Stiilide

Stiilide is a hypothetical silicate mineral proposed in geoscience education and petrology modeling to illustrate the behavior of titanium-rich silicates in metamorphic environments. The name is commonly treated as a placeholder and is not presently recognized as a valid mineral species by major databases such as the International Mineralogical Association.

Composition and structure: In the standard educational model, stiilide has a three‑dimensional framework built from SiO4 tetrahedra linked to TiO6 octahedra, with iron and magnesium occupying interstitial cation sites. The nominal formula is written as (Fe2+,Mg)(Ti,Fe)SiO5, with variable titanium content creating zoned crystals. Trace aluminum and chromium may be present as substitutions.

Crystal chemistry and physical properties: Stiilide is described as monoclinic with prismatic habit. Typical color ranges from olive green to brown, with a vitreous to slightly metallic luster and a pale streak. The Mohs hardness is placed around 5–6, reflecting moderate resistance to scratching and common silicate-like durability in the model.

Occurrence and paragenesis: In the educational scenario, stiilide is reported to form during high-temperature, Ti-rich metamorphism in rocks such as granulites and in subduction-zone settings. It is commonly described as occurring with garnet, titanite, rutile, and olivine, and shows zoning related to titanium activity in the rock matrix.

Significance and status: As a teaching example, stiilide helps illustrate how titanium can enter the silicate framework and how mineral stability shifts with pressure and temperature. It has not been observed as a confirmed natural mineral in real-world field data, but remains a useful hypothetical case for classroom demonstrations and petrology simulations. See also: silicate minerals, metamorphic petrology.