hilliten

Hilliten is a mineral species described in the field of clay and phyllosilicate minerals. It is characterized as a potassium- and hydrogen-bearing aluminosilicate with a layered, mica-like structure. The name derives from the Hill region where the type specimen was found. Chemically, hilliten is variable, but analyses typically yield a formula that reflects interlayer cations such as potassium and hydronium balancing the aluminosilicate layers.

Structurally hilliten belongs to the sheet silicate group, consisting of stacked tetrahedral silica sheets linked to octahedral alumina sheets. This mica-like arrangement allows interlayer cations to occupy sites between the sheets, contributing to its defining optical and physical properties. In practice, hilliten is described as monoclinic with perfect basal cleavage and a pearly to dull luster.

Physical properties commonly cited for hilliten include a Mohs hardness in the 2.5–3 range, a pale yellow to brown color, and a white to pale streak. It is typically translucent to opaque and has a specific gravity around 2.6–2.8 g/cm3. The mineral is most often associated with other clay minerals such as illite and muscovite and tends to form in weathered volcanic ash deposits and other low‑grade metamorphic or clay-rich sedimentary environments.

Occurrence and distribution are currently limited in published literature, with the type locality in Hill County. Some analyses and reviews treat hilliten as a distinct mineral species, while others propose it as a variety or intergrowth within illite, reflecting ongoing discussions about its precise classification.

See also: illite, muscovite, phyllosilicate minerals.