186-225A
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.
corundum
dolomite baking soda garnet
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.
Fe2O3, Al2O3 are
based on XO6
polyhedra.
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 AlO bonds in corundum.
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
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.
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.
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
A2B3(SiO4) 3
Large cations in
AO8 polyhedra
share edges with
BO6 and SiO4
groups:
• no good cleavage
• dense
• hard
• rounded grains
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.
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)SiO4
• XO6 share edges with each
other and with SiO4
• low silica content
• dense
• moderate hardness
• no good cleavage
• grains equant, subrounded
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.
Single crystals of
feldspars break with
blocky outlines that
are quasiperpendicular.
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.
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.
phyllosilicates: sheets of SiO4 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.