The mineral in the above pictures is calcite, a common mineral in earth’s crust that is the main component of the sedimentary rock limestone. The stack of samples (top) exhibit a physical characteristic known as cleavage. The cleavage of calcite causes it to break into a rhombus-shape (see picture).
Cleavage is the tendency of a crystalline substance, such as a mineral, to break along parallel planes that reflect the internal arrangement of the atoms in the crystal. All crystals, by definition, have a uniform atomic arrangement. To illustrate this property, I’ve included a second picture (bottom), borrowed from Dr. Cathy Sutton, that shows an extremely magnified calcite crystal. The repeating rhombus-shapes in the picture are individual calcite molecules. Basically, cleavage is the outward expression of the internal structure of a mineral.
The samples on the left were collected from Midwest Lime Quarry, Batesville, Arkansas.
Even though they might look like it, those crystals in the picture above didn’t come out of a dog’s mouth. They are crystals of dogtooth calcite. Calcite (CaCO3) is the primary mineral that makes up limestone. It occurs in several crystal shapes. The two most commonly found in Arkansas are 6 sided rhombohedrons and the scalenohedral shape you see above. When it forms in this scalenohedral crystal structure it is called “dogtooth spar”.
Calcite is a very common mineral, but this particular crystal form of the mineral is typically only found in Arkansas in conjunction with the minerals sphalerite (zinc ore) and galena (lead ore) in the lead and zinc districts. Calcite is also a polymorph, like the mineral brookite from a previous geo-pic. This means calcite has “sister” minerals with the same chemical composition, but differing crystal structures. The three polymorphs of CaCO3 are: calcite, aragonite, and vaterite.
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.
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
Pictured above is an exposure of Prairie Grove Sandstone near Durham, Arkansas, southeast of Fayetteville. The ribbed, planar faces that are central in the photo resulted from a weathering phenomenon called zebra weathering.
Zebra weathering occurs in sandstones cemented with calcite – a soluble mineral. Calcite is common in marine sediment and, in the tidal environment where this rock was deposited, marine sediment mixed with insoluble sand from the continent. The ratio of marine sediment to sand changed continuously in that environment due to seasonal and climatic cycles. Today, the beds of sandstone weather at different rates depending on their calcite content. As the rock weathers, the sandier beds stand out in relief since they wear away more slowly than the soluble beds between them. Hence, the banded zebra pattern.