Amazing Story - Part 2:
The haystack: kimberlite pipes
Octahedral diamond crystal on crushed kimberlite.
Dissolution of the mineral is a very real hazard during the next-most necessary step to the mining of diamonds: getting them within reach. The primary source of diamonds in Canada is the rock kimberlite: molten kimberlite forms at greater depths than diamonds, and sometimes brings diamonds along on its rush up to the Earth's surface.
Kimberlite has been found in many places in the world. Wherever kimberlite eruptions have occurred, they have done so in clusters. The magma was moved, by pressure, toward the surface only where there were weaknesses in the crust. Moving at approximately 10 to 30 km per hour, the magma would have passed through the crust to the surface in a few hours.
As it neared the surface, the magma's temperature and the pressure would have lessened. Wherever this happened too slowly, any diamonds that might have been in the kimberlite dissolved. In other eruptions, diamonds made it through this stage intact. Most kimberlite deposits do not contain diamonds. Fewer than one per cent of those with diamonds have enough to encourage further tests for economic potential.
Nearer the surface, the force of the kimberlite magma's final push typically blasts a path, called a pipe, that is shaped like a funnel. The widest point is at the surface, where the gases and magma exploded into the open. It is likely that the material blown out of the pipes formed a cone around the crater, but subsequent erosion has removed the cones from all known kimberlite pipes. The kimberlite in the pipes eventually cooled and solidified, leaving carrot-shaped deposits of the distinct rock. Pipes extend hundreds of metres below the current surface.
The cluster of kimberlite pipes that provided Nature's Canadian diamonds blasted up from the depths about 52 million years ago. Geologists are interested in them for more than their diamonds; these pipes are unique because layers of sediment and kimberlite volcanic materials suggest that the eruptions that made the pipes did not immediately fill them with kimberlite magma. It seems the upper portions of the pipes remained empty for an unknown amount of time. Eroded surface material eventually fell into the pipes (possibly from the cones), and later eruptions of kimberlite filled them. Geologists also think the pressure of the initial eruptions was stronger than those elsewhere: comparatively few fragments of blasted crust were found mixed in with the solidified kimberlite, suggesting that the eruptions blew the overlying rock sky-high.
Our Amazing Story Continues... Eureka!