186225A

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186-225A Properties of Minerals Week 3 cleavage
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• 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.

corundum
dolomite
garnet
baking soda
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Tendency of minerals to break along flat planes
is called CLEAVAGE. You can only notice cleavage
in specimens where single crystals are large
enough (gt 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.
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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.
Fe2O3, Al2O3 are based on XO6 polyhedra.
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Cleavage is well developed in solids showing a
wide range of bond types. It is absent only in
purely metallic solids.
halite, NaCl
fluorite CaF2
sylvite, KCl
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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.
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Structures with similar type of bonds (here,
ionic bonds) but different bond orientationss
give rise to different types of cleavage.
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In halite, the planes of weakness are
perpendicular to 3 perpendicular axes. The
cleavage is cubic, i.e. along three perpendicular
planes.
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Orientation of Ca-F bonds towards body diagonals
of a cube result in an octahedral cleavage.
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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 gt Na) gradually takes a more metallic
character.
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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.
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Sulfur breaks with a conchoidal to uneven
fracture.
The S8 rings are not aligned predominantly along
certain planes within the structure. The weak Van
der Waals forces are not restricted to any
particular direction.
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Diamond, a crystalline polymorph of carbon
Tetrahedral coordination gives diamond its
octahedral cleavage. These planes cut across less
C-C bonds per unit area.
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Cutting a rough diamond into a gemstone requires
knowing where to cleave it.
A sharp blow is all thats needed to split the
stone.
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Silicate minerals show combinations of bonds that
are partly covalent and ionic.
Si-O bonds are the strongest bonds in their
structures.
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Garnet group A2B3(SiO4) 3

• Large cations in AO8 polyhedra share edges with
  BO6 and SiO4 groups
• no good cleavage
• dense
• hard
• rounded grains

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Quartz is a tectosilicate. All SiO44- 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.
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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.
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olivine (Mg, Fe)SiO4

• XO6 share edges with each other and with SiO4
• low silica content
• dense
• moderate hardness
• no good cleavage
• grains equant, subrounded

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Feldspars KAlSi3O8 NaAlSi3O8 CaAl2Si2O8
tectosilicates
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.
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Single crystals of feldspars break with blocky
outlines that are quasi-perpendicular.
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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.
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metallic luster
Molybdenite, MoS2, also shows micaceous cleavage
and softness. Only VanderWaals bonding between
adjacent layers of S ions. It is used as a
lubricant, like graphite.
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phyllosilicates sheets of SiO4 units are weakly
linked by Van der Waals forces or K-O bonds
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Micas can be cleaved into thin sheets. Mica
flakes are used to prepare mica paper which is
an insulating material for electric equipment.