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Solid substances fall into two general classes, crystalline and amorphous. Those whose atoms show long-range order, like the squares on a chessboard or the loops in a chain-link fence, are crystalline; those whose atoms are arranged in no particular, repeating pattern are amorphous. Naturally occurring amorphous solids are also termed mineraloids.

Amorphous solids are made of the same elements that produce crystalline solids, often mixed in the same ratios. For example, pure silicon dioxide (silica; SiO2) occurs both in a crystalline form (e.g., quartz); and in an amorphous form (e.g., glass). The difference between the two forms is one of atomiclevel organization. Given sufficient time, as when precipitating atom by atom from a hydrothermal solution or solidifying slowly from a pure melt, silicon and oxygen atoms assume an orderly, crystalline arrangement because it is a lower-energy state and therefore more stable, as a pencil lying on its side has less energy and is more stable than a pencil balanced on its eraser.

However, if cooled suddenly, the silicon and oxygen atoms in, for example, molten silica have no time to line up in orderly crystalline ranks but are trapped in a random solid arrangement. Natural glasses (lechatelierites) are in fact produced in large quantities when silica-rich lava is quenched suddenly in air or, as during undersea eruptions, in water.

Although few amorphous solids beside glasses occur naturally, an amorphous form of virtually any substance can be manufactured by sufficiently rapid quenching of the liquid phase or by depositing atoms from the vapor phase directly onto a cool substrate. Vapor deposition is used to build up the amorphous silicon films found in all integrated electronic circuit chips.

Most natural amorphous solids are formed by fast quenching, but not all. The precious stone opal (SiO2 [1] nH2O) is a mineraloid formed by the solidification of a colloidal solution (fine-particle mixture) of silica and water—in essence, opal is very firm silica jello. Minerals formed by solidification of colloids, like opal, are termed gel minerals. Limonite (Fe2O3 nH2O) is another gel mineral.

A crystalline solid may be transformed into an amorphous solid by alpha-particle radiation emitted by uranium or thorium atoms contained in the crystal itself. Each alpha particle that passes through the crystal strikes a tiny but violent blow against its atomic structure, slightly scrambling the orderly ranks of atoms. A once-crystalline mineral whose crystal structure has been obliterated by alpha radiation is termed a metamict mineral