Why do minerals cleave

Each direction signifies the two opposite sides of a three-dimensional figure, since opposite sides will always exhibit the same cleavage properties. If a mineral has cleavage in three directions, then every side of the mineral has cleavage i.

If a mineral occurs in modified crystals with more than six sides i. Combining the cleavage level together with the number of sides will measure the cleavage of a mineral. For example, if a mineral has Good Cleavage, Two Directions , this means that it has good cleavage on four out of six sides while the other two sides exhibit no cleavage. If a mineral has Perfect Cleavage, One Direction; Poor Cleavage, Two Directions , this means that the mineral has perfect cleavage on two sides, and poor cleavage on the other four.

In this guide, cleavage quality is measured in numbers, then the amount of sides, separated by a comma. If the cleavage of a mineral is written as 1,2 the mineral has perfect cleavage in two directions.

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 two directions, and poor cleavage in one other direction.

If a mineral exhibits indistinct or no cleavage, Indiscernible or None is written in the cleavage field. Different habits of cleavage exist on different minerals, depending on their mode of crystallization. These forms of cleavage are:. Basal cleavage: Cleavage exhibited on a horizontal plane of the mineral by way of its base.

Minerals with basal cleavage can sometimes be "peeled". An example of basal cleavage are the mica minerals. Cubic cleavage: Cleavage exhibited on minerals of the isometric crystal system that are crystallized as cube s. In this method of cleavage, small cubes evenly break off of an existing cube.

An example is Galena. Octahedral cleavage: Cleavage exhibited on minerals of the isometric crystal system that are crystallized as octahedron s. In this method of cleavage, flat, triangular "wedges" peel off of an existing octahedron.

An example is Fluorite. Prismatic cleavage: Cleavage exhibited on some prismatic minerals in which a crystal cleaves as thin, vertical, prismatic crystals off of the original prism. An example is Aegirine. 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. Cleavage terms only use if cleavage planes can be recognised : Perfect - Produces smooth surfaces often seen as parallel sets of straight lines , e.

Fracture terms use in all other cases : Conchoidal - Fracture surface is a smooth curve, bowl-shaped common in glass ; Hackly - Fracture surface has sharp, jagged edges; Uneven - Fracture surface is rough and irregular; Fibrous - Fracture surface shows fibres or splinters. Notes : The number of cleavage planes can differ from mineral to mineral. Mica e. Since some bonds are weaker than other bonds, each type of mineral is likely to break where the bonds between the atoms are weaker.

For that reason, minerals break apart in characteristic ways. Cleavage is the tendency of a mineral to break along certain planes to make smooth surfaces. Halite breaks between layers of sodium and chlorine to form cubes with smooth surfaces Figure below.

One reason gemstones are beautiful is that the cleavage planes make an attractive crystal shape with smooth faces. Fracture is a break in a mineral that is not along a cleavage plane. Fracture is not always the same in the same mineral because fracture is not determined by the structure of the mineral. Minerals may have characteristic fractures Figure below. Metals usually fracture into jagged edges. If a mineral splinters like wood, it may be fibrous.

Some minerals, such as quartz, form smooth curved surfaces when they fracture. Some minerals have other unique properties, some of which are listed in Table below. Hint: It is most likely found on your dinner table. Skip to main content. Search for:. Mineral Identification Lesson Objectives Explain how minerals are identified.

Describe how color, luster, and streak are used to identify minerals. Explain how the hardness of a mineral is measured. Identify additional properties that can be used to identify some minerals. Vocabulary cleavage fracture hardness luster mineralogist streak Introduction Minerals can be identified by their physical characteristics.

How are Minerals Identified? This mineral has shiny, gold, cubic crystals with striations, so it is pyrite.