Mineral Cleavage and Mineral Cleavage and FractureFracture

What is cleavage?What is cleavage?

How minerals break under stress on a How minerals break under stress on a particular particular plane plane Determined by the atomic arrangement of Determined by the atomic arrangement of the mineralthe mineralCleavage is often measured by three Cleavage is often measured by three factors:factors:1)1) Quality of Cleavage Quality of Cleavage2)2) Number of Sides Exhibiting Cleavage Number of Sides Exhibiting Cleavage3)3) Cleavage Habit Cleavage Habit

Quality of CleavageQuality of Cleavage Quality of cleavage can be categorized into five Quality of cleavage can be categorized into five

qualities:qualities:– Perfect – Perfect – cleaves with no rough surfaces; perfectly cleaves with no rough surfaces; perfectly

smooth breaksmooth break– Good – Good – cleaves smoothly, but with some rough surfacescleaves smoothly, but with some rough surfaces– Poor – Poor – smooth crystal edge is barely visible, because smooth crystal edge is barely visible, because

the rough surface dominatesthe rough surface dominates– Indiscernible (Indistinct) – Indiscernible (Indistinct) – the mineral exhibits the mineral exhibits

cleavage, but it is so poor that it’s unnoticeable cleavage, but it is so poor that it’s unnoticeable – None – None – fractured and rough surfaces when broken; fractured and rough surfaces when broken;

does not have cleavage; does not have cleavage; Not an official measurement used by Not an official measurement used by

mineralogistsmineralogists May have other names like “excellent” or May have other names like “excellent” or

“distinct”“distinct”

Number of Sides Exhibiting Number of Sides Exhibiting CleavageCleavage

This is a measure of how many planes the mineral exhibits cleavage This is a measure of how many planes the mineral exhibits cleavage on. They are labeled as:on. They are labeled as:

One directionOne direction Two directionsTwo directions Three directionsThree directions All directionsAll directions

Each direction includes the sides opposite of the 3-dimensional Each direction includes the sides opposite of the 3-dimensional figure (like a cube)figure (like a cube)Cleavage directions can be combined with cleavage quality like this:Cleavage directions can be combined with cleavage quality like this:

Good Cleavage, Two DirectionsGood Cleavage, Two Directions, means that the mineral has good , means that the mineral has good cleavage on four out of six sides (while the other two sides exhibit no cleavage on four out of six sides (while the other two sides exhibit no cleavage).cleavage).

Perfect Cleavage, One Direction; Poor Cleavage, Two DirectionsPerfect Cleavage, One Direction; Poor Cleavage, Two Directions, , means that the mineral has perfect cleavage on two sides, and poor means that the mineral has perfect cleavage on two sides, and poor cleavage on the other four. cleavage on the other four.

Cleavage HabitCleavage Habit

The 3-dimensional shape of the The 3-dimensional shape of the mineral after it breaksmineral after it breaks

Depends on the crystal structureDepends on the crystal structure

Basal cleavageBasal cleavage

Cleavage exhibited on a horizontal plane Cleavage exhibited on a horizontal plane of the mineral by way of its base. Minerals of the mineral by way of its base. Minerals with basal cleavage can sometimes be with basal cleavage can sometimes be "peeled"."peeled".

An example of basal cleavage are the mica An example of basal cleavage are the mica minerals.minerals.

Cubic cleavageCubic cleavage Cleavage exhibited on minerals of the Cleavage exhibited on minerals of the

isometric crystal system that are isometric crystal system that are crystallized as cubes. In this method of crystallized as cubes. In this method of cleavage, small cubes evenly break off of cleavage, small cubes evenly break off of an existing cube.an existing cube.

An example is Galena or HaliteAn example is Galena or Halite

Octahedral cleavageOctahedral cleavage Cleavage exhibited on minerals of the Cleavage exhibited on minerals of the

isometric crystal system that are isometric crystal system that are crystallized as octahedrons. In this method crystallized as octahedrons. In this method of cleavage, flat, triangular "wedges" peel of cleavage, flat, triangular "wedges" peel off of an existing octahedron.off of an existing octahedron.

An example is Fluorite.An example is Fluorite.

Prismatic cleavagePrismatic cleavage

Cleavage exhibited on some prismatic minerals in Cleavage exhibited on some prismatic minerals in which a crystal cleaves as thin, vertical, prismatic which a crystal cleaves as thin, vertical, prismatic crystals off of the original prism.crystals off of the original prism.

An example is Tremolite.An example is Tremolite.

Rhombohedral cleavageRhombohedral cleavage Cleavage exhibited on minerals Cleavage exhibited on minerals

crystallizing in the hexagonal crystal crystallizing in the hexagonal crystal system as rhombohedrons, in which small system as rhombohedrons, in which small rhombohedrons break off of the existing rhombohedrons break off of the existing rhombohedron.rhombohedron.

An example is Calcite.An example is Calcite.

Dodecahedral cleavageDodecahedral cleavage

Breaks off in a direction parallel to the Breaks off in a direction parallel to the direction of the rhombic faces of the direction of the rhombic faces of the existing dodecahedron. existing dodecahedron.

An example is garnet.An example is garnet.

Mineral FractureMineral Fracture There are several terms to describe the various There are several terms to describe the various

mineral fractures:mineral fractures:– ConchoidalConchoidal - Fracture resembling a semicircular shell, with a - Fracture resembling a semicircular shell, with a

smooth, curved surface. An example of conchoidal fracture can smooth, curved surface. An example of conchoidal fracture can be seen in broken glass. (This fracture is also known as be seen in broken glass. (This fracture is also known as "shelly" in some reference guides.)"shelly" in some reference guides.)

– UnevenUneven - Fracture that leaves a rough or irregular surface. - Fracture that leaves a rough or irregular surface.– HacklyHackly - Fracture that resembles broken metal, with rough, - Fracture that resembles broken metal, with rough,

jagged, points. True metals exhibit this fracture. (This fracture jagged, points. True metals exhibit this fracture. (This fracture is also known as "jagged".)is also known as "jagged".)

– SplinterySplintery - Fracture that forms elongated splinters. All fibrous - Fracture that forms elongated splinters. All fibrous minerals fall into this category.minerals fall into this category.

– Earthy or crumblyEarthy or crumbly - Fracture of minerals that crumble when - Fracture of minerals that crumble when broken.broken.

– Even or smoothEven or smooth - Fracture that forms a smooth surface. - Fracture that forms a smooth surface.– SubconchoidalSubconchoidal - Fracture that falls somewhere between - Fracture that falls somewhere between

conchoidal and even; smooth with irregular rounded corners.conchoidal and even; smooth with irregular rounded corners.