Tourmaline (#17)

A group of lustrous gem to gemmy tourmalines of various shades of green, ex DeTrin Minerals.
South America Pederneira mine, Minas Gerais, Brazil 3 ¼" x 1 ½" x 1"

 

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You may have recognized that well-formed crystals have flat sides, sharp edges, and distinctive shapes. These shapes {or "crystal habits") may vary in detail for different specimens of the same mineral, but there are consistencies for all crystals of the same mineral that reveal its "crystal system." There are six {or seven, depending on how you classify them) crystal systems. Each system can be defined by the distinctive geometry of its characteristic unit cell - an imaginary box that contains all of the chemical composition and crystal structure necessary to build the entire mineral, simply by stacking unit cells next to one another. Since each unit cell is a box, each has three dimensions or unique sides: a, b, and c. The angles between the sides are a (between b and c), 13 (between a and c), and y (between a and b). The crystal systems have unit cells defined as:

Isometery system a = b = c, α = ß = γ = 90°
Hexagonal system a = b ≠ c, α = ß = y = 120°
Tetragonal system a = b ≠ c, α = ß = y = 90°
Orthorhombic system a ≠ b ≠ c, α = ß = y = 90°
Monoclinic system a ≠ b ≠ c, α = ß = 90°, y > 90°
Triclinic system a ≠ b ≠ c, α ≠ ß ≠ y ≠ 90°

Unit cells are mere Angstroms on a side (remember, an A is 0.00000001 cm), so you can't see them. But if you stack many millions of unit cells together, then you get a crystal with a shape conforming to one of the crystal systems. The specific crystal faces (the flat sides) of two specimens may be different, but the overall form of the crystal will be consistent with one of the systems and the shape of its unit cell.