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Nenadkevichite with ancylite crystals on the side
Category Cyclosilicate
(repeating unit)
Strunz classification 9.CE.30a
Crystal system Orthorhombic
Crystal class Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space group Pbam
Color Rose-pink, very light pink, light yellow, brown; dark brown due to inclusions
Cleavage Poor/ indistinct
Fracture Irregular/ uneven
Tenacity Brittle
Mohs scale hardness 5
Luster Vitreous, dull
Streak White, very light rose-pink
Diaphaneity Transparent, translucent, opaque
Specific gravity 2.78 - 2.885 g/cm3

Nenadkevichite is a rare silicate mineral containing niobium with formula: (Na,Ca)(Nb,Ti)Si2O7·2H2O. It forms brown to yellow to rose colored orthorhombic dipyramidal crystals with a dull to earthy luster. It has a Mohs hardness of 5 and a specific gravity of 2.86.

It was first reported in 1955 from a nepheline syenite pegmatite in the Kola Peninsula. In addition it has been reported from Mont Saint-Hilaire, Canada; the Ilimaussaq complex, Greenland; Windhoek District, Namibia; and Zheltye Vody, Ukraine. It was named after Konstantin Avtonomovich Nenadkevich (1880–1963), Russian mineralogist and geochemist.