Thursday, November 21, 2013


Pisolitic bauxite from Arkansas.  Width ~4.1 cm.

Many CSMS members have been to the State of Arkansas on a collecting trip, most likely to collect quartz crystals and dabble in the diamond mine.  I would be interested in meeting someone who actually has collected a diamond in the State Park!  However, there are many other really interesting minerals in Arkansas that offer some collecting opportunities.

One of these “minerals” that Colorado rocks do not offer is bauxite.  Now, I don’t know much about bauxite except that it: 1) is an ore of aluminum; 2) forms in a tropical to subtropical environment; 3) occurs as small to large oolites; and 4) is mined in Arkansas (and elsewhere).  I learned all of these facts in a grade school geography class!  What I later learned as a geology student is that bauxite is not really a “mineral” but is a composite of several different aluminum-rich minerals: [gibbsite—Al(OH)3, diaspore-AlO(OH), boehmite-ALO(OH)],  a couple of iron oxides [hematite-Fe2O3,  goethite- FE3O(OH)],  and a clay mineral [kaolinite-Al2(Si2O5)(OH)4].

Bush (2007) described the formation of Arkansas bauxite as lateritic weathering and leaching of alumina and silica from intrusions of nepheline syenite (lots of nepheline and sodium/potassium feldspars) exposed during the Tertiary in what is now Pulaski and Saline Counties in the Gulf Coastal Plain.  These rocks (nepheline syenite), rather rare in the U.S., are alumina-rich and quartz-poor and evidently were intruded during the Late Cretaceous as the old Reelfoot Rift was reactivated.  Later in the Tertiary, intense weathering in a tropical environment allowed precipitation of the alumina-rich minerals.  Actually bauxite is an ancient soil and Harder (1948) noted its stratigraphic importance as marking “long periods of emergence, quiescence, nondeposition, and subaerial weathering”.
Bauxite ore has been mined in Arkansas since ~1896 and reached peaks during World War I and II. Major companies abandoned mining in ~1991 but a smaller company still operates a small mine (I think).
I once collected several pieces of bauxite; however, I mostly added them to a university collection that was lacking specimens and only one small piece remains.
Bauxite specimen from Pulaski County, Arkansas collected late 1960’s. Width ~6.5 cm.
OK, here is the story that goes with bauxite.  The mineral nepheline (Na3KAl4Si4O16), last seen in my blog on Black Hills phonolite (9-5-13), is a feldspathoid, a fairly rare group of minerals with a very low silica content.   Nepheline is related to leucite, lazurite, sodalite and others. As I understand it, nepheline does/can not exist with free quartz in nature (as with all feldspathoids).  Rockhounds usually find nepheline in the above-mentioned intrusive rock, nepheline syenite, and its extrusive equivalent, fine-grained phonolite.  I really never understood, in my mineralogy and petrology classes, the genesis of nepheline-rich rocks!  Were they the result of a segregated magma or did they form in a completely separate magma?  Of course, I don’t understand much about even stranger feldspathoid rocks such as the leucite-bearing, lamproite volcanic rocks in the Leucite Hills of southwestern Wyoming!  A field trip leader tried to explain it to me one time in my youth.  However, I recollect it was a very hot and windy day and my mind was on a cold adult beverage waiting for the group in Rock Springs.  At any rate, the U.S. has those aluminum-rich rocks in Arkansas because of the presence of nepheline syenite.  
The other related story concerns the Reelfoot Rift, an ancient rift system that is currently buried under what is now the Mississippi River from about Cairo, Illinois, south to the Gulf.  The rifting of the early Paleozoic proto- North American continent was probably most active in the Cambrian.  The exact cause of rifting is not well understood but perhaps that area of the continent was situated over a hot spot—a mantle-derived plume developed along the boundary of two old Precambrian terranes (Csontos and others, 2008).   As the continent split the resulting down-dropped rift basin filled with several thousand feet of Early Cambrian clastic, non-marine sediments overlain by a thick sequence of Cambrian and Ordovician marine rocks.  Then some weird things begin to happen and these “things” lasted throughout the Paleozoic.  Although the rift seemed to fail in the early Paleozoic, the basin still accumulated Paleozoic marine sediments during times of continental “quietness” but every time some tectonic mass along the east coast (current direction and location) bumped into the continent the rift area was squeezed and somewhat pushed up.  Geologists know these bumps as the Taconic (mostly Ordovician), Acadian (mostly Devonian) and late Paleozoic Alleghenian (or Appalachian) orogenies.  So, many of the mid- to late Paleozoic rocks are missing from the rift basin (absence also partially due to later erosion in the early/mid Cretaceous).  Sometime in the mid- to late Cretaceous the rift was reactivated and marine waters reached far to the north (~Cairo) and essentially received sediments, off and on, throughout the late Mesozoic and Cenozoic—the Mississippi Embayment or Mississippi trough was born.  Paleocene (Midway) and Eocene (Wilcox, Claiborne, Jackson) marine incursions  also reached far to the north while Oligocene, Miocene and Pliocene marine rocks are restricted to more southern states like Louisiana and the modern Gulf Coast. 

Diagram Reelfoot Rift.  Courtesy USGS.
There are a number of faults associated with the rift system and they seem the cause of the many seismic events in the region (for example the New Madrid earthquake).  In addition, the faults allowed magma to rise toward the surface and some of that magma cooled into nepheline syenite.  So, if it were not for a failed rift my grade school geography class would not have produced a lesson on mining aluminum in the U.S.  Although this factoid may seem trivial, lessons like this whetted my appetite for learning more about rocks and minerals and eventually resulted in a career.

Bush, W. V., 2007, History of Bauxite in Arkansas: Arkansas Geological Survey AGES Brochure Series 003.

Csontos, R., R. Van Arsdale, R. Cox and B. Waldron, 2008, Reelfoot Rift and its Impact on Quaternary Deformation in the Central Mississippi River Valley: Geosphere, v. 4, no. 1.

Harder, E. C., 1948, Stratigraphy and Origin of Bauxite Deposits: Bulletin of the Geological Society of America, v. 60, no. 5.