Life in the USA is not normal. It feels pointless and trivial to be talking about small looks at the fascinating natural world when the country is being dismantled. But these posts will continue, as a statement of resistance. I hope you continue to enjoy and learn from them. Stand Up For Science!
Vivianite is a pretty straightforward mineral, chemically – it’s just hydrated iron phosphate, Fe2+Fe2+2(PO4)2 · 8H2O. It’s not exceptionally rare, but it does take some unusual circumstances for the iron and the phosphate to come together in the right ways to make vivianite.
Vivianite is naturally colorless (or nearly so), but exposure to light moves some of the protons around in its structure, oxidizing the iron and changing the mineral to green and blue shades. That can happen in minutes to hours, so there is little colorless vivianite out there in collections. Apparently the change is more or less the result of Fe+2 converting to Fe+3.
This piece is from a well-known locality on the Kerch Peninsula, the eastern arm of the Crimean Peninsula in the Black Sea, where the iron ores have been exploited since the 7th century B.C.
The most likely explanation for the source of the oolitic iron is an “exhalative” process, in which hot iron-rich fluids percolate up from depth and precipitate iron and other minerals when they interact with sea water. This is the process in the deep sea where “black smokers” precipitate metals on the sea floor. At Kerch, the process may be related to and enhanced by the presence of mud volcanoes in the region, and that may contribute as well to the levels of phosphorous present. During the Pliocene, only about 4 or 5 million years ago, the iron-rich waters came up in a coastal area that was rich in life, especially bivalves and brachiopods, but apparently also enough animals with phosphatic shells to contribute significant phosphorous to the system.
So this is an unusually young iron deposit. A conventional explanation for the source is that the iron is ultimately a result of erosion from Archaean rocks to the north in Russia, part of the famous Kursk Magnetic Anomaly, one of the most iron-rich locations on earth. One problem with Kursk as the source of iron at Kerch is that they are about 700 km apart, but then there’s been a lot of erosion for a long time, so it’s possible. Outcrops of iron formation around Krivoy Rog in Ukraine could also be sources for the iron.
The Kursk Magnetic Anomaly in southwestern Russia, about 500 km south of Moscow, is the most intense magnetic anomaly in the earth’s crust. Magnetic intensity depends on the distance from the source to the sensor, but in this map of airborne data, the overall intensity is at least 12,000 nanoTesla. For comparison, typical strong magnetic features, such as the Boulder Batholith of Montana, have intensities on the order of 400 to 600 nanoTesla. The Kursk anomaly is easily sensed with magnetometers on satellites, where it stands out against the generally broadly varying whole earth’s field, which ranges from about 25,000 to 60,000 nanoTesla.
The ultimate cause of almost all significant magnetic variations in the earth’s crust is variations in magnetite content. Magnetite, an iron oxide, is the only common and widespread mineral that is magnetic. This part of Russia is an area where the ancient core of Europe (variously called the Baltic or Fennoscandian Shield, or the East European Craton) reaches the surface. Not surprisingly, it’s a region where lots of iron is present, in the form of banded iron formations like those of the Lake Superior area of Michigan, Wisconsin, and Minnesota.
Banded iron formations are mostly older than about 1.8 billion years and probably represent the oxidation of iron in the early oceans. Once most of the iron oxide had precipitated out (as banded iron formation), free oxygen could accumulate in the atmosphere, ultimately leading to the development of complex life on earth.
The iron ores at Kursk are dated to about 2,400 million years ago, making them among the older banded iron formations. That’s Precambrian time, just slightly younger than the 2,500-million-year point that is taken as the boundary between Archaean (older) and Proterozoic (younger) rocks. For comparison, the Lake Superior ores are around 1,850 million years old. And the iron reserves at Kursk are estimated to be 30 times those of the Lake Superior region.
Oxygen isotopes suggest that the precipitation and reduction of iron in the rocks at Kursk were at least mitigated by some kind of organic material: life (Belykh and others, 2007, Physicochemical formation conditions of banded iron formations and high-grade iron ores in the region of the Kursk Magnetic Anomaly: Evidence from isotopic data: Geology of Ore Deposits, 49 (2) 147-159). At that time, the only significant life on earth was probably cyanobacteria, and it is their action in helping iron oxides precipitate to which we owe our oxygen-enriched atmosphere.
The data in this map were acquired by the Soviet Union mostly in the 1960s. I had their published aeromagnetic maps digitized for a year-long analysis I did of the former Soviet Union for oil and natural gas exploration in 1990. The Kursk rocks themselves are not prospective for hydrocarbons, but the Precambrian crust not too far away was broken by a rift system during Devonian time, about 365 million years ago, and that rift (called the Dnieper-Donets Graben) does contain natural gas deposits that are valuable to the Ukraine.
Whatever the origin of the iron to make the vivianite, the result is spectacular sheaves of vivianite and other minerals (more than 70 different minerals are known from Kerch), sometimes within the empty cavities inside clam or brachiopod shells. Sometimes the shell itself is faithfully replaced by vivianite. There’s manganese in the system (no surprise, since it’s a common associate with iron), and rarely manganese carbonate (pink rhodochrosite) replaced brachiopod, pelecypod, and gastropod shells, making some of the most unusual fossils in the world.
The sheaf of vivianite in my specimen here is about 18 mm long. It’s perched on a substrate of yellow-brown limonite (a mixture of iron oxides and hydroxides) and black botryoidal material which is probably either goethite (iron hydroxide) or one of the manganese oxide minerals.
The mineral was named in 1817 for John Henry Vivian (1785-1855), the Welsh-Cornish politician, mine owner, and mineralogist who discovered it. The type locality is Wheal Kine, Cornwall.