Vivianite

vivianite

metavivianite

parasymplesite

ludlamite

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Formula: Fe2+3(PO4)2.8H2O
Hydrated phosphate, vivianite group
Crystal System: Monoclinic
Specific gravity: 2.67 to 2.69 measured, 2.696 calculated
Hardness: 2
Streak: White to faintly light blue
Colour: Green to blue
Solubility: Readily soluble in hydrochloric acid and sulphuric acid; slightly soluble in nitric acid
Common impurities: Mn,Mg,Ca
Environments:

Pegmatites
Sedimentary environments
Hydrothermal environments

Vivianite is a secondary mineral which is a weathering product of primary iron and manganese phosphates in granite pegmatites, in the oxidation zone of metal ore deposits, particularly associated with gossan, in the oxidation zone of hypothermal (high temperature) hydrothermal veins, in clay sediments, and in recent alluvial deposits replacing organic material, peat, lignite, bog iron ores and forest soils (Mindat, Dana, Webmin, HOM). Associated minerals include metavivianite, limonite, ludlamite, pyrite, pyrrhotite, santaclaraite and siderite (HOM, Mindat).

Alteration

Pure vivianite is colourless, but when exposed to light, in a matter of minutes it changes to a brilliant transparent green, with the colour in transmitted light becoming a cobalt blue when rotated at a certain angle; further exposure changes the colour more slowly to a darker and darker green and finally black. The reason for this is oxidation.
Initially a photon of visible light strikes the crystal and changes vivianite Fe2+3(PO4)2.8H2O into metavivianite Fe2+2Fe3+(PO4)2(OH).7H2O (in this article, but see below **). This alteration is accompanied by a progressive colour change from the colourlessness of absolutely pure, fresh, end-member vivianite to a brilliant transparent green.
The proton knocks a proton (a hydrogen nucleus) out of one of the 8 water molecules in the vivianite molecule, which converts the water molecule into a hydroxyl ion (OH) having a negative charge. This extra negative charge must be balanced by the oxidation of one iron atom, whose valence state then changes from 2+ (ferrous) to 3+ (ferric). No interaction with the atmosphere was involved. No oxygen entered the structure from outside, and no water molecule escaped to the outside; we are dealing with a completely internal reaction (oxidation by deprotonation). The liberated hydrogen (the proton) easily migrates through the crystal lattice.
Theoretically, if a second proton gets knocked out of the molecule, we get ferrostrunzite Fe2+Fe3+2(PO4)2(OH)2.6H2O, but this does not happen as easily as the change from vivianite to metavivianite.
(**NB the IMA has metavivianite as a paramorph of ferrostrunzite, both Fe2+Fe3+2(PO4)2(OH)2.6H2O**).
Knocking out a third proton would leave us with Fe3+Fe3+2(PO4)2(OH)3.5H2O, which corresponds to the two species ferristrunzite (triclinic) and santabarbaraite (amorphous). Since santabarbaraite occurs as pseudomorphs after vivianite, it would seem that these reactions really do happen in nature, although light may not play a role in these later stages (MinRec 54.2.285-289).

Localities

At Llallagua, Bolivia, vivianite crystals to 10 cm occur on a goethite matrix derived from the alteration of pyrite and marcasite. Associated minerals include childrenite, cronstedtite, pyrrhotite, franckeite and sphalerite (Minrec 37.2.156).

At the Cigana mine, Minas Gerais, Brazil, good vivianite crystals occur on a matrix of muscovite, some with druses of pyrite (Minrec 35.3.252).

At Trepča, Kosovska Mitrovica, District of Mitrovica, Kosovo, vivianite crystals to 10 cm occur, resting on pyrrhotite or pyrite, and in some cases on quartz or carbonate (MinRec 38.4.290).

The type locality is Wheal Kine, St Agnes Consols, St Agnes, Cornwall, England, UK.

At Burdell Gill, Caldbeck Fells, Cumbria, England, UK, vivianite-parasymplesite occurs as tiny sheaves of blades to about 0.2 mm on gossanous quartz. It is often associated with corroded bariopharmacosiderite from which it appears to have formed. The vivianite and parasymplesite appear to have been confined to the isolated blocks that also carried the switzerite and rhodochrosite (JRS 8(1).1-9).

At the Blackbird mine, Lemhi county, Idaho, USA, some of the finest specimens of vivianite have been found. They occur to 22 cm on altered schist and on white quartz. Associated minerals include ludlamite, quartz and siderite (Minrec 41.4.366-369).

At the Bayport limestone quarries, Bellevue, Eaton County, Michigan, USA, vivianite has been found in a pyrite-rich shale breccia in limestone (R&M 93.3.230).

At the Chickering Mine, Walpole, Cheshire County, New Hampshire, USA, vivianite occurs rarely in the pegmatite. When found, it is as crystals or fibrous masses in zones of hydrothermally altered triphylite. Associated minerals include siderite, quartz, fluorapatite, ludlamite and, rarely, whitmoreite (R&M 90.5.421).

At the Keyes Mica Quarries, Orange, Grafton County, New Hampshire, USA, the pegmatites are beryl-type rare-element (RE) pegmatites.
The Number 1 mine exposed a pegmatite that shows the most complex zonation and diverse mineralogy of any of the Keyes pegmatites. Six zones are distinguished, as follows, proceeding inward from the margins of the pegmatite:
(1) quartz-muscovite-plagioclase border zone, 2.5 to 30.5 cm thick
(2) plagioclase-quartz-muscovite wall zone, 0.3 to 2.4 metres thick
(3) plagioclase-quartz-perthite-biotite outer intermediate zone, 0.3 to 5.2 metres thick, with lesser muscovite
(4) quartz-plagioclase-muscovite middle intermediate zone, 15.2 to 61.0 cm thick
(5) perthite-quartz inner intermediate zone, 0.9 to 4.6 meters thick
(6) quartz core, 1.5 to 3.0 metres across
The inner and outer intermediate zones contained perthite crystals up to 1.2 meters in size that were altered to vuggy albite-muscovite with fluorapatite crystals. This unit presumably was the source of the albite, muscovite, fluorapatite, quartz and other crystallised minerals found in pieces of vuggy albite rock on the dumps next to the mine.
The middle intermediate zone produced sheet mica with accessory minerals including tourmaline, graftonite, triphylite, vivianite, pyrite, pyrrhotite, and beryl crystals to 30.5 cm long and 12.7 cm across.
Vivianite occurs with triphylite and other phosphates at the Keyes No. 1 mine. Most of it is massive or roughly crystallised, but sharp 1 to 2 mm deep blue vivianite crystals have been found on a 9-mm specimen juxtaposed with pyrite and hydroxylherderite crystals. A cabinet-sized mass of chalky blue vivianite with minor pyrite has been collected from the dump of the Keyes No. 2 pit (R&M 97.4.326-327).

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