What is cleavage?How minerals break under stress on aparticular planeDetermined by the atomic arrangement ofthe mineralCleavage is often measured by threefactors:1) Quality of Cleavage2) Number of Sides Exhibiting Cleavage3) Cleavage Habit
Quality of Cleavage Quality of cleavage can be categorized into five qualities: – Perfect – cleaves with no rough surfaces; perfectly smooth break – Good – cleaves smoothly, but with some rough surfaces – Poor – smooth crystal edge is barely visible, because the rough surface dominates – Indiscernible (Indistinct) – the mineral exhibits cleavage, but it is so poor that it’s unnoticeable – None – fractured and rough surfaces when broken; does not have cleavage; Not an official measurement used by mineralogists May have other names like “excellent” or “distinct”
Number of Sides Exhibiting CleavageThis is a measure of how many planes the mineral exhibits cleavageon. They are labeled as: One direction Two directions Three directions All directionsEach direction includes the sides opposite of the 3-dimensionalfigure (like a cube)Cleavage directions can be combined with cleavage quality like this: Good Cleavage, Two Directions, means that the mineral has good cleavage on four out of six sides (while the other two sides exhibit no cleavage). Perfect Cleavage, One Direction; Poor Cleavage, Two Directions, means that the mineral has perfect cleavage on two sides, and poor cleavage on the other four.
Cleavage Habit The 3-dimensional shape of the mineral after it breaks Depends on the crystal structure
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 cubes. In this method of cleavage, small cubes evenly break off of an existing cube. An example is Galena or Halite
Octahedral cleavage Cleavage exhibited on minerals of the isometric crystal system that are crystallized as octahedrons. 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 Tremolite.
Rhombohedral cleavage Cleavage exhibited on minerals crystallizing in the hexagonal crystal system as rhombohedrons, in which small rhombohedrons break off of the existing rhombohedron. An example is Calcite.
Dodecahedral cleavage Breaks off in a direction parallel to the direction of the rhombic faces of the existing dodecahedron. An example is garnet.
Mineral Fracture There are several terms to describe the various mineral fractures: – 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.) – 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.