DENSITY AND LOSS ON IGNITION AS INDICATORS OF THE FOSSILIZATION OF SILICIFIED WOOD
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98 IAWA Journal
IAWA 37 (1),
Journal 37 2016: 98–111
(1), 2016
Density and loss on ignition as indicators of
the fossilization of silicified wood
George E. Mustoe
Geology Department, Western Washington University, Bellingham, WA 98225, U. S. A.
E-mail: mustoeg@wwu.edu
ABSTRACT
Measuring density of silicified wood and determining weight loss after 450 °C
heating provides useful data for interpreting the process of permineralization.
These simple gravimetric methods do not replace X-ray diffraction, electron mi-
croscopy, polarized light microscopy, Raman spectroscopy, and other specialized
techniques for studying fossil wood, but they can be performed rapidly, and
require minimal laboratory facilities. Woods mineralized with opal have densi-
ties of 1.9–2.1 g/cm3, compared to 2.3–2.6 g /cm3 for wood mineralized with
chalcedony or quartz. Weight loss after 450 °C heating, commonly described as
“loss on ignition” can be used to roughly estimate the % of original organic matter
that remains in chalcedony or quartz-mineralized wood, using the density of extant
taxa for comparison. For opalized wood, 450 °C weight loss mostly represents de-
hydration of the hydrous silica. Data from specimens from 20 localities reveal two
characteristics: 1) silicified woods typically consist either of opal or chalcedony /
quartz, not an intermediate mixture of the two silica polymorphs; 2) the percent-
age of organic matter that remains after petrifaction is usually very small.
Keywords: Silicified wood, petrified wood, organic matter, density, loss on
ignition.
[In the online version of this paper Figure 4 and 6 are reproduced in colour.]
INTRODUCTION
Wood density varies widely among different tree species, ranging from less than
0.2 g/cm3 for Ochroma pyramidale (Balsa) to 1.35 g /cm3 for Krugiodendron ferreum
(Black Ironwood). Density varies at different trunk heights within a single tree (Panshin
& De Zeeuw 1980). For petrified wood, density is related to the mineral content, and the
possible presence of voids or unmineralized cells. Density measurements of silicified
wood can serve two purposes. First, density provides an indication of the particular
silica minerals that are present. Second, in combination with 450 °C loss on ignition
(LOI), the percentage of the original organic matter that remains after fossilization
can be roughly estimated for wood mineralized with chalcedony or quartz. 450 °C LOI
values for opalized wood mostly represent dehydration.
Silica mineralogy can be determined with greater precision using X-ray diffraction,
scanning electron microscopy, polarized light microscopy, and Raman spectroscopy,
© International Association of Wood Anatomists, 2016 DOI 10.1163/22941932-20160123
Published by Koninklijke Brill NV, Leiden
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but these specialized techniques require facilities that are not available to most paleo-
botanists. Methods described in this paper can be performed rapidly and with minimal
laboratory facilities.
Two pieces of evidence are required to estimate the % of organic matter that remains
after fossilization: 1) the density of the fossil wood, the % weight loss when a powdered
sample has been heated for several hours at 450 °C to destroy organic constituents,
and 2) an estimate of the density of the original wood based on evidence from nearest
living relatives. Evaluating silica mineralogy only requires measurement of petrified
wood density. Although calculation of relict organic matter based on 450 °C LOI yields
only an approximation of the actual value, the results are useful for evaluating the de-
gree of permineralization.
MATERIALS AND METHODS
Density vs. specific gravity
Density is the mass per unit volume. In the metric system, common units are g/cm3
or kg/m3. This study uses the former. Specific gravity is the ratio of the density of a
substance relative to a second material, typically water. The formulas for the two
parameters are:
Density: ɠ = g/cm3 or ɠ = kg/m3
Specific gravity: S.G. = ɠ Sample / ɠ H2O
Specific gravity is a ratio, and is therefore dimensionless. In this report, densities cal-
culated as g/cm3 are used.
Density measurement
For both modern and fossil woods, density can be determined by sawing the sample
into a rectangular block. Volume is determined by measuring the dimensions of the
block using a machinist’s caliper for small samples or a ruler for large ones. Weighing
the sample and dividing the mass by the volume yields the density.
Specific gravity of petrified wood can be obtained by any of several common labo-
ratory techniques: Jolly balance, pycnometer, or laboratory balance equipped with a
hydrostatic weighing accessory.
Specific gravity is calculated by the displacement method as follows:
S.G. = weight of sample in air/(weight of sample in air – weight of sample in H2O)
The volumes of sawn slabs that have parallel top and bottom faces can be determined
by measuring the surface area of the slab using an electronic digitizing tablet, and
measuring the slab thickness with a caliper. Weighing the sample quickly yields the
density. In the absence of a digitizing tablet, the area of a slab can be calculated by
tracing the outline of the slab on a sheet of paper, and carefully cutting out the pattern.
Weigh the paper pattern, and calculate the weight of 1 cm2 by measuring the weight
of a rectangular piece of known size. The weight of the paper outline divided by the
weight of 1 cm2 of paper yields the area of the sawn slab in cm2. Multiplying this value
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by the measured slab thickness yields the volume in cm3. This method of graphical
integration has largely been forgotten in the modern digital age, but the method remains
very useful. Regardless of method, the important requirement is that sample mass and
volume can accurately be calculated at a precision of three significant figures.
Silica mineralogy as evidenced by density
The method is based on the observation that silicified wood may contain any of
four silica polymorphs, forms of SiO2 that have different properties of crystallinity.
These forms are comprised of opal-A (amorphous), opal-CT (incipient crystallization
of tridymite and cristobalite, chalcedony (micro-fibrous quartz), and microcrystalline
quartz (Fig. 1). Opal-A is well-known as a form of silica that encrusts and permeates
wood in modern hot springs, and it is a common constituent of gem grade “precious
opal”, but opal-A has rarely been reported in petrified wood (Scurfield & Segnit 1984).
5 µm 10 µm
20 µm 20 µm
Figure 1. Silica polymorphs present in silicified wood. – A: Opal-A encrusted on modern Pinus
contorta (Lodgepole Pine) twig, Yellowstone National Park, Wyoming. – B: Opal-CT permin-
eralizing tracheids, Pliocene Quercinium (oak), Glenns Ferry Formation, Clover Creek, Idaho. –
C: Chalcedony permineralizing Miocene conifer wood, Virgin Valley Formation, northern Ne-
vada. – D: crystalline quartz mineralizing tracheids, Late Cretaceous Cupressoxylon (Cypress
family), Trinity Formation, Montague Co, Texas. — Photomicrographs were made at Western
Washington University using a Tescan Vega scanning electron microscope, operated at a beam
voltage of 15 Kv. Specimens were mounted to 1-cm-diameter aluminum stubs with epoxy ad-
hesive, and sputter-coated with Pd to provide electrical conductivity. All specimens from the
author’s research collection at Western Washington University.
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Instead, opalized wood almost always contains opal-CT, the silica polymorph present
in “common opal”. The density of opal varies from 1.9–2.3 g/cm3, with an average
value of 2.09 g/cm3 (Eckert 1997). In contrast, chalcedony and quartz in pure form
both have a density of 2.60 g/cm3. For wood permineralized with either of these min-
erals, density may be slightly reduced because of the presence of structural voids or
compositional impurities. Detailed analysis of mineralization requires a method such
as electron microscopy (Fig. 1), X-ray diffraction (Fig. 2), but for most samples den-
sity alone is sufficient to distinguish between opalized wood and agatized wood, the
two descriptive categories popular among petrified wood aficionados. Density can
also be used as a preliminary search for the uncommon instances when petrified wood
contains more than one silica phase.
Relative intensity
chalcedony/quartz
opal-CT
opal-A
5 10 15 20 25 30 35 40 45 50 55
Degrees two-theta - Cu K alpha
Figure 2. X-ray diffraction patterns of silica polymorphs present in silicified wood. Opal-A:
modern Pinus contorta (Lodgepole Pine) twig from a hot spring at Yellowstone National Park,
Wyoming. Opal-CT: Miocene wood, Mineral County, Nevada. Chalcedony/quartz: Miocene,
Grant County, Washington.
Determining % of original organic matter
This method uses weight loss after 450 °C heating as a proxy for the amount of relict
organic matter. This value is compared with the original amount prior to fossilization,
based on the density of the ancient wood as estimated from nearest living relatives.
As a hypothetical model, consider wood from an ancient oak tree that was buried in
a depositional environment favorable to silicification. Let us assume that the wood
originally had a density of 0.77 g/cm2, and that after complete permineralization with
chalcedony the petrified wood has a final density of 2.6 g/cm2. During this time, only
10% of the original wood tissue remains, the rest having been destroyed by gradual
degradation. A 1-cm3 fragment of this petrified wood would therefore have a mass of
2.60 g, which includes 0.077 g of relict wood = 2.96 weight %.
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These calculations can be reversed to estimate the % of relict organic matter remain-
ing in silicified wood. The density of the fossil wood can be determined by accurately
weighing a specimen of known volume, and calculating the density as described below.
The amount of relict organic matter can be estimated by measuring the loss in weight
of a known volume of powdered silicified wood after two hours heating at 450°C.
Both parameters can be measured with accuracy. Hydrous crystalline silicate minerals
(e.g., mica and clay) contain hydroxyl ions as components of the lattice framework,
and they are an unlikely source of analytical error because removal of this structural
water requires temperatures of approximately 900 °C (Frondel 1982).
The main analytical difficulty is estimating the density of the original wood prior
to mineralization. The method uses average density values for extant genera that are
considered to be the nearest living relatives of fossilized woods. The uncertainty arises
because density values of wood vary in accordance with anatomical and ecological fac-
tors. Bark, limb and trunk tissue may all have different densities, and fast-growing trees
may have lower density than slow-growing trees. Density may vary significantly within
species comprising a single genus. These issues are described in detail by Wiemann
and Williamson (1989), Muller-Landau (2004) and Williamson and Wiemann (2010).
Loss on ignition is commonly used for determining organic matter in soils (Schulte &
Hopkins 1996; Combs & Nathan 1998). More precise determinations could be made
using mass spectrometry or gas chromatography, but these refinements would add
considerable complexity to the analytical process. For this study, silicified wood sam-
ples were pulverized to fine powder using a Model 8000 Mixer Mill (Spex Industries,
3880 Park Ave, Edison, New Jersey 08820-3012, USA). Samples were dried over-
night at 110 °C to remove adsorbed moisture. A dry sample was added to a pre-weighed
10 ml porcelain crucible; the exact weight of crucible & sample was determined using a
Sartorius model 1700 analytical balance. The weight was measured after 450 °C heating
for 2 hours, allowing the sample to cool to 110 °C prior to weighing. The exact details
of this process are not of critical importance; the essential issues are that the amount
of sample needs to be relatively small, to allow good air circulation during heating,
otherwise organic matter may be reduced to charcoal rather than combusted. Pulver-
izing samples to a particle size of approximately 0.074 mm (#200 U.S. Standard screen
mesh size) produces a large surface area that facilitates rapid combustion. Density and
450 °C loss on ignition (LOI) were measured for 20 specimens of silicified wood that
were identified to the genus level. These included conifers and angiosperms, ranging
in age from Devonian to Pleistocene. The mineralogy of every sample was determined
by X-ray diffraction using a Rigaku Geigerflex diffractometer using Ni-filtered Cu-Kα
radiation, and by scanning electron microscopy using a Tescan Vega SEM. Samples
included specimens mineralized with opal-CT, chalcedony, or quartz. Five specimens of
uncertain taxonomy were analyzed in order to include additional examples of opalized
wood. Density was measured using a Sartorius model 1700 analytical balance equipped
with a model 6080 hydrostatic weighing accessory. Densities of modern woods are
from Zanne et al. (2009).
Several alternate methods are possible. The density of fossil wood can be determined
by inference based on anatomical characteristics, e.g., the fiber to lumen ratio (Martinez-
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Cabrera et al. 2012) and the percentage of cell wall in a cross-sectional area (Wheeler
et al. 2007). These techniques are considerably more difficult to perform than 450 °C
loss on ignition.
Example
Miocene oak from Swartz Canyon, north central Oregon mineralized with chal-
cedony, has a measured density of 2.58 g/cm3. A powdered sample had a 450 °C loss
on ignition of 0.78%. Densities of 24 species of modern oak from North America range
from 0.520 g/cm3 to 0.89 g/cm3, with a mean value of 0.610 g/cm3. This value was
used to estimate relict organic matter for fossil oak wood in Table 2.
One cm3 of this silicified oak has a mass of 2.58 g, so the 0.78% LOI suggests that
the remaining organic content = 2.58 g/cm3 × 0.0078 = 0.0201 g/cm3. Assuming an
original wood density of 0.61 g/cm3, the calculated proportion of original wood that
remains is (0.0201 g/cm3)/(0.61 g/cm3) = 0.0330 = 3.30% (Table 1).
Table 1.
Assumed density Species Calculated % relict organic matter
remaining after silicification
.510 Quercus kelloggii 3.94 %
.890 Quercus minima 2.26 %
.610 Mean value of 24 species 3.30 %
The degree of analytical uncertainty is indicated by the results obtained when the
lowest, highest, and mean densities reported for modern Quercus from North America
are used. The results suggest that calculations for relict organic matter remaining after
silicification are only semi-quantitative, but the data clearly show that for this example
only a very small proportion of the original organic matter remains.
RESULTS
Percentage of original organic matter
The calculated percentage of organic matter remaining after petrifaction varied from
0.65% to 21.71% (Table 2). The values have no obvious correlation with taxonomy or
geologic age, as evidenced by the variation among three samples each of Palmoxylon,
Quercinium, and Sequoioxylon from different locations. These data are consistent with
visual evidence. For example, the Sequoioxylon wood from Yellowstone River, Montana
has a dark brown color that suggests the presence of relict organic matter. The 3.94%
LOI yielded is equivalent to preservation of a calculated 21.71% of the original wood;
the powdered sample changed from dark brown to light brown after heating, evidence
that the LOI represented combustion of organic matter.
Relatively high LOI values were measured for wood specimens mineralized with
opal, resulting in estimations of organic matter that were significantly higher than
values determined for chalcedony and quartz petrifactions. Possibly these LOI val-
ues represent combustion of organic matter, consistent with the common belief that
opalization represents the earliest stage of silicification, when wood is less degraded
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than in later stages of mineralization. A more likely explanation is that the higher LOI
values are mostly caused by loss of structural water from the hydrous opal (Fig. 3). This
interpretation is supported by the observation that the opalized wood specimens are
light colored, and the color remained unchanged after heating. In their study of silica
from geyser discharge aprons, Day and Jones (2004) reported that a water content of
1.2–9.8 wt. % for opal-A, and 3.8–8.2 wt. % for opal-CT.
Silica mineralogy as evidenced by density
The densities of silicified wood have a bimodal distribution, reflecting the differences
in the physical properties of silica polymorphs (Fig. 1). Despite these complexities,
which are described later, density provides a clear way to distinguish opalized wood
from quartz/chalcedony wood. Amorphous opal-A and weakly-crystalline opal-CT
typically have densities in the range of 1.9–2.1 g/cm3. Density does not distinguish
between opal-A and opal-CT, but as noted earlier, except in geothermal environments
Table 2.
Estimated Calculated %
Age Location Genus Mineralogy Density % LOI original original
g/cm3 450
density wood
Devonian Murray, OK Callixylon chalcedony 2.49 0.41 – –
Triassic Holbrook, AZ Araucarioxylon chalcedony 2.62 0.19 0.52 0.96
Cretaceous Montague Co., TX Cupressinoxylon quartz 2.53 0.55 0.45 3.09
Eocene Leesville, LA Palmoxylon chalcedony 2.58 0.14 0.56 0.65
Eocene Eden Valley, WY Palmoxylon chalcedony 2.50 1.39 0.56 6.21
Eocene Watertree River, SC Palmoxylon chalcedony 2.32 2.20 0.56 9.11
Oligocene Panama Palmoxylon chalcedony 2.57 1.24 0.56 5.69
Paleocene North Dakota Metasequoia chalcedony 2.60 0.33 0.45 1.91
Oligocene Rapid City, SD Metasequoia chalcedony 2.62 0.27 0.45 1.57
Eocene Florissant, CO Sequoioxylon chalcedony 2.53 0.36 0.45 2.02
Eocene Florissant, CO Sequoioxylon chalcedony 2.41 0.43 0.45 2.30
Eocene Gallatin Co., MT Sequoioxylon quartz 2.48 3.94 0.45 21.71
Miocene Yakima, WA Platanus chalcedony 2.37 0.83 0.56 3.51
Miocene Yakima, WA Ulmus opal-CT 1.95 2.18 0.60 7.09
Miocene Madras, OR Quercinium opal-CT 2.01 2.80 0.74 7.61
Miocene Swartz Canyon, OR Quercinium chalcedony 2.58 0.78 0.61 3.30
Pliocene Bliss Co., Idaho Quercinium opal-CT 1.93 1.65 0.74 4.30
Pleistocene Florida Taxodium chalcedony 2.54 0.55 0.48 2.91
Eocene Cache Creek, BC unknown opal-CT 2.07 4.49 0.45 20.65
Miocene Miller Mtn., NV unknown opal-CT 2.07 3.75 0.45 17.25
Miocene Yakima Co., WA Cupressinoxylon opal-CT 1.99 3.28 0.45 14.50
Miocene Washoe County, NV unknown opal-CT 1.86 2.77 0.45 11.45
Miocene Rawhide, NV unknown opal-CT 1.85 4.68 0.45 19.24
Miocene Lake Tahoe, CA unknown opal-CT 1.90 3.25 0.45 13.72
Neogene Columbia unknown opal-CT 1.90 2.66 0.45 11.23
Miocene Nye County, Nevada conifer opal-CT 1.90 1.61 0.45 6.80
*Mineralogy determined by X-ray diffraction and scanning electron microscopy.
**Density estimated from extant relatives.
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10 µm 2 µm
Figure 3. Radial view of Miocene opalized conifer wood from Nye County, Nevada. Silicifica-
tion preserved anatomic details, but no visible organic matter remains. At high magnification, a
pit can be seen to consist of microcrystalline opal-CT, and energy-dispersive X-ray fluorescence
analysis shows no detectable carbon. The 1.61 % weight loss after 450 °C heating presumably
represents loss of moisture from the hydrous opal.
opalized wood is almost always mineralized with opal-CT. Chalcedony and quartz both
have densities of 2.57–2.62 (Fig. 4). They cannot be distinguished by density, but visual
appearance provides useful clues. Silicified wood with density >2.3 that has waxy or
vitreous luster and conchoidal fracture is probably mineralized with chalcedony. Wood
mineralized with microcrystalline quartz may have a rather dull luster, but sometimes
show sparkling microcrystals lining cavities.
opal-CT chalcedony/quartz
1.8 2 2.2 2.4 2.6
Density g/cm3
Figure 4. Opal-CT and quartz/chalcedony mineralized woods plot in two density categories.
Data are from Table 2.
Silicified wood densities of 2.30–2.56 may be caused by several phenomena. One
possibility is that the wood contains carbonaceous matter that reduces the density
(Fig. 5A), or wood may contain non-silica minerals (Fig. 5 B). Another possibility is that
the wood is not completely mineralized. A common example is when tracheids have
been silicified, but other anatomical regions remain empty (Leo & Barghoorn 1967;
Mustoe 2015). These voids may include lumina (Fig. 5 C, 5 E), intercellular spaces
(Fig. 5 D, 5E), or areas of decay (Fig. 5 F). A third possibility is that the wood contains
more than one form of silica (Fig. 6).
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10 µm 25 µm
20 µm 20 µm
250 µm
25 µm
Figure 5. Features in silicified wood that may affect density. – A: Carbonized cell walls, lumen
filled with opal-CT, transverse view, Miocene, Rainbow Ridge Mine, Virgin Valley, NV. –
B: Tracheids mineralized with opal-CT, with blocky zeolite crystals derived from alteration of
tuffaceous matrix, oblique transverse view, Miocene, Rainbow Ridge Mine, Virgin Valley, NV. –
C: Silicified cell walls, with empty lumen, transverse view, Miocene, Rainbow Ridge Mine,
Virgin Valley, NV. – D: Open intercellular spaces, opal-CT mineralizing cell walls and lumen,
transverse view, Miocene, Norita Mine, Virgin Valley, NV. – E: Opal-Ct mineralized oak wood,
with botryoidal chalcedony partially filling vessels, Pliocene, Clover Creek, Bliss Co., ID. –
F: Chalcedony mineralization of rotted wood, radial view, Miocene, OR /NV border near
McDermitt, NV. Thin white linear features are silicified fungal hyphae.
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55 mm
cm
3 mm
Figure 6. Miocene wood from Blowout Mountain, Humboldt County, Nevada contains three
forms of silica. This wood was partially rotted prior to fossilization. Organic matter is mineral-
ized with opal-CT. Open spaces contain quartz crystals that have been overgrown by botryoidal
chalcedony. – A: Ordinary transmitted light. – B: Polarized light. Dark opaque areas = opal-CT,
banded botryoidal zones = chalcedony, angular shapes at lower right = quartz crystals. Each
silica polymorph represents a different episode of mineral deposition.
DISCUSSION
This report describes analytical methods, but the reported data have several implications
for understanding the petrifaction process. Density values are clustered in two distinct
groups: opal-CT and quartz/chalcedony. A long-accepted model for wood silicification
presumes that mineralization follows a transformational sequence, where opal is the
initial silica phase, with chalcedony and quartz originating as successive transforma-
tions during long burial (Buurman 1972; Scurfield 1979; Stein 1982; Scurfield & Segnit
1984). If gradual mineral transformation is a common process during petrifaction, it
would seem likely that many deposits would contain petrified wood that contains both
opal and chalcedony. The bimodal distribution (Fig. 2) is evidence that specimens of
intermediate composition are rare. This mystery deserves more study. Mustoe (2008)
described late Eocene specimens from the Florissant, Colorado fossil forest where opal
and chalcedony coexist, but different silica phases appear to have been formed during
separate stages of mineral deposition, not as a transformative sequence. Figure 6 shows
Nevada Miocene wood that contains multiple phases of silica deposition. Density of
this specimen cannot accurately be determined because of the many voids. Complex
silicification pathways in Neogene wood are described in detail by Mustoe (2015).
Although the calculated % of original wood is only an approximation, the data are
significant: in every quartz/chalcedony sample, the amount of relict tissue that remains
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is very small. The data cast doubt on the popular belief that petrified wood represents
two different processes, replacement and permineralization. The replacement hypoth-
esis proposes that mineral precipitation can produce detailed preservation of cellular
anatomy, with complete loss of the original organic constituents. Permineralization
assumes that the original cells remain entombed within a mineral matrix. The two
hypotheses have long appeared in paleontology textbooks and popular publications.
For example, in their article preparation of modern and fossil wood for microscopic
examination, Abbott et al. (1982) claim that transverse sections of silicified wood
can be sectioned with a sharp knife after prolonged soaking in strong hydrofluoric acid
solution, a method that is likely to be successful only for woods in very early stages
of mineralization. The only researcher who has actually conducted an investigation
was St. John (1927), who attempted to visually evaluate the amount of preserved relict
tissue by dissolving blocks of silicified wood in hydrofluoric acid and examining the
remains with a microscope. The preservation of cellular tissue was highly variable.
One of the best examples of organic preservation in St. John’s study was a sample of
silicified wood from the fossil forests at Yellowstone National Park, correlative with
the strata that yielded the Yellowstone River specimen that gave a 450 °C LOI value
of 3.94% in my study, yielding a calculated 21.71% relict organic matter (Table 2).
Other samples contained lesser amounts of cellular remains, some containing none.
St. John (1927) concluded that wood petrifaction was a variable process, and that
even in a single specimen some areas could have silica filling cellular and intracellular
spaces of intact tissue, while other zones would have complete loss of organic matter.
A limitation of St. John’s method is that it is a difficult analytical task to dissolve a
large amount of silica to release a small amount of organic matter, preventing quantita-
tive measurements. The combination of LOI and density measurements described in
this report provides a safe, quick way to approximately estimate the amount of relict
tissue. The disadvantage is that the form of the organic matter cannot be visualized.
2.65
2.6
2.55
Density g / cm 3
2.5
2.45
2.4
2.35
2.3
0 1% 2% 3% 4%
450 °C Loss on ignition
Figure 7. Plot of density vs. 450° loss on ignition for fossil woods mineralized with quartz /
chalcedony.
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Plotting density versus 450 °C loss on ignition (Fig. 7) shows a weak trend for LOI
to be lower in relation to decreasing density values. This trend probably results because
during gradual silicification, permineralization and tissue degradation are occurring
simultaneously. In early stages of petrifaction, the amount of original organic matter
is relatively high compared to the amount of precipitated silica.
The percentage of original wood that remains after petrifaction shows very low
correlation with the age of the fossils (Fig. 8). When density is plotted in relation to
geologic age (Fig. 9), the lowest densities are associated with late Tertiary woods, which
are commonly mineralized with opal, in contrast to the chalcedony/quartz compositions
0
Approximate age - millions of years
50
100
150
200
250
0 5 10 15 20 25
% Original wood
Figure 8. Plot of geologic age of fossil wood vs. % original wood.
0
Approximate age - millions of years
50
100
150
200
250
300
350
400
1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8
Density g /cm3
Figure 9. Plot of geologic age versus fossil wood density.
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of older samples. These data suggest that wood petrification is primarily influenced by
physicochemical conditions after burial, rather than geologic age. A common question
asked of paleobotanists is “how long does it take for wood to become petrified?” The
question has no simple answer. Ancient wood buried in environments that inhibits
microbial decomposition, but which lacks exposure to mineral-bearing groundwater,
may remain unfossilized even after millions of years (Basinger 1991; Bigras et al.
1995; Wolfe et al. 2012; Yancey et al. 2013; Bardet & Pournou 2014). Conversely, a
twig that has fallen into a modern hot spring may become rapidly silicified (Hellawell
et al. 2015).
ACKNOWLEDGEMENTS
Specimens from Florissant Fossil Beds National Monument were provided by park paleontologist Dr.
Herbert Meyer, under authorization of National Park Service Permit FLFO-00403. The manuscript
greatly benefited from comments suggested by IAWA Journal editor Pieter Baas, reviewer Chris
Ballhaus, and my colleague Mike Viney.
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Accepted: 7 September 2015
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