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.
In the picture above, large black rectangular aegerine crystals are prominent in a rock type known as a pegmatite. Pegmatites are igneous rocks characterized by extremely large crystals. Sometimes they also contain unusual mineral species. This sample was collected from Magnet Cove, Arkansas. Magnet Cove, which is approximately 10 miles east of Hot Springs, is one of the few places in Arkansas where igneous rock is exposed at the surface.
Between 84 and 100 million years ago, magma was injected into the earth’s crust under central Arkansas where it slowly cooled and crystallized into igneous rock. Millions of years of erosion eventually unearthed that rock. Despite only being exposed over approximately 5 square miles, the rocks of Magnet Cove have yielded more than 100 different minerals. Rare minerals have been discovered there including a new variety of zirconium-rich garnet called Kimzeyite.