard, J. M., 2012, Rockhounding Arkansas: Magnet Cove at http://www.rockhoundingar.com/magcove.php
 
Howard, J. M. and A. Chandler, 2007, Magnet Cove: A Synopsis of its Geology, Lithology and Mineralogy at: http://www.geology.ar.gov/pdf/pamphlets/magnet_cove.pdf

mike
PHOTOMICROGRAPH OF BROOKITE. LENGTH ~9 MM.

PHOTOMICROGRAPH OF BROOKITE ADJACENT TO TERMINATED QUARTZ CRYSTAL.  LENGTH OF BROOKITE ~5 MM.
 

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