FMF - Friends of Minerals Forum, discussion and message board

[Cesar M. Salvan]

[Philip G]

Hi all,

I have always found this specimen quite interesting.

The pictures show what i believe to be hematite encasing a solitary rugose coral but leaving the fossil itself totally unhematised. The small cavity in the coral contains micro crystals of dolomite.

From the Carboniferous limestones, Florence Mine, Egremont, West Cumbria, England.

Philip

[alfredo]

Fossils could be divided into two types:

1) The dead organism's original organic material is preserved without replacement, as for example with Siberian mammoths frozen for 20,000 years, or giant ground sloths merely desiccated in dry caves, or human beings pickled in the acid waters of peat bogs. These fossils are NOT minerals, obviously.

2) The much more common type of fossil in which the original organic matter is replaced by mineral matter. These are obviously both fossils and minerals, and any distinction is merely semantic and due to the collection preferences of the person who has it. I'm sure most MINERAL collectors would not consider a fossil to be a mineral SPECIMEN unless it contained either visible crystals or some interesting replacement species (opal, vivianite, etc). A fossil composed of clay minerals is of course also a mineral, but few mineral collectors would consider it to be a mineral "specimen".

The definitions of "mineral" and "fossil" are pretty well established and it would be redundant to rehash them here, so what this discussion is really about is the definition of "specimen"!

[Cesar M. Salvan]

Alfredo, you forgotten the common case of fossils in which organic material hasn't been replaced by inorganic materials, but without preservation of original organic components. Is the case, for example, of amber (not mineral) or, again, fichtelite (a mineral).

[Elise]

Hi Pete!

Maybe that was Bob Hazen's work? See our thread here: https://www.mineral-forum.com/message-board/viewtopic.php?p=13282#13282

Cheers
Elise

[Ru Smith]

That is a fascinating coral enclosed in hematite, Philip. Brian Young has a paper in the Journal of the Russell Society (2012) which shows a colonial coral (?Siphonodendron sp.) preserved in hematite from the old Florence Mine dump. "The skeletal elements of the original coral are preserved in quartz surrounded by massive hematite."

My favourite example of mineralization of fossils is the replication of soft parts by very early precipitation of pyrite, such as is seen in the Ordovician of New York or the Devonian of Bundenbach in Germany.

[Riccardo Modanesi]

[Don Lum]

[Don Lum]

[Carles Millan]

[Carles Millan]

[Pierre Joubert]

[Roger Warin]

Hello,
Without giving a lesson, I think fossilization is actually a pseudomorphosis whose shell (for example) gives the form.
A conventional mineral pseudomorphosis is the gradual transformation of azurite into malachite. The material turns without transport. The resulting crystal of malachite retains the shape of the initial azurite. But malachite is not a single crystal.
A fossil is a residue of an organic, a plant or animal skeleton. Silica impregnated wood fibers whose soft material disappears.
A shell will fill with sediment, calcite, pyrite, etc…
In general, a fossil will be a differentiated rock or a mineral depending on the composition.
I'll post another interesting topic, I think.
Roger.

[Roger Warin]

Hi,
At the 2002 Munich Show, my friend Roger Titeux asked me to buy two fossil specimens. I refused for lack of money. But I regret it now, and for a good reason.
Both fossils were made of emerald. I think the emerald has replaced the calcite filling the shell, in a flow of hydrothermal waters.
These fossils come from the famous Valley of Colombia, and I think remember from Gachala.
I think in this case, we can say that the fossil is a mineral.
Here are the photos.
Roger.

[Don Lum]

Roger,

That is an amazing specimen and great photos. Thank you.

Don