FMF - Friends of Minerals Forum, discussion and message board

[Carles Millan]

[Cesar M. Salvan]

A cautionary note:

The use of term pseudomorphosis to define the replacement of a mineral after a biological remain, even if the remains are of inorganic nature (as bones or shells) is a misuse. The substitution of a biomineral or an organic structure by a new mineral (silica after phosphate or after wood's lignin) is a replacement, not a pseudomorphosis. The pseudomorphosis refers to the maintaining of the crystal shape of an euhedral or idiomorph original crystal. I remark, crystal shape. Again (maybe the words of the south europeans are worth less), the sentence "pyrite after ammonite" suggesting a pseudomorphosis is not correct and should not be used in labels.
Although from an etymological view, pseudomorphosis could refer to any replacement that maintain the former shape, from a mineralogical view, the use of pseudomorph is clearly defined and not mixed with fossilization or replacement terms. Replacement is the general term that encompass the other and, in doubt, it should be used.

[Roger Warin]

Hi,
Your point is well taken. But as long ago as I left the academic way, which makes me also to address a literary public by taking some liberties with the specific discipline. So I chose to use the term "pseudomorphosis" according to the Greek signification. Roger Titeux’s emerald has the form of a shell, but it is not. This is obviously a crystallographic impossible because even the helicity of the shell is respected (dextrorotatory or screw right).
Excuse my reply, but I do not think anyone knows the sequence of processes that led to the formation of this beryl, apart from the general context of a hydrothermal type. The "replacement" is certainly not trivial.
Roger.

[Cesar M. Salvan]

Hi Roger,

Indeed, the beryl fossilized gastropods from Gachala are very interesting. I'm not expert in these deposits, but to my eyes are not very surprising taking into account that the hydrothermalism of these deposits is epigenetic and the deposit is associated to marine black shales. The metasomatism of organic rich black shales, with presence of macrofossils, by hydrothermal fluids could explain the replacement of the original material of gastropod by beryl. I think that, in this case, the major mystery is not the "berylized" fossils, that, to my eyes appears as an usual replacement (in this case, beryl instead of a more common silica, after carbonate? or after previously silicified remains, that could explain that the original shape survived the hydrothermalism with only partial dissolution?). More difficult to explain is the origin of a remarkable beryllium anomaly.

As I said I'm not expert in these deposits and this is a mere 'back envelope' hypothesis and I should revise all published references. Anyway, as you said is not trivial, but still I wouldn't define these "emerald snails" as a case of pseudomorphosis. I hope that an expert in the Gachala and related deposits read my brief explanation and can elaborate it.

[Don Lum]

[Carles Curto]

The process of fossilization involves the substitution molecule by molecule of most of the original animal compounds. In this sense, fossilization is, at the same time, a mineralization.
But, must we include fossils in mineral collections? I think the answer is yes and it is not (in another word: it depends!).

Would you include a calcitic ammonite in your collection? Surely no.

But, and this (see the picture)?

It is a silicified (chalcedony) Pleurotomaria sp., from Germany.

[Carles Curto]

By other hand, fossils are present as a base for finely recrystallized minerals (i.e. some phosphates as apatite, vivianite or anapaite, carbonates as calcite and aragonite, oxides and hydroxides, as goethite, or sulphides as pyrite and marcasite).

See the picture of this anapaite from Crimea (Krim), the base of the crystals is a valve of a fossil bivalve shell, not easaily visible on the photo, but it is complete.

[Carles Millan]

[Roger Warin]

Hi Cesar,
Thank you for your very nuanced reply. I think with you it was good to stay rigorous even for the general public. But the scientific popularization is a difficult art.
Roger.

[Carles Curto]

US Supreme Court are not botanists (evidently).

[Don Lum]

"Knowledge is knowing the tomato is a fruit, wisdom is not putting in your fruit salad.”

Miles Kington

[Carles Millan]

[Carles Curto]

Pyritized fossils (why not opalized fossils, i.e?) were usually admitted in most of the collections in the XIX and XX centuries. Currently, they are not so commonly included.
Some peripheral aspects of the mineralogy, as artificial and synthetic material, faceted and polished minerals, mining lamps and artifacts, old (or modern) photos and other documents, stamps… are freely added to the any collection.
I want especially note this aspect of “freely collecting”. Anybody having his own baggage of knowledge, aesthetic perception, interest, etc., the most important is to enjoy collecting, so, admit some “clearly mineralized” fossils in the collection must be a personal election, not a rule.

[Carles Millan]

[Singingstone48]

Carles,
Here are a couple examples from my own collection. I collected a bryozoan fossil from an outcrop near Kansas City. Bryozoans normally coat other objects on the seafloor. I accidently dropped it, and one corner broke off. When I looked inside I could see the internal structures of a brachiopod which had re-crystallized as calcite rhombs. Sitting in the middle of this hollow space was a 2 cm long white calcite scalenohedron. So this is a case of a fossil on a fossil with a mineral inside. Second, when I was in graduate school in Indiana I collected geodes near Bloomington. Often they were recognizable as crinoids, corals or brachiopods, but they were still geodes with interesting quartz crystals and occasionally calcite or Baryte inside.