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.
Pyrite, also known as Iron Pyrite (FeS2), is the most common sulfide mineral. Its most frequent crystal structure is cubic, as seen in the picture above. It also forms octahedral (8 sided) and dodecahedral (12 sided) structures. Its brassy-yellow color and metallic luster can sometimes cause it to be mistaken for gold, hence the nickname “fool’s gold”. While it may look like gold, it is much lighter and harder. Typically pyrite cannot be scratched with a knife.
Pyrite is found in many counties in Arkansas. It is used in the production of sulfuric acid, although its use is declining. The primary value of this mineral currently is as a collectible specimen. Individual crystals are commonly found up to 1 inch in diameter.
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.
(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.
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.
As a naturally occurring, inorganic, solid, crystalline substance with a constant chemical formula, ice is in fact considered a mineral. By the same logic, water, which is the liquid form, is lava. This picture was taken earlier this week near Prim Arkansas. Rock hammer for scale.