(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.
Zinc ore collected in 1943 from the Rush Creek Mining District, Marion County, Arkansas. The brown mineral is sphalerite: an ore of zinc. The pink mineral is dolomite – it’s pretty, but not economically valuable. They were both deposited on the gray dolostone; you can just make it out on the right, in back.
Zinc deposits are found throughout northern Arkansas, commonly with the lead mineral, galena. They’re most abundant in Marion County, in a two mile stretch of rugged terrain, along Rush Creek, where 4 faults come together. That area was mined for lead and zinc in the late 1800s and early 1900s.
It’s typical to find rich ore deposits in rock that’s been fractured by faulting. The fractures facilitate migration of mineral-rich ground water which deposits the ore minerals in the fractures. It’s hard to see in the picture, but the fractured dolostone rock, in this specimen, is bound together by the sphalerite and dolomite minerals.