“Big Red”
The story of an exceptional petrified
wood specimen at
the Houston Museum of Natural
Science, number 587
by Neal Immega
Member of the Houston Gem & Mineral Society
Originally Published November 2002 in The Backbender's Gazette
One of the most interesting pieces
of petrified
wood we have
seen anywhere is at the Houston Museum of Natural Science, mounted on
the wall
under the tail of the Diplodocus. It is informally known as “Big Red”
because
of its beautiful coloration. Amazingly, all the red, yellow, black, and
peach
colors are from iron impurities in various oxidation states of
different oxides
and hydroxides in a range of grain sizes and combinations. If you look
at any
piece of petrified wood with a 100-power microscope, you would see that
most of
the color comes from colored pancakes and puffballs floating in a sea
of clear
chalcedony. The red ones are hematite, the black ones are really dense
red
ones, and the yellow, orange, and brown ones are hydrated iron oxides
like
limonite.
This tree is an
Araucaria species,
a
ancient type of
conifer that can be traced from living trees in Chile
to fossils of the
Pennsylvanian period, 290+ million years before the present. People who
study
wood tack the suffix –xylon onto species names to indicate a
fossil
tree, so the label says Auricarioxylon. Some related modern
trees are
the Monkey Puzzle tree and the Norfork Island Pine.
Our piece comes
from the Chinle
formation,
Triassic in age
(about 225 million years ago) from Arizona.
When the army first explored the area, a dry wash that cuts through was
named
“Lithodendron,” or “Stone Trees.” General Sherman (of Civil War fame)
had the
army send a couple of logs to the Smithsonian in 1887. About 1890, an
industrial facility was built near Holbrook to grind the giant stone
logs to
make sandpaper. In 1906, President Teddy Roosevelt made a portion of
the area a
national monument, Petrified Forest,
to
preserve it for future visitors. Herbert Zuhl became interested in
petrified
wood when he stopped at a gravel pit on a ranch outside the park and
watched a
backhoe load petrified wood onto a truck. The rancher offered him logs
at the
rate of 10 cents a pound. Mr. Zuhl took the risk that he could find a
way to
cut and polish the logs and that someone would want the results. Mr.
Zuhl later
donated 200 other specimens to the Museum. Mr. Zuhl uses another slab
cut from
the same log as his office desk!
Big Red
deserves a close-up look.
Although the
pore space is
filled with silica, the original wood is still present. You can
actually
dissolve away the silica with hydrofluoric acid and see the cellulose.
All
confers have very small wood cells, so the individual cells do not show
up
without magnification. The polish on our piece is excellent because it
is
thoroughly mineralized.
There is
something odd about the
slab. It is oval
and has
almost no growth rings in the center area. The center has almost no
structure—just like a modern log in the forest that is largely rotted.
After
our log was buried, its outer layers began to petrify before the log
was
crushed by the overburden. The crushing caused the outer layers to
break off
and be pushed into the center. You can see the compression line across
the long
axis of the oval where the petrified layers are broken.
Look carefully
and you can see
that some of the
cavities are
filled with agate. The whole piece was cracked after petrification and
cemented
back together with white agate. Mr. Zuhl told me that some of the
damage is a
lot more recent, though. When the slabs were delivered to his showroom,
the
movers managed to break the end off each piece. The vertical line a
third of
the way across from the right is where the slab was glued back together
before
being repolished.
The images
below are closeups of various parts of the slab.
