Recognising a micrometeorite, and telling it from industrial spherules
Almost every tiny magnetic ball you find will be man-made or natural Earth material. Knowing the look-alikes is the most important skill a micrometeorite hunter can learn.
Genuine cosmic spherules are usually dark, shiny and patterned with fine crystals, sometimes with stripes, a metal bead or a dimple. Industrial spheres are often perfectly smooth and mirror-like, hollow, bubbly or covered with tiny stuck-on particles. Under a good microscope many can be ruled out, but a confident identification usually needs an electron microscope that measures the chemistry.
Key facts
- Most common look-alikes
- Welding and grinding sparks, fly ash, brake and rail dust, fireworks
- Good signs
- Barred or patterned surface, magnetite network, metal bead, dimple, olivine texture
- Warning signs
- Mirror-smooth surface, hollow shell, many bubbles, bright colours, perfect symmetry
- To confirm
- Chemistry: magnesium, iron and silicon in chondritic proportions, with nickel
Why look-alikes are everywhere
Any process that melts tiny droplets of metal or rock and lets them freeze in the air will make spheres. Cities and roads are full of such processes. Grinding and welding steel throws out showers of sparks, each one a glowing droplet that freezes into a magnetic ball. Coal and oil burning in power stations and boilers produces fly ash, which includes hollow glass spheres and magnetic iron-oxide spheres. Brakes, wheels on rails, exhaust systems and fireworks all add more. In roof sediment, these outnumber genuine micrometeorites by thousands to one or more.
Nature makes spheres too: volcanic ash, lightning striking sand, and molten droplets from large meteoroids and impacts. Beware also of round mineral grains, such as rounded magnetite in sand, and small seeds, eggs and resin beads, which can look surprisingly like spherules under a hand lens.
What genuine cosmic spherules look like
- Colour and lustre: usually black to dark grey with a glassy or metallic shine; some glassy S-types are dark brown, green or amber when lit from behind.
- Surface pattern: a fine network or mosaic of magnetite crystals, often visible as a delicate texture under strong magnification.
- Barred texture: thin parallel ridges or stripes across the surface, from barred olivine crystals inside. This is one of the most convincing features.
- Metal bead or dimple: a bright metal bead at one side, or a smooth hollow where a bead escaped.
- Shape: round or slightly flattened, sometimes teardrop-shaped, but usually with no tail or attached blobs.
- Size: most finds are 0.1 to 0.5 mm. Spheres larger than 1 mm are seldom micrometeorites.
Signs of industrial or Earthly origin
- Mirror-smooth, featureless surface, like a ball bearing. Many welding and grinding spheres look like this.
- Hollow or broken shells, or many gas bubbles throughout. Typical of fly ash and slag.
- Satellites: tiny spheres stuck to the surface of a bigger one, which happens when droplets collide in a cloud of sparks.
- Bright colours such as vivid blue, red or iridescent rainbow films, often from heat-tinted steel.
- Rough, spiky or very irregular shapes, or a long tail.
- Composition: iron spheres with chromium, manganese or no nickel at all are almost certainly from steel; glass rich in aluminium, calcium and potassium usually comes from ash or slag.
Confirming an identification
An optical microscope can rule out many look-alikes, but it cannot measure chemistry. The standard test is a scanning electron microscope with an X-ray detector, which shows the surface in fine detail and measures the main elements. A genuine stony spherule typically has magnesium, iron and silicon in roughly the proportions of chondritic meteorites, with little aluminium, calcium or sodium, and its iron is often accompanied by small amounts of nickel. Cutting and polishing a spherule reveals its interior texture and any relict crystals, and oxygen-isotope analysis can tie it to a meteorite family.
Universities, museums and some geology societies have such instruments. If you think you have found a convincing candidate, take careful photographs, keep it safely mounted, and contact a local museum or university geology department to ask whether anyone works on cosmic dust.
Unmelted particles: almost impossible for amateurs
Unmelted and scoriaceous micrometeorites are irregular, dark and look like ordinary grit. In roof or soil sediment they cannot be recognised without an electron microscope and expert knowledge. Amateur hunters should expect to find cosmic spherules, if anything, and treat any irregular particle as Earthly unless proven otherwise.
Did you know?
A few seconds of angle-grinding steel can throw out thousands of magnetic spheres, each one a frozen spark.
Some hunters keep a reference collection of known industrial spheres, labelled by source, to compare with their candidates.
Spheres from ordinary steel usually contain manganese and little or no nickel, so a quick check of their chemistry rules most of them out at once.