Vivianite tabular crystal, transparent, with a deep green color. Crystal size: 82 mm × 38 mm × 11 mm. From Huanuni mine, Dalence Province, Oruro Department, Bolivia
Vivianite (Fe(II) 3(PO 4) 2·8H 2O) is a hydrated iron(II) phosphate mineral found in a number of geological environments. Small amounts of manganese Mn2+, magnesium Mg2+, and calcium Ca2+ may substitute for iron Fe2+ in its structure.[5] Pure vivianite is colorless, but the mineral oxidizes very easily, changing the color, and it is usually found as deep blue to deep bluish green prismatic to flattened crystals. Vivianite crystals are often found inside fossil shells, such as those of bivalves and gastropods, or attached to fossil bone. Vivianite can also appear on the iron coffins or on the corpses of humans as a result of a chemical reaction of the decomposing body with the iron enclosure.[6]
It was named by Abraham Gottlob Werner, the "father of German geology", in 1817, the year of his death, after either John Henry Vivian (1785–1855), a Welsh-Cornish politician, mine owner and mineralogist living in Truro, Cornwall, England, or after Jeffrey G. Vivian, an English mineralogist.[7] Vivianite was discovered at Wheal Kind, in St Agnes, Cornwall.[3]
Note: Metavivianite, that vivianite readily alters to, is not a member of the vivianite group because it contains trivalent Fe3+ cations.
Structure
In pure end member vivianite all the iron is divalent, Fe2+, but there are two distinct sites in the structure that these ions can occupy. In the first site, the Fe2+ is surrounded by four watermolecules and two oxygens, making an octahedral group. In the second site, the Fe2+ is surrounded by two water molecules and four oxygens, again making an octahedral group. The oxygens are part of the phosphate groups (PO43−), that are tetrahedral. The vivianite structure has chains of these octahedra and tetrahedra that form sheets perpendicular to the a-crystal axis. The sheets are held together by weak bonds, and that accounts for the perfect cleavage between them.[5]
The crystals are monoclinic, class 2/m, space group C 2/m, with two formula units per unit cell (Z = 2). The approximate values of the unit cell parameters are:
a = 10.024(6) Å, b = 13.436(3) Å, c = 4.693(4) Å, β = 102.30(5)°[10]
Appearance
The mineral may occur as crystals, or as masses or concretions.[5] The crystals are usually prismatic parallel to the c-crystal axis, and flattened perpendicular to the b-axis. Equant crystals are rarer.[2][3][5] They may also occur as stellate (star-shaped) groups, or encrustations with a bladed or fibrous structure.[5]
Unaltered specimens are colorless to very pale green, but they oxidize on exposure to light (and possibly also in situ) to blue, then darker green, brown, purple and purplish black. The streak is white, altering to dark blue or brown. Crystals are transparent to translucent with a vitreousluster, pearly on the cleavage surface, or dull and earthy.[2][3][4][5]
Birefringence: δ = 0.050–0.059[3] or 0.0470–0.0730[4]
The refractive indices increase with increasing oxidation, the birefringence decreases, and the pleochroism on {010} becomes stronger.[3][5]
The angle between the optic axes, 2V, has been measured as between 63° and 83.5°; it can also be calculated from the refractive indices, giving a value between 78° and 88°.[3][4] The dispersion of the optic axes is weak, with r<v,[2][3][5] or non-existent.[4]
Vivianite is pleochroic with X= blue, deep blue or indigo-blue; Y= pale yellowish green, pale bluish green or yellow-green; Z= pale yellowish green or olive-yellow. X is parallel to the b-crystal axis and Z is inclined to the c-crystal axis at an angle of 28.5°.[2][3][5] It is not fluorescent.[3][4]
Physical properties
Vivianite is a soft mineral, with Mohs hardness only 1+1⁄2 to 2, and specific gravity 2.7. It splits easily, with perfect cleavage perpendicular to the b-crystal axis, due to the sheet-like structure of the mineral. It is sectile, with a fibrous fracture, and thin laminae parallel to the cleavage plane are flexible. It is easily soluble in acids.[3][5]
It has a melting point of 1,114 °C (2,037 °F),[3] it darkens in color in H2O2,[3] and is not radioactive.[4]
The type locality is Wheal Kind (Wheal Kine), West Wheal Kitty group, St Agnes, St Agnes District, Cornwall, England.[3]
Photo-oxidation
Oxidation of vivianite is an internal process; no oxygen or water enters or leaves the mineral from the outside. A visible light photon knocks a proton out of a water molecule, leaving a hydroxide ion (OH−). In turn, a divalent iron Fe2+ loses an electron to become Fe3+, i.e., it is oxidized and balances the charge. This process starts when visible light falls on the vivianite, and it can occur within a few minutes, drastically changing the color of the mineral. Eventually, the vivianite changes to a new species, metavivianite Fe2+2Fe3+(PO4)2(OH)·7H2O, which usually occurs as paramorphs after vivianite.[12]
Pigment
Vivianite was known as a pigment since Roman times, but its use in oil painting was rather limited.[13] It has been found in Vermeer'sThe Procuress in the blue-grey parts of the carpet in the foreground.[14]
Johannes Vermeer, The Procuress, 1656
Localities
Brazil. Cigana Mine, Galileia, Minas Gerais, with muscovite and pyrite.[15] Typically wedge-shaped crystals of vivianite to 11 cm across, of medium lustre, smoke-blue color and good transparency on matrix of sharp silvery muscovite plates, some with druses of pyrite microcrystals.[16]
Japan: At Nagasawa, Iwama-machi, Ibaraki Prefecture, vivianite was found along fractures in rocks rich in graphite, pyrite and pyrrhotite. The vivianite is intimately associated with pyrite and occurs as very thin tabular crystals, up to 10 cm in length.[10]
Kosovo. Trepča Mines, Stari Trg. Thick prismatic crystals up to 10 cm long and 2 cm thick, relatively stable. Deep green in color and transparent, commonly resting on pyrrhotite or pyrite, and in some cases on quartz or carbonates.[18]
US: In green sand at Middletown, New Castle County, Delaware.[5]
US: Blackbird Mine, Lemhi County, Idaho. Crystals in shades of pink, green, greyish blue, purple and purplish black, as well as colorless. The unique deep purple color of some Blackbird mine specimens is characteristic of the locality. Some single crystals have both purple and green zones. Vivianite crystals from the Blackbird Mine are usually elongated and blade-like. They occur as singles and groups on dark altered schist and on white quartz. Associated minerals include ludlamite, quartz and siderite.[22]
^Although mindat.org claims "J. G. Vivian" is a typo for "J. H. Vivian", there is at least one reference that gives a full first name. The original description of Vivianite in Abraham Gottlob Werner, Letztes Mineral-System, Freiberg/Wien, 1817, p. 42 reads „Der Name ist vom Hrn. B. R. Werner zu Ehren des Hrn. J. Vivian aus Truro in Cornwall, dem Er die Kentnis des Fossils verdankt, gebildet.“ [“The name is formed by Mr. B. R. Werner in honour of Mr. J. Vivian from Truro in Cornwall, to whom he owes the memory of the fossil.”] It is ambiguous (and puzzling in its reference to B. R. Werner). As for Jeffrey G. Vivian, no other trace of him can be found in Google Books.
^H. Stege, C. Tilenschi und A. Unger. Bekanntes und Unbekanntes – neue Untersuchungen zur Palette Vermeers auf dem Gemälde „Bei der Kupplerin“. In: Uta Neidhardt und Marlies Giebe (Ed.), Johannes Vermeer – Bei der Kupplerin, Ausstellungskatalog Dresden 2004, pp. 76–82.
^ abHandbook of mineralogy. 4: Arsenates, Phosphates, vanadates. Tucson, Ariz: Mineral Data Publ. 2000. p. 632. ISBN978-0-9622097-3-4.
^Calvo, Miguel (2015). Minerales y Minas de España. Fosfatos, Arseniatos y Vanadatos. Escuela Técnica Superior de Ingenieros de Minas de Madrid. Fundación Gómez Pardo. Madrid, Spain. p. 297. ISBN978-84-95063-96-0.
^Cochran, U., Goff, J., Hannah, M., and Hull, A. (1999) Relative stability on a tectonically active coast: paleoenvironment during the last 7000 years at Lake Kohangapiripiri, Wellington, New Zealand, Quaternary International, 56, 53–63
