microcrystals

Microcrystals are crystals whose dimensions lie in the micrometer range, roughly from one micrometer up to a few hundred micrometers along at least one dimension. They may appear as single crystals visible under a light microscope or as small polycrystalline aggregates in powders. The term is used across mineralogy, chemistry, and materials science to distinguish these entities from macroscopic crystals and from nanoscale crystallites.

Microcrystals form by crystallization from a solution or melt, or by solid-state transformation during cooling or aging. Growth conditions such as supersaturation, temperature, and impurities influence final size and habit. Some minerals naturally occur as microcrystalline masses (for example chalcedony, a microcrystalline form of silica) rather than as large, euhedral crystals. Microcrystalline materials can have well ordered lattices but less well defined external facets than large crystals.

Characterization relies on optical microscopy, scanning or transmission electron microscopy, and X-ray diffraction. XRD patterns reveal crystalline structure and degree of crystallinity; electron microscopy assesses morphology, size distribution, and defects. The crystallinity fraction, particle size, and shape influence properties such as surface area and dissolution behavior.

Applications span pharmaceuticals, catalysis, pigments, ceramics, and model systems for crystallization studies. In drug formulation, crystal size can affect dissolution rate and bioavailability; in materials engineering, microcrystalline powders can enable controlled packing, sintering, and optical or catalytic behavior.

See also: chalcedony, X-ray diffraction, crystallography.