Petrified Wood Colors and Petrification

Which minerals produce the colors found in the petrified wood? Red and pink colors are produced by the presence of hematite, a form of oxidized iron - Fe₂O_3. The intensity of the color depends on the quantity of hematite present in the petrified wood.

Process: Iron dissolves in ground water when no oxygen is present. The ground water becomes re-oxygenated as it moves though the tree trunks causing oxygen to bond with the iron. The iron then precipitates to produce a solid form of iron called hematite. This hematite is incorporated into the log's cell walls. The same process occurs when iron stains porcelain sinks. The soluble iron in ground water becomes oxidized into a solid form when it comes in contact with air, causing a reddish stain.

Yellow, Brown, and Orange colors are produced by the presence of goethite - HFeO₂ and Fe₂O₃. Goethite is a hydrated iron oxide that is derived by weathering from iron-bearing minerals. It crystallizes into tablets, scales, needles, radial and concentric aggregates.

Green colored petrified wood is produced by pure reduced iron that is a magnetic, malleable mineral. The chemical composition is Fe. Referred to as native iron, it is quite rare in terrestrial rocks but common in meteorites. Native iron combines with chlorophyll to give tree leaves and plants their green colors but rarely attaches to wood cells.

White is produced by pure silica - SiO₂. Silicon, Si, and oxygen, O, are the two most abundant elements in the earth's crust. Silica group minerals are common worldwide. Free silica, SiO₂, referred to scientifically as silicon dioxide, occurs most commonly as quartz. Quartz is the principal element of glass. In many respects quartz is the most interesting of all minerals. It has a larger number of distinct varieties with wider differences than any other mineral. Petrified wood, also referred to as silicified wood, is a common illustration of a quartz pseudomorph. Wood is slowly replaced, cell by cell, by silica until not a trace of the original material remains.

Process: The structural arrangement of silicon and oxygen creates an open bonding structure that permits other ions such as various forms of iron to occupy interstitial positions in the molecule and bond to it, thus producing variations in color.

Organic carbon or pyrite - FeS₂ (iron sulfide), the most abundant and widespread sulfide mineral, produces Black. Because it was mistaken for gold, it is often called "fool's gold." Pyrite translated freely means "fire mineral," a reference to the sparks given off when struck.

Process: The wood was affected as hydrogen sulfide from decaying organic matter interacted with iron, forming pyrite.

Purple and blue are produced by manganese dioxide - MnO₂. This is a secondary material formed when water leaches manganese from igneous rock and redeposits it as a concentration of manganese dioxide. As a result, it occurs more often as coatings on other minerals than as large crystals. Manganese is very important in the manufacture of steel.

Tan indicates silica dioxide is the predominate replacement mineral. This color is most often seen in permineralized wood. In permineralization, the wood's cell structure is better preserved, giving it the appearance of real wood. It should be noted that not all petrified wood in the park is permineralized. Permineralization also happens when wood is preserved with calcite rather than silica. No calcified wood occurs in the park.

How the very fine detail of the cell structure is preserved is not well understood. It would appear that less than cell-sized gradients in acidity created very small reprecipitation gradients and replaced the wood on almost an atom-by-atom basis.