By BBC News Online's Damian Carrington

UK meteorite hunters have tracked down two extremely rare space rocks which will help probe the deepest parts of the Earth.

The samples were spotted in the arid wastes of the Namib desert. They contain minerals that have never been seen in terrestrial rocks and yet probably make up most of our planet.

The reason for this apparent paradox is that the minerals, called ringwoodite and majorite, only form under extreme pressure - deeper than 400km below the Earth's surface. Rocks from this depth rarely come to the surface but, as the Earth's rocky mantle extends to 2,900km down, the minerals will nevertheless be very abundant.

Now that the cosmic coincidence of a meteorite fall has served up a sample to mineralogists, a deeper understanding of the region of the Earth where earthquakes and volcanoes originate may follow.

"Having a real sample can tell you more about how the mineral forms in nature," says expedition member, Dr Sara Russell at the Natural History Museum in London.

"You can synthesise the minerals, but out of the sterility of the lab we can see how impurities affect the minerals in ways we have not previously understood.

"And these meteorites have the same bulk composition as the Earth, so the mineralogical changes in these do have direct relevance to understanding the planet."

Titanic collision

The ultra-high pressure minerals in the meteorites are believed to have a dramatic origin - titanic collisions between asteroids in the earliest days of the Solar System.

At that time, billions of years ago, the asteroid belts were much more congested than now and collisions were more common. The shock of the most intense impacts partially melted the asteroids, and the new minerals grew from the liquid rock.

Aeons later, the altered space rocks were caught by the Earth's gravity and dragged to ground. The samples discovered in the Namib desert appear to be a pair, found just over a kilometre apart. About 20% of the rock was melted in the primeval collision.

The two fragments found weigh just 136 g and 80 g. These weights are too small for the meteorites to have had sufficient momentum to form the shock minerals on impact with Earth.

The expedition to Namibia yielded a total of 13 meteorites, making it the most successful British trip for 20 years. The rare mineral discoveries will be reported at a conference at Nasa's Johnson Space Center (JSC) later this year.

Dr Russell, with colleagues Matt Genge, Phil Bland and, from JSC, Mike Zolensky, spent several weeks searching the parched ground for meteorites.

The dry conditions mean that fallen meteorites, which are often rich in metallic iron, do not rust away. Many of the samples are thought to have landed on Earth over 10,000 years ago.

Although the meteorites will not be sold, their commercial value is three times greater than the cost of the expedition.

"In Namibia, we only scratched the surface of what we could find - we'd love to go back," says Dr Russell.