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Identity HelpSericite on Kona Dolomite?

12th Dec 2018 20:50 UTCRobert Darabos

Banded Chert and/or Dolomite from Marquette County, Michigan

A few specimens I found had a very shiny sheen on it and was curious what it could be.

Mostly found on a clean break of the material.

A super close up looks kind of like muscovite, perhaps Sericite?

I know Sericite is found somewhere in the area, but was unsure if it was tiny pieces on matrix (as shown) or if they were much larger.

13th Dec 2018 01:14 UTCPaul Brandes 🌟 Manager

Does a powdered piece of this effervesce w/ acid?

Where exactly in Marquette County does this come from?

13th Dec 2018 04:00 UTCDana Slaughter 🌟 Expert

I think this represents the stress upon the dolomite from the fracture or break of the specimen and does not represent a different mineral like muscovite (v. sericite)---common at the Champion mine in Marquette Co. for example. I've seen this feature in some "hard" or dense carbonates and personally believe it to be the effect of stress. Just a thought---could be way off!

13th Dec 2018 12:14 UTCReiner Mielke Expert

"Banded Chert and/or Dolomite" ? Seems to me you need to determine which before even considering anything else. It's fundamental.

14th Dec 2018 01:40 UTCDana Slaughter 🌟 Expert

It certainly looks like classic Kona dolomite to me.

14th Dec 2018 02:10 UTCGregg Little 🌟

Robert;

Could you either draw a steel needle across the sample or apply some acid so we know whether it is a carbonate or chert. While you are at it, touch the needle to the white stuff you are speculating is sericite. It should be very soft if it has mica in it.

Before the test comes in determining rock type, I would side with Dana's explanation. There is a weak conchoidal fracture pattern as well on the fracture surface which, along with the white material (micro-fracturing), is typical of dense fine grained limestone and dolomite.

14th Dec 2018 03:29 UTCPaul Brandes 🌟 Manager

One thing to note; if it truly is Kona Dolomite, you won't see any sericite in it, but the Kona does contain phlogopite, another member of the mica group.

14th Dec 2018 15:16 UTCRobert Darabos

I'm guessing its Kona as well, I just haven't seen any banded quite like this before, so thought it could have been banded chert.

The white mineral in question is very powdery, i can rub it off on my finger with enough force and have shiny residue on my hands.

14th Dec 2018 15:41 UTCReiner Mielke Expert

"I'm guessing its Kona as well" Why are you guessing? Test it and make sure.

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