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Fossil Information Sheet F007

Fossilised Wood

Fossilised, or "Petrified" wood consists of a large variety of minerals: silicates, carbonates, oxides, phosphates etc. The most common petrified wood, however, consists of silica in the form of opal (a hydrated form of silica) or chert (a micro to crypto-crystalline variety of quartz). It is usually found within sands, silts and muds deposited by rivers and streams that have hardened to sandstones, siltstones and shale or within accumulations of volcanic ash Tuff and Breccia (for example in the petrified forests of the Yellowstone National Park).

Petrified tree in Petrified Forest National Park, USA - photo by Stefan Pauli

Many examples of silicified wood originate from the Miocene era (up to 25 million years ago) and are found in Arizona as well as areas of Louisianna and Texas.

From the volcanic sediment in groundwater the silicic acid attaches to the cellulose of the wood and in time dehydrates into silica gel. The loss of water turns this into an amorphous silica (opal) and within 10 to 40 million years this further dehydrates and crystallises into microcrystalline crystal Chert. Temperature and pressure may hasten or delay the process and during the change the woody texture may be retained or lost.

Oxidisation, organic matter, clay, manganese and many other factors and elements may cause colour changes in the silicified wood. Because each piece is petrified in its own geo-chemical environment the trace and major minerals vary greatly even within the same stratum, therefore trace element analysis is generally of no help in sourcing petrified wood.

Petrified wood has the same hardness of Quartz 7

Sources range from Argentina, USA, Canada, Czech, Greece,Australia and India.

See also my FAQ How does wood turn to stone ?

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