Pictured above is a mineralized vug (approximately 3 inches long) in chert. A vug is a void or open space in a rock. Many vugs are filled with minerals after water that is saturated with a certain mineral flows through the rock. This mineralization can happen in multiple stages. The vug above was initially filled with silica-rich fluid therefore quartz precipitated out of solution and lined the walls of the vug. Afterwards calcite precipitated, as is evident from the larger crystal on the interior left of the vug.
This vug is present in a section of ornamentally banded chert. Chert is a sedimentary rock made up of microcrystalline quartz. It can be a variety of colors or banded and quite beautiful. The chert above is Devonian age (416-359 million years ago) from northwest Arkansas.
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.
Above is a picture of lodestone, which was collected from Magnet cove, Arkansas. Lodestone is composed of iron and oxygen and is just like the mineral magnetite except, lodestone is naturally magnetic.
Magnetic minerals are rare on earth but, they have been a crucial part of human evolution. Discovery of magnetic lodestone by ancient people led to the invention of the compass. The compass, in turn, revolutionized navigation, and lead to the spread of technologically advanced cultures around the world.
Scientists are still uncertain how lodestone became magnetized but, the most accepted theory holds that it forms when magnetite is struck by lightning, which has a strong magnetic field. The fact that lodestone is primarily found at the earth’s surface supports this theory and scientists have been able to produce rock identical to lodestone by exposing magnetite to lightning.
The above picture shows several varieties of kryptonite…. not really! These are various minerals that display a natural phenomenon known as fluorescence. Fluorescent minerals contain particles that glow when exposed to ultraviolet light, a type of light outside of the spectrum of light we can see. Actually, these minerals also fluoresce in visible light such as sunlight, but the visible wavelengths drown out the glow so we don’t notice it.
This collection of fluorescent minerals, along with other educational, geology-related exhibits, is on display at the Arkansas Geological Survey’s learning center in Little Rock. If you would like to visit the learning center and see for yourself, it’s available for touring by appointment. You can find out more by contacting the Arkansas Geological Survey.
Above is a picture of the State Mineral of Arkansas, quartz. Quartz crystals are found in the Ouachita Mountains from Little Rock to Oklahoma. The crystals grew in fractures and vugs in the sandstone and shale as hot, mineral-rich water from the compression of the Ouachita Mountains circulated through the bedrock around 260 million years ago.
Like all crystals, quartz grows in a distinct shape. The six-sided shape of quartz is due to the arrangement of the molecules (SiO2) that comprise it, which pack together in this shape naturally. Quartz crystals are prized for their beauty, but are also useful in devices, such as radios and clocks, because of their electrical properties.
Mineral collectors flock to Arkansas for a chance to find world-class quartz crystals. There are 7 locations around the Ouachitas where you can prospect for quartz crystals for a fee.
For more views of Arkansas quartz crystals click here