is a mineral of iron and sulfur, and it comes out of the ground in almost perfect cubes with straight corners. Nobody cut them. How does a rock arrange itself into that shape?
From the inside out. Its atoms repeat in a cubic , so a symmetry at atomic scale shows up in a shape you can hold. During growth, atoms attach at of the lattice, and here a counter-intuitive principle applies: the fast-growing faces disappear and the slow ones stay exposed and set the shape. In the right conditions they meet at right angles.
An almost perfect cube coming out of rock looks like something a person made, and it was built from atoms outward.
This mineral is made of iron and sulfur, and its atoms repeat in a cubic unit cell. Every crystal is that unit in billions of copies, so a symmetry at atomic scale shows up in a shape you can hold.
During growth, atoms attach at different rates in different directions of the lattice. Here a counter-intuitive principle applies: the faces that grow fast disappear, and those that grow slowly stay exposed and set the shape.
In the right conditions they meet at right angles, and a cube comes out. confirms the lattice really is cubic, which is why the shape is used to identify minerals. The mineral is not always a cube: a change in temperature, in chemistry and in the space available for growth also produces lumps and many-sided forms.
wat. A pyrite cube is the large, visible copy of a cubic symmetry repeating in its atomic lattice.
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The shape of a crystal is a visible clue to an arrangement of atoms nobody can see, and it is used to identify minerals.
How do we know?
X-ray diffraction reveals the atomic lattice, and microscopy follows the relation between the conditions of growth and the shape of the faces.
What is still unknown
Pyrite is not always a cube; a change in temperature, in chemistry and in the space available for growth also produces lumps and many-sided forms.
Learn something?
Where to next
What lives on the rocks that deep-sea mining would collect?minerals
Where does the water come from in a frost flower on sea ice?crystals
Why does a thin spike of ice sometimes grow out of an ice cube?crystals