The scientists investigated how a process called 'fluidised spray granulation' can occur during kimberlite eruptions to produce well-rounded particles containing fragments from the Earth's mantle, most notably diamonds. This physical process is similar to the gas injection and spraying process used to form smooth coatings on confectionary, and layered and delayed-release coatings in the manufacture of pharmaceuticals and fertilisers.
Kimberlite volcanoes are the primary source of diamonds on Earth, and are formed by gas-rich magmas from mantle depths of over 150 km. Kimberlite volcanism involves high-intensity explosive eruptions, forming diverging pipes or 'diatremes', which can be several hundred metres wide and several kilometres deep. A conspicuous and previously mysterious feature of these pipes are 'pelletal lapilli ' — well-rounded magma coated fragments of rock consisting of an inner 'seed' particle with a complex rim, thought to represent quenched magma.
These pelletal lapilli form by spray granulation when kimberlite magma intrudes into earlier volcaniclastic infill close to the diatreme root zone. Intensive degassing produces a gas jet in which the seed particles are simultaneously fluidised and coated by a spray of low-viscosity melt.