(FOV approx. 2 mm, photo courtesy of Stephen Stuart)
The wedge-shaped crystal in the photo above is the mineral titanite. This calcium titanium silicate (formula CaTiSiO5) is commonly found as an accessory mineral in igneous intrusions similar to those present at 3M and Granite Mountain quarries near Sweet Home. This sample was collected from 3M Quarry.
Titanite gets its name from its titanium content, but it was more commonly known by the name “sphene” until 1982 when the new name was officially adopted by the International Mineralogical Association. Sphene was derived from the Greek word “sphenos”, meaning wedge.
Crystals of titanite have a higher dispersion than diamonds. Dispersion is the measurement of refractive properties of a gemstone. The higher the dispersion, the more “sparkle” from the gem. However, gem quality samples of titanite are very rare, and the mineral is relatively soft compared with other gemstones.
Pictured above is a little piece of geologic history known as a basal conglomerate. that’s a rock formed after a period of erosion that marks the boundary between two geologic time periods: in this case, the Mississippian (359-318 million years ago) and the Pennsylvanian (318-299 million years ago).
318 million years ago sea level subsided, bedrock was exposed, and the Mississippian Period came to an end. When exposed to erosion at the earth’s surface, pieces break off from bedrock. Flowing water in rivers, streams and oceans wears the edges of those rock fragments till they’re rounded. Once ocean level rises and deposition resumes, the rounded gravel gets mixed with newly accumulating sediment and forms a rock which is made partly of fragments of the older bedrock. Geologists call this type of rock a basal (at the base) conglomerate (containing round gravel) because it is the first bedrock signaling the beginning of a new period of geologic time.
The photo above illustrates surface iridescence. Iridescence is a play of colors caused by the interference of light waves. This phenomenon is also called thin film interference. You have probably seen this effect on soap bubbles and oil sheens. Light reflecting from a thin coating of iron oxide on the piece of novaculite above is producing the play of colors. Light waves are reflected from the top of the iron coating and the base of the iron coating producing multiple waves. A color is seen when the waves interfere constructively. The resultant color is dependent on the thickness of the coating and consequently, streaks and bands of differing color develop since the thickness of the iron oxide coating varies. The colors also change when the angle of reflection is changed.
The grooved surface pictured above is a slickenside. Slickensides indicate the relative direction of movement between fault blocks (hanging wall moved up, down, laterally, etc..).
Slickensides form when fault blocks move against each other. The natural irregularities on each scratches grooves into the other. The grooves are parallel to movement; for instance in this example, movement was either to the right or the left. To tell whether it was right or left, you can rub your hand along the slickensides. They feel smooth in the direction the fault moved and rough in the opposite direction – it’s like petting a dog from tail to head. Slickensides are a valuable tool because determining fault movement can be a challenge when there are no easily-recognized beds that can be correlated across the fault to show the sense of offset.
The shale above was photographed in Big Rock Quarry, North Little Rock, AR. It’s a part of the Jackfork Formation (Pennsylvanian).
Pictured above is one of many faults, closely spaced together, in an outcrop of the Atoka Formation, near Lake Fort Smith, Arkansas. The fault pictured extends from the upper right to the lower left and is highlighted. This type of faulting is called syn-depositional faulting, meaning it occurred at about the same time the rock was being deposited. It results in disturbed-looking outcrops like this one.
Around 300 million years ago, plate tectonic forces were deforming the Ouachita Mountains in south central Arkansas. Those forces also caused faulting in the southern Ozark Plateaus, as the sediment that composes this rock outcrop was being deposited. The freshly deposited sediment wasn’t fully consolidated when the faulting took place and the rock surrounding the fault got contorted by the stress.
Some of the deformed features of the outcrop are labeled above. The Zone of Soft-Sediment Deformation is the area surrounding the fault where the rock has been deformed by shearing: there is no recognizable bedding in that zone. The soft clay-rich Deformed Shale was squeezed plastically between the fault blocks in that soft sediment deformation zone. The bedding orientations surrounding the deformation zone (indicated by magenta lines) vary greatly, because the soft bedrock was broken and heaved around by the fault.
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.