|South America||Minas Gerais, Brazil||23" x 8 ¾" x 6½"|
The idea of unit cells - the imaginary tiny boxes that are repeatedly stacked next to one another to form an entire crystal - was introduced in Learn More for specimen #17. This concept began to take shape in the late 17th and early 18th centuries as scientists of the day made careful measurements on crystals.
Examine carefully the shapes of the amethyst (purple quartz) crystals in this specimen. They are quite similar, but not all exactly identical. Now look at the shapes of the quartz crystals on the large centerpiece of this display - again, though similar, they are not exactly the same. The cross-sections of all of these crystals have six sides, and you probably assume (correctly) that the ideal shape would be a perfect hexagon.
If every quartz crystal has the same ideally shaped hexagonal unit cell, you could stack more in one direction than another and distort the shape of the crystal from ideal, but one thing would remain constant: the angles. Each crystal face would be 120° from the adjacent faces because the unit cells, given their shape, could make only 120° angles at the corners.
A Danish scientist, Nicolas Steno (1638-1686), made careful measurements on many different sorts of crystals and showed that angles between corresponding faces on crystals of the same mineral are the same for all specimens. This led, among other things, to the eventual recognition that there must be tiny building blocks of crystals, and that the shapes of these blocks (unit cells, nowadays) govern the angles between crystal faces.