Halyyle

Halyyle is a hypothetical mineral frequently cited in speculative geology and science fiction as a halide-bearing silicate that forms under extreme metamorphic conditions. The term is used in worldbuilding contexts to illustrate a mineral with unusual optical and magnetic properties, and it is not recognized as a real mineral by major mineralogical databases.

Formation and occurrence: In the narratives, halyyle forms in subduction-zone environments where high pressure and saline fluids drive metasomatic reactions between silicate rocks and halide-rich fluids. It is often described as coexisting with serpentine, chlorite, and halite at garnet-rich assemblages.

Composition and structure: Proposals vary, but halyyle is commonly depicted as a layered halide silicate with interlayer halide ions (chloride or fluoride) and a framework of silicon-oxygen tetrahedra linked to metal cations. The exact composition is intentionally left variable in many stories to allow worldbuilders to adapt.

Physical properties: Descriptions include a pale blue-to-violet color, glassy to pearly luster, and a hardness roughly moderate on Mohs scale (around 4–6). Some accounts assign halyyle luminescence or anisotropic optical properties, particularly under ultraviolet light.

Potential uses and research: In fiction, halyyle is imagined for advanced optical materials, radiation shielding, or lightweight energy storage, but there is no experimental synthesis or measurement reported in real science.

Status and references: Halyyle remains a fictional or speculative entry rather than a validated mineral.