metavivianite

{{Infobox mineral |boxbgcolor=#8a8f82| name = Metavivianite | boxtextcolor = #fff | category = Phosphate mineral | image = Metavivianite Pavel-Kartashov.jpg | caption = A 6x4 cm specimen of paramorphosed olive-green to green Metavivianite after oxidized vivianite. From Kerchenskoe deposit, Crimea Oblast, Ukraine. Photo and collection specimen by Pavel Kartashov. | formula = | IMAsymbol = Mviv | molweight = 499.548 g/mol | strunz = 8.DC.25 (10 ed)7/C.14-20 (8 ed) | dana = 40.11.9.4 | system = Triclinic | class = Pinacoidal (1) | symmetry = P1bar | unit cell = a = 7.989(1) Å, b = 9.321(2) Å c = 4.629(1) Å; α = 97.34(1)°; β = 95.96(1)°; γ = 108.59(2)° | color = Dark blue to blue-black; Dark green to green-black | habit = Bladed crystals, often with irregular acute multiple terminations. | twinning = {110} | cleavage = Perfect on {110} | tenacity = Sectile | mohs = 1.5–2 | luster = Sub-vitreous, Resinous, Greasy, Dull | streak = Blue or greenish blue | diaphaneity = translucent | gravity = 2.69 | density = | polish = | opticalprop = Biaxial (+) | refractive = nα = 1.600 – 3.000, nβ = 1.640 – 3.000, nγ = 1.685 – 3.000 | birefringence = δ = 0.050 – 0.085 | pleochroism = Visible; X = blue to blue-green; Y,Z = yellow-green | 2V = Measured: 85° (5), Calculated: 90° | dispersion = Very weak | extinction = | length fast/slow = | fluorescence= Not fluorescent | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | references = }}

Metavivianite () is a hydrated iron phosphate mineral found in a number of geological environments. As a secondary mineral it is typically formed from oxidizing vivianite. Metavivianite is typically found as dark blue or dark green prismatic to flattened crystals.