MINERAL OF THE MONTH: Spring 2011 – Petrified Wood

Petrified Wood can be thought of as a type of fossil. Fossils are formed from a petrification process, which is a geology term describing the method by which organic living material is converted into stone. Usually, this happens when the organic remains are buried in lava or sediments before they can decay. Petrification can take place in two related ways: replacement and permineralization. Replacement occurs when water dissolves the original hard parts and replaces them with mineral matter. The most common replacement minerals are calcite, silica, pyrite, and hematite. When the original organism is replaced quickly, the fossil usually loses the detailed structure, leaving behind just the original shape. Permineralization occurs when ground water carrying dissolved minerals infiltrates the microscopic pores. The minerals in this case replace the detail of the original organism.

The most famous example of petrification is the Petrified Forest in northern Arizona. However, this is not the only area known for petrified wood since it is also found in nearly every state and in many foreign countries. It is not known for certain who the first Europeans were to see the great display of petrified wood in Arizona. It was probably Spanish explorers during their expeditions in the 1500s. The earliest written record, however, dates from 1851 when an army officer mentioned petrified wood in a report.

At one time the northeast part of Arizona was lowland with numerous rivers and streams. In the basin there was a lush forest with conifer trees up to nine feet in diameter and over 200 feet tall. During the Triassic Period (200 to 250 million years ago), the area that is now Arizona was located near the equator. At that time, all of the continents had combined to form one super-continent called Pangea. Over time, trees in the area died. Rivers deposited the deceased trees in their flood plains or buried them in streambeds. Most of the trees decomposed, but a few were buried so deep that there was not enough oxygen to allow decay. To the west of this area were massive volcanoes that spewed ash into the atmosphere. Wind currents carried the ash and deposited it with the silt that buried the trees. Ground water dissolved silica from the ash and carried it into the buried logs. Over time, the silica in solution either replaced cell walls, crystallizing as quartz, or deposited in the air spaces within the wood tissue. This petrification process explains how cell structure, annual rings, and other features of the original trees were preserved.

As the petrification process continued, other minerals combined with quartz to create the brilliant rainbow of colors often found in petrified wood. In some cases minerals infiltrated later during the millions of years of burial as a secondary deposit in the cracks, checks, or other openings in the petrified or partially petrified wood. Iron oxides produced the great variety of shades of red, brown, and yellow. The black color was probably due to manganese oxide or carbon.

At first, the now petrified trees remained buried under 3,000 feet of sediment. However, around 60 million years ago this area was uplifted along with the Rocky Mountains. With the uplifting came erosion from streams and rivers that removed the overburden layers, exposing the petrified trees.

Some scientists classify petrified wood as agate; others do not. The determination comes down to two issues. First, the type of quartz needs to be identified. In some cases, most of the quartz in petrified wood is macrocrystalline, although there may also be some microcrystalline replacement. For those specimens that contain mostly microcrystalline crystals, the second issue is whether petrification was a replacement process, or whether it may have involved an agatization process. If the wood tissue was directly replaced by silica so that the wood fibers defined the structure, then petrified wood is not agate. If, on the other hand, each of the individual pore spaces self-organized and filled independently, thus creating individual agatized pockets, you could then classify petrified wood as a colony of agates. Depending on the specimen, there could be some of both structures.


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