Petrification Of Wood

[Jdeutsch]

I'm a bit confused with terminology. I always assumed that the fine micro crystalline wood was made through replacement, and therefore the micro structure was very fine, and the specimen was very hard..Picture 1 Where as the coarser fossil wood was formed through silicification and this wood is generally softer and younger than specimens formed by replacement.Picture 2. ( In fact, when looking at the specimen in picture 2 in hand, it is hard to believe this isn't just wood, although it does have pockets with crystals (picture 3). )

Are there obvious features to determine replacement versus silicification, and are older specimens more likely to be formed through replacement, as it seems to be more durable and less likely to degrade?

[Auspex]

"Replacement" and "silicification" are nearly synonymous, as silification alludes to replacement by silica. There are infinite intergrades of fineness of preserved detail and hardness, all dependent on myriad geologic factors (minerals present, depth and pressure, time...).

[ashcraft]

Have been doing a little reading on mechanisms for fossilization of wood. It generally doesn't occur by replacement, it is more of infill, similar to what occurs in a geode. Wood has alot of space in it, and is fairly polar. Silica dissolved in the water is attracted to this polarity, and over time, it fills the voids, leaving outlines of everything. Initially the infill is opal, which is fairly soft, but turns to chert over time, which is much harder. All of this is dependent on many factors, including pH, mineral in the water, oxygen exposure, etc.. Sometimes it can be very rapid, in months in some mineral springs, or much slower. Some samples millions of years old when heated and exposed to oxygen, lose mass, presumably from carbon being ignited.

Brent Ashcraft

[Jdeutsch]

When the opal turns to chert, is this chert synonymous for jasper, agate, chalcedony, etc?

[ashcraft]

I don't know much about minerals, but most, if not all, of those you mentioned are made up of silicon dioxide, with different 3-D rearrangement of the molecules, or same structure but different impurities. Maybe, I think.

Brent Ashcraft

[Mediospirifer]

The line between jasper and agate is not well defined. As I understand it (based on a recently-read book on the subject), the rock is called jasper if it's mostly opaque, and agate if it's largely translucent. I'm not sure how chert or flint are different, although I think they're thought to have formed under higher pressure than jasper or agate.

Chalcedony is a highly translucent silicon dioxide mineral that is a frequent component of jaspers and agates.

All three (agate, jasper, and chalcedony) have been found in petrified wood. I think most petrified wood is infilled by jasper.