Mineraloid

Mineraloids are mineral-like materials that fail the crystalline test - their atoms don’t organize into the regular, long-range repeating order that defines true minerals. ^[1] They’re substances that otherwise are like minerals in chemistry and occurrence but lack an ordered atomic arrangement. ^[2] Despite not being minerals by strict definition, they fall within the domain of mineralogy. They fall into two main types: amorphous solids and natural glasses. ^[1]

The term also applies to liquids like water and mercury, which also lack internal order. ^[2] Glacial ice meets the criteria for a mineral, whereas liquid H₂O fails the test. Similarly, the fluid nature of naturally occurring mercury forces its exclusion under a rigorous application of the solid-state rule. ^[2]

Amorphous Solids

Amorphous solids lack long-range atomic order but may have short-range order in the range of approximately 10-100 Å. ^[1] Without a crystal lattice, such materials produce no coherent X-ray diffraction and develop no crystal faces. Solids that lack an ordered atomic arrangement are called amorphous. ^[2]

Several natural solids are amorphous: ^[2]

Due to an inconsistent chemical makeup and an absence of internal crystalline geometry, volcanic glass falls outside the strict definition of a mineral. ^[2] It is the most common amorphous natural solid. ^[1]

Limonite is a hydrous iron oxide that lacks a crystalline structure. ^[2]

Allophane is a hydrous aluminum silicate that also lacks ordered structure. ^[2]

As perhaps the most famous mineraloid, opal is composed of polymerized silica gel that frequently forms microscopic spheres. ^[1] Chemically it is essentially SiO₂ - the same composition as quartz - but the silica never organized into a true lattice, making it amorphous. ^[2]

Metamict Minerals

Metamict minerals take a different path. They start as true minerals, then lose their crystalline structure over time. The crystalline structure of U- and Th-bearing minerals such as zircon may be extensively disrupted by radioactive decay of those elements, and the term metamict describes these disrupted structures. Once a mineral’s structure becomes metamict, it is properly considered a mineraloid. ^[1]

Several metamict minerals qualify as examples. Radiation emitted by decaying isotopes within these minerals breaks down their initial lattice, progressively erasing their crystalline nature. ^[2] These include:

• Microlite ^[2]
• Gadolinite ^[2]
• Allanite ^[2]

Each decay event displaces atoms from their lattice positions, and over geological time the cumulative damage destroys the original order entirely. What remains is structurally amorphous even though the material began as a well-ordered crystal.

Natural Glasses

Natural glasses also qualify as mineraloids. ^[1] In each case a melt is quenched too rapidly for any organized lattice to develop, producing a rigid but disordered solid.

Pseudotachylite forms in fault zones from frictional heat during intense shearing. ^[1] The surrounding rock quenches the frictional melt almost instantaneously, preserving a glass that marks the slip surface.

Impact glass forms when a large meteorite strike releases enough energy to melt the target rocks. ^[1] Small glass bodies called tektites are interpreted as solidified impact melt ejected from the crater, carried ballistically through the atmosphere, and frozen before landing. ^[1] Tektite strewn fields can be traced back to specific impact events.

Fulgurites form when a lightning strike heats soil or rock enough to melt it, producing a glassy tube that preserves the path the lightning took through the ground. ^[1]

Ash glass / clinker forms when burning coal beds generate enough heat to fuse the surrounding rock into a slaglike glass. ^[1]

In every case - volcanic, fault, impact, lightning, or combustion - a brief intense heat event melts material that cools too fast to crystallize.

References

1. Nesse, W. D. (2017). Introduction to Mineralogy, 3rd ed. Oxford University Press.
2. Klein, C. (2002). Manual of Mineral Science, 22nd ed. John Wiley & Sons.

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