DietrichIAWA et al. Journal – Natural 36 (2),and 2015:artificial 167–185 silicification 167 PETRIFACTIONS AND WOOD-TEMPLATED CERAMICS: COMPARISONS BETWEEN NATURAL AND ARTIFICIAL SILICIFICATION* Dagmar Dietrich1, Mike Viney2, and Thomas Lampke1 1Chemnitz University of Technology, Surface Technology/Functional Materials Chair, Institute of Materials Science and Engineering, 09107 Chemnitz, Germany 2 Corresponding author: Colorado State University, College of Natural Sciences, Education and Outreach Center, Teacher-in-Residence, Poudre School District Blevins MS Science Chair; e-mail: [email protected] *Dedicated to the memory of the palaeontologist Johann Traugott Sterzel April 4, 1841 in Dresden (Missouri, USA) – May 15, 1914 in Chemnitz (Saxony, Germany) ABSTRACT Fascination with petrified wood has stimulated interest in understanding the process of natural petrifaction. Early attempts of modeling natural petrifaction in the laboratory have been limited to mimicking incipient permineralization resulting in the creation of silica casts of pore spaces and inner cell walls. Silica lithomorphs produced through artificial silicification provided a possible avenue for studying microstructure of wood. More recently artificial petrifaction is motivated by the goal of creating advanced ceramic materials for engineering applications. The concept of using wood as a biotemplate has led to the crea- tion of porous ceramics by cell wall replacement. To some extent artificial and natural petrifaction processes are comparable; although, some of the materials and procedures used in the laboratory are not found in nature. Research focused on the composition and structure of fossil wood from different-aged deposits is compared with research focused on the development of wood-templated porous ceramics. Differences and similarities in the pathways of natural silicification and creation of biomorphous ceramics are discussed. The comparison between artificial and natural silicification highlights the particular significance of the degree to which (de)lignification is needed for silica permeation. Keywords: Petrifaction, silicification, wood, biotemplating, porous ceramics. INTRODUCTION Petrified wood is an important source of data for reconstructing ancient environments and biodiversity. Petrifaction is the replacement of organic material into minerals like silica, calcite, pyrite, siderite, and apatite through processes of mineralization. Accord- ing to Scurfield and Segnit (1984) the silicification of wood includes the processes of permineralization, infiltration, replacement and recrystallization. Silicified wood may retain anatomical detail of original wood structure down to the microscopic level. © International Association of Wood Anatomists, 2015 DOI 10.1163/22941932-00000094 Published by Koninklijke Brill NV, Leiden Downloaded from Brill.com10/10/2021 02:22:53PM via free access 168 IAWA Journal 36 (2), 2015 A prominent example is the Chemnitz Petrified Forest, a 291 Ma old ecosystem with a variety of arborescent plants including seed ferns (Medullosa sp.), tree ferns (Psaronius sp.) and conifers (Dadoxylon spp.) (Cotta 1832; Sterzel 1875; Rößler 2001). The delicate preservation of anatomical features has spurred interest in understanding the conditions under which natural petrifaction occurs. Artificial silicification experi- ments have been designed in an attempt to better understand natural silicification and have provided insights into the early stages of the silicification process (Drum 1968a,b; Leo & Barghoorn 1976; Persson et al. 2004a). In addition such experiments have provided methods that may enhance our understanding of wood structure and cellular connections in extant plants. Recently, materials scientists have designed advanced procedures for the replication of wood in order to develop hierarchical structured ceramics (Paris et al. 2013). Bioinspired materials research is a continuously growing field in advanced materials science and engineering. Using wood as a biotemplate, ceramics with specific porosity have been produced for applications in acoustic and heat insulation structures, as filters and catalyst carriers at high temperatures and for medical implant structures (Greil 2002; Van Opdenbosch et al. 2011). A review of previous studies provides a framework for a discussion of the similarities and differences between pathways leading to natural silicified wood and procedural steps designed for the creation of wood-templated ceramics in the laboratory. Compositional and structural studies of natural petrifactions Arborescent petrifactions of Devonian to Pleistocene age help researchers in their quest to reconstruct evolutionary pathways, ancient environments, and past diversity. Optical microscopy in association with petrographic thin-sections and acetate peels after hydrofluoric acid etching are common methods used to study arborescent petrifactions. The minute anatomical details revealed by these methods provides impetus to unravel the mystery behind pathways in nature that lead to the formation of fossil wood and to better understand the process of natural silicification. Robert Hooke (1635–1703) was asked to examine petrified wood under his micro- scope at a meeting of the Royal Society not long after his discovery of plant cells. He came to the conclusion that it had the same structure as living wood and “… That this petrify’d Wood having lain in some place where it was well soak’d with petrifying water (that is such water as is well impregnated with stony and earthy particles) did by degrees separate, either by staining and filtration, or perhaps, by precipitation ...” as published in his “Micrographia” (Hooke 1665). Bachofen von Echt summarized early attempts at quantitative chemical analysis and loss-on-ignition studies (Bachofen von Echt 1867). Constituent minerals like silica, alumina, carbonates of calcium and mag- nesium, iron oxides and organic remains were detected. Optical microscopy (St. John 1927; Arnold 1941) has been supplemented by scanning electron microscopy com- bined with X-ray spectroscopy and by X-ray diffraction, thus improving information about morphology, composition and structure of the fossils (Buurman 1972; Furuno et al. 1986a,b). Early transmission electron microscopic studies combined with rep- lica techniques (Eicke 1952) revealed details such as bordered pits and the fibrillar structure of cell walls. The analytical techniques applied to determine the silicification Downloaded from Brill.com10/10/2021 02:22:53PM via free access Dietrich et al. – Natural and artificial silicification 169 mode of the petrified wood from fossil deposits in America, Europe, Asia, Africa and Antarctica are summarized in Table 1. In evaluating the results of previous studies the constant development and improvement of analytical techniques during the past decades should be taken into consideration. The application of field-emission cathodes and the development of detectors for backscattered electrons used in modern scanning electron microscopes facilitate an enhanced resolution and have lowered detection limits. Improved techniques such as orientation imaging microscopy in the scanning electron microscope provide localization of crystalline phases already detected by X-ray diffraction but not clearly localized (Dietrich et al. 2013). As early as 1933 Stromer found substantial evidence for infiltration as the key process of petrifaction, but even in 1941 Arnold had to refute the competing model, which held that minerals in solution replace organic wood content molecule-by-molecule. Buur- man (1972) differentiated between permineralization as the cast process of open spaces (lumina, pits, intra-cellular spaces) and silica impregnation resulting in the replacement of cell walls. Leo and Barghoorn (1976) described petrifaction as a multi-stage process of infiltration and subsequent impregnation with the wood serving as an active template for silica deposition. The authors inferred that silica complexes in aqueous solutions undergo chemical bonding with the functional groups of wood constituents, especially cellulose. Scurfield and Segnit (1984) described the formation of silicified wood as a five-stage process that included permineralization and cell wall replacement. In their model cell walls act as a framework for silica deposition and are slowly replaced as wood degraded. In their detailed study they found evidence for the transformation of opal-CT to chalcedony and chalcedony to quartz. They also hypothesized that the rate of cell wall breakdown may determine whether opal-CT or chalcedony is the initial replicating substance. Lynne et al. (2005) studied siliceous sinters and found a progres- sive alteration of non-crystalline opal-A into paracrystalline opal-CT/moganite and to microcrystalline quartz. Smith (1998) provides the following definitions: non-crystalline opal-A is disordered; paracrystalline opal-CT and opal-C contain ordered domains of stacked sequences of cristobalite and tridymite sheets. The “disordered tridymite” evidenced by Buurman (1972) seems to correspond with paracrystalline opal-CT since X-ray patterns show features similar to cristobalite as well as to tridymite. Numerous researchers have suspected that wood petrifactions and silica residues, like siliceous sinters or marine sediments, undergo a similar progressive mineralogical transformation from opal-A→opal-CT→chalcedony by a diagenetic process (Stein 1982). There is also evidence for the formation of silica polymorphs originating