Same Mineral, Same Chemistry, Completely Different Shape. Why?
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The external shape a crystal takes is not fixed by its internal structure, and the same substance can grow as needles, cubes or plates depending on the conditions it grew in.
Shape against structure
A crystal's internal arrangement of atoms is fixed for a given substance and form, and repeats identically throughout. The external shape is a separate matter, determined by how fast different faces grew while the crystal was forming. A face that grows quickly shrinks and eventually disappears, because material added to it moves the face outward until neighbouring slower faces meet. The faces that survive are the slow ones. The shape you see therefore records growth rates rather than the underlying geometry, which is why one mineral takes many shapes.
What controls the growth rates
Several conditions during formation change the outcome:
- •How quickly the crystal grew overall
- •Temperature and pressure during growth
- •How concentrated the solution or melt was
- •The presence of impurities, which stick to particular faces
- •Available space, and whether growth was obstructed
- •How much the conditions changed while growth continued
The impurity trick
Small quantities of a foreign substance change shapes dramatically, which is both a natural phenomenon and an industrial tool. An impurity that attaches preferentially to one type of face slows growth there, so that face becomes larger while others shrink. Common salt grown from pure solution gives cubes and grown with a trace of urea gives octahedra, from the same chemistry. Manufacturers exploit this deliberately, since the shape of crystals determines how a powder flows, how fast a drug dissolves, how a fertiliser cakes and how easily a product is filtered and dried.
The words used for the shapes
Mineralogy has a specific vocabulary for these forms and a few terms cover most specimens. Needle-like describes long thin crystals, and hair-like describes finer ones still. Bladed describes flattened elongated forms. Tabular describes flat plates and equant describes roughly equal dimensions in every direction. Botryoidal describes rounded lumps resembling a bunch of grapes, formed where many crystals grew outward from scattered centres. Dendritic describes branching patterns that look like plants and frequently fool people into reporting fossils. The terms describe shape only and carry no implication about the mineral involved.
Why geologists care
Shape is evidence about history, which is what makes it worth recording rather than merely describing. Long thin crystals generally indicate rapid growth or growth along a preferred direction in a flowing environment. Broad flat ones indicate growth between confining surfaces. Well formed crystals with complete faces on all sides grew freely in a cavity or a melt, while irregular masses grew crowded against neighbours. Fibrous forms indicate growth in a crack that was opening as the crystal filled it. A hand specimen therefore carries information about the conditions that no chemical analysis provides.
The takeaway
Internal atomic arrangement is fixed for a substance, and external shape is decided by which faces grew slowest, since fast growing faces shrink and vanish. Growth rate, temperature, concentration and especially impurities all change the result, so salt gives cubes or octahedra from the same chemistry. Shape therefore records the conditions of growth and is read as evidence.