No category of fossil preservation is so misunderstood as petrifaction (sometimes spelled petrification). Everyone has heard of petrified wood. The word "petrified" comes from the Greek word petros, meaning "stone," and petrifaction literally means "turned to stone." Unfortunately, many persons consider any fossil petrified. But strictly speaking, a fossil is petrified only when additional minerals have been deposited in pores or cavities in the fossil, or when the fossil is entirely replaced by other material. Consider a piece of wood. It can petrify in three distinct ways, each with a distinctive result with a distinctive name:
1. By filling the empty spaces with some mineral, as water fills the empty spaces in a sponge. This is called permineralization. Dissolve this mineral, and the original piece of wood remains.
2. By filling the empty spaces with mineral, then dissolving the cellulose and wood fibers and replacing them with mineral matter, often of a different color. The result is a piece of stone that faithfully reproduces
Wood replaced by quartz, found in Utah. Growth rings are indicated by alternating brown and white quartz. Cracks filled by clear quartz suggest wood dried before silicification.
every cell and detail. But dissolve away the mineral matter and there is nothing. This is called histometabasis, or more commonly, replacement.
3. By surrounding the wood with mud or sand that hardens around it. When the wood decays or is dissolved, a mold is left which fills with
Once driftwood, now quartz. This piece of petrified wood from New Mexico floated long enough to wear away bark and soft wood before sinking and becoming a fossil.
mineral matter. The result is a piece of what looks like wood on the outside, but inside may be banded agate or even a geode with sparkling amethyst crystals, showing no cellular detail at all. This type of replacement is often called pseudomorph ("false form"). This also occurs in minerals where one mineral has faithfully replaced another mineral crystal, such as a calcite crystal replaced by quartz that retains the crystal shape of the calcite.
All three of these quite different processes are correctly termed petrification or petrifaction.
Bone, plant materials, and many shells are porous enough for perminerali-zation or replacement to occur. The best example is the coal ball, found in some coal mines. This is nothing more than a mass of Coal Age plant fragments and seeds that has become permineralized by calcite or sometimes by iron sulfide (pyrite or marcasite). When the coal ball has been permineralized by calcite, the collector can perform the interesting opera-
Chonetes fragilis, a brachiopod from the Devonian shale of Sylvania, Ohio. Like most brachiopods from this area, it is replaced by pyrite.
tion of peeling off a thin layer of the actual woody material of these 275-million-year-old plants. (Details are explained in Chapter 10).
Permineralization is common in petrified wood, but too often the mineral filling the empty spaces is quartz, which can be readily dissolved only by hydrofluoric acid. Peels, however, have been made of silicified wood in the same way coal-ball peels are made. Quartz and calcite are
The small straight cephalopod Pseudorthoceras knoxense is often found brightly pyritized like this one in Pennsylvanian black shales. Omaha, Nebraska.
the minerals most commonly found in permineralized fossils, but others include pyrite, marcasite, barite, selenite, opal, and manganese oxide.
Replacement is often met with. Coal mines sometimes produce beautifully "pyritized" snails, clams, and brachiopods. Actually, most of this "pyrite" is marcasite, the less stable form of iron sulfide. It is hard to draw the line between a replacement and a pseudomorph, especially in fossil shells, although, strictly speaking, in replacement only the shell is replaced; in a pseudomorph both the shell and its filling are replaced. Beautifully pyritized brachiopods have been found in the Silica shale (Devonian) of Ohio. Silica-replaced brachiopods occur in New Mexico, and silica-replaced corals, stained a beautiful red, are found in Utah. Large colonies of Lithostrotionella, a Mississippian coral replaced by colorful quartz, are found in southeast Iowa. These retain the fine detail of the original organism. Another colonial coral, Hexagonaria, occurs as handsome specimens filled with calcite in Devonian formations in Michigan. When washed up, rounded by the waves, on Lake Michigan beaches, these are
known as "Petoskey stones" and are eagerly collected. This soft, tan material, which polishes well to display the inner workings of the fossil, has been declared the official state rock of Michigan.
True pseudomorph plants are found in lava flows in the Pacific Northwest, where trees engulfed by hot lava were burned away but left a faithful mold in the hardened lava. Even a mold of an unfortunate rhinoceros, formed in this fiery furnace, is known from there. Such cavities may become filled with agate. A slab of this agate does not look like a slab of petrified wood from the Petrified Forest in Arizona; it looks like any banded agate because it had a chance to replace only the shape of the tree, not the grain of the wood.
Agate pseudomorphs of coral are dug from the bottom of Hillsborough Bay at Tampa, Florida. They show traces of coral structure on the outside but the interior cavity is lined with vividly colored layers of chal-
cedony or with drusy quartz crystals; the original coral had completely dissolved before the chalcedony arrived.
The process of petrifaction can most easily be understood by analogy to a water pipe in a house. Most water has some degree of "hardness" — that is, it contains minerals it dissolved while percolating through rock and soil underground. Given enough time, water will dissolve almost any mineral. The process is hastened when water absorbs carbon dioxide gas and becomes a weak acid. It picks up carbon dioxide while falling through the air as rain, or by release of the gas from decaying organisms in the soil. Furthermore, water draining slowly through soil will pick up such organic ammunition as tannic or humic acid. This acid water seeps slowly into the ground, where it dissolves minerals in its underground passage.
Water can hold only so much dissolved mineral matter before it is saturated. As hot saturated water cools or evaporates, the excess mineral matter crystallizes on any object in contact with the water. This may be a water pipe. Each surge of water passing through the pipe deposits an imperceptible layer of mineral. This is especially true in hot-water pipes. Sometimes pipes become so filled with mineral deposits that the plumbing must be replaced.
Wood and bone, and to a lesser extent shells, are composed of closely connected pipes through which water can move. When these organic remains are buried under a lake or ocean, water has a chance to move through these natural pipes and to deposit layer after layer of mineral in the channels or cells. The cell filling may become brightly colored by tiny amounts of dissolved metals; the bright reds, yellow, greens, and browns in petrified wood are caused by iron. Blacks are often made by trace amounts of manganese, and copper or nickel can create greens. Little by little the tubes are plugged up, and the result is a permineralized fossil. If a mold is filled, it is a pseudomorph.
Petrified wood and bone are commonly filled by quartz-bearing solutions, especially when the fossil is buried in volcanic ash, which is rich in silica. Water percolating through the ash, which may remain warm for a long time, dissolves the silica and immediately deposits it in the wood or bone fibers. Whether this takes place in a relatively short time is uncertain. Nobody has ever been able to petrify wood effectively in a laboratory.
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