One giant leap for lunar minerals

US and Russian scientists discover new mineral in moon rocks.

The moon is probably the most extensively studied planetary body in the solar system, not least because it is the only one that mankind has actually set foot on. However, it is still capable of throwing up the odd surprise, such as the new lunar mineral recently discovered by a team of US planetary scientists and Russian chemists.

The mineral, a species of iron silicide (Fe₂Si), was discovered in a lunar meteorite found in the Dhofar region of Oman in January 2000. The US team from the University of Tennessee, Knoxville, and the Carnegie Institution of Washington, and the Russian team from the Vernadsky Institute of Geochemistry and Analytical Chemistry, Moscow, were investigating the meteorite with an electron probe when they identified some tarnished metal grains as compounds of iron silicide.

This is the first time that iron silicides have been found in lunar rocks and the first time that Fe₂Si has been found naturally. The researchers also discovered two other species of iron silicide in the meteorite, FeSi (known as fersilicite) and FeSi₂ (known as ferdisilicite), which had previously been discovered in terrestrial rocks.

The researchers were then faced with trying to explain the process by which the minerals had formed. The composition of the surface of the moon is a result of a range of physical and chemical processes known as space weathering, which mostly consists of impacts from meteorites, micrometeorites (10-150?m in size) and cosmic particles.

The researchers doubted whether the iron silicides had been delivered by a meteorite and thought it more likely that they had formed from chemical reactions on the moon. Similar minerals form on the Earth as a result of the high temperatures caused by lightning strikes, and the team hypothesised that an equivalent process may have occurred on the moon, but involving micrometeorites rather than lightning.

They propose that micrometeorites hitting the lunar surface melt and vaporise iron and silicates present in the soil, which then combine and condense to produce iron silicides. The researchers have named the new iron silicide mineral hapkeite, after Bruce Hapke, the US scientist who first predicted the presence of impact-induced iron in the lunar soil.

Jon Evans