An Iron Snowflake from the Past
November 28, 2001
Understanding the big picture of Earth system history sometimes requires examining the tiniest bits of evidence. A scanning electron microscope produced the images above of a tiny three-dimensional 'snowflake', less than one tenth of a millimetre across. But this crystal was born in an inferno rather than a freezing cloud, about 35 million years ago. The core is an octahedral grain of the spinel mineral chromite (iron chromium oxide), and is overgrown by branching whiskers of nickel-bearing magnetite (Fe3O4). The structure comes from the interior of a small, squashed sphere of weathered glass, found in limy clay, in a quarry near the Italian city of Ancona. The lower image is presented as an anaglyph, allowing it to be viewed in three dimensions with red/green glasses.
A clue as to the crystals' origin lies in the nickel, derived from the metal of an asteroid whose explosive impact created the gigantic fireball from which this spinel crystallised. Where is the crater? Two major craters of late Eocene age have been found: at Popigai in Siberia, and under Chesapeake Bay on the eastern seaboard of the USA, and may be responsible for the global extinctions and fallout of microscopic iron minerals at the end of the Eocene.