Vivianite
Chemical formula: Fe₃(PO₄)₂·8H₂O
Vivianite is a rare phosphate mineral, known for its intense color, which changes under light, and characteristic elongated crystals.
Description
## Characteristics Vivianite is a hydrated iron phosphate, often forming elongated, columnar, or acicular crystals that can reach significant sizes. Freshly extracted, the mineral is usually colorless or pale green, but upon exposure to sunlight and air, it oxidizes, taking on intense shades of blue, navy blue, and even black. Crystals can be transparent to translucent, and their surface often shows distinct striations along the elongation. ## Physical Properties The hardness of vivianite on the Mohs scale ranges from 1.5 to 2, making it a very soft mineral. It has perfect cleavage in one direction, which makes it brittle and easily delaminated. The luster of vivianite is vitreous to pearly on cleavage surfaces. The density ranges from 2.6-2.7 g/cm³. ## Colors and Varieties Typical colors of vivianite include colorless, pale green, blue-green, indigo, dark blue, and even black. The color change is a result of the oxidation of Fe²⁺ iron ions to Fe³⁺. No specific commercial varieties are distinguished, but specimens with an intense, deep blue color are the most prized. ## History and Name Vivianite was first described in 1817 by the German mineralogist Abraham Gottlob Werner. The mineral's name comes from the English mineralogist and mineral collector, John Henry Vivian (1785-1855), who discovered this mineral in Cornwall, England. ## Uses Vivianite has no significant industrial uses. It is primarily a collector's mineral, valued for its aesthetic qualities, especially its intense color and well-formed crystals. It is also a subject of scientific research due to its crystallochemical properties and oxidation processes.
Diagnostic features
## Identification Vivianite can be identified by its characteristic, often intensely blue or blue-green color, which develops upon exposure to light. Elongated, columnar crystals with perfect cleavage along their length are also helpful. The softness of the mineral (it can be scratched with a fingernail) is also a diagnostic feature. ## Distinguishing from Similar Minerals Vivianite is sometimes confused with lazurite, azurite, or indicolite (blue tourmaline) due to similar coloration. However, vivianite is much softer and has perfect cleavage, which the aforementioned minerals lack. The color change under light is also a unique feature of vivianite. ## Crystal Forms Vivianite most often occurs as elongated, columnar, acicular, or fibrous crystals. It often forms radial or stellate aggregates. It can also occur as compact masses, concretions, or as coatings and incrustations.
Geological environment
## Genesis Vivianite is a secondary mineral, forming in reducing environments rich in phosphates and iron. It often forms in weathering zones of iron ore deposits, in lake and bog sediments, in peat bogs, and also in the bones and teeth of fossilized animals. It can also occur in pegmatites and hydrothermal veins. ## Mineral Associations Vivianite often co-occurs with other phosphate minerals, such as phosphorite, apatite, as well as with pyrite, marcasite, siderite, and limonite. In sedimentary environments, it can be associated with organic matter. ## Locations Significant vivianite deposits occur in many places around the world. The most important include: Bolivia (Siglo XX mine, Llallagua), where some of the largest and most beautiful crystals have been found; Russia (Kerch, Taman Peninsula); United States (Colorado, New Jersey); Australia (Queensland); England (Cornwall); Germany (Saxony); and Cameroon.
Rarity
Rare
Collector aspects
## Quality Criteria The most prized vivianite specimens are those with an intense, deep blue color, high transparency, and well-formed, undamaged crystals. Crystal size is also important, with larger, aesthetic specimens being more sought after. Minimal inclusions and absence of cracks are also crucial. ## Popular Localities The most sought-after vivianite specimens come from the Siglo XX mine in Llallagua, Bolivia, famous for its large, intensely blue crystals. Other valued localities include Russia (Kerch) and Australia.
Care and storage
## Cleaning Due to its softness and brittleness, vivianite requires gentle cleaning. It is best to use a soft, dry cloth or brush to remove dust. For heavier soiling, a cloth lightly dampened with distilled water can be used, but prolonged contact with water should be avoided. ## What to Avoid Vivianite is very sensitive to sunlight, which accelerates the oxidation process and darkening of the mineral. Exposure to direct sunlight and strong artificial lighting should be avoided. The mineral is also sensitive to high temperatures and sudden temperature changes, which can lead to cracks. Due to its softness, it should be protected from scratches and impacts. Contact with acids and strong bases should also be avoided. ## Storage Vivianite specimens are best stored in a dark, cool, and dry place, away from light sources. Airtight containers or display cases that protect against dust and moisture are ideal. It is worth considering storage in containers with moisture-absorbing material to prevent its accumulation. Due to its brittleness, each specimen should be secured against movement and contact with other minerals.