Phosphates: Vivianite

Diagnostic card.

Fe ₃ (PO ₄ ) ₂ * 8 H ₂ O
Singonia monoclinic
Hardness 1.5-2
Specific weight 2,6-2,7
Cleavage is very good
Beam breaking, brittle
Colorless, white
Color in powder white
Glitter glass, bold, pearly

Depending on the formation conditions, this mineral can form crystals longer than 1 m. This fairly common phosphate is represented by prismatic crystals with excellent cleavage. They are often rounded or pointed; The dimensions are huge (crystals reach 1 m). There are tabular individuals, sometimes combined in star aggregates. Powder-like raids, inclusions, rounded bonds in clays are also common.

Chemical composition-content (in%): FeO-43; P2O3- 28.3; H2O: 28.7; Varieties: partially oxidized with Fe2 + and Fe3 + -b-cupricite, with predominance of Fe3 + - a-kerchenite completely oxidized without Fe2 + - oxy-cerchenite; Enriched with Mn, Mg and Ca - paravivianite. Glitter - glass to pearl on cleavage planes. Prismatic view of symmetry. Cleavage is perfect by (010).

Long-columnar crystals, most often overgrown; Usually in radiant, fibrous aggregates, rosettes, spheres and kidneys, as well as tabular (Colorado, USA); Often friable and earthy (blue iron earth). Cleavage is perfect, gypsum-like; Density of 2.6-2.7; Fresh colorless, under the influence of atmospheric oxygen, the crystals are immediately colored in light blue, blue, dark blue or black; The ferrous iron contained in the crystal lattice is converted into a trivalent iron. The thin plates are flexible, the layers of the crystal lattice gliding like graphite and gypsum.

Diagnostic signs.
In its fresh form, Vivianite is colorless, but due to secondary changes it quickly acquires a blue, bluish-green and light blue color. Many crystals are mostly transparent or translucent. The color is unchanged - light shades and even colorless, with oxidation in the air - grayish-blue, grayish-green, dark blue to black-blue (Kerchenits - in the photo).

Origin.
Vivianite can form as a secondary mineral in the roof of many sulphide deposits, as well as in secondary changes in the primary phosphates of pegmatites. It is formed, in addition, in the clay sediments of fresh lakes as a result of the interaction of iron-enriched water with phosphorus-containing organic material. Vivianite is also found in deposits of lignite and sedimentary iron ores of freshwater basins.

Vivianite is formed in areas close to the surface, under the influence of oxygen in the air. Circulating phosphate-rich waters affect minerals that contain iron (mostly bivalent), such as pyrite, siderite, fayalite, pyrrhotite, and wash away some of the iron. When the water evaporates, the vivianite crystallizes. In pegmatites, iron phosphates can be converted to vivianite, absorbing water. In this way very beautiful crystals were formed, in particular in Cornwall (England); Very large (up to 15 cm) dark green samples in Bolivia, Colorado mines and many other places in the USA; Exceptional beauty crystals originate from Southern Serbia, Eastern Bavaria (Germany) and Peru. In the form of fine dust, the mineral is widely distributed in clay sedimentary rocks. Impressive crystals longer than 1 mm come from the clay quarries of Cameroon.

Deposits and applications.
The largest crystals in the world (maximum size about 130 cm), either single or combined in radiant druses, are mined in the Anlo lake lakes near Ngaunder in Cameroon. Beautiful transparent crystals were found at the deposits of Ljalagua in Bolivia, Bingham in Utah and Idaho. Vivianite is of great interest for collecting. It can also be used (if it forms large clusters) in the manufacture of paints. Initially, the mineral was called blue iron earth, but in 1817 A.G. Werner gave the mineral a name in honor of the English mineralogist D.G. Viviana. It is used as an inexpensive dye; Is of interest to collectors.

Vivianit. Kerch mine, Crimea, Ukraine. Photo: © А.А. Evseev.

Vivianit. Kerch mine, Crimea, Ukraine. Photo: © А.А. Evseev.

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