Nucleation, then growth
Every crystal starts as a nucleus, a tiny cluster of atoms that happens to hold together in the right pattern. Nucleation can happen on its own in a liquid, or, more easily, on a surface such as a cavity wall or another crystal. Once a stable nucleus exists, new atoms attach to it and the pattern spreads outward, much faster than the nucleus formed.
Real crystals grow faster than perfect ones would, because defects in their structure offer easy places for atoms to attach. Impurities can slow growth or change the crystal's shape, which is one reason the same mineral can grow as needles in one deposit and blocky prisms in another. Slow, stable conditions favour fewer, larger crystals; fast changes give many small ones.
Four ways nature makes crystals
Most collector crystals come from one of these four routes:
- From molten rock: as magma cools, minerals such as feldspar and quartz crystallise from the melt.
- From hot water: hot, metal-rich water circulating through fractures in the crust deposits minerals as it cools or reacts with the rock. Many ore veins form this way, some of them hosted in limestone and dolomite.
- From weathering near the surface: rain water sinks, oxidises the original sulfide ores and carries their metals down, where they recombine as new, secondary minerals. Geologists call these supergene processes.
- From evaporation and living things: as water evaporates, dissolved minerals such as gypsum crystallise out; organisms build crystals too, like the aragonite of pearls, coral and shell nacre.
How Moroccan specimens formed
Morocco's classic specimens are a field guide to these processes.
| Specimen | How it formed |
|---|---|
| Vanadinite, cerussite, wulfenite, anglesite | Galena (lead sulfide) weathered near the surface; its lead recombined with vanadium, carbonate, molybdate or sulfate in the oxidised zone. Mibladen's deposits sit in Lower Jurassic limestones and dolomites. |
| Azurite and malachite | Copper ores weathered in the oxidised zone; azurite can later turn into malachite |
| Erythrite | Cobalt arsenides such as skutterudite weathered at Bou Azzer, releasing cobalt and arsenic that recombined as erythrite |
| Desert rose | Gypsum crystallised in damp sand above a sulfate-rich water table as the desert heat evaporated it |
| Geode | Mineral-rich water seeped into a hollow in rock and lined it with crystals from the walls inward |
| Aragonite | Calcium carbonate in a different structure from calcite; unstable at the surface, it slowly turns into calcite |
Why this matters when you buy
Knowing how a mineral forms helps you spot what does not belong. Secondary minerals like vanadinite grow on the surfaces of cavities and on minerals such as barite in the oxidised zone; a vanadinite cluster sitting on a rock it could never have grown on is a sign of a glued composite. Geodes line from the walls inward, so a 'geode' with identical points on a perfectly smooth shell deserves a closer look.
Questions collectors ask
How do crystals form?
Atoms collect into a small ordered nucleus, then more atoms attach and the pattern grows outward. In nature this happens as magma cools, as hot water deposits minerals, as surface water weathers ore into new minerals, and as water evaporates.
How long do crystals take to form?
It varies enormously with the process, from hours in a lab to geological time in rock. Slow, stable conditions generally produce larger crystals.
Why are some crystals bigger than others?
Large crystals need slow, stable growth with few nuclei competing for the same atoms. Fast changes produce many small crystals.
What is a secondary mineral?
A mineral formed by the alteration of an earlier one, such as vanadinite and cerussite forming from weathered galena.