When is a fossil not a fossil ....
The term "fossil" is used a lot among rockhounds. But it's a very broad term and doesn't tell you much about a rock. Geologists and dictionaries cling to the definition that a fossil has to be "any preserved remains, impression, or trace of once-living thing from a past geological age" (Wikipedia). But astrogeobiology.org from Lund University has an extended discussion about "Fossil Meteorites". They were never living. I've long described the Utah agatized barite crystal nodules as fossils and there is nothing biological about them either.
Recently, I appealed to readers here and at Reddit RockhoundExchange to help me identify a type of rock I was finding in my dad's collection. (I'm now calling this rock, "Yellowcat Bark" and you can read about it at: Introducing Yellow Cat Bark.) I knew that it was a "fossil" of some kind right from the beginning. That didn't help much because it could have been a "fossil" of a lot of things.
My fiction story, "The Last Mammoth" (read the story here) was inspired by the discovery of the remains of a Mammoth in Huntington Canyon, Utah, now on display at the prehistoric museum in Price, Utah. There's a great article about it here: Price, Utah Prehistoric Museum. The mammoth bones discovered in Huntington Canyon are consistently described as "remains". The word "fossil" only appears once; it's literally the "last word" in the article: The mammoth bones "were not fossilized."
I wonder if the paleontologists at Lund University and the Utah prehistoric museum ever talk to the geologists? No wonder the rest of us get confused.
So, what is a fossil? If I was writing a dictionary, I would change the definition to this:
Any impression or trace of a discrete object from a past geological age that preserves identifying properties of the object.
Criticism of my new definition is welcomed! Leave a comment! A leaf impression in shale would be a "fossil" even though no part of the leaf itself is preserved. My dad's Utah agatized barites and the Swedish meteorite fossils are clearly fossils too.
When they think of fossils, rockhounds usually think of the classics: petrified wood and petrified dinosaur bone. For these classic fossils, geologists would use a more technical term: "permineralization". This term is used to describe the slow, molecule-by-molecule replacement of an object, usually by silica (which results in agate or jasper) but sometimes by calcite, pyrite, or even other materials.
Rockhounds are also familiar with the term, "cast": A replacement of only the shape of the object. (That's what I believe my Yellowcat Bark turned out to be.) The idea of a "cast" also helps to understand permineralization. An organic object, such as a bone or tree, consists of a lot of chemically different things. A plant cell wall is mainly cellulose. Animal bone cell walls are mostly collagen, a completely different material. Ground water reacts differently to these different materials.
Permineralization starts when organic cells are "mineralized" like a very small "cast". As Wikipedia puts it:
Crystals begin to form in the porous cell walls. This process continues on the inner surface of the walls until the central cavity of the cell, the lumen, is completely filled. The cell walls themselves remain intact surrounding the crystals.
With big, open cells like bone, the effect is striking. My dad's "butterfly" pins are a magnificent example. But xylem and phloem in plants seldom show up well in petrified plant fossils. There are a few exceptions. My dad's "algae-on-a-stick" has the most perfect replacement I have ever seen for plant cells. (If you're a paleontologist and you would like to see for yourself, send me a message.) Blue Forest wood from Eden Valley, Wyoming and Utah Tempskya Fern can also have detailed plant cell replacement.
The "wood grain" in Utah Henry Mountain wood is beautiful, but usually isn't as detailed as a fossil. Utah Yellow Cat wood and Wiggins Fork, Wyoming wood are just casts with no interior wood detail at all.
In general, it takes millions of years for silica to replace the organic material in a fossil, but the age of fossils varies a lot. Scientists have been able to measure permineralization starting in organic material in as little as three years. The Morrison Formation in the Colorado Plateau, rich in dinosaur and wood fossils, is about 150 million years old. The famous Utah Rugosa red horn coral fossils are about 450 million years old; three times older than the dinosaur fossils. Utah's famous trilobite fossils in the West Desert are about 100 million years older than that. But the grand prize probably goes to Mary Ellen Jasper, a fossil of ancient algae found in the Mesabi Iron Range in northern Minnesota. These massively ancient fossils can max out at 2 billion years. If you're keeping track, that's four times older than trilobites and Rugosa coral and over ten times older than dino bone and petrified wood.
In this case, age and beauty go together. Here's a massive Mary Ellen jasper from my dad's collection.