Pick up a quartz point or a pyrite cube and the first thing you notice is that nature has produced flat faces and straight edges, apparently without tools. It looks designed. It is not. The flatness is a direct consequence of how atoms stack, and once you know the rule, you can read a crystal's shape the way you read a word.
The four plates in the Euhedral series are four minerals that each show a different version of the rule. The series is named for the geologist's word for a crystal with well-formed faces: euhedral, from the Greek for "good" and "seat". A crystal that grew cramped by its neighbours and shows only some faces is subhedral; one with no faces at all is anhedral. Most crystals in most rocks are anhedral or subhedral. The ones in this series had room.
A crystal is a lattice, and a lattice has planes
In a crystal, atoms or ions sit in a pattern that repeats in all three directions, like a stack of identical boxes. Any flat slice through that stack along a direction where the pattern lines up is a plane of atoms, and the planes with the densest packing are the ones that end up as faces.
The reason is a race. A growing crystal adds atoms to every exposed surface, but not at the same rate. Faces that grow fast push outward and shrink until they disappear at an edge; faces that grow slowly are left behind and get larger. The shape you see is the set of slowest-growing planes. That is why the same mineral grown under the same conditions comes out the same shape every time, and why a different mineral, with a different lattice, comes out differently.
Mineralogists call the characteristic shape the habit. Cubes, octahedra, prisms, needles, blades, and the branching forms called dendrites are all habits, and each one is the lattice telling you which planes it prefers.
Pyrite: the cube, with handwriting on it
Pyrite is iron sulfide, FeS₂, and it crystallises in the cubic system. Its commonest habit is the cube, often several cubes grown through one another. Look closely at a cube face and you will usually find fine parallel lines, the striations. They come from the crystal alternating, layer by layer, between two habits, the cube and a twelve-sided form called the pyritohedron; the striations on neighbouring faces run at right angles to one another, which is a quick way to tell a pyrite cube from a galena one.
It has been called fool's gold since prospectors first mistook its brass-yellow lustre for the real thing. The two are easy to separate: pyrite is hard (6 to 6.5 on the Mohs scale, enough to scratch glass), gold is soft; pyrite leaves a greenish-black streak on unglazed porcelain, gold leaves a gold one. The Pyrite plate draws the interpenetrant cubes with their striations, as exact as the mineral is.
Fluorite: the cube that breaks into octahedra
Fluorite is calcium fluoride, CaF₂, also cubic, and it grows as cubes so clean they look cut. But hit a fluorite cube with a hammer and it does not break into smaller cubes. It cleaves along four directions at an angle to the cube faces, and the fragments are octahedra. Habit and cleavage are two different properties: habit is the set of planes the crystal prefers to grow, cleavage is the set of planes along which its bonds are weakest. In fluorite they do not coincide, and a single specimen can show both.
Pure fluorite is colourless. The purples, greens, blues and yellows come from traces of other elements and from defects in the lattice called colour centres, which is why a single cluster can carry two colours side by side. Fluorite sits at 4 on the Mohs hardness scale, where it is one of the ten reference minerals, and it gave its name to fluorescence: many specimens glow under ultraviolet light. The Fluorite plate shows green and purple cubes on their matrix.
Bismuth: when the edges win the race
Bismuth is an element, not a compound, and a crystal of it looks like a staircase of hollow square steps in oil-slick colours. Both features have a physical cause.
The stepped shape is a hopper crystal. When a crystal grows fast from a melt, atoms are supplied more readily to its edges and corners than to the middle of each face, so the edges race ahead and the face centres fall behind, leaving each face as a hollow square frame with a smaller frame inside it. The colours are thin-film interference: a bismuth oxide layer a few hundred nanometres thick forms on the surface as it cools, and light reflecting from the top and the bottom of that film interferes, cancelling some wavelengths and reinforcing others. The thickness varies from place to place, so the colour does too. The same physics colours a soap bubble.
Bismuth melts at 271 °C, low enough that the hopper crystals in collections are usually grown on a kitchen stove. Its native lattice is rhombohedral. The Bismuth plate draws one crystal with its staircase and its spectrum.
Copper: a metal that grows like frost
Native copper, copper as the pure metal in rock, sometimes grows as a dendrite, a branching form that looks like a fern or a frost pattern on glass. The mechanism is the hopper crystal's logic pushed further. When growth is limited by how fast atoms can diffuse to the surface, any tip that happens to stick out a little reaches fresh material first and grows faster, sticks out more, and grows faster still. Branches sprout branches. Snowflakes form the same way, and so do the dendrites that grow inside a cooling casting of any metal.
Copper is dense, 8.9 grams per cubic centimetre, and a good dendrite is heavy for its size. The Copper plate is one arborescent specimen, the metal branching as it grew.
Reading a crystal
Put the four together and you have most of a first lesson in mineralogy: lattice, habit, cleavage, hardness, twinning, and two kinds of growth where the simple rule breaks down. Each plate carries the mineral's formula, its crystal system and its defining number, so the shirt can be checked against a textbook.
Every design in the series comes as a tee, a hoodie and a desk mat, and the mineral-shaped sister series is Valence, which draws the bonding that makes a lattice in the first place. Start with the Euhedral series and pick the one whose growth story you like best.
