Thames & Kosmos Wow in the World: Break-Open Crystal Geodes handleiding

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Thames & Kosmos

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Handleiding

“Volcanoes

are geode

factories”

HOW DO CRYSTALS FORM?HOW DO CRYSTALS FORM?

The Great Pyramid of Giza in Egypt, a 4,000-year-old wonder

of the world, is like a

gigantic model of a crystal.

It can help us

understand the structure of a crystal, which is too small to see

with the naked eye. This pyramid was built from over

two miion

rectangular blocks

stacked on top of one another in such a way

that they form the pyramid’s shape. This is a lot like how crystals are

formed, except in their case the building blocks are much tinier —

not much bigger than atoms. These are known as unit ces.

Unit cells can be assembled in various ways depending on the

type of material involved. Some consist of

molecules,

or atoms

connected to each other in a certain way (one example being

rock candy).

With other substances, they can consist of

nothing but identical

atoms

(this applies to

diamond

crystals, for example, which are composed only

of carbon atoms). In addition, unit cells can be

made of

ions,

which are electrically charged

atoms or groups of atoms. Substances made of ions

are known as

salts

— like

table salt,

which is made of

sodium and chloride ions.

Unit

ce of

pyramid

LANDING

SITE

PYRAMID

UNIT

CE

Landing

SITE

ATOMS IN A CRYSTAL LAICE

A crystal forming in a solution

(or

cooling from molten rock) is like a

highly

active construction site.

Strong physical

forces of araction

are drawing the unit cells

together. The unit cells are constantly collecting

on all parts of the growing crystal, staying for a bit,

and then zooming away again. This happens mostly

on the edges and corners, and a little less on the

at surfaces. Usually, the arriving unit cell doesn’t

t well into its

landing site,

so it quickly its off.

Sometimes, though, the right unit cell joins just

the right place on a growing

crystal structure,

or laice, and it is held tight there. That is how a

crystal grows, layer by layer.

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