Vivianite is a hydrated iron phosphate mineral [Fe3++ (PO4)2-8H20] that is a crystal of many colors, and in fact, can change color over its lifetime. Freshly exposed vivianite is generally colorless but with time oxidizes to green to bluish green to blue crystals. Continued oxidation of the iron from Fe++ (ferric) to Fe+++ (ferrous) will produce crystals so dark blue they appear black. Many crystals have a vitreous luster although they can grade into pearly or dull specimens. Colorless crystals are transparent while lighter colored specimens become translucent and massive specimens generally are rather opaque. As with the color, mineral streak ranges from colorless to various shades of blue. Vivianite is quite soft, ~2.0 or less (Mohs). The best “showy” specimens have prismatic (elongated along the C Axis) or flattened/bladed (along the B Axis) crystals and often form in stellate cluster; however, there are a variety of other morphological forms.
Vivianite is thought to occur as: 1) as a secondary mineral in metallic ore deposits; 2) in pegmatites as an alteration product of primary phosphate minerals; or 3) as a mineral associated with the phosphate found in sedimentary deposits. However, Petrov (2008) noted the mineral is not characteristic of the oxidized zone but of “deep unoxidized levels of ore deposits.” Most vivianite specimens collected in the western states, or observed in rock and mineral shows, are secondary in nature or from the phosphate minerals in pegmatites. Collected specimens, when first exposed to sunlight and oxygen, are often a beautiful blue and prismatic along the C-axis.
Very rarely do Colorado rockhounds come upon vivianite collected from organically rich unconsolidated clays and other sediments/rocks (mostly Cenozoic in ages). In the U.S. most of these sedimentary vivianites come from the Central Florida Phosphate District (AKA Florida Platform) where iron and water, along with original phosphatic material, has allowed vivianite to form: Fe3++ (PO4)2-8H20. The original phosphorus is thought to have been derived from precipitation in marine waters, and from the skeletons/shells and the waste products of animals living in these waters.
The basement rocks of the Florida Platform are a fragment of the African Plate that remained attached to the North American Plate when rifting occurred in the Jurassic and range in age from late Precambrian-early Cambrian to mid-Jurassic (Barnett 1975).
A sedimentary sequence rests uncomfortably on top of the basement rocks, and is composed of Middle Jurassic to Holocene evaporite, carbonate, and siliciclastic sediments. This sedimentary sequence is the result of deposition on the relatively stable, passive margin of the North American Plate (Scott, 1989, 2016).
During the Cenozoic concentrations of silt to sand-sized phosphate pellets, mixed with carbonates and clastic sediments, were deposited in shallow water environments over much of the Florida Platform, in a broad range of carbonate and clastic sediments. During the Miocene and Pliocene phosphate was particularly concentrated in several basins in the Central Florida Phosphate District and these were the areas where a major phosphate industry begin development in the late 1800s. By 1893, production had expanded to 1.25 million tons and Florida became the world's leading producer of phosphate for the next century. By 2015/2016 the U,S, had dropped to the 3rd largest producer of phosphate behind China and Morocco and production had expanded from the “Eastern Phosphate Fields” of Florida and North Carolina to the “Western Phosphate Fields” of Utah and Idaho. However, in 2021 Florida still produced ~75% of the U.S. production of ~24 million tons. The Eastern Field operations use open pit mining to extract the ore from Miocene and Pliocene sediments/rocks. The Western Fields mine phosphate from limestone in the Permian Phosphoria Formation (Scott, 2016).
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A well-formed, terminated crystal of glassy and gemmy, blue-green, vivianite, ~4 mm in length, collected from Clear Spring Mine, Homeland, Central Florida Phosphate Mining District, Polk Co., Florida. Light patch is carbonate matrix. A backlight would show transparency. Collection of Art Smith 1980.
REFERENCES CITED
Barnett, R. S., 1975, Basement structure of Florida and its tectonic implications: Gulf Coast Association of Geological Societies Transactions, Vol. 25.
Scott, T.M., 1989, The Geology of Central and Northern Florida with Emphasis on the Hawthorn Group, in Scott, T.M., and Cathcart, J.B., AGU 28th International Geological Congress, Field Trip Guidebook T178.
Scott, T. M., 2016, Geologic overview of Florida in Hurst, M. V. (Ed.), Central Florida Phosphate District Third Edition: Southeastern Geological Society Field Trip Guidebook No. 67.
Virtually everything you might want to know about Florida phosphate may be found in the Hurst guidebook referenced above and available as a PDF file: http://www.segs.org/wp-content/uploads/2010/01/SEGS-Guidebook-67.pdf
AND NOW FOR THE REALLY INTERESTING STORY FROM CHRIS DRUDGE October 25, 2016 at: The Vivid Blue Mineral That Grows on Buried Bodies and Confuses Archaeologists - Atlas Obscura
IN 1861, a railway engineer by the name of John White passed away, was buried in a cast iron coffin, and began a slow transformation from White to blue.
The explanation for this spooky color change, which has occurred on numerous occasions all over the world, lies in the composition of the human body. Among the molecules contained within us is phosphate, a central phosphorus atom bound on four sides to atoms of oxygen. Phosphate is present in the hard bits of bones and teeth (as part of the mineral hydroxylapatite), helps hold together strands of DNA and RNA, and is used by cells to store and move energy around as well as to organize their many protein-driven activities.
If a dead person ends up buried somewhere waterlogged, lacking in oxygen, and loaded with iron, the phosphate leaking from their decaying remains can slowly combine with the iron and water to form a mineral called vivianite. It starts out clear and colorless, but will rapidly turn progressively darker shades of blue upon exposure to air as the iron within it reacts with oxygen. The formation of vivianite (also known as blue ironstone) is helped along by bacteria which act to dissolve iron out of soil and phosphate out of bodies while also directing the growth of the blue crystals.
In the case of Mr. White, in keeping with the styles of the time, his coffin had a glass window installed in the front so his face could be seen by mourners when the lid was shut. At some point after burial, the glass had broken, allowing groundwater to seep inside and react with the cast iron coffin and phosphate-rich body. The end result was a corpse surround by blue vivianite crystals, revealed when the coffin was exhumed as part of an archaeological rescue excavation over a century after being buried.
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