Last week, we talked about Kepler’s observations on the lattice structure of snowflakes, Steno’s law, and Rene Just Huay’s fundamental unit hypothesis.
Crystal Classification
In 1830, a few decades after Huay’s fundamental unit hypothesis, Johann F. C. Hessel used geometry to derive the possible unit structures of crystals. He knew that crystals could only have certain types of rotation, two-fold, three-fold, four-fold, and six-fold. That means, starting from the center of the unit, drawing lines that split the crystal into two, three, four, or six equal sections. Think of it like a circle; starting from the center, the number of lines to the edge of the circle you draw while making sectors of the same shape is the number of rotational symmetry. Because lattices have a limited number of rotational axes, even without a microscope, Hessel was able to describe all possible fundamental unit symmetries. Auguste Bravais described many of these symmetries more specifically as types of lattice structures.
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