Mineral growths in Dawsonoceras fossil

[aek]

Hi, I am wondering what the white globular mineral growths are on this dolomitized Dawsonoceras mold. Calcite? Thanks for any help.

[Innocentx]

I think calcite is correct. See here (from Wisconsin), first pic.  http://wgnhs.uwex.edu/minerals/calcite/

 

[aek]

@Innocentx Thanks!

[Innocentx]

8 hours ago, aek said:

dolomitized Dawsonoceras mold

Welcome. Now I'm curious to see whole thing.

[ynot]

Calcite is a good possibility, but there are other minerals it could be.

Can You isolate some of the mineral and do a fizz test? (place in vinegar)

[Ludwigia]

Aragonite would be another possibility.

[aek]

@Innocentx @ynot @Ludwigia  Ok, I did a fizz test and nothing happened, so not calcite. After looking at this small brachiopod through the scope and seeing the same material, I'm thinking it's original shell material highly diagenetized. What do you think? 

 

[ynot]

I think Ludwigia is correct with aragonite.

[Innocentx]

1 hour ago, aek said:

Ok, I did a fizz test and nothing happened, so not calcite.

Then @Ludwigia has most likely got the answer with aragonite. Your brachiopod has nice beekite on it. 

[paleoflor]

1 hour ago, Innocentx said:

Then @Ludwigia has most likely got the answer with aragonite. Your brachiopod has nice beekite on it. 

Why? Aragonite should also fizz upon exposure to acid.

[ynot]

1 hour ago, paleoflor said:

Why? Aragonite should also fizz upon exposure to acid.

Whoops, You are correct. Aragonite should fizz.

What about selenite?

[paleoflor]

How was the "fizz test" performed? The results of an acid test depend on many parameters, including the type of minerals present, the particle size of the minerals, the type and concentration of the acid used, and the temperature of the acid. You can have false negatives, e.g. if the acid used was weak and the carbonate contained impurities, which seems not unreasonable considering the association with dolomite (has dolomite been ruled out yet?) Sulfate minerals such as gypsum could be an option, though I'm not familiar with gypsum exhibiting this particular spheroidal morphology. Any chance we can get a higher magnification image?

[Ludwigia]

The fizz test would narrow it down to Gypsum/Selenite as the nearest possibility.

[aek]

1 hour ago, paleoflor said:

How was the "fizz test" performed? The results of an acid test depend on many parameters, including the type of minerals present, the particle size of the minerals, the type and concentration of the acid used, and the temperature of the acid. You can have false negatives, e.g. if the acid used was weak and the carbonate contained impurities, which seems not unreasonable considering the association with dolomite (has dolomite been ruled out yet?) Sulfate minerals such as gypsum could be an option, though I'm not familiar with gypsum exhibiting this particular spheroidal morphology. Any chance we can get a higher magnification image?

I used a tiny chunk of the mineral about 3mm long and dropped it into 5%vinegar at room temperature, no fizz, left it in over night , remained unaltered. Here are a few higher magnification pics. Thanks for your help.

 

40X

 

100X

 

 

[aek]

@paleoflor Sorry..these pics might be a little better.

 

40X

 

100X

 

[paleoflor]

Reaction to vinegar is generally very mild compared to the response to 10% hydrochloric acid. You might need to use a hand lens to observe the fizzing, or you could try to powder the mineral first (increasing reactive surface area makes the reaction more vigorous). See for example: https://geology.com/minerals/acid-test.shtml

 

Thanks also for the photographs. I will take a closer look tomorrow, on my pc screen.

[paleoflor]

11 hours ago, Ludwigia said:

The fizz test would narrow it down to Gypsum/Selenite as the nearest possibility.

Would agree, the morphology suggests some sort of efflorescence. It must be a relatively insoluble salt (was tested in excess vinegar), and then gypsum efflorescence seems a good option. You see similar structures in sulphate leeching of concrete.

[Innocentx]

17 hours ago, paleoflor said:

sulphate leeching of concrete

I read about his a little. Interesting and thanks for your explanations.

[aek]

Thanks all for your info-  greatly appreciated!