Picture by Paul V. Heinrich
Petrified wood consists of a wide variety of minerals including silica, silicates, carbonates, sulfates, sulfides, oxides, and phosphates. They all can permineralize wood to form petrified wood (Adams 1920). However, petrified wood most commonly consists of silica (Si02) in the form of either opal or chert (Stein 1982).
Silicified wood is usually found within one of two types of strata. First, it occurs within accumulations of volcanic ash, tuff, and breccia, e.g. the petrified forests of Yellowstone National Park. Second, silicified wood also occurs within sands, silts, and muds deposited by rivers and streams that have hardened to sandstones, siltstones, and shale. Typically, the sandstones, siltstones, and shales that contain silicified wood also contain redeposited tuffaceous materials or volcanic ash. The silicified wood found in Petrified Forest National Park in Arizona and the Miocene strata of Louisiana and Texas occur within such strata (Knauth 1972:44).
Silicified wood forms in these deposits, because of the presence of dissolved silica within the groundwater. The silica is derived from the dissolution of the volcanic material by the groundwater within the volcanics or sediments. This dissolved silica in the form of monomeric silicic acid attaches itself to the lignin and cellulose of the wood. With time, a layer of the monomeric silicic acid forms on the exposed woody tissues. The monomeric silicic acid dehydrates into silica gel. Additional layers of the monomeric silicic acid attach to this silica gel eventually filling and encasing the wood with silica gel. A rapid loss of water converts the silica gel into amorphous silica (opal) (Leo and Barghoorn 1976; Scurfield and Segnit 1984).
Within 10 to 40 million years, the opal of the silicified wood further dehydrates and crystallizes into microcrystalline quartz (chert). Factors such as temperature and pressure may speed or slow the process, but eventually the opal of the silicified wood becomes chert (Stein 1982). During the change from opal to chert in silicified wood, the relict woody texture may either be retained or lost.
During the silicification process, various materials and minerals may be incorporated into the silica gel. For example, manganese dioxides, iron oxides, organic matter and authegenic clay can alone or in combination color the opal or chert that forms silicified wood. Because each piece of wood becomes silicified in its own local geochemical environment, the trace and major materials and elements that it contains will vary greatly from piece to piece even within the same stratum (Knauth 1972:45; 1981). Therefore, trace element analysis is generally useless in tracing the source of any silicified wood.
Within the Tertiary strata that outcrop in southeast to southwest Texas and into Louisiana, three types of silicified wood can be recognized. They are rather nondescript silicified wood, palm wood, and a massive silicified wood. The nondescript silicified wood consists of silicified wood that possesses a recognizable woody structure. The relict woody structure is fine and nondescript. It is not identifiable without oriented thin sections, specialized references, and comparative material (Blackwell et al. 1983:2). Therefore, this type is best described just as "silicified wood".
The second type of silicified wood is commonly called including the State Fossil "petrified palm wood" by geologists and rock collectors. Palm wood is a group of fossil woods that contain prominent rod-like structures within the regular grain of the silicified wood. Depending upon the angle at which they are cut by fracture, these rod-like structures show up as spots, tapering rods, or continuous lines. The rod-like structures are sclerenchyma bundles that comprise part of the woody tissues that gave vertical strength to the Oligocene and Miocene tree genera, Palmoxylon (Blackwell et al. 1983:4-5).
The third type of silicified wood is massive silicified wood. The silicification of the wood or subsequent transformation of silica gel to opal or chert has obliterated any trace of the grain of the former wood. The destruction of the structure of the silicified wood has resulted in a massive opal or chert that lacks any visible indication of its origin. Because of its variable, massive nature and heterogeneous trace element composition, many investigators, e.g. Jolly (1982) and Jolly and Kerr (1984), have often failed to recognize the nature of this material. The Eagle Hill Chert found within West-Central Louisiana is a local, massive silicified wood (Heinrich 1984). A similar massive silicified wood called "golden palm" occurs in equivalent strata within East-Central Texas (Patterson 1985).
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A very nice page on petrified wood is a page about Petrified Wood From Western Washington By Ed Strauss.
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