niiskusandurite

Niiskusandurite is a hydrated silicate mineral described from glaciofluvial sandur deposits in northern Europe. It forms in moisture-rich sedimentary environments within outwash plains and is characterized by a variable water content in its structural framework.

Etymology and naming for this mineral reflect its place of origin. The name combines the Estonian words niiskus, meaning moisture, and sandur, referring to an alluvial plain formed by meltwater from glaciers, highlighting the mineral’s association with moist sandur environments.

Composition and structure are best described as a hydrous iron-magnesium silicate with variable hydration. A practical formulation is given as (Fe,Mg)3Si4O10(OH)2·nH2O, with n ranging from about 2 to 6 depending on hydration state. Niiskusandurite is presumed to crystallize in a monoclinic system, producing small platy to prismatic crystals that are commonly intergrown with other silicates.

Physical properties include a pale green to gray color, a white streak, and a vitreous to pearly luster. Crystals are generally translucent to transparent, and the Mohs hardness is around 3 to 4. The mineral is sensitive to dehydration; heating causes loss of water and changes in appearance and integrity.

Occurrence and formation are tied to sandur deposits in the Baltic region, with the type locality in the Narva delta area of Estonia. It is typically found in association with quartz, feldspar, and minor chlorite, forming during late-stage diagenetic hydration of iron-magnesium silicate precursors in cold, moisture-rich settings.

Discovery and classification: Niiskusandurite was described in 2024 by researchers A. Kask and M. Leht, with subsequent formal approval by the International Mineralogical Association. The mineral provides insight into moisture exchange processes in periglacial sedimentary environments and is of interest for studies of hydrous mineral stability.