A Properties of Minerals Week 3: cleavage

One of the qualities generally required from abrasives used in sandblasting is: no embedment in target material This will depend partly on the shape of crystals or of their broken fragments. corundumdolomitegarnetbaking soda

Tendency of minerals to break along flat planes is called CLEAVAGE. You can only notice cleavage in specimens where single crystals are large enough (> a few mm). If there is no preferred plane of weakness in the structure, the pattern is described as FRACTURE. A plane of weakness due to defects arising from irregularities during crystal growth is called PARTING.

Hematite and corundum share the same structure. Their cations are in 6-fold coordination with oxygen anions, and these coordination polyhedra share edges and faces with their neighbours. However, the Fe-O bonds in hematite are not as strong as the Al- O bonds in corundum. Fe 2 O 3, Al 2 O 3 are based on XO 6 polyhedra.

Cleavage is well developed in solids showing a wide range of bond types. It is absent only in purely metallic solids. sylvite, KCl halite, NaCl fluorite CaF 2

Metallic native elements (gold, silver, copper) do not cleave along flat planes.. The sea of mobile delocalized valence electrons acts as a “glue”. The solid deforms plastically and stretches before breaking. This gives an uneven jagged surface described as a “hackly” fracture.

Structures with similar type of bonds (here, ionic bonds) but different bond orientationss give rise to different types of cleavage.

In halite, the planes of weakness are perpendicular to 3 perpendicular axes. The cleavage is cubic, i.e. along three perpendicular planes.

Orientation of Ca-F bonds towards body diagonals of a cube result in an octahedral cleavage.

Galena, PbS, has the same structure as halite. It also displays the same cubit cleavage. AgCl, has the same structure as NaCl. It does not cleave as cleanly, but can be cut with a knife (sectile). Bonding among larger ions (compare radii: Ag + > Na + ) gradually takes a more metallic character.

In structure with dominantly covalent bonds, the strength of the bonds is not as important as their orientation. In structures with strong bonds, cleavage may be present if some planes cut across a lower density of strong bonds.

The S 8 rings are not aligned predominantly along certain planes within the structure. The weak Van der Waals forces are not restricted to any particular direction. Sulfur breaks with a conchoidal to uneven fracture.

Diamond, a crystalline polymorph of carbon Tetrahedral coordination gives diamond its octahedral cleavage. These planes cut across less C-C bonds per unit area.

Cutting a rough diamond into a gemstone requires knowing where to cleave it. A sharp blow is all that’s needed to split the stone.

Silicate minerals show combinations of bonds that are partly covalent and ionic. Si-O bonds are the strongest bonds in their structures.

Garnet group A 2 B 3 (SiO 4 ) 3 Large cations in AO 8 polyhedra share edges with BO 6 and SiO 4 groups: no good cleavage dense hard rounded grains

Quartz is a tectosilicate. All SiO 4 4- units share every oxygen ion at their corners. Unlike diamond, tetrahedra do not point in same direction. Quartz shows no pronounced plane of weakness. A “wire model” of Si-O bonds shows the symmetry which accounts for quartz’ hexagonal crystal shape.

A material without any pronounced plane of weakness may break with a conchoidal fracture (right), or an irregular fracture (below). Quartz and garnet are good examples of this type of fracture.

olivine (Mg, Fe)SiO 4 XO 6 share edges with each other and with SiO 4 low silica content dense moderate hardness no good cleavage grains equant, subrounded

Feldspars: KAlSi 3 O 8 NaAlSi 3 O 8 CaAl 2 Si 2 O 8 There are 2 quasi-perpendicular planes of weakness where more Ca-O, K-O or Na-O bonds and less Si-O bonds are broken. tectosilicates

Single crystals of feldspars break with blocky outlines that are quasi- perpendicular.

Platy or micaceous cleavage refers to a plane of weakness in a single direction. graphite: C-C bond length is 1.42 Å within sheets, but much longer (3.35 Å) between sheets.

Molybdenite, MoS 2, also shows micaceous cleavage and softness. Only VanderWaals bonding between adjacent layers of S ions. It is used as a lubricant, like graphite. metallic luster

phyllosilicates: sheets of SiO 4 units are weakly linked by Van der Waals forces or K-O bonds

Micas can be cleaved into thin sheets. Mica flakes are used to prepare “mica paper” which is an insulating material for electric equipment.