Program Speaker: Jane Ward, Diamond Expert
By Nancy Attaway
Jane R. Ward presented a fascinating and in-depth account regarding the geologic significance of the diamonds found in Ghana. Physical and geochemical analysis of the inclusions found in the diamonds of the Tarkwa and Akwatia deposits indicate the existence of two distinct, highly altered, kimberlite events. This information will further define the diamond potential in Ghana and help direct future exploration.
Situated in the tropics, Ghana is known for its diamonds, its rubber
plantations, and its watermelons. The diamonds of Ghana found in the Tarkwa
and Akwatia deposits have been dated at 1.9 to 2 billion years of age,
making them the world’s oldest known diamond deposits. Discovered in 1919
by a British geologist, the diamond deposits of Ghana were not developed
by DeBeers into a significant world diamond producer until 1925. During
the 1960’s, the diamond deposits yielded three and one half million carats
Not recognizing its geologic importance, DeBeers withdrew from the diamond deposits of Ghana in 1995. Geochemical studies conducted in 1996 revealed further mining potential. Global Positioning System (GPS) satellite readings proved crucial in locating the host ore kimberlite pipes. A joint venture between Canadian and U.S. mining companies conducted feasibility studies on the kimberlite tailings.
The diamond deposits of Ghana range through the famous Ashanti Gold Belt. Miners sift gold, blue spinels, spessartine garnets, and chrome diopside along with the diamonds using hand-held mesh trays they dip in water holes. Local mining lore spins tales of the evil diamond spirits who eat gold. The superstitious miners place the first rough diamond found that day inside their mouths and later put it into a vial of holy water to ward off the evil spirits. Men work primarily as miners, but men and women, not married to each other, have also formed diamond mining partnerships. Diamond sifting is best performed after one of the many rains, where the constant erosion weathers out the diamonds. A nearby processing plant sifts the diamonds using modern equipment and operates under very tight security.
The origins of the two kimberlite pipes in the Akwatia and Tarkwa regions indicate eclogitic and peridotite geology. The diamonds appear specific to each area by color and size. The physical attributes studied include weight, color, secondary morphology, and fluorescence. The unique patterns and contents of inclusions reveal the history of two different kimberlite geologic events in the life of the diamonds. Scientists study the surface features and the inclusions using microprobe analysis with scanning electron microscope photographs to yield clues to the origin of the host ore.
Studies also show that the diamonds of Tarkwa are of better quality, in both color and clarity, than the diamonds from Akwatia. Akwatia diamonds are more suited for industrial uses. In terms of physical size, the diamonds from Tarkwa range from one to three points, while the diamonds from Akwatia run from 20 to 30 points. Tarkwa diamonds fluoresce in every color, even vivid red tones, while only half of the Akwatia diamonds fluoresce.
Besides chemical differences, scientists study the primary morphology, growth, and resorption of the diamonds from Ghana. Most of the diamonds form as octahedrons rather than as cubes. The trigons on the octahedrons show negative orientation to the octahedral face. The grain or hardness follows the octahedral face. The presence of lamination lines indicate graining. An experienced diamond cutter knows to cut a diamond perpendicular to the trigons. Along this crystal plane, the diamond has a Mohs hardness of 8, while parallel to the trigons is hardness 10. Crystalline inclusions, such as garnet, pyroxene, and olivine, are much rarer in Tarkwa diamonds than in the diamonds from Akwatia.
Surface features on the diamond crystals yield tremendous insight into the geologic conditions present when the kimberlite pipes transported the crystals to the surface. Corrosion markings on the diamond crystal faces indicate a highly corrosive environment in the magma during transport. The diamonds from Tarkwa are found as broken or fractured crystals, indicating a rough ride from the kimberlite source to the final location miles away. In contrast, the diamonds from Akwatia show more physical features from weathering and are less broken, implying that they were found closer to the original source.
The many industrial uses of diamonds include optical applications, coatings
on magnetic discs used for data storage, and as transducers. Diamonds are
also used in acoustic applications, such as speakers. Sound waves can reach
very high velocities in diamonds, because the diamond crystal structure
does not significantly impede the acoustic wave as it travels through it.
The result is clean and crisp sounds from the speakers.
Because diamonds were formed at great temperatures and pressures within the earth, the diamond crystals themselves can withstand great temperatures and pressures. The manufacture of cutting tools and household cooking ware utilizes this feature of diamonds. Cockpit windows on commercial airlines feature diamond coatings as standard equipment. Diamond is also very inert to chemical attack at temperatures below 427 C. Our technology has advanced to now permit the deposition of thin layers of diamond on a variety of materials. Used on ships and other marine equipment, this application prohibits salt water corrosion and barnacle formation.