Paragenesis of silica in silicified woods of the Whitsett formation (Eocene), in Texas



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Fifty-eight silicified wood specimens were collected from 6 locations is the Whitsett formation, Jackson Group, (Upper Eocene) over a distance of about 400 miles across the Texas Gulf Coastal Plain. The silicified woods were for the most part found on the surface, weathered out of the enclosing rocks. Devitrification of tuffaceous material in the Whitsett formation may have been the source of free silica that was dissolved in the ground water and reprecipitated as it permeated the wood. Polymorphs of silica found in the silicified woods include: low-quartz (and its variety chalcedony)5 "opaline" material with mixed-layer structure; possibly lussatite (fibrous low-cristobalite), and virtually amorphous opal. Attempts to separate quartz from other polymorphs by. means of heavy liquids were unsuccessful because of the fine grain-size of the intergrowth. A principal objective was to gain some understanding of the silicification process by relating the crystal structure of the silica to other features and properties of the silicified wood. Most of the X-ray diffractograms show a strong broad, "hump" from about 4.92 A° to 3-56 A°, and a smaller "hump" at about 2.50 A°, which are produced by the Interlayering of Silica sheets having low-cristobalite and low-tridymite configurations. Samples were arbitrarily divided into 5 groups, based in part on structural ordering as indicated by the strength and sharpness of these "opaline" peaks, and in part on the amount of associated quartz. A study of the diffractograms and thin- sections reveals association of various types of "opaline" material (optically amorphous mixed-layer low-cristobalite-tridy mite) with chalcedony and microgranular quartz. In some thin- sections, quartz veinlets cut and ramify into the "opaline" material. In the few specimens which are zoned, quartz occupies the core and "opaline" material occupies the rim. A substance that "glows" under crossed polarizers occurs"in more than half the thin-sections. Presence or absence of this substance cannot be correlated with any particular type of "opaline" structure; it may occur with any of the X-ray structure-types except that of pure quartz. However, abundance of the substance appears to be related to Increasing sharpness of the "opaline" hump, a factor which suggests that the cryptocrystalline "glowing" material is fibrous, low-cristobalite (lussatite). These observations suggest, but do not prove, a progressive ordering and crystallization of silica after deposition. Indices of refraction of the isotropic samples, which range from 1.444 to 1.466, do not appear to correlate with the crystal structure as revealed by X-ray diffraction. The "glowing" material is, however, associated with the lower indices of refraction. The optically isotropic, material could not be separated into true opal versus "opaline" material by optical examination. 1 Steps leading to the silicification of the wood were probably as follows: 1. Deposition of wood by stream and ocean currents, accompanied by sedimentation of tuffaceous material 2. Devitrification of the tuffaceous material, which released free silica into the ground water. 3. Destruction of all or most of the wood, cellulose, allowing precipitation of silica in its place. The structural nature of thig original silica is unknown. It may have been precipitated as amorphpus silica, as interstratified low-cristo- balite-tridymite, or even as quartz, 4. The amorphous silica, if deposited as such, gradually crystallized to interstratified low-cristobalite-tridymite. Ordering of silica tetrahedra continued in some cases until the conversion to quartz was complete. Why some silicified woods are better crystallized and contain more quartz than others is not understood. Perhaps a faster rate of precipitation of. the silica in some specimens tended to include more foreign ions, or structural defects thus inhibiting crystallization. If so, the more rapidly precipitated silica would be expected to persist in the "opaline" state.