Sciencemadness Discussion Board

Making vivianite

Arcaeca - 14-12-2019 at 12:15

So I learned that "blue ochre" exists, a naturally-occurring blue iron-based pigment historically obtained by grinding up the mineral vivianite, a hydrated iron phosphate. Which is odd, given that iron compounds usually stay in the brownish-orangish to yellow to green range.

According to one study I found (DOI: 10.1007/s00339-010-5637-9):
Quote:
The interest in blue iron minerals as pigments for painting was greatly enlarged in the early tenth century A.D. with the use of vivianite, which in medieval Europe became a suitable alternative to the very expensive commodity imported from Afghanistan—lapis lazuli or lazurite, with general formula (Na, Ca)4(AlSiO4)3(SO4, Cl, S). This mineral is a member of the ultramarines, a family of closely related pigments based on the framework structure of sodalite [1] and whose blue color is due to the presence of sulfur polyanions (S3−) partially replacing sulfate or chloride anions in cages within the tetrahedral aluminous-silicate framework. Conversely, vivianite—a mineral with ideal formula Fe3(PO4)2·8H2O — owes its diagnostic indigo-blue color to intervalence charge transfer (IVCT) between iron ions filling dimers of edge-sharing octahedra in the crystal structure.


So it gets its blue color from iron ions constantly swapping between the Fe+3 and Fe+2 oxidation states. Great. So this isn't just a simple hydrated iron(II) phosphate after all, which is ordinarily orangey-yellow. It's extraordinarily dependent on the crystal structure that confines the possible charge transfers.

From another study that mentions synthetic vivianite (DOI: 10.1016/s0040-6031(02)00505-1):
Quote:
A blue encrustation was found on the repatriated remains of three US servicemen listed as missing in action (MIA) from Vietnam after 28 years. The identification and origin of the blue material was determined. Scanning electron microscopy with energy dispersive analysis and powder X-ray diffraction identified the material as the mineral vivianite, Fe3(PO4)2·8H2O [2]. It is possible that the minerals can be formed from aqueous solutions from ground waters and precipitated from solutions in sediments and soils...

The natural vivianite mineral was obtained from BK minerals suppliers and was composed of large deepblue prismatic crystals. The mineral originated from the Ukraine. Synthetic vivianite iron(II) phosphate Fe was prepared by the slow addition of the 3.5×10-3 M ferrous sulphate solution to a very dilute 5.0×10-3 M sodium phosphate solution using a peristaltic pump at 70 ◦C. The hydrated synthetic vivianite precipitated from the solution and were filtered and dried.


I have trisodium phosphate on hand and can pretty easily get iron(II) sulfate from the gardening section of a hardware store, but has anyone... [I]tried[/I] this? This seems too easy. Vivianite is known to be pleocroic and occur in many colors; did you actually get blue? Any idea why it would have to be so dilute and added so slowly?

Alternatively, on hand I have: 10% ammonia, 3% hydrogen peroxide, 6% acetic acid, 31% HCl, 5% NaClO, iron metal (zinc plated, mostly), copper metal, tin metal, zinc metal, lead metal, trisodium phosphate, sodium carbonate, calcium carbonate, sodium metabisulfite, sulfur, solid NaOH, copper(II) sulfate, oxalic acid and acetone.

Anyone see a path using those materials to hydrated iron(II) phosphate? My first instinct is to oxidize iron metal into solution with the peroxide and then add TSP, but that would probably overoxidize the iron and give me iron(III) phosphate.

Note that since my intention is to use this as a pigment, the color is most important to me. It can be the purest iron(II) phosphate on Earth but if it's not blue, it's useless to me.

Dan Vizine - 16-12-2019 at 14:33

The color variability of synthesized Vivianite is your clue as to why the reagents need to react as they do. Particle size is associated with the observed color of many inorganic compounds. If you significantly change the reaction conditions, you'll likely get a different-colored product.