Accessory minerals are minerals found in igneous rocks that are not used for the classification or naming of the rock. These minerals may be commonly present in a type of rock, but the absence of the mineral would not change the general classification geologists give to the rock.
The two accessory minerals in the center of the picture above are greenish-black needles of aegirine (AY-jur-EEN) and orangish-pink analcime (uh-NAL-seem) crystals. These minerals are frequently found together in igneous intrusions of syenite like the one present at Granite Mountain, where this sample was collected.
Accessory minerals give important clues to geologists when trying to determine details about how a rock formed and how it changed over time. They can make up a substantial portion or a fairly insignificant portion of a rock. Some accessory minerals make up a sufficient portion of the rock to be included as a modifier in the name, such as “biotite syenite”. Adding such a modifier gives geologists quick and useful information about how this rock differs from standard syenite.
(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.
As a naturally occurring, inorganic, solid, crystalline substance with a constant chemical formula, ice is in fact considered a mineral. By the same logic, water, which is the liquid form, is lava. This picture was taken earlier this week near Prim Arkansas. Rock hammer for scale.
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