Petrifactions and Wood-Templated Ceramics: Comparisons Between Natural and Artificial Silicification*

Home , Fossil wood, Petrifaction, Petrified wood, Psaronius

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 creation 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.

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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 experiments 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 microscope 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 combined 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 replica 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 progressive 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 indepen- dently, rather than from progressive diagenetic transformation. Mustoe (2008) contrasts the concept of transformation of opal-A to higher ordered forms of silica over time with the idea of independent formation of opal-CT and chalcedony in samples from the Eocene Florissant fossil forest. Petrifaction occurred in several stages, beginning with precipitation of amorphous silica on cell wall surfaces. Cell lumina later became filled with opal-CT and chalcedony. A final phase of silica deposition is evidenced by chalcedony-filled fractures that crosscut some specimens. Supported by observations on Pliocene and Miocene samples, four phases for silicified wood formation were sug-

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2000 2004 et al. et al. 2015 et al. Authors Stein 1982 Stein 1982 Stein 1982 Stein 1982 Buurman 1972 Buurman 1972 Akahane Hellawell Furuno et al. 1986a,b Furuno et al. 1986a, b Kuczumow Leo & Barghoorn 1976 Leo & Barghoorn Leo & Barghoorn 1976 Leo & Barghoorn Leo & Barghoorn 1976 Leo & Barghoorn Brezinova & Süss 1987 Karowe & Jefferson 1987 Karowe & Jefferson (continued on the next page) µ SRD = synchrotron-based

SRD SEM SEM SEM OM, SEM SEM, EDX SEM, EDX XRD, SEM XRD, SEM XRD, SEM SXRF, µ SXRF, XRED, SEM RAMAN, AAS RAMAN, µ XRD, OM, SEM XRD, OM, SEM Analytical techniques XRD, OM, SEM, WDX XRD, OM, SEM, XRD, OM, SEM, WDX XRD, OM, SEM, XRD, SEM, WDX, TEM, WDX, XRD, SEM,

Quartz Opal-A Opal-A Quartz, Opal-CT Opal-CT Silica-gel µ SXRF = synchrotron-based X-ray microfluorescence, Silicification Silica gel cast, Opal-CT+quartz Silica precipitations, Opal (no specification) Opal (no specification) mostly lignitic remains Quartz, lignitic remains partially lignitic remains partially abundant lignite Chalcedony + cryst. quartz, opal+quartz, lignitic remains different moderate wood decay different in cell walls, corners empty, Opal-A with cellulose orientation Opal-A Opal-CT in lumen, quartz+cellulose Opal-CT

Taxon Unknown Unknown Unknown Unknown Unknown angiosperm angiosperm Angiosperm Gymnosperm Gymnosperm Gymnosperm Gymnosperm Gymnosperm Gymnosperm Gymnosperm Gymnosperm Gymnosperm, Gymnosperm,

Locality Japan, Asia Japan, Japan, Asia Japan, Japan, Asia Japan, Poland, Europe Indonesia, Asia Indonesia, Hungary, Europe Hungary, Czech Republic, Europe Montana, North America Montana, North Colorado, North America Colorado, North Colorado, North America Colorado, North Wyoming, North America North Wyoming, Wyoming, North America North Wyoming, America North Wyoming, California, North America California, North Washington, North America North Washington, New Mexico, North America New Mexico, North

34–23 23–5.3 1.75–0 5.3–1.75 Age [mya] Pliocene Miocene Oligocene Quaternary Epoch / period / Epoch

Table 1. Silicification modes of fossil wood of different ages and localities. 1. Silicification modes of fossil wood different Table X- energy-dispersive = EDX spectroscopy, X-ray dispersive wave-length = WDX microscopy, electron scanning = SEM microscopy, optical = OM diffraction, X-ray = XRD Note: spectroscopy, + microscopy cathodoluminescence = CL spectrometry, RAMAN = RAMAN diffraction, backscatter electron = EBSD fluorescence, X-ray = XRF spectroscopy, ray EPR = electron paramagnetic resonance, GC-MS = gas chromatography – mass spectrometry, thermal analysis, X-ray microdiffraction, = differential = instrumented neutron-activation analysis, TG = thermogravimetry. INAA AAS = atomic absorption spectroscopy, DTA

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2009 et al. 2013 et al . 2004 et al. 1982 Authors Stein 1982 El-Din 2003 Arnold 1940 Mustoe 2008 Stromer 1933 Jefferson 1987 Jefferson Buurman 1972 Buurman 1972 Buurman 1972 Buurman 1972 Beck Witke Sigleo 1978, 1979 Dietrich Sweeney et al. Matysova et al. 2010

OM, OM, SEM GC-MS, INAA OM, SEM, EDX XRD, OM, SEM XRD, OM, SEM XRD, OM, SEM XRD, OM, SEM XRD, OM, SEM XRD, OM, SEM chemical analysis OM, CL, RAMAN SEM, EDX, EBSD XRD, OM, CL, WDX XRD, OM, CL, Analytical techniques XRD, SEM, WDX, EPR, XRD, SEM, XRD, OM, CL SEM, XRF XRD, OM, CL

Chalcedony, Chalcedony, Silicification lignitic remains lignitic remains lignitic remains lignitic remains cryptocryst. quartz anthracitic remains anthracitic remains Chalcedony, quartz Chalcedony, Chalcedony+quartz Chalcedony-quartz, Chalcedony+quartz, Chalcedony+quartz, Chalcedony+quartz, Chalcedony+quartz, Chalcedony+quartz, Opal-CT, chalcedony Opal-CT, quartz, lignin remains Chalcedony, cryptocryst. Chalcedony, Silica, polymorphs unknown Silica, polymorphs unknown Chalcedony+moganite+quartz, Lignitic remains, coal, charcoal

Taxon Psaronius Lycopods Unknown Unknown angiosperm angiosperm Callixylon Gymnosperm Gymnosperm Gymnosperm Gymnosperm Gymnosperm Gymnosperm, Gymnosperm, Gymnosperm, Gymnosperm, Gymnosperm, Medullosa, Psaronius Medullosa, Psaronius

Locality Egypt, Africa Egypt, Oman, Africa Oman, France, Europe France, Europe England, Europe Belgium, Europa Germany, Europe Germany, Germany, Europe Germany, Arizona, North America Arizona, North Colorado, North America Colorado, North Colorado, North America Colorado, North Kentucky, North America North Kentucky, Alexander Island, Antarctica Alexander Island, Germany, Czechia & France, Germany, New Mexico, North America New Mexico, North Europe, Brazil, South America, Europe, Brazil, South

56–34 66–56 135– 65 203–135 250–203 359–299 299–250 419–359 Age [mya]

Eocene Triassic Jurassic Permian Devonian Paleocene Cretaceous Carboniferous Epoch / period / Epoch

(Table 1 continued)

Downloaded from Brill.com10/10/2021 02:22:53PM via free access 172 IAWA Journal 36 (2), 2015 gested by Furuno et al. (1986a,b): silica filling of the lumen, formation of casts and loss of the cell wall, partial replacement of the cell wall by silica infiltration, cell wall replacement and loss of the intercellular substance in some cases (Furuno et al. 1986b). Beck (1982) studied the silicified wood of Callixylon erianum and found no silicification of cell walls but silica casts of lumina and pits. Jefferson (1987) stated an oppos- ing silicification sequence with cell wall impregnation occurring before lumen filling in Cretaceous fossil wood found in Antarctica. This observation was associated with a fungal or bacterial induced cell wall delignification. Obviously, taphonomic pathways leading to petrifaction are influenced by extrinsic factors such as microbial activity and sedimentology. Sweeney et al. (2009) focused on wood permeability and chemistry as intrinsic factors that affect the mode of fossil wood preservation in the middle Cretaceous Moreno Hill formation. By far the most intriguing specimens from this location are preserved with competitive modes of preservation, coalification and mineralization. Specimens that retain excellent cell structure have cell walls mineralized with microcrystalline silica and apatite, with little organic matter remaining. An exception is charcoal that retains three-dimensional cell structures with the organic fraction highly resistant to decay. Many of the xylem elements of coalified specimens in this study are permineralized with calcite but with no cell wall replacement. Fossil wood of the Moreno Hill Formation illustrates multiple pathways to preservation within the same taphonomic setting. Sweeney et al. (2009) concluded that permeability within the wood tissue and surrounding sediments triggers mineralization by allowing access of mineral bearing fluids to the functional groups exposed on cell walls. Studies of wood alteration and decay over centuries and millennia extend the scope of investigations into the formation of fossil wood. Karowe and Jefferson (1987) studied wood buried in Mt. St. Helens lahars dated at 1980, 1885, 1450–1550, and 36,000 years. They found increasing fungal decay associated with cell wall breakdown and the beginning of silica precipitation. Grosser et al. (1974) studied coalification of 28,000–100,000 year old subfossil wood samples and observed a specific degradation pattern with humic substances filling the tracheid lumina. Lignin is the component that lasts longest during thermochemical decomposition and coalification. In fact lignin remnants have been detected in Triassic-aged petrifactions from Arizona (Sigleo 1978). The breakdown of cell wall components is also affected by fungal attack (e.g., Blanchette 2000). White-rot fungi break down the lignin in wood, leaving the cellulose behind, while brown-rot fungi disintegrate hemicelluloses and cellulose. Wood in aquatic environments or buried in saturated soils is usually attacked by bacteria. Distinct patterns of wood attack due to bacterial activity have been observed, but a comprehensive understanding is not presently available (Blanchette 2000). Siliceous hot springs, weathering of labile minerals in alluvium, and volcanic eruptions can all act as a source of silica (Murata 1940; Buurman 1975; Sigleo 1978, 1979; Karowe & Jefferson 1987; Channing & Edwards 2009; Matysova et al. 2010). Fossil plant petrifactions are most often associated with volcanic sources. Laboratory autoclave processing of obsidian demonstrates the effectiveness of this silica source in the incipient silicification of wood (Ballhauset al. 2012). Deposition of amorphous

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Figure 1. Transverse views of silicified wood of different age with replication of the multi- layered cell walls. – a: Pinus contorta from Puff’n’Stuff Geyser, Norris Geyser Basin, showing precipitation of silica on cell wall with lumina remaining open (Courtesy of Western Washing- ton University Geology Department). – b: Permian Medullosa sp. from Chemnitz showing chalcedony in the secondary cell wall (S2), quartz in the cell corner (CC) and silica filling of different density in the lumen (L), magnification of the inset ×2. silica in the lumina and/or the walls of wood cells is also observed in modern hot springs (Akahane et al. 2004; Channing & Edwards 2009; Hellawell et al. 2014). Hot springs provide evidence of very rapid silicification, but the chemical environment is characterized by strongly oxidizing conditions causing rapid destruction of the organic matter. The most common form of silicification from hot springs is the deposition of an opaline crust on the outer surface of the wood. Individual tracheids become coated and cell walls penetrated by opal. The exceptionally high concentration of dissolved silica results in silica precipitates of varying texture; some specimens show beautiful examples of opal-A microspheres (Channing & Edwards 2009; Hellawell et al. 2014). Figure 1 compares a Holocene wood silicified in hot spring water with a Permian wood from Chemnitz. Lodgepole pine (Pinus contorta) wood from a Yellowstone Park thermal pool (Fig. 1a) shows silica precipitated on cell walls with lumina remaining open. Additional evidence suggests silica has permeated the cell walls of this specimen. Medullosa sp. from Chemnitz (Fig. 1b) shows chalcedony in the secondary cell wall, quartz in the cell corner and silica fillings of different density and crystallinity in the lumina. Some trends for modes of silicification can be inferred from the data in Table 1. At first glance, the data reveal a pattern of silica recrystallization over time supporting the work of Buurman (1972) and Stein (1982). Wood in modern hot springs is encrusted and permeated by opal-A. Silicified woods found in Cenozoic deposits are preserved with opal-CT, chalcedony and quartz. Silicified woods found in Mesozoic and Paleozoic deposits are preserved with chalcedony and quartz. The same relationship of silicifiation and time was found in a study of 75 Australian fossil wood specimens from Cenozoic to Permian by Scurfield and Segnit (1984). Their findings could not be integrated in the table because the exact ages of the specimens were not clear. Scurfield and Segnit (1984) conclude that petrifaction involves penetration of the wood, permeation of cell walls, enlargement of the micropore system due to cell wall components break down, and con- tinuing deposition of silica. Scurfield and Segnit (1984) suggest that deposition in cell

Downloaded from Brill.com10/10/2021 02:22:53PM via free access 174 IAWA Journal 36 (2), 2015 lumina is sometimes a separate and later event. The final stages of petrifaction in- volve loss of water and silica digenesis. Apart from transformation of an opaline precursor, the independent formation of higher ordered forms of silica is suggested by the studies of Mustoe (2008), Sweeney et al. (2009) and Dietrich et al. (2013). Transport and precipitation of silica is influenced by polymerization, concentration gradients of silicic acid, and permeability of sediments and woody tissue as well as the state of cell wall breakdown. Hence applying a diagenetic model for all silicified wood deposits is too simplistic. It is a curious fact that opalized wood is not found in older formations. Opaline preservation of wood has taken place contemporaneously with the formation of common and precious opal. Watkins et al. (2011) demonstrate opal was being formed, eroded and redeposited as part of a dynamic process during a period from the Cretaceous to the Late Oligocene-Early Miocene. The authors conclude that opal formation was influeced by biochemical weathering and temperature as well as geologic conditions. There is still considerable research potential in examining the different possible pathways that lead to the formation of silicified wood. In the following section we discuss whether recent developments in creating wood-templated ceramics through artificial petrifaction may provide potential clues to understanding natural silicification.

Artificial petrifaction – from casting to cell wall replacement by ceramics Drum (1968a,b) described attempts of laboratory silicification of twigs by soaking in sodium metasilicate (24 h, room temperature) and subsequent wet-ashing in chro- mic acid. Single cells up to small aggregates were replicated for microscopic studies. Drum suggested that these silica lithomorphs could be used to study microscopic wood structure. Leo and Barghoorn (1976) used water-dispersed tetraethoxysilane (TEOS) for vacuum impregnation of wood after preconditioning by boiling in water. Via sol-gel transformation TEOS easily converts into silica by addition of water (hydrolysis) under condition of neutral pH and moderate temperatures below 100 °C. Treatment with nitric acid and potassium chlorate was used to remove wood and retrieve silica lithomorphs, which were more substantial than Drum’s. The authors compared their results to natural petrifactions and hypothesized a sequence of infiltration, permineralization, replacement and recrystallization. Persson et al. (2004a) improved the method as a tool for morphological studies. Softwood (spruce) and hardwood (birch) were preconditioned by extraction in ethanol and impregnated by TEOS dispersed in hexadecyltrimethyl ammonium chloride, ethanol and hydrochloric acid. Calcination at 575 °C produced brittle silica cast replicas. Götze et al. (2008) studied impregnation with different silica sources (sodium metasilicate, tetraethoxysilane, methyltriethoxysilane, colloidal silica suspension and silica sols) after wood extraction by acetone-soaking and oven drying (50 °C, 24 h). Filling of the tracheid lumina with varying depth was achieved. This work is an example of materials research with the aim of improving the physical properties of wood as a construction material by increasing mechanical and chemical resistance, reduced water uptake and increased surface hardness. Persson et al. (2004b) achieved complete infiltration of voids between cellulose microfibrils in chemically separated cellulose fibers (pulp). The use of delignified wood as a template for creating an oxide ceramic with the inherent porosity of the wood

Downloaded from Brill.com10/10/2021 02:22:53PM via free access Dietrich et al. – Natural and artificial silicification 175 became a novel approach of advanced ceramic research during the last decade (Greil 2001). Hitherto, numerous authors (Ota et al. 1995; Greil et al. 1998; Shin et al. 1999; Vogli et al. 2001; Vogli et al. 2002) have reported the replacement of cell wall structure in charcoal by SiC. The process involves infiltration of silicon powder, conversion to a melt or a gaseous compound and calcination between 1400–1700 °C. Charcoal from softwoods and a variety of hardwoods is selected according to the intended pore size in the SiC. Close attention has to be paid to the temperature of the charring or calcination procedure since it affects the integrity of the middle lamellae that supports the cohesiveness of the obtained ceramic. The range of wood-templated ceramics has been extended to oxidic ceramics like titania (Ota et al. 2000), zirconia and alumina (Mizutani et al. 2005). Acetylated wood serves as a template and liquid organic metal compounds as precursors. Liu et al. (2008) prepared porous zinc oxide ceramic using softwood as template extracted by ammonia solution, infiltrated by zinc nitrate in ethanol, hydrolyzed and calcinated. Almost coinciding with the publications of metal oxide replications, Shin et al. (2001) achieved a detailed silica replication of wood templates. Following the experiments of Leo and Barghoorn (1976), the authors used TEOS as silica source. An additional surfactant (cetyltrimethylammonium chloride) was used instead of wood extraction. The hydrolysis rate of the sol-gel process was adjusted by the quantity of the solvent (ethanol) and the acidity (hydrochloric acid) of the sol in order to avoid bulk precipitation. Lignin was leached out during the two-month long infiltration. Silica species filled the tracheids and penetrated the cell walls. The surfactant initiated the formation of interconnected nanopores in the silica casts of the tracheids, thus providing pathways for the volatile decomposition products of the organic matter during calcination. Hence, the structural integrity of the lithomorph could be maintained. A sophisticated example for a hierarchically structured silica replica is shown in Figure 2 by scanning electron and transmission electron microscopy. The entire template, a pine wood cube after extraction and delignification, detail of early- and latewood tracheids and detail of cellulose microfibrils in the secondary cell wall are shown (left). The silica replica (right) exhibits shrinkage experienced during processing. The cell walls are replicated by silica that intercalated the cellulose microfibrils. The transmission electron microscopic image clearly shows a layered microstructure of the replicated secondary cell wall originating from different orientations of the cellulose microfibrils in the vicinity of the middle lamella (Van Opdenbosch et al. 2011). Adapted pre-treatment steps of the template have been established to achieve improved infiltration and complete replacement of the cell walls (Zollfrank et al. 2004; Deshpande et al. 2006; Van Opdenbosch et al. 2011; Fritz-Popovski et al. 2013; Paris et al. 2013) resulting in the following processing scheme: Template cutting Wood extraction Wood delignification Surface functionalization Infiltration of metal organics and sol-gel processing Calcination

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Figure 2. Wood-templated hierarchically structured silica replica with nanometer precision. – a & b: Photographs on millimeter paper. – c & d: SEM micrographs. – e & f: TEM micrographs of a, c, e: organically extracted, lignin-reduced pine wood used as template for the preparation of b, d, f. The differences in scale are due to the shrinkage during processing. (Courtesy of Daniel Van Opdenbosch, Biogenic Polymers, Technische Universität München.)

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Template cutting Preparing a sample size with dimensions measured in millimeters or centimeters. Wood extraction Extraction of low-molecular weight substances, formerly achieved by boiling water (Leo & Barghoorn 1976) or acetone (Götze et al. 2008), is accomplished by ethanol or toluene (Van Opdenbosch et al. 2011; Fritz-Popovski et al. 2013). The application of surfactants (e.g. cetyltrimethylammonium chloride) can substitute for this step (Shin et al. 2001). Wood delignification The hydrophobic nature of lignin interferes with the formation of the hydrogen bonds between the silanol groups (Si-OH) and the cellulose (and hemicelluloses). Hence, delignification is crucial for the chemical bonding of silanol groups. Shin et al. (2001) triggered delignification by the acetic components of the infiltrated solution and applied a long-term infiltration process. Persson et al. (2004b) used a mixture of sodium hydroxide, sodium sulfide and sodium sulfate adopted from the Kraft process. More recently, the oxidative removal of lignin was achieved by sodium chlorite (Van Opdenbosch et al. 2011; Fritz-Popovski et al. 2013). Care has to be taken because the lignin-rich middle lamella is essential for the cohesiveness of the template. Surface functionalization Functionalization, i.e. additional formation of cross-links in the cell wall, is achieved by acetylation or by esterification using maleic anhydride (Van Opdenbosch et al. 2011; Paris et al. 2013) with the aim of reducing hygroscopicity and stabilizing geometric dimensions of the template. Infiltration of metal organics and sol-gel processing Infiltrated liquid metal organics as silica sources are subjected to a sol-gel process. A series of condensation reactions converts metal organics like TEOS into a mineral- like solid. TEOS may be dispersed in water (Leo & Barghoorn 1976), but ethanol and hydrochloric acid are more efficient (Shin et al. 2001; Persson et al. 2004a; Van Opdenbosch et al. 2011) because the sol-gel conversion rate is sensitive to the presence of acids, bases and ethanol. Calcination The final step is the removal of the organic template by wet-ashing (Drum 1968a,b; Leo & Barghoorn 1976) or calcination (Shin et al. 2001; Persson et al. 2004a; Van Opdenbosch et al. 2011; Fritz-Popovski et al. 2013; Paris et al. 2013). The temperature should be sufficiently low to retain an amorphous or nanocrystalline microstructure of the ceramic, which is necessary for a successful replication of the original nanometer scale porosity of cell walls (Van Opdenbosch et al. 2011).

Pathways of natural petrifaction in comparison to process steps of wood-templated ceramics In general, artificial silicification supports the idea that incipient wood permineralization occurs due to bonding of silica with wood components. The extent to which

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Figure 3. Psaronius sp., preservation by different silica polymorphs. – a: Transverse section of a trunk with a rather thick root mantle (sample and photo courtesy of Ronny Rößler, Museum of Natural History Chemnitz). – b: Optical micrograph of one of the actinostelic vascular bundles inside the root mantle. – c: SEM image showing preservation of thick-walled sclerenchyma cells by fibrous microcrystalline quartz. – d: SEM image of six tracheary elements, showing cell wall preservation by crystalline quartz and partially non-crystalline fillings in the lumina. the artificial processing scheme can simulate natural pathways depends on substances and geological processes found in natural settings.

Template cutting Comparing the established processing scheme of artificial silicification with potential routes in nature, the first noticeable difference is sample size. Ceramic wood replications have dimensions in the scale of millimeters or centimeters (Fig. 2); petrified forests all over the world consist not only of twigs and branches but also of large trunks.

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A rather small example is shown in Figure 3. The trunk with a root mantle is a Permian treefern (Psaronius sp., Chemnitz, Germany, Zeisigwald Tuff Horizon, stratigraphic position corresponding to the upper Asselian/lower Sakmarian) (Fig. 3a) and has a diameter of about 11 cm. The optical micrograph (Fig. 3b) shows an actinostelic vascular bundle inside the root mantle. The boxes mark the position of SEM images (Fig. 3c, d) that are discussed later.

Wood extraction In a lab setting wood extraction is achieved through the use of organic solvents. In nature wood extraction may result from environmental conditions encountered during burial. A pyroclastic flow event with a high-temperature gas-and-ash mix or hydrothermal circulations in the vicinity of volcanic processes can be inferred from geologic settings. An alternation between drying and immersion in ground water might also achieve wood extraction to a certain extent. In fact different levels of void filling observed in natural silicification may be due to low-molecular substances remaining in the wood (inter alia Furuno et al. 1986b).

Wood delignification Delignification has proved to be a fundamental and essential process step in artificial silicification because the removal of hydrophobic lignin provides silicic acid greater access to hydrogen bonding sites on the cellulose. In natural settings wood may be subjected to thermal or microbial decomposition before as well as during natural silicification. Clear evidence of fungal induced delignification has been found in some cases. Jefferson (1987) emphasized that biogenic delignification is an important part of the preservation process. Non-biogenic delignification is expected especially under conditions of hot spring water. In other cases fossil wood exhibits a preferential disin- tegration in the locations of the middle lamella (Eicke 1952; Dietrich 2013). This seems to be similar to a kind of thermo-mechanical pulping process used in papermaking that does not remove lignin but alters and softens the middle lamella. In most cases delignification is not assumed to be substantial under natural conditions since lignin is the wood component that is last to thermally decompose.

Infiltration of metal organics and sol-gel processing The metal organics used for ceramic formation are technical products that do not occur in nature. The silicification in hot springs seems to be most likely comparable to artificial silicification, but does not prove to be a very reliable model for wood silicification in non-hot spring environments. In natural settings partly polymerized silicic acid is the most likely silica source. The sol-gel processing is most likely triggered by the hydrogen ion activity (pH), which changes from a basic condition in volcanic ash to an acidic condition by deacetylation of hemicelluloses during the initial stage of thermal decomposition of wood (Ballhaus et al. 2012).

Calcination Disassembling the prepared ceramics from the wood-template is the last laboratory step that is quite different from natural petrifaction. In nature the degradation of wood

Downloaded from Brill.com10/10/2021 02:22:53PM via free access 180 IAWA Journal 36 (2), 2015 and the emplacement of silica are assumed to be concurrent and may be long-lasting over geological time. The balance between templating wood with minerals and removing wood components determines the quality of petrifaction. It seems that the decomposition of hemicellulose and cellulose must not occur too quickly if cellular detail is to be replicated with high fidelity. Silicification preserves the state of cell degradation as stated by Sweeney et al. (2009).

Discussion Natural silicification most likely occurs through numerous pathways. However, the inception of silicification occurs when silica solution in the form of a fairly concen- trated polysilicic acid permeates wood tissue and precipitates at the inner cell walls. The silica gel recrystallizes to more stable forms. Leo and Barghorn’s (1976) model of silicification includes two key concepts. First, wood acts as a template for silica even as it slowly degrades and is replaced. Second, silica sol is the silicifying agent that forms hydrogen bonds with hydroxyl groups exposed on cell walls. Leo and Barghorn’s (1976) key concepts have been incorporated into every silicification model proposed since the publication of their classic paper. Scurfield and Segnit (1984) refined Leo and Barghorn’s model by inferring that the rate of cell wall break down may determine whether opal-CT or chalcedony is the primary replicating substance because no evidence was found for the conversion of opal-A to opal-CT. Furuno et al. (1986a,b) have contributed the idea that penetration of silica into cell walls can occur after the filling of silica into lumina and pit chambers. Numerous authors have contributed evidence for the existence of multiple pathways that lead to petrifaction following incipient silicification. Evidence for the classic diagenetic sequence of silica (Buurman 1972; Stein 1982; Jefferson 1987), for additional hydrothermal conversion (Witke et al. 2004), for opal-CT and chalcedony formation independent of an opaline precursor (Mustoe 2008), and for the role microenvironments play within fossil wood (Sweeney et al. 2009), serve as examples of different preservation pathways. A major difference between the models for silicification is the timing during which cell lumina are filled by silica – at the beginning or in the end. If the conditions are appropriate silica penetrates the cell walls at the beginning and leaves the lumina open. This is the case for wood exposed to hot springs where organic matter is decomposed by the strong oxidizing conditions and the high silica concentrations cause rapid preservation by opal (for example in Fig. 1a). In contrast, cell walls of the Permian seed fern example are preserved by chalcedony while the tracheid lumina are often filled by cryptocrystalline or microcrystalline silica (Fig. 1b). A density gradient across the lumen filling is observed which can be interpreted as evidence for a diagenetic process or changes in silica sol concentrations during deposition. It should be noted that the initial silica gel layer inside or the gel cast filling of the tracheids is crossed by a pore system that allows a continued influx of silica sol. The preservation of Chemnitz Permian wood occurred under conditions of moderate temperatures (Götze et al. 2001), slight alkalinity and low concentration of the silica sol (Rößler 2001). A secondary silicification under hydrothermal conditions is assumed (Witke et al. 2004).

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Considering hydrophobicity and decay resistance of lignin, the following silicification pathway for Permian taxa of Chemnitz Petrified ForestDadoxylon spp. and Medullosa sp. (Fig. 3) with cell wall organization similar to conifers has been proposed (Dietrich et al. 2013). Degradation of the wood starts with deacetylation of the hemicelluloses followed by cellulose thereby leaving space for silica precipitation and growth; however, the hydrophobicity of resistant lignin delays the permeation of silica sol into the cell wall. The silica sol intercalates the cellulose fibrils and precipitates as crystalline silica, potentially as chalcedony, starting in layer S2 of the secondary cell wall, where lignin concentration is lowest. This suggestion is substantiated by the studies of Heaney (1993) who proposed different crystallization modes of repeated chalcedony-quartz sequences because the polymerization of silica-containing fluids appears to create silica species with discrete populations of uniformly sized silica polymers down to silica monomers. In contrast to chalcedony preservation, the quartz preservation of the cell walls in the vascular bundles of the tree-fern Psaronius sp. (Fig. 3b, d) appears quite different showing larger, equiaxed crystals. However, the highly lignified sclerenchyma tissue surrounding the vascular bundles is preserved by chalcedony (Fig. 3c). Anthra- cite as a lignin remnant in the sclerenchyma has been substantiated by Witke et al. (2004). Based upon this evidence, the formation of chalcedony due to delayed silica permeation by highly lignified parts of the layered secondary cell walls is suggested for the Permian taxa of Chemnitz. In summary, the comparison of cell wall preservation provides evidence that differences in tissue structure affect which form of silica is the primary replicating material (Dietrich et al. 2013).

Summary Laboratory development and production of ceramics retaining structural details of complex biological templates remains a challenge. An additional benefit of the biotemplating technology is the insight that it provides to the initial stages of natural petrifactions. Two main paths of artificial petrifaction can be identified. One path is artificial casting achieved by infilling lumina to make replicas of inner cell walls and pits. The second path aims to completely replace cell walls by various inorganic compounds by oxides including SiO2 leaving lumina empty. The product is a ceramic material with hierarchical structures and porosity similar to wood. The successful creation of a fibrillar silica matrix replicating the oriented cellulose fibrils into nanopores of similar diameter essentially depends on the delignification pretreatment of the wood template. Some similarities between processing steps in the lab and natural silicification can be noted. The chemical bonding to wood is the same, but the artificial sol-gel-process involves the use of metal organics dissolved in ethanol and acids as a silica source. The conditions of wood silicification in modern hot springs are the most comparable natural process to laboratory silicification. Delignification, cell wall impregnation, and cell wall decomposition are separate laboratory process steps essential for cell wall replication in wood-templated ceramics. In nature these processes might occur almost simultane- ous with lignin commonly as the last wood component to disintegrate. The appropriate balance between the decomposition of wood and the precipitation of silica is a key to replicating cell structures with high fidelity in nature. Factors that affect what form of

Downloaded from Brill.com10/10/2021 02:22:53PM via free access 182 IAWA Journal 36 (2), 2015 silica serves as the primary replicating material include the degree of polymerization in the silica solution and the organization of the plant tissue, particularly the propor- tion of cellulose and lignin as well as the degree of wood decay. In nature buried fossil wood undergoes a diagenetic sequence over geologic time. Although many models of wood silicification share the basic framework proposed by Leo and Barghoorn (1976), numerous studies provide evidence to suggest that multiple pathways to petrifaction exist, making a general model for mineralization elusive. Further work will be needed in order to achieve a more comprehensive understanding of natural petrifaction. Sedimentology, taphonomy and mineralogy contribute to the refining of our understanding of natural silicification pathways. The mutual exchange of knowledge between paleontologists and engineers may advance our understanding of fossil wood preservation as well as the development of advanced ceramics.

Acknowledgements

We are grateful to the reviewers for their constructive comments. Together with Elisabeth Wheeler they provided valuable support to the improvement of the manuscript. George Mustoe, Western Washington University, Daniel Van Opdenbosch, Biogenic Polymers, Technische Universität München and Ronny Rößler, Museum of Natural History Chemnitz are also greatly acknowledged for providing figures.

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Accepted: 1 December 2014

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