PETRIFIED WOOD IN THE UPLAND COMPLEX: A MOGANITE MYSTERY

Moganite and quartz are the only minerals observed in petrified wood logs obtained from Upland Complex (UC, Pliocene) gravel in the Mississippi Embayment. Moganite is a relatively rare mineral. Is the unusual composition of these samples due to a rare mode of petrifaction or is it a consequence of improved technology? To answer these and related questions XRD and petrography were used to analyze the mineralogy, mineral texture, and wood texture of 35 logs from Upland Complex gravel. UC results were compared to samples from associated Eocene deposits, samples of unknown provenance, associated chert-replaced limestone boulders and 25 samples from museums. Upland Complex logs are well-rounded dense chert. Moganite content ranges from absent to 15 percent with no gap in range. None has tridymite or cristobalite. The quartz Crystallinity Index ranges from 10 to one with no gap. Thin sections revealed quartz, chert and length-slow chalcedony. Moganite is present in three of seven Eocene samples; six of thirteen unknowns, and two of five chert-replaced limestone, none has tridymite or cristobalite. Studies of petrified wood prior to the year 2000 do not report moganite. However, XRD analysis of petrified wood from the UC and samples from a variety of other sources suggests that moganite is common, albeit in small amounts. Conclusion - the presence of moganite in UC petrified wood is neither unique nor rare. UC samples trap the petrifaction process at a stage not recognized until advances in technology revealed it.