Cleavage
Stones ability to split
into 2 flat surfaces b/c well-defined planes of weak atomic bonding, breakage
occurs along these atomic planes
Occurs along a plane of
weak bonding within lattice, occurs more easily then in other directions
Cleave parallel to
possible crystal face
Cleaves along perfect planes: perfect
Cleaves along poorly defined planes: poor.
Perfect
cleavage can also be seen as steps, splitting may occur along different but
parallel layers of atoms
Break
along structural planes, between individual crystals
Plane of weakness in the crystal structure, but it is along planes
that are weakened by some applied force
Not
consistent like cleavage, can not be reproduced in same way
Caused
by pressures applied to crystal or by twinning, pressure breaks crystal on a
plane of weakness, pressure parting the break was formed long before
exacavation
Seen
as incomplete fracture lines, visible as striations or planes of inclusions
The
way in which a mineral breaks, other then parting and cleavage plane, in random
directions,
Bonds
between atoms are equal in strength and break randomly
Occurs
in all minerals (cleavage will diminish appearance of fracture surfaces)
Concoidal:
Most common, clam shell looking
Subconcoidal:
Similar to concoidal but shapes less curved
Uneven
Jagged:
Sharp points or edges
Splintery:
Fibrous or fine acicular minerals, usually stronger in one directions then in
others
Earthy:
found in massive minerals usually
The resistance to breaking, crushing, bending or tearing
Manner in which mineral deforms under stress
brittle--if a mineral powders easily
malleable--if a mineral can be hammered into sheets
sectile--if a mineral can be cut into thin shavings with a knife
ductile--if a mineral can be drawn into wire
flexible--if a mineral is bent but does not resume the original shape
elastic--if a mineral bends and resumes the original shape
Measure
of strength of structure of mineral relative to the strength of chemical bonds
Resistance
in which a mineral exhibits while being scratched and the ease of such
scratching
Minerals
with small atoms and tightly packed covalent bonds are harder and
stronger. Softest have metallic bonds
or Van der Waal.
Consistent
test as chemical compostion is generally consistent
Principle
is that a harder material can scratch a softer material but not vice versa
As
hardness increases, the difference in hardness greatly increases as well
(absolute hardness)
Massive
specimens are generally softer then individual crystals
Range of hardness within
minerals is due to presense of impurities or subsitition of different atoms
kerf
The shallow groove scratched into the surface of a diamond, in order to place a chisel to strike and cleave the stone
Parallel
to octahedral planes or cubic faces
Four
planes of cleavage (forms triangular cleavage faves) i.e. Fluorite
Two
planes of cleavage, occurs parallel to major axis/prism faces i.e. spodumene,
feldspar
Perpendicular
to major axis of mineral or parallel to pinacoid i.e. topaz
1
plane of cleavage, flaky or flat with ragged edges i.e. muscovite
3
planes of cleavage intersecting at right angles i.e. halite, galena
Parallel
to rhombohedral faces
3 planes of cleavage not intersecting at right angles i.e. calcite
The
attractive forces between molecules of same substance (not what holds the
molecule together)
Very
weak and allow slippage
Graphite
is composed of parallel layers of hexagonal arranged carbon atoms all linked up
together by covalent bonds, 3 of 4
bonds are covalent, the 4th is a van der Waals bond. The layers are
held together by the attractive forces of van der Waal bonds
Moh's Relative Scale
1. Talc
2. Gypsum
3. Calcite
4. Fluorspar
5. Apatite
7. Quartz
8. Topaz
9. Corundum
10. Diamond
talented guys can frequently
allow orgasms quickly to come
4. 5 types of fracture
i.
Concoidal
(quartz, glass, garnet)
ii.
Splinerty
(nephrite, jadeite, ivory)
iii.
Jagged
(metals, copper)
iv.
Earthy
(generally massive materials)
v.
Uneven
(anhydrite)
vi.
Subconcoidal
(andalusite)
vii.
Smooth
(some diamonds)
Fracture
is the way in which a mineral breaks other than cleavage and parting, in random
direction. Bonds between atoms are more
equal in strength and breakage is random.
5. How do you distinguish between fracture, a cleavage surface, and a polished facet of a gem?
Examine the crystal. Look for significant characteristics.
|
FRACTURE |
CLEAVE |
FACET |
|
Texture/surface contour |
Fresh, glistening |
Not parallel to
cleavage plane (difficult and will be uneven) |
|
Random directions |
May have many faces
parallel i.e. steps |
Smooth |
|
Not reproducible so you
may not see exact same plane twice |
Smooth/flat breaks in
specific directions |
Glasslike |
|
Not smooth/flat |
Reproducable |
Polishing lines |
|
|
Internal reflection
planes parallel to one another, relflecting light back similtationously |
Precision, truncated
points |
|
|
Matches minerals
symmetry |
Sharp edges |
|
|
Specific angles for
mineral |
|
|
|
Generally can’t be seen
in facetted materials |
|
6.
Describe the cleavage and incipient cleavage planes of calcite and topaz.
Topaz
has 1 direction of perfect basal cleavage parallel to its base and
perpendicular to its major axis. It is so smooth and perfect that occasionally
lapidaries will use this surface as the table of the cut gem.
Calcite
has perfect rhombohedral cleavage meaning that it has three directions of
cleavage that intersect at angles other than 90°.
Transparent
minerals show incipient cleavage breaks that look like flat ghosts or
interference colors inside the mineral, parallel to the cleavage.
The cleave is
so perfect that it may cause little small cracks called incipient cleaves.
3. Describe the considerations taken
by a lapidary when fashioning diamond
7. What in crystalline material causes cleavage and what causes parting?
Directional
hardness
Incipient cleavage