FMF - Friends of Minerals Forum, discussion and message board

[Elise]

Hi,
Just a vague question about the three terms which I hoped might generate a little discussion and sharing of images. While writing about a Miocene pearl which someone gave me to study and a contemporary aragonite concretion which completely replaces a snail shell, I've been thinking about the fascinating specimen Jordi posted earlier of calcite growing on a shell https://www.mineral-forum.com/message-board/viewtopic.php?p=10755&highlight=#10755 Maybe seemingly unrelated but indicative of the way my mind wanders around, fascination was piqued again when a couple weeks ago I worked on an assignment for a journal abstracting the long article on Andradite in the Madagascar issue of MinRec in which I found a brief mention of andradite replacing shells and corals. Things like this are a good springboard for great discussions at work -- one thing leads to another and progressively more problems are dissected....the question of what is a "fossil" was a bit like the recent copal/amber thread here https://www.mineral-forum.com/message-board/viewtopic.php?t=1429 (my mentor-in-all-things-mineralogical's fossil definition: "well, does it smell?")(I never know when he is pulling my leg). In reading about these types of fossils in the andradite article, I also find conflicting descriptions of how they form and the line between pseudomorphs and perimorphs seems blurred.

In extraLapis No. 2 Emeralds in the article by Deitmar Schwarz and Gaston Guiliani there is the Van Pelt portrait of Ron Ringsrud's emerald gastropod shells (Matecaña, Mine, Colombia) the description of which says that the shells were completely replaced by microcrystalline beryl (I have Dietmar Schwarz's book Esmeraldas, but I can't read it! - maybe there is mention in there for those who can). Those emerald gastropods are also in Ron's wonderful book with some discussion and a reference to a study by Pierre Viullet (I don't find this). Some excerpts from that section, hopefully accurate when taking them out of context: "....the formation was one of carbonate destructive replacement; a molecule by molecule replacement of grains of the shell structure by molecules, or unit cells of emerald. Gastropods can also be called pseudomorphs..." "....it appears that the crystallization started at the outside of the sea shell and proceeded into the interior..." ".......the existence of these emerald gastropods validates the low-temperature mineralization of the Columbian emerald." And apparently albite pseudomorphs have been found in the same region (I am not sure if he means as such fossils).

When looking online for more information about the Antetezambato andradite fossils, I found this very interesting thread on MinDat with many nice images. https://www.mindat.org/forum.php?read,15,195371,page=1 One poster included the following papers which I am trying now to access. I do read Norwegian, though not at an academic level...I have a good Norsk/Engish technical dictionary to limp along with, but maybe if one of the Norwegian FMF members has read them, they could give some insights:

"Garnet and vesuvianite replacment of silurian fossils (mostly gastropods or brachiopods) are well known from the the classical contact metamorph skarns in the Oslo area, Norway . A relatively recent article on these is published in :

Hurum, J.H & Svensen, H. (2001): Granatiserte fossiler fra Drammen-området [Garnetisized fossils from the Drammen area].Norsk Bergverksmuseum Skrift, 18: 13-17. (In Norwegian].

A find of a fossil brachiopoda replaced by coronadite from the same geological province is just published in : Folvik, H.O (2010): En brakiopode fra Skjerpemyr. STEIN 37 (3): 30-31 [also in Norwegian] "

From the MinRec issue: Andradite from Antetezambato, North Madagascar. F. Pezzotta. Mineralogical Record, Vol. 41, No. 3, 2010, pp 209-229. Under the article's overview of the geology of the area and its deposits, it reads: "The metasomatic process allowed the preservation of much of the original structure of the sedimentary rock, and even fossils have been replaced locally by the fine-grain skarn minerals. In a few cases, recognizable fossil shells, corals and even ammonites have been replaced by garnet and quartz, and form the matrix for demantoid crystals. " And in a caption for an image: "Fossil shell transformed into garnet skarn (fine grained grossular-andradite with minor quartz)."

Andradite is a calcium rich garnet; does that lend itself to the possibility of replacing the carbonates in the shells and corals, analogous to Jordi's specimen above? The emerald snail and these Antetezambato fossils are microcrystaline rather than the macro crystals of the calcite specimen - temperature related?

The term perimorph is new to me and in further admitting my ignorance I can say my first exposure to the term pseudomorph was while watching Jeff Scovil image an "opal pineapple" several years ago - he assumed I knew what he meant when he was talking about it....while I was thinking- that's a pineapple???!!! (I've come a long way since then). Re Alberto's comment in the Mindat thread: "...mineralogists might not be interested because there are no good crystal faces. ...or maybe I'm just a pessimist and there is an academic group somewhere totally enamored of fossils replaced by rare minerals?" that would be us....it brought a lot of chuckles yesterday; Bill Basset and I are completely obsessed with the weird and unusual here....

Cheers!
Elise
PS: that opal pineapple portrait is published in Si and Ann Frazier article in the Lithographi No. 10 Opal issue, page 32, in which they write: "It is entirely possible that the pineapples are "double" pseudomorphs: ikaite transitioned to calcite which weathered away leaving a cast to be filled with opal."
PSS: am I going to get docked for excessive bandwidth usage with my questions?

[Jordi Fabre]

[dtkasper]

I had a chance to study and photograph the famous Klement pineapple opal recently. I like the interpretation that it is opaline mineralization of glauberite. The striations on the individual crystals clearly show it is not after ikaite. The look of ikiate I have collected is totally different. Sulfates like Baryte, etc commonly seem to have that type of incised crystal morphology.
Sites of ikiate form in cold lakes, so if you find crystals they are going to be found by the thousands. The Australian pineapples, by comparison are extremely rare. I find it untenable there was a broad depositional environment involved just because of that. It took a dramatic change of climate to go from sulfates to opaline silica replacement, and relates to why those structures are so rare. The opal is hydrothermal, and ikiates form in very cold water, so there is no match.

[Elise]

I should have searched FMF for the terms before starting a new thread - Gail's 2008 posting "Endomorph or Pseudomorph?" (which was recently resurrected) answers many questions; especially with John's R&M article https://www.mineral-forum.com/message-board/viewtopic.php?p=1906&highlight=#1906 Also, should just add that the Fraziers' article "White Cliffs in New South Wales: Opals and Pineapples" which I mentioned above referenced an unpublished monograph on ikaite by Keith Harschbarger in their discussion. As I understand it, glauberite is a sodium calcium sulfate (gaylussite and gypsum were also candidates) and ikaite is a hydrated calcium carbonate; the latter turns to calcite and water at room temperature which is why they proposed that it might be a "double" pseudomorph of sorts. I was looking at Jeff's photo of it and can't make out striations; neither do I after several years remember that detail of the actual specimen -- I was probably still thinking of the fruit anyway!
Cheers!
Elise.

[dtkasper]

My Ikaite from CA looks like dogtooth calcite that someone shot with a wagner spray gun. It is covered in tiny splatterings of carbonate that get built up enough they oozed down like a caliche looks. For that western site at least, the look is very distinctive and nothing like a pineapple opal. The bench has them by the thousands, probably tens of thousands, so when someone stated they found 100 pineapple opals, I considered it highly unlikely a large, open, Pleistocene lake system made them. The Ikaites at this CA site often fell over and crystallization restarted on the original long axis. The result is crystals perpendicular to the original crystal direction. They also formed in dense mats. Resolving identification often takes looking at populations of specimens and the geologic setting, particularly when hand identifying specimens.