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MINERAL PROPERTIES and identification procedures CLEAVAGE, PARTING and FRACTURE
Cleavage, fracture, and parting all have to do with the positioning of atoms in a mineral and how it breaks when put under stress. These three properties are listed on the same page due to their comparability, but are each individually discussed.
In mineral terms, cleavage describes how a crystal breaks when subject to stress on a particular plane. If part of a crystal breaks due to stress and the broken piece retains a smooth plane or crystal shape, the mineral has cleavage. A mineral that never produces any crystallized fragments when broken off due to stress has no cleavage. Cleavage is measured by two factors: quality and number of sides exhibiting cleavage. Quality of cleavage can be categorized into four qualities:
Minerals with perfect cleavage cleave without leaving any rough surfaces; a full, smooth plane is formed where the crystal broke. Minerals with good cleavage also leave smooth surfaces, but often leave over some rough surfaces. In minerals with poor cleavage, the smooth crystal edge is barely visible, since the rough surface is dominant. Minerals with no cleavage (none) never exhibit any cleavage, thus broken surfaces are jagged and rough. If a mineral exhibits cleavage, but it so poor that it is hardly noticeable, it has "indistinct" cleavage. Categorization of cleavage qualities is not scientifically affirmed. The above categorization is used by most mineral references, but some references categorize cleavage in three or five different groups, and may give them different names, such as "excellent" and "distinct". Many minerals exhibit cleavage only on one side, and some may exhibit different quality cleavage on different sides. We may expect to find the following criteria when analyzing the cleavage of any particular mineral:
These tell us how many crystal "directions" or planes of a mineral exhibit cleavage. Each direction means two opposite sides of a three-dimensional figure, (since opposite sides exhibit the same cleavage properties). If a mineral has cleavage in three directions, then every side of the mineral has cleavage. If a mineral often occurs in modified crystals with more than six sides (i.e. octahedron) and exhibits cleavage on all the sides, than we call that cleavage "all directions". When we combine the cleavage level together with the number of sides, we have measured cleavage. For example: A book states that the cleavage of a particular minerals is: Good, Two Directions. This informs us that the mineral has good cleavage on four out of six sides (while the other sides exhibit no cleavage). If the book tells us a different mineral’s cleavage is: Perfect, One Direction; Poor, Two Directions, it means that the mineral has perfect cleavage on two sides, and poor cleavage on the other four. In this guide, cleavage is measured in numbers, first the quality, then the number of sides, separated by a comma. If the cleavage of a mineral is written as 1,2 , the mineral has perfect cleavage. If the word Indiscernible is written in the cleavage field of a mineral, then the cleavage of the mineral is so poor it is hardly recognizable. If all sides of mineral have the same cleavage, and the mineral often occurs in modified crystals with more than six sides, than All Sides is written instead of a number. If a mineral exhibits different cleavage on different crystal planes, there will be two cleavage indicators separated by a semi-colon (;). For example, if the cleavage of a mineral is written as 1,2-;- 3,1, than it has perfect cleavage in one direction, and poor cleavage on two other directions. If a mineral exhibits no cleavage, None is written in the field. Different forms of cleavage exist on different minerals, depending on the mode of a minerals crystallization. These forms of cleavage are: Basal cleavage Cubic cleavage Octahedral cleavage Prismatic cleavage Pinicoidal cleavage Rhombohedral cleavage
Parting is characteristically similar to cleavage. It is easily confused with cleavage, and is often present on minerals that do not exhibit any cleavage. There are two causes of parting:
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" 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. There are several terms dubbed for various mineral fractures: Conchoidal - Fracture resembling a semicircular shell, with a smooth, curved surface. An good illustration of a conchoidal fracture is a large chip in a piece of glass. This fracture is also known as "shelly" in some references. Uneven - Fracture that leaves a rough or irregular surface. Hackly - Fracture that resembles broken metal, with rough, jagged, points. True metals exhibit this fracture. This fracture is also known as "jagged". Splintery - Fracture that forms elongated splinters. All fibrous minerals fall into this category. Earthy or crumbly - Fracture of minerals that crumble when broken. Even or smooth - Fracture that forms a smooth surface. Subconchoidal - Fracture that falls somewhere between conchoidal and even; smooth with irregular rounded corners. Some references may describe additional fractures not mentioned above, but those terms are either synonymous or simply used as a verbal depiction of the authors inference. Almost all minerals have a characteristic fracture. Some minerals of the same species may exhibit a different fracture, but this is rare.
A specimen should not be broken to check its fracture habit or cleavage. Rather, it should be checked for areas of stress where it could have broken or chipped. Fracture marks are rarely present on minerals with good or excellent cleavage. Minerals with poor cleavage will fracture more often than those with good or perfect cleavage.
Observe the mineral to see if it has any cleaved surfaces or fractured edges. If it has cleaved surfaces, the quality of the smoothness on the surface should be noted. If there are no visible cleaved surfaces, it does not mean the mineral does not exhibit cleavage. It is possible that particular specimen did not cleave. On such a specimen, it is only possible to check cleavage by chipping off a piece. This should be done gently and carefully in an area which will not degrade its value. If there is a noticeable fracture on the mineral, it is a signification that the mineral probably has poor cleavage or none at all. Observation should be able to distinguish between a cleaved crystal and a crystal that parted.
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