Tag Archives: #geoblog

Geopic of the week: Arkansas Wavellite

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The above picture shows two examples of the mineral wavellite: an aluminum phosphate mineral prized by rock and mineral collectors and fairly common in the Ouachita Mountains.  The green sample on the right is the typical color of wavellite, whereas the blue sample is a rare form.  The green color is due to the presence of vanadium.  These samples were both collected from an abandoned quarry a couple of miles northwest of Mt. Ida, Arkansas.

It’s easy to see why mineral collectors would be interested in wavellite as it comes in a variety of attractive colors – rarely white, yellow and black in addition to the blue and green examples shown here. It also has a visually interesting growth habit; it grows in botryoidal (grape-like) spheres that internally consist of a radiating array of slender crystals that resembles an eye.  Examples of this eye-like structure can be seen by clicking on the “more views” link below.

Wavellite is also quite sparkly and Christmassy.  I even hear it’s the official mineral of the north pole.  Ho ho ho!

To see more views of Arkansas wavellite click here

Geopic of the week: La Petite Roche (Little Rock)

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Pictured above is about all that remains exposed of the rock outcrop that is the namesake of the capitol of Arkansas, La Petite Roche or The Little Rock.  The outcrop was given its name by early french explorers to the area who first arrived in 1722.  Though unimpressive in stature, it is notable because it is the first exposure of solid rock that one sees when navigating up the Mississippi and Arkansas rivers, starting at the Gulf of Mexico.

The geologic explanation for this is simple;  The land between New Orleans and Little Rock Arkansas along that route is in the Mississippi River Valley, where the bedrock is buried beneath river-lain and wind-blown sediment.  The city of Little Rock straddles the boundary between the lowlands of the Mississippi River Valley and uplands of the Ouachita Mountains, which stretch west well into Oklahoma.  Therefore this little sandstone and siltstone outcrop marks that physiographic boundary. 

If you boated all the way from New Orleans to Little Rock without seeing a single rock you would probably be impressed by it too.