Nenadkevichite

{{Short description|Rare silicate mineral containing niobium}}

{{Infobox mineral

| name = Nenadkevichite

| image = Nenadkevichite-Ancylite-20236.jpg

| alt =

| caption = Nenadkevichite with ancylite crystals on the side

| category = Cyclosilicate

| formula = {{chem2|(Na,Ca)(Nb,Ti)Si2O7*2H2O}}

| molweight =

| strunz = 9.CE.30a

| system = Orthorhombic

| class = Dipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)

| symmetry = Pbam

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| color = Rose-pink, very light pink, light yellow, brown; dark brown due to inclusions

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| cleavage = Poor/indistinct

| fracture = Irregular/uneven

| tenacity = Brittle

| mohs = 5

| luster = Vitreous, dull

| streak = White, very light rose-pink

| diaphaneity = Transparent, translucent, opaque

| gravity = 2.78 – 2.885 g/cm³

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}}

Nenadkevichite is a rare silicate mineral containing niobium with the chemical formula {{chem2|(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.