Sienille

Sienille is a fictional mineral species described in speculative mineralogy. In this article, sienille refers to a hydrated aluminosilicate with a sheet-like crystal structure and distinctive optical properties, typically presenting a pale blue‑green color and a vitreous to pearly luster.

Etymology and history. The name sienille is derived from a fictional region called Sien and the diminutive suffix -ille, used in early literature to denote a mineral closely related to the broader silicate family. The term first appeared in print in 1989 in a speculative geology journal and has since circulated in fictional case studies and teaching materials.

Properties. Sienille forms tabular crystals in pegmatitic pockets and exhibits a Mohs hardness of about 4 to 5. It is moderately transparent, with a refractive index near 1.60 and notable birefringence under crossed polars, which can produce characteristic interference colors in thin sections. The mineral is typically found with associated quartz, muscovite, and albite, and may display faint copper-induced greenish tints due to trace impurities.

Occurrence and locality. In the fictional geology, the type locality is described as the Lirin Mountains in the country of Valara. Sienille is said to form under relatively low-temperature metamorphic conditions and is commonly hosted by granitic or pegmatitic rocks.

Applications and research. Within the imagined context, sienille is used as an ornamental stone and by lapidaries for its color and luster. Some speculative studies have suggested potential photonic or optical applications owing to its unique crystal orientation, though practical use remains theoretical.