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.
Continuing with our previous theme “Sharkansas”, this week’s geo-pic is on Arkansas corals. Of course, corals don’t live in Arkansas today, but from about 480 million years ago, up until roughly 40 million years ago, coral would have been a fairly common sight in the natural state.
The picture above is of a tabulate coral: a now-extinct variety of colonial coral. Each hexagonal corallite chamber housed a simple, individual animal, called a polyp, that could protrude and retract to filter food from the water. The chambers in this fossil are in-filled with the mineral calcite, but that occurred after the coral died and was incorporated into the rock. It was photographed in the Ozark Plateaus, in the Prairie Grove Member of the Hale Formation.
Other varieties of coral are found in the rocks of Arkansas. For more views of Arkansas corals click here
You wouldn’t know it to look around now, but Arkansas, at times in the distant past, was teaming with sharks (and other marine fish). Indeed, Arkansas was in part or wholly covered by ocean many times in the past. One such time was 250 million years ago, during the Carboniferous Period. The fossilized Cladodus tooth pictured above belonged to a primitive shark that had sharp teeth with multiple points of varying size that it used to gig fish before gulping them down. The long point at the middle of the tooth is broken off and displaced to the right in this picture.
This particular specimen was found near West Fork, Arkansas. It was collected from the Prairie Grove Member of the Hale Formation, a limey sandstone. However, shark teeth can be found locally, throughout other parts of the state, in marine rock layers spanning hundreds of millions of years.
Pictured above is Dardanelle Rock located on the south side of the Arkansas River between the towns of Dardanelle and Russellville. The white truck in the lower right corner shows the scale of this outcrop. It was designated a Natural Area by the Arkansas Natural Heritage Commission in 1976.
The Arkansas River Valley is north of the Ouachita Mountains and is characterized by gently folded sedimentary rock that was subject, to a lesser extent, to the stress that folded the Ouachita Mountains. The rock pictured here is the south limb of a broad syncline, or down-warped fold. The north limb is about two miles to the northeast. The bedrock dips to the north (toward the white truck), goes sub-surface beneath the Arkansas River, then reverses dip direction and rises back to the surface just southwest of Russellville. If you could see a cross-section of the folded rock, it would look like giant a smiley face with the middle of the smile underground and the corners sticking up in opposite directions, two miles apart.
This picture gives perspective to the colossal size of geologic features geologists study. Folds like this one, which can trap upward-migrating fluid, are sometimes rich oil and gas reservoirs.
The sedimentary rock in the picture above is a sandstone with pebble molds. If the pebbles were present, this rock would be considered a conglomerate. Conglomerates consist of 2 mm or larger rounded fragments of rock, or clasts, surrounded by finer-grained sediment which geologists call “matrix”. The clasts in the rock above were pebble sized, 2-64 mm, and the matrix is sand sized.
Even though many of the clasts have been removed by erosion, we can tell that they were primarily shale pebbles. The sandy matrix was more resistant to erosion than the softer shale pebbles, so we are left with cavities where the pebbles were (pebble molds) on the rock’s surface. This creates an interesting optical illusion. Did you see the cavities as pebbles or as molds when you first looked at the picture?
This type of conglomerate is deposited by energetic and dynamic water, such as is found in rivers and waves. During higher flow periods, only large clasts are deposited. When flow is lower, finer-grained sediment settles in between the larger clasts.
(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.