Specimens of plagioclase and calcite , and particularly good specimens of some other minerals, may show twinning. Other minerals e. Introduction Properties Common rock-forming minerals Economic minerals. With 2 or more cleavage planes present, it is important to pay attention to the angle of the cleavage planes.
To determine the angle of cleavage, look at the intersection of cleavage planes. Be cautious when you see a flat surface on a mineral — not every flat surface is a cleavage plane. Crystal faces can be flat, but remember they form as a mineral grows, while cleavage forms as a mineral breaks.
The crystal form of quartz is a hexagonal prism, with nice flat sides. But when quartz is hit with a rock hammer, it breaks in an irregular fashion and does not exhibit cleavage. By way of examples, the micas have a perfect cleavage in one direction that is easy to produce; calcite has a perfect cleavage in three directions that is also easy to produce; the feldspars have a perfect cleavage in one direction that is easy to produce and a good cleavage in another direction that is hard to produce; and diamond has a perfect cleavage in four directions that is easy to produce.
Sphalerite has perfect cleavages in six directions, some of which are easy to produce, others hard - hence you won't always see all six cleavage surfaces in any given sample of the material. Microcline feldspar, South Dakota, 4cm long. Perfect cleavage is the right face. Good cleavage is the top face Cleavage may also be described in terms of crystallographic type: cubic galena.
These are usually referenced to what are called crystallographic forms, usually using a shorthand known as Miller Indices. This Key does not get that advanced, but the guides many collectors use often have this information in them. Next is whether or not there are two or more cleavage surfaces present at angles to one another and, if so, the quality of the various cleavages. Pinicoidal cleavage: Cleavage exhibited on some prismatic and tabular minerals in which a crystal cleaves on the pinacoidal plane, which is the third dimension aside from the basal and prismatic sides.
An example is Barite. Rhombohedral cleavage Cleavage exhibited on minerals crystallizing in the hexagonal crystal system as rhombohedron s, in which small rhombohedrons break off of the existing rhombohedron.
An example is Calcite. Parting is characteristically similar to cleavage. It is easily confused with cleavage, and it may be present on minerals that do not exhibit any cleavage. There are two causes of parting: Two separate pressures pushed toward the center of a crystal after its formation, causing the crystal interior to evenly dislodge on a flat, smooth plane.
Twinned crystals that separated from one another, leaving a flat, smooth plane. With enough perception, a distinction can be made between parting and cleavage. If fracture marks are present on a crystal in addition to a cleaved plane, the "cleaved" surface is usually the result of parting, not cleavage. An outline of a crystal etched in a mineral is also the result of parting, in the form of twinned crystals that separated.
In general, one need not worry about confusing parting with cleavage. Parting is uncommon, and it can usually be determined by the distinguishing characteristics mentioned above. Fracture is the characteristic mark left when a mineral chips or breaks.
Cleavage and fracture differ in that cleavage is the break of a crystal face where a new face resulting in a smooth plane is formed, whereas fracture is the "chipping" shape of a mineral.
All minerals exhibit a fracture, even those that exhibit cleavage. If a mineral with cleavage is chipped a certain way, it will fracture rather than cleave. Conchoidal - Fracture resembling a semicircular shell, with a smooth, curved surface. An example of conchoidal fracture can be seen in broken glass. This fracture is also known as " shelly " in some reference guides.
Hackly - Fracture that resembles broken metal, with rough, jagged, points. True metal s exhibit this fracture. This fracture is also known as " jagged ". Splintery - Fracture that forms elongated splinters.
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