Formselite

Formselite is a fictional silicate mineral commonly used in teaching and speculative mineralogy to illustrate the properties of complex sheet- and framework-silicate structures. The name is a constructed term used in educational contexts to demonstrate how minerals with layered arrangements can combine with octahedral or tetrahedral sheets.

In terms of composition and structure, formselite is described as a hydrated aluminosilicate that features alternating sheets of tetrahedrally coordinated silicon and aluminum joined by octahedral magnesium or iron cations. The interlayer region may accommodate cations such as sodium or calcium and hydrogen-bearing species, giving the mineral variable chemistry. The idealized structure is often presented as a phyllosilicate-like framework with unusual interlayer chemistry, allowing discussions of cation interchange and hydration states without referencing a real mineral with the same name.

Occurrence and formation are also hypothetical in nature. Formselite is depicted as forming through low-to-moderate grade metamorphic alterations of feldspathic rocks or via hydrothermal processes in controlled laboratory analogs. Its imagined environments emphasize the role of temperature, pressure, and fluid composition in determining layer stacking, interlayer spacing, and hydration level.

Physical properties commonly attributed to formselite in textbooks include a pale to colorless to light-green appearance, a vitreous to pearly luster, and a Mohs hardness typically cited around 4 to 6. It is described as brittle with a tendency to cleave along one dominant plane, consistent with its layered structure.

Because formselite is a didactic construct, it has no mining value and is used primarily to illustrate mineralogical concepts such as sheet silicates, interlayer coordination, and the effects of hydration on crystal structure.

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