Hunting and collecting

Caring for meteorites

A meteorite has survived billions of years in space, but Earth's air is hostile to it. Oxygen and water attack its metal, and salts picked up from soil make the damage spread. Good storage stops that. This guide explains how to store, handle and display meteorites so they stay as they are.

Why meteorites rust

Most meteorites contain iron-nickel metal that formed where there was almost no oxygen or water. On Earth that metal reacts with damp air and turns to rust, just as a steel nail does. Stony meteorites rust too, from the inside, because their small metal grains rust and the rust stains the rock brown and swells in cracks.

Scientists grade how far a meteorite has weathered on Earth. A fresh fall picked up within days shows no rust; a desert find that has lain for thousands of years can be rusted right through. Your job as an owner is to stop the clock where it is.

Lawrencite and the weeping meteorite

The worst enemy of an iron meteorite is chloride. Many finds absorb chlorine from soil water while they lie in the ground. It collects as iron chloride, traditionally called lawrencite, along cracks and at the boundary between metal and rust.

Iron chloride draws moisture from the air. Even in a room that feels dry, it forms tiny droplets of acidic liquid, which collectors describe as the meteorite 'weeping' or 'sweating'. The liquid attacks fresh metal and produces a rust mineral called akaganeite, which takes up more space than the metal it replaces. Flakes pop off, cracks widen and, in bad cases, a specimen splits apart. Signs of active chloride rust are orange-brown spots or powder that keep coming back, yellow droplets, or flakes on the shelf beneath.

Some meteorites are notorious. Many Campo del Cielo irons and many pallasites, where metal surrounds olivine crystals, are prone to it. Others, such as Gibeon and most Sikhote-Alin pieces, are usually more stable.

Humidity: the number that matters

Rust needs water, so the key to care is dry air. Relative humidity measures how much water vapour the air holds compared with the most it could hold at that temperature.

Below about 40 per cent: most iron meteorites and stony meteorites with metal stay stable. Many homes are drier than this in a heated winter but damper in summer.

Below about 15 per cent: specimens with active chloride rust need very dry air, because iron chloride stops drawing in water only at these low levels. Ordinary rooms never get this dry, so such specimens need a sealed box with fresh desiccant.

A cheap digital hygrometer or a humidity indicator card inside the box shows you whether your storage is working.

Desiccants and sealed boxes

A desiccant is a material that soaks up water vapour. The usual choice is silica gel, sold in sachets or loose beads. Indicating types change colour, often from orange to green, when they are full. Avoid the older blue type that turns pink, because it contains a toxic cobalt compound. Molecular sieve desiccants dry the air even further and suit the most troublesome irons.

Put the meteorite and desiccant together in a container that seals well: a plastic box with a gasket lid, a heavy zip-lock polyethylene bag inside a box, or a glass jar with a good seal. Check the indicator regularly and dry or replace the desiccant when it changes colour, following the maker's instructions; drying it in an oven is a job for an adult. A box that is opened every day needs its desiccant renewed more often.

Handling

Skin carries salt and oil, and fingerprints on cut iron can rust within days. Handle meteorites with clean, dry hands, or better, with nitrile gloves or a soft cloth. Hold specimens over a table with a soft mat, so a dropped stone does not shatter; fragile meteorites such as carbonaceous chondrites can crumble.

Do not stick magnets on meteorites. Scientists study the faint natural magnetism locked in meteorites, and a strong magnet can overwrite it. Never wash a meteorite in water, scrub its crust or use household cleaners. Some meteorites contain minerals that dissolve in water, and the delicate black fusion crust is part of what makes a specimen valuable.

Cleaning and treating rust

Dust can be removed with a soft, dry brush or a gentle puff of air. Anything more is conservation work, and for a valuable or scientifically important meteorite, ask a museum conservator first.

Collectors who care for cut and etched irons usually keep them dry and protect polished faces with a thin layer of conservation-grade microcrystalline wax or a light oil. Badly affected irons are sometimes soaked by specialists in dry alcohol or treated to draw out chloride, but these treatments can change the specimen and should be done only by an experienced adult. Children should never cut, heat, wash or put chemicals on meteorites.

Remember that coatings and treatments contaminate a meteorite for chemical study. If a specimen may have scientific value, leave at least part of it untreated and write down anything that was done to it.

Storage materials

Store meteorites in clean, inert materials: polyethylene or polypropylene boxes and bags, acid-free card and archival foam. Avoid unsealed wooden drawers and cabinets, especially oak, because many woods give off acid vapours that corrode metal. Avoid soft PVC, rubber bands and cotton wool, which can shed fibres or release harmful chemicals. Small specimens fit well in membrane boxes or gem jars, which hold them still without rubbing.

Keep a label with every specimen: its name, type, mass, where and when it fell or was found, and where you obtained it. This record, called provenance, is what makes a meteorite trustworthy to science and to future owners.

Displaying meteorites

A display case that seals reasonably well, with a hidden tray of silica gel, keeps meteorites dry while you enjoy them. Place it away from radiators, sunny windows, kitchens and bathrooms, and off cold outside walls, where daily swings in temperature can cause condensation inside the case. LED lighting gives little heat. Rotate troublesome irons into a dry box and display stable ones instead.

For schools and clubs, a sealed box with a clear lid lets people see a meteorite without touching it, and a separate robust handling specimen, such as a piece of a common ordinary chondrite, can be passed around.

If you find a fresh fall

A meteorite collected within hours or days of landing is rare and precious to science, because Earth has not yet changed it. Do not wash it, test it with a magnet or keep handling it. Pick it up with a clean plastic bag turned inside out over your hand, or wrap it in clean aluminium foil, and keep it dry and cool. Note the exact place, time and the landowner's details, and photograph where it lay. Then contact a national museum or university as soon as you can. The Winchcombe meteorite of 2021 was collected within about 12 hours and kept in clean conditions, which made it one of the most pristine meteorites of its kind ever studied.

Glossary: Lawrencite, Rust bloom, Desiccant storage, Fusion crust, Provenance