Vivianite is a delicate, pale mineral that transforms into a distinctive deep blue-to-green color upon exposure to air. This hydrated iron phosphate has the chemical formula Fe3(PO4)2 · 8H2O. It is generally considered rare and fragile in its pure, unoxidized state. The mineral is highly sought after by collectors due to its striking color and tendency to form well-defined, elongated crystals.
The Essential Chemical Conditions for Formation
The formation of Vivianite is dependent on a precise chemical recipe: the availability of dissolved iron and phosphate ions in an aquatic or subterranean environment. The iron must be in its reduced state, specifically the ferrous ion (Fe2+), which is necessary to fit into the mineral’s crystal structure. This requirement means that Vivianite can only precipitate in environments with very low or zero oxygen, often referred to as reducing or anoxic conditions.
Phosphate ions (PO4 3-) are commonly sourced from the dissolution of other phosphate-bearing minerals or, more frequently, from the decay of organic matter. Once these two components reach sufficient concentration, they combine with water to form the colorless or pale green Vivianite. When a freshly formed crystal is exposed to the atmosphere, the ferrous iron quickly oxidizes to ferric iron (Fe3+). This oxidation causes a change in the electron configuration within the crystal lattice, resulting in the characteristic deep indigo or blue-green color that identifies the mineral.
Primary Deposits in Hard-Rock Environments
Vivianite occurs as a secondary mineral in high-energy, hard-rock environments, forming through the alteration of pre-existing minerals rather than direct sedimentation. It is frequently encountered in the oxidation zones of metal ore deposits, where it develops as a byproduct of weathering. These deposits often involve sulfide minerals, such as pyrite or pyrrhotite, which release iron as they decompose.
The mineral is also found in decomposed granite pegmatite veins, which are coarse-grained igneous rocks that sometimes contain primary phosphate minerals. In certain hydrothermal vein systems, the mineral can precipitate directly, often yielding the clearest and largest prismatic crystals. These crystals are typically found lining cavities or embedded within the matrix rock of the deposit.
Low-Temperature Finds in Sedimentary Contexts
The most common occurrences of Vivianite are in low-temperature, sedimentary settings where biological activity plays a significant role in its formation. It is frequently found in waterlogged soils, lake beds, and marine sediments where decaying organic material consumes oxygen, creating the necessary reducing conditions.
Peat bogs and marshy areas are particularly conducive to Vivianite formation, sometimes yielding massive, soft, earthy concretions known as “bog iron.” A specialized occurrence involves the mineral acting as a replacement substance in organic remains, a process known as authigenesis. This leads to the discovery of Vivianite crystals encrusting or replacing fossilized bone, ancient wood, and even inside fossil shells. In archaeological contexts, the mineral has been found within ancient burial sites, where phosphate from the remains reacts with iron in the surrounding soil.
Noteworthy Global Geographical Locations
Bolivia is internationally recognized for producing some of the world’s most spectacular Vivianite crystals, especially from the Huanuni mine in the Oruro Department. These Bolivian specimens are prized for their transparency and large, well-formed, deep blue-green prisms.
In Europe, Germany has deposits where Vivianite is found in limonite ores and in the weathered pegmatites of Bavaria, illustrating both the hard-rock and secondary ore deposit origins. Historically, the type locality where the mineral was first described is Cornwall, England, which is known for its association with oxidized metal-mining veins. Russia also hosts notable occurrences, including the Kerch Peninsula, where it is found in iron ore deposits.
North America features several important locations, such as the Blackbird Mine in Idaho, United States, where Vivianite crystals are found in association with cobalt and copper ores. In Canada, significant finds have been reported in the Yukon Territory, where the mineral occurs as nodules, and in Quebec, associated with bog iron deposits.