Answer:
Microscopically, a single crystal has atoms in a near-perfect periodic arrangement; a polycrystal is composed of many microscopic crystals (called "crystallites" or "grains"), and an amorphous solid (such as glass) has no periodic arrangement even microscopically.
Most inorganic solids are not crystals but polycrystals, i.e. many microscopic crystals fused together into a single solid. ... The third category of solids is amorphous solids, where the atoms have no periodic structure whatsoever. Examples of amorphous solids include glass, wax, and many plastics.
In condensed matter physics and materials science, an amorphous (from the Greek a, without, morphé, shape, form) or non-crystalline solid is a solid that lacks the long-range order that is characteristic of a crystal. In some older books, the term has been used synonymously with glass.
Explanation:
This chapter highlights mesocrystals as an interesting example of particle‐mediated, non‐classical crystallization processes. Mesocrystals — the shortened name for mesoscopically structured crystals — are superstructures composed of nanoparticles, being arranged three‐dimensionally in crystallographic register. Mesocrystals are often only intermediate structures in a non‐classical crystallization pathway leading to a final single crystal by nanoparticle fusion. Therefore, they are difficult to detect. Although mesocrystals were initially described for synthetic systems, recent investigations have revealed an increasing number of bio‐mineral systems which appear to be mesocrystals, but which so far have been considered to be single crystalline, including nacre and sea urchin spines. This chapter briefly defines non‐classical crystallization processes, provides some examples of synthetic mesocrystals and mesocrystals in biomineralization, and attempts to provide some insight into their formation mechanisms, despite their being as yet largely unexplored.
