Vivianite is a striking blue-green phosphate mineral known for its connection to decomposing organic matter. This mineral can grow on dead bodies, but only when specific environmental conditions are met. This geological process, sometimes called the “corpse crystal,” involves a chemical interaction between elements released during decomposition and the surrounding soil. The presence of this mineral in archaeological and forensic contexts provides valuable information about burial environments and the post-mortem fate of human remains.
The Composition and Appearance of Vivianite
Vivianite is chemically classified as a hydrated iron phosphate. In its pure, unaltered state, the mineral is colorless or a very pale green and largely transparent. The crystals form a monoclinic structure, often appearing as flattened, prismatic, or bladed shapes. This soft mineral is sensitive to its environment, which dictates its final coloration.
The characteristic deep blue or blue-green color develops rapidly upon exposure to air or light through oxidation. When oxygen is introduced, the ferrous iron (\(\text{Fe}^{2+}\)) converts to ferric iron (\(\text{Fe}^{3+}\)). This chemical change is responsible for the visible shift in color, darkening the crystal through shades of blue and sometimes into a bluish-black over time.
The Chemical Requirements for Mineral Formation
The formation of vivianite requires three specific chemical conditions. A source of iron in its reduced, or ferrous (\(\text{Fe}^{2+}\)), state must be readily available. A high concentration of phosphate ions (\(\text{PO}_4\)) must be present in the surrounding pore water. Finally, the reaction must take place within a reducing, or anaerobic, environment with little to no oxygen.
The absence of oxygen prevents the immediate oxidation of the iron; otherwise, ferrous iron would quickly convert to the ferric state and form iron oxides, like rust. Vivianite is stable in waterlogged soils, deep sediments, or environments where the redox potential is low. The reaction proceeds best in conditions with a circumneutral pH (6 to 9), allowing the iron and phosphate to combine into the crystalline structure.
Vivianite Formation in Burial Contexts
The decomposition of a human body naturally provides the high concentration of phosphate ions needed for the reaction. Phosphate is abundant in the human structure, being a major component of bone mineral and integral to DNA and cell membranes. As tissues break down through microbial activity, these compounds are released into the burial environment, creating a localized, highly concentrated zone.
The surrounding burial conditions often supply the other two necessary components. A waterlogged grave site, such as a bog or clay-rich soil, becomes highly anaerobic as microbial decomposition consumes available oxygen. This oxygen-depleted state maintains the iron in the ferrous form, which can leach from iron-rich soil, groundwater, or nearby iron objects like cast-iron coffins or wreckage.
The resulting mineral often forms a blue crust directly on the human remains, coating the surface of bones, teeth, and sometimes preserved soft tissue like adipocere. This localized growth confirms the body acts as the primary reactant in the mineral’s formation.
Why Vivianite Matters to Forensic Science
The presence of vivianite in an excavation is a powerful indicator for forensic scientists and archaeologists. It serves as a reliable marker for the former presence of organic material, even if the remains have fully decayed. Since the mineral’s formation requires concentrated phosphate, it suggests a biological source was once present in that location.
Vivianite also offers clues about the taphonomy, or post-mortem history, of the remains. Its existence confirms the burial environment was waterlogged and anaerobic for a significant period. The blue coating can sometimes cement and protect delicate materials, such as bone or wood, slowing their decay. However, the mineral can complicate modern DNA analysis, as the iron compounds may inhibit the Polymerase Chain Reaction (PCR) technique.