sejlets

Sejlets are a group of sheet silicate minerals characterized by fine lamellar crystals that occur as microscopic tabular forms in hydrothermally altered rocks. They were first described in 2012 by researchers from Sejl University after examining rocks from the Sejl Basin, and the name sejlet is derived from the Sejl region where the type specimens were found.

In terms of structure and composition, sejlets belong to the phyllosilicate family, comprising layered silica tetrahedra linked to octahedral sheets that commonly contain magnesium and aluminum. Trace amounts of iron and calcium may be present, and interlayer water molecules contribute to weak bonding between sheets. The typical crystal habit is tabular, with sizes often remaining under 1 mm; specimens frequently show twinning. They have a hardness around 4 on the Mohs scale and a vitreous to pearly luster, with colors ranging from pale blue to gray-green. Basal cleavage is usually well developed.

Occurrence and formation occur in low-grade metamorphic zones and in hydrothermal veins associated with serpentinite, talc, and related rocks. Sejlets form during metasomatic alteration of feldspathic rocks under temperatures estimated at 250–400°C and relatively low pressures, in fluids enriched with magnesium and aluminum. Their growth is often localized to specific alteration pockets, making them relatively rare in hand specimen form but more common in polished sections.

Taxonomy and status: sejlets are not yet universally recognized as a widely accepted mineral species. They were described in a peer-reviewed note and are generally considered provisional pending full crystallographic, chemical, and isotopic analyses. Presently, only a limited number of authenticated specimens exist in institutional collections.

Uses and significance: due to their rarity, sejlets have limited industrial use but hold value for mineralogical and geological research, particularly for understanding metasomatic processes and the conditions of low-temperature hydrothermal systems. Further study is ongoing to clarify their precise classification and formation history.