STSE #3 – Diamond Formation and Properties
 Diamonds are:
- made of pure carbon (C);
- formed deep within Earth’s upper mantle (150 km down so in the
asthenosphere);
- valued based on carats (1 carat = 0.2 grams and 1 carat is divided
into 100 points);
- found in an intrusive (plutonic) igneous rock called kimberlite;
- more transparent than all other minerals;
- harder than all other minerals;
- extremely useful (e.g. drill bits for mineral industry, petroleum
industry, and dentist profession); and
- extremely good conductors of heat due to their high thermal
conductance (good semiconductors).

It is easy to process diamonds. Here is how you do it:
- Roll the crushed ore over a greased table. The ore needs to be
mixed with water. The diamonds will repel water and be attracted
to the grease on the table. The water washes away all of the waste
materials.

Diamonds are not forever. Let me explain!
- Diamonds may be the hardest mineral, but they are not the
toughest mineral (fair to moderate ranking). There are areas of
weak bonding in the crystal lattice of a diamond and as a result, the
atomic arrangement of the carbons can change over time,
particularly when the diamond has reached the surface and the
pressure is greatly reduced. This happens over geologic time
though!

Facts about Conditions of formation:
- Conditions have to be favourable (i.e. certain temperature and
pressure conditions).
- Graphite (also pure carbon) will be produced is temperature is
below 600 degrees Celsius and pressure is below 4 gigapascal.
- Such favourable conditions can be found in the upper mantle just
below continental cratons.
- A craton is the stable foundation of a continental landmass that is
made of very old rocks.
- Temperatures are not quite as high (so a little colder) in the upper
mantle beneath cratons.
- If temperature and pressure conditions are too high, then the
diamonds could be destroyed.
The SWEET SPOT!

How do diamonds get to the surface?
- Thick kimberlitic magmas called pipes force their way up through
the upper mantle and cratons and picks up diamonds along their
way.
- Kimberlitic magmas are ultramafic in composition.
- The process is explosive so the diamonds survive.
- The expanding gases keep the pressures high.
- Upward speed is between 10 and 30 km/h (otherwise graphite
would form).
- Pipes are carrot-shaped due to the fact that gases escape as they
move upwards.
CARROT-SHAPED!
Expanding gases! Explosion.
Kimberlite Annimation:
http://www.bcminerals.ca/i/video/kimberl
ite-anim.swf
- Diamonds have also been found in association with subduction
zones at convergent plate boundaries (plate subducts down below
craton into upper mantle).
- They get brought down into the upper mantle by subduction and
get brought back to the surface by upwelling magmas (volcanoes).
This occurs about 80 km down and at temperatures of about 200
degrees Celsius.

Diamonds at the surface:
- Look for kimberlite! It is rare and may or may not contain
diamonds.
- Kimberlite will look different from surrounding rock (i.e. host or
country rock).
- Look for indicator minerals such as garnet, olivine, ilmenite,
diopside, and chrome pyrope. These form at the same conditions as
diamond. The G10 garnet is very significant!
- Kimberlite can be weathered and eroded so diamonds (and
indicator minerals) can be found in fluvial and glacial
environments.
- It is hard to find kimberlites within stable cratons. Kimberlite
weathers faster than surrounding host rock so looking for circular
depressions is key! Lakes often form in these circular lows.
Kimberlite pipe found by
geophysical means.
“Bulls-eye”!
Kimberlite has been
mined out, but you can
see the circular
depression.
Diamond Properties:
Property
Category
Formula
Colour
Cleavage
Crystal Shape
Fracture
Toughness or Tenacity
Hardness on Moh’s Scale
Lustre
Fluorescence
Other Properties
Diamond
Native nonmetal
Carbon (C)
Colourless to yellow or brown, can sometimes be pink,
orange, green, blue, or grey
Octahedral (Perfect)
Octahedral, spherical, or cubic
Conchoidal (ropy)
Fair to good
10
Adamantine (diamond-like)
Exhibits fluorescence
Good electrical insulator, good heat conductor, extremely low
compressibility, nonreactive with most acids and bases, highly
transparent, combustible if oxygen is available and
temperatures are extremely high, high density, high specific
gravity
Structure of diamond
Questions:
1.
What possible igneous textures could the rock kimberlite exhibit
given the location of crystal formation and cooling rate?
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2.
Determine the weight of the 3106 carat Cullinan Diamond in grams [1
carat = 0.2 grams].
3.
Could diamonds be discovered along mid-ocean ridges (e.g. MidAtlantic Ridge), located along divergent plate boundaries? Explain.
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4.
At which location would you expect to discover a kimberlite pipe?
Explain your reasoning.
A: the middle of the landmass of Australia
B: the middle of the Pacific Ocean
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5.
Pretend you are a prospector with the Government of Newfoundland
and Labrador and your job is to find diamonds in northern Labrador.
Briefly describe two techniques or methods that you could use (or
follow) to find them.
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6.
Predict the colour of the rock kimberlite to be based on its described
composition.
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7.
List three indicator minerals and explain why they are given the term
“indicator”.
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8.
Of all the properties of the mineral diamond, why should geologists
not focus on colour?
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How do geologists describe diamonds’ extremely low
compressibility?
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9.
10.
Explain how graphite (extremely soft carbon) could be formed instead
of diamond (extremely hard carbon)?
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Questions (Answers):
1. Most diamonds crystallize slowly deep inside Earth so the
crystals are large. The kimberlite magma then explodes to
the surface where it cools quickly, thereby causing
crystallization of the remaining magma to occur quickly.
The end result is large crystals (i.e. phenocrysts)
surrounded by a fine-grained matrix (i.e. groundmass). This
would be called a porphyritic texture. The other possibility
is an aphanitic texture if the kimberlitic magma reaches
the surface (becoming lava) (containing or not containing
diamonds) whereby it cools quickly resulting in all small
crystals.
2. Solution = 3106 carats X 0.2 grams = 621.2 grams.
Remember that 1 carat = 0.2 grams.
3. No since there are no cratons along mid-ocean ridges. No
cratons mean no locations for favourable conditions for
diamond formation. Temperatures in the upper mantle
would be too high (without cratons) and diamonds would be
destroyed. There is also no subduction along divergent
plate boundaries.
4. A: the middle of the landmass of Australia. This landmass
comprises of a craton, which acts as its foundation.
Beneath the craton in the upper mantle is where conditions
are favourable for diamond formation. Kimberlitic magmas
would pick up the diamonds on their ascent. In doing so,
the kimberlitic magmas would create carrot-shaped pipes
at various locations across the landmass.
5. (1) Use geophysical surveys and try to identify bull’s-eyes
or round targets. (2) Try to locate from the air circular
depressions (or lows) that are filled with water (i.e. lakes).
(3) Look for indicator minerals, which are minerals that
form at the same conditions as diamond. You could look for
all of these minerals in fluvial and glacial environments
(past and present) due to the fact that kimberlite rocks
can also be weathered and eroded.
6. You would think that since kimberlite came from the upper
mantle that it would be really dark in colour (similar to
peridotite which is ultramafic). As the kimberlitic magma
burns up through a continent, which is mostly granitic (i.e.
felsic), you get a mixing of felsic magma with ultramafic
magma. As a result, I would expect kimberlite to be
intermediate in composition (i.e. mixture of dark and light
minerals). Similar to diorite and Andesite.
7. Garnet, olivine, ilmenite, diopside, and chrome pyrope. All
these minerals form at the same temperature and
pressure conditions as diamond. So if you find these
minerals around a kimberlite pipe or in a fluvial or glacial
environment, then the chances are high that you could also
find diamonds.
8. Colour is not a reliable physical property since trace
impurities (i.e. elements) in the crystal lattice of a mineral
such as diamond could cause the colour to vary/change.
9. Diamonds may be the hardest mineral on Earth; however,
they are not the toughest (fair to moderate). This is due
to areas of weak bonding in the crystal lattice. As a result,
diamond has a low compressibility.
10. If temperature conditions are below 600 degrees
Celsius and pressure conditions are below 4 gigapascals,
then graphite (pure carbon) will form instead of diamond
(pure carbon).