Cosmic spherules: space dust melted into tiny balls
Cosmic spherules are micrometeorites that melted completely as they entered the air and froze again as tiny balls. They are the most common micrometeorites in most collections and the kind amateur hunters are most likely to find.
A cosmic spherule began as a speck of space rock or metal. For a second or two it became a white-hot droplet, then it cooled so fast that its crystals froze in striking patterns. Most are dark, shiny balls between 0.1 and 0.5 mm across. They are sorted into S-types (stony), I-types (iron) and G-types (in between).
Key facts
- Size
- Mostly 0.05–0.5 mm; larger ones are rare
- Shape
- Spheres or slightly squashed spheres, sometimes with a dimple or a metal bead
- Main types
- S-type (stony), I-type (iron oxide and metal), G-type (glass with iron oxide)
- S-type textures
- Porphyritic olivine, barred olivine, cryptocrystalline, glass, CAT
How a spherule forms
When a grain heats past roughly 1,300 °C, it melts. Surface tension pulls the liquid into a sphere, just as it pulls a raindrop round. Gases bubble out and some of the elements evaporate. Within a second or two the droplet has slowed and starts to cool, and it freezes so quickly that its crystals have no time to grow large and neat. Instead they grow as bars, branches, feathers and fine needles, or not at all, leaving glass.
As the droplet cools, iron at its surface reacts with oxygen and grows tiny crystals of magnetite, often forming a delicate net or a patchwork over the surface. That magnetite is one reason most cosmic spherules stick to a magnet.
S-type spherules
S-type, or stony, spherules are by far the most common. They have roughly the chemistry of chondritic meteorites, rich in magnesium, silicon, iron and oxygen. They are divided by their internal texture, which depends on how hot the droplet became and how fast it cooled:
- Porphyritic olivine: many small, separate crystals of olivine in glass, often with dark magnetite between them. These were heated least and sometimes still contain relict grains that never melted.
- Barred olivine: one or a few sets of thin parallel plates of olivine, like the bars of a grille, sometimes visible as fine stripes on the surface. They are among the most recognisable and attractive spherules.
- Cryptocrystalline: crystals too small to see with a normal light microscope, giving a smooth, often patterned surface.
- Glass: no crystals at all. These were heated more strongly and cooled before crystals could form. They are often dark but can be brownish or greenish and translucent at the edges.
- CAT spherules: so strongly heated that most of their iron, silicon and magnesium evaporated, leaving them rich in calcium, aluminium and titanium. They are rare.
I-type spherules
I-type, or iron, spherules are made mostly of the iron oxides magnetite and wüstite, sometimes around a core of nickel-iron metal. They form from metal grains, perhaps from metal-rich chondrites or tiny fragments of iron meteoroids. They are dense, black to steel-grey and often look polished, with a fine, interlocking pattern of crystals on the surface.
Many contain a minute nugget of platinum-group metals, such as platinum, iridium and osmium, which collected together as the surrounding iron was oxidised. Finding such a nugget is strong evidence that a spherule came from space, because these metals are extremely rare in Earth's crust but much more abundant in meteoritic metal.
G-type spherules
G-type spherules sit between the two. They contain a lot of magnetite, typically as branching, tree-like crystals called dendrites, set in silicate glass. They are uncommon and are thought to form from grains that were a mixture of metal or sulphide and silicate.
Metal beads and dimples
Some S-type spherules carry a bright bead of nickel-iron metal at one side. Metal and silicate liquid do not mix, and as the droplet decelerates the dense metal can be thrown towards the front. Sometimes the bead escapes altogether, leaving a smooth dimple or hollow. A spherule with a metal bead or such a dimple is a classic sign of a micrometeorite, although these features alone do not prove one.
Weathering
Once on the ground, spherules slowly change. Glass is attacked by water, and in sea-floor mud olivine dissolves or is replaced by clay over hundreds of thousands of years, until some spherules are left as hollow shells of magnetite. I-types survive longest. The freshest, least weathered spherules come from Antarctic snow and from city roofs, where they may have landed only a few years before they were collected.
Did you know?
The first cosmic spherules recognised by science were black magnetic balls dredged from the floor of the deep ocean in the 1870s.
Barred olivine spherules have the same striking texture as barred olivine chondrules in meteorites, which also formed as molten droplets in space 4.6 billion years ago.
Some I-type spherules have been found to contain wüstite, an iron oxide that forms at high temperature and is rare in ordinary rocks at the Earth's surface.