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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
Luminescence: Not fluorescent under UV
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).
Localities
Type Locality:ⓘ Wheal Kine (Wheal Kind), St Agnes Consols (Polberro Consols), St Agnes, Cornwall, England, UK
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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