FMF - Friends of Minerals Forum, discussion and message board

[Jim Robison]

I posted this several times on another forum. Having gone several days day with no comments, thought I would try again in another forum.

One of Tsumeb's oddities is a wide variety of dolomite epimorphs (yes, I know some would argue that term), more loosely known as 'dolomite casts.' These are formed when a layer of dolomite is deposited over a layer of calcite crystals, or some other unknown soluble mineral, and the underlying base is subsequently dissolved leaving a shell of dolomite. These may remain quite thin, or other minerals, including calcite, more dolomite, smithsonite, and a host of other goodies, may be deposited either on top of, or on the underside of, the shell or 'cast'.

I will be putting a case of Tsumeb dolomite in at the Main Show in Tucson this year, including a numer of 'casts,' and am looking for a simple explanation of why and how the casting process and dissolution of the supporting base occurs. Information on the subsequent formation of calcite and other minerals also would be helpful, especially since calcite dissolved away in the first place. Google and Wikipedia searches have not been helpful. I know the 'what,' it's the 'how' and 'why' that interests me.

I am hoping that somebody can explain in relatively simple chemical terms what is really going on. I'm actually interested in a detailed explanation as well, so will take either level of response.

Tsumeb is not the only location where these 'casts' occur, and the coating minerals may also include siderite, amongst others. If you also want to address this, that would be great.

[Jordi Fabre]

Hi Jim,

Amazing, but no one has responded to Jim yet. As in this moment is not the best time for me to write a detailed explanation, considering that my English language is not so good, I would ask you if you can please post a photo of one of these casts. Maybe with the photo, one of the great experts who visit this forum can answer you better than me.

[Pete Modreski]

Just checking this thread, and I see that no one indeed has tried to answer yet.

My comment is, that I think that in many cases, there just is no simple, known answer as to why and how one mineral forms a pseudomorph after another. In a few cases, it is straightforward and logical; pyrite becomes pseudomorphed to limonite/goethite because oxidation takes place, and azurite becomes altered to malachite because malachite is more stable under surface or near-surface conditions. But in many other caes, all we can say is that "after the dolomite was deposited, for whatever reason, conditions changed and the calcite was dissolved away, but not the dolomite. There are such a lot of physical and chemical variables (temperature, pH, salinity, composition of everything else in the solution), and sometimes, apparently very insoluble minerals (quartz, Baryte, fluorite) become dissolved or pseudomorphed.

All I can think of to perhaps add, is that we do know that dolomite does tend to be less soluble in acid than calcite--e.g., calcite dissolves rapidly in dilute HCl, dolomite, only slowly; hence it seems logical that calcite might be dissolved while dolomite remains.

Best of luck,
Pete Modreski

[Jean Sendero]

Jim,

You may want to read the following article. It describes the solution equilibria between calcite and dolomite. It does not provide all the answers but will allows you to better understand the role of the Ph and the content of Ca in solution etc....

https://www.columbia.edu/~ps24/PDFs/Mineral%20Solution%20Equilibria%20in%20Sparingly%20Soluble%20Mineral%20Sys.pdf
(link normalized by FMF)

hope it helps you.

cheers

Jean

[Jim Robison]

Jordi

Jordi, I've attached two pictures of casts. Have been traveling for the last week so couldn't respond sooner. The first piece is approximately 14 by 10 cm in size, the second 12 by 9 cm. .

Pete

It is indeed difficult to figure out pseudo's! One of the things that makes the calcite-dolomite mixes so interesting at Tsumeb is their complexity in terms of composition, color, and shape. Most places I am familiar with the pseudo's tend to follow a single pattern. Not so at my favorite mine.

Jean

I have printed out the article and will spend some time with it, Thanks. I'm not a chemist or geochemist and diagrams such as those in the article tend to go over my head, but I will give it a go.

In the meantime, Malcom Southwood responded to my inquiry on Mindat, and I'm attaching below a reference to that correspondence, which includes some information I received since the original question was posted.

https://www.mindat.org/mesg-15-202241.html

The bottom line indeed is that it is very difficult to determine the history of pseudomorphs, but I think I know enough now to make a couple of simple illustrations that will help explain them.

[Jim Robison]

Dolomite casts

The following notes were in the Tsumeb cast case titled 'greatly simplified explanation ... dolomite casts" The simple fact is that nobody I have talked to or corresponded with can say exactly why the casts form, only make some educated suppositions based on mineral solubility and mineralizing fluid likely compositions. The casts raise lots of questions beyond the basic how did they form? Some form multiple layers with small gaps in between. Why? Unknown. What led to some being pseudomorphed, often with smithsonite? Unknown. What were some of the other rocks upon which the casts formed? Unknown. Etc.

What is known is that dolomite, epimorphs (casts), and pseudo's after same are found in many different forms at Tsumeb. While casts are found in many locations throughout the world, Tsumeb is - to my knowledge - the premier location as far as variety of form, color, associated minerals, etc. But then, after all, it is Tsumeb.



DOLOMITE - CALCIUM & MAGNESIUM CARBONATE

CALCITE - CALCIUM CARBONATE (MORE SOLUBLE)

MINERAL FLUID WITH EXCESS CALCIUM OVER MAGNESIUM GIVES CALCITE

MINERAL FLUID WITH ‘BALANCE’ OF CALCIUM AND MAGNESIUM GIVES DOLOMITE

MINERAL FLUID WITH DEFICIT OF CALCIUM RESULTS IN CALCITE DISSOLVING

EPIMORPH IS ONE MINERAL DEPOSITED ON TOP OF ANOTHER. BOTTOM MINERAL MAY LATER BE DISSOLVED, RESULTING IN LAYER OF TOP MINERAL ONLY (THE “CASTS” - ALSO SOMETIMES CALLED SHELLS)

PSEUDOMORPH (pseudo) - REPLACEMENT OF ONE MINERAL FORM WITH ANOTHER MINERAL


THE HOST ROCK AT TSUMEB IS MASSIVE DOLOMITE. IT IS THUS NOT SURPRISING THAT DISSOLVED DOLOMITE CRYSTALIZED IN THE FAMOUS OREBODY.

AT TSUMEB, CALCITE OR DOLOMITE CRYSTALLIZED - OR RE-DISSOLVED - ACCORDING TO CHANGES IN THE ACIDITY, THE TEMPERATURE, AND THE RELATIVE CALCIUM AND MAGNESIUM CONTENTS OF THE MINERALIZING SOLUTIONS. SUBTLE VARIATIONS IN THESE CONDITIONS RESULTED IN MANY DIFFERENT TYPES OF PSEUDOMORPHS AND EPIMORPHS INVOLVING THESE TWO MINERALS.

THIS CASE INCLUDES EXAMPLES OF DOLOMITE, DOLOMITE EPIMORPHS (“CASTS”), AND PSEUDOMORPHS OF MINERALS FORMED FROM DOLOMITE “CASTS.”




THE ‘USUAL’ “CAST” SEQUENCE IS: CALCITE FORMS, THEN DOLOMITE FORMS ON TOP, THEN CALCITE DISSOLVES (AND MAY LATER RE-FORM). CASTS WERE ALSO FORMED ON OTHER MINERALS. THE FORMATION MECHANISM IS NOT WELL DEFINED OR UNDERSTOOD, BUT THE RESULTS OF THE PROCESS ARE SOMETIMES VERY SPECTACULAR.

THE COMPLEX MINERALIZING FLUIDS ALSO CONTAINED A WIDE RANGE OF OTHER ELEMENTS WHICH AFFECT THE COLOR AND MORPHOLOGY (SHAPE) OF THE MINERALS, AND CREATE ADDITIONAL MINERALS.

“PURE” DOLOMITE IS UNCOMMON AT TSUMEB. COPPER, ZINC, LEAD, IRON, CADMIUM, COBALT, AND OTHER ELEMENTS PROVIDE COLOR TO OTHERWISE WHITE DOLOMITE.

ADDITIONAL MINERALS IN THIS CASE INCLUDE SMITHSONITE, ROSASITE, CERUSSITE, MOTTRAMITE, MIMETITE, DUFTITE, WULFENITE, AND OTHERS.

(Note, I put five of my best Tsumeb calcites on the top shelf of the case with the following note above them. Hope you appreciate the intended humor. Jim)

CALCITE IS A COMMON AND SOMETIMES VERY BEAUTIFUL MINERAL FROM TSUMEB. THE EXAMPLES ON THIS SHELF ARE ALL FROM THERE.

JUST THINK, SOMETHING LIKE ONE OF THESE SPECIMENS MAY HAVE ‘GIVEN ITS LIFE’ TO FORM ONE OF THE DOLOMITE “CAST” SPECIMENS EXHIBITED HERE!!!



Sorry this is so long, but it seems the best way to respond to the question.

Jim

[Gail]

Thanks Jim, you are..as always...wonderful!

[Jim Robison]

The following pics scan the Tucson case of dolomite, etc. There are two shelves of specimens, and some overlap between pictures.

[Jim Robison]

sorry about the mis-start above. Forgot how to add multiple images

[robynahawk]

Thanks Jim - your case caught my eye but in the rush (I only had one day to do the TGMS show) I never got a shot of the information...and then I kept hearing the word "cast".

Wasn't sure what that meant - thanks so much it makes sense now.

Robyn

[Debbie Woolf]

I really enjoyed your photos & especially the top shelf :)