Tag Archives: mineralogy

Geo-pic of the Week: Brookite

brookite(FOV approx. 1.5 mm, photo courtesy of Stephen Stuart)

The metallic crystal in the center of the photo above is a mineral known as brookite. It was collected in Magnet Cove, AR. This particular crystal is approximately 0.5 mm in diameter.

Brookite is one of three forms of titanium oxide (TiO2) that naturally occur in Arkansas. These three forms are what are known as “polymorphs”. Polymorphs are minerals that have the same chemical composition but their atoms are arranged differently creating differing crystal structures. It’s the mineral equivalent of being a fraternal twin instead of an identical twin!

The three types of TiO2 crystal found in Arkansas are brookite, anatase, and rutile. When geologists talk about a mineral’s stability, they are talking about how much of a change in temperature and/or pressure (stress) is necessary to change the crystal structure or composition. The more stress required to change it, the more stable the mineral. Brookite is the least stable of the three forms and therefore the rarest. Typically, brookite crystals are yellowish or reddish brown in color, but the variety found in Arkansas is commonly black which is due to the presence of the element niobium (Nb) as an impurity.

This mineral usually occurs around metamorphic rocks or igneous intrusions similar to the intrusion at Magnet Cove.

 

Geopic of the week: Arkansas Wavellite

DSCN0480

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