Thursday, February 18, 2016

Lucapa Diamond

The Perth-based Lucapa Diamond Company found a 404 carat diamond at its mine in Angola, a republic in southern Africa. It is the biggest diamond found to date in Angola and has been valued at about $20 million.

The mass of gemstones such as diamonds is measured in carats. 1 carat equals 200 mg.
1 carat is subdivided into 100 points, so a point is equal to a mass of 2mg.

Diamonds form at depths of 150 - 200 km in the upper mantle where temperatures range from 900 to 1300oC and the pressure is about 50,000 times atmospheric pressure. Under these extreme conditions, carbon atoms come together to form the diamond structure in which each carbon atom makes 4 covalent bonds to other carbon atoms. In the diagram shown, each black ball represents a carbon atom and each line represents a covalent bond.

Diamonds are brought to the earth's surface from the upper mantle  in a dyke which geologists refer to as a kimberlite pipe.

 The density of naturally occurring diamonds varies between 3.15 and 3.53 g cm-3, with the purest diamonds having a density closer to 3.52 g cm-3.
Type I diamonds include diamonds in which nitrogen is present as an impurity. A colourless Type 1 diamond has very little nitrogen impurity. As the amount of nitrogen present increases, the diamond becomes more yellow.
Type II diamonds have  no measurable nitrogen impurity. The Lucapa diamond is a Type II diamond and is colourless because it contains no measurable nitrogen or other impurity, and, the structure has not been changed significantly from the "pure" diamond structure shown above during formation.
Some Type II diamonds are coloured pink, red or brown as a result of changes to the structure of the diamond during formation so that light is scattered in such a way as to produce these colours.
The presence of boron as an impurity results in a light blue coloured diamond, whereas the presence of black impurities such as graphite or sulfides produces a black diamond.
The presence of measurable quantities of hydrogen as well as structural changes during formation can result in purple diamonds.


Further Reading:
Mass Conversions

Suggested Study Questions:
  1. Convert 200 mg to a mass in
    • grams
    • kilograms
    • micrograms
  2. The diamond found at Lucapa is reported to be a 404 carat diamond. What is the mass of this diamond in
    • grams
    • kilograms
    • micrograms
  3.  The value of the Lucapa diamond is estimated to be $20 million. What is the value of the diamond:
    • per carat
    • per point
    • per milligram
    • per gram
  4. Assuming the Lucapa diamond to be a pure diamond, its density is 3.52 g cm-3 . What is the volume of this diamond? 
  5. Imagine pouring 500 mL of water in a 1000 mL measuring cylinder at 101.3 kPa and 25 oC, then dropping the Lucapa diamond into the water. What would the new volume of water be in the measuring cylinder? (assume the density of water is 1 g cm-3)
  6. Coal is also made up largely of carbon atoms, with tiny amounts of oxygen and impurities such as sulfur.  It is usually found as a vein or seam of coal within sedimentary rocks. Coal has an average density of about 1.2 g cm-3. If a lump of coal had the same volume as the Lucapa diamond, what would its mass in grams be?
  7. Given the difference in the density of diamond and the density of coal, what does this tell you about the structure of diamond and coal?
  8. If you were given 5 grams of coal and 5 grams of diamond, which would have the greatest volume?
  9. Draw a diagram to represent the 3-dimensional structure of diamond. On your diagram:
    • draw a red line to indicate a covalent bond
    • use a blue pencil to show a carbon tetrahedron
    • use a black pencil to show one carbon-carbon bond angle
  10. What is the angle between two adjacent carbon atoms in the diamond structure?

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