Confirm Rock ID - Mica Schist on Calcite

[Westaus]

Hi All

I have brought back some samples from our last expedition and was curious on the positive ID of it. This is only a small sample of what I have in various base rocks. It is very hard to get a good photo so hopefully the below will help for ID.

Some details - Very, very metallic, extremely brittle, when breaking away tit does have some flex before it breaks, it also splits many times down the layered plane. Some of the flakes can be quite large (never seen this big) plus in bulk formation this with it and going in different directions. It is also quite heavy for its size. Though I have not done a SG on any.

The other questions I have is can it be cleaned? It seems so brittle.
Is there any market for items like these? eBay etc...
I have panned a few samples and found no gold, is there ANY chance that these may contain any gold?

There are a couple of different pieces shown, and a 5c piece in some for size reference. Any information is appreciated.

 

[Westaus]

Additional pics -

 

[shivan]

It reminds me of biotite (a type of mica) in the first few photos. Is it soft enough to be scratched with a fingernail?

 

[AtomRat]

Like shivans noted, should be mica, a symbol of regional metamorphasism. Schists, gneiss are probably close by too i guess. Mica was once used as a flame proof screen before glass windows on fire places. Do you see much of an oxidation / limonite anywhere closeby

 

[Westaus]

It reminds me of biotite (a type of mica) in the first few photos. Is it soft enough to be scratched with a fingernail?

Yes it scratches with the fingernail, it forms into minute little glitter pieces. It does break away if too much pressure it is very soft. It has similar consistency to fish scales average about the size of the 5c piece there were others that were much larger.

Like shivans noted, should be mica, a symbol of regional metamorphasism. Schists, gneiss are probably close by too i guess. Mica was once used as a flame proof screen before glass windows on fire places. Do you see much of an oxidation / limonite anywhere closeby

I cannot say for certain, however it was spread across an area of approx .5km with 1 large open cut mine approx 100mt long. The second was cut into the side of a rise and was about more circular. I think was more old timers (early 1900's), it was not as clean? The newer open cut would have been machinery for sure I think around 1970's-80's? In this cut I could see older greenstone flows, plus some extremely hard granite like rock. I believe the limonite could have been in this area. What we don't know is what they actually mined? As this material is not there!

This material seemed to be waste and not wanted as there was too much of it left behind. Less than 3km as the crow flies is a known gold mine and gold deposits.

I will look over the geo in the area to see if I can get more info as well.

Is there a burn/flame test?

Thanks for the help so far.

 

[shivan]

From the hardness and cleavage i would say its a biotite, though i could be wrong.
Though its interesting that you say that its heavy, i don't think that would be the biotite. When you say you have panned a few samples is that of the black mineral or the quartz as well?

 

[blisters]

Maybe a pegmatite, look for gems.
Jon

 

[user 4386]

Looks like a mica schist but it could be a number of other minerals. Colour seems wrong for biotite unless very weathered (which it could be as it looks like limonite present). Can you split it easily into layers with a pocket-knife blade? Now scratch a soft face (not across the book-like pages but on a flat surface) - how soft is it compared with your fingernail, a $2 coin, the knife blade, a lump of quartz? Peel a very thin layer off - if you bend it, does it snap back into shape or keep the bent shape? Hold the thin layer up and look at a light through it - is it clear or light brown? Could there be two flaky minerals not one?

 

[Westaus]

From the hardness and cleavage i would say its a biotite, though i could be wrong.
Though its interesting that you say that its heavy, i don't think that would be the biotite. When you say you have panned a few samples is that of the black mineral or the quartz as well?

It does look like biotite, though it appears a lot more shiny and metallic than this reference - http://geology.com/minerals/biotite.shtml

I have also samples very similar to this that was in the same vicinity - http://geology.com/rocks/schist.shtml

Maybe a pegmatite, look for gems.
Jon

It could be a version. I looked up and found this "To be called a pegmatite, a rock should be composed almost entirely of crystals that are at least one centimeter in diameter. The name pegmatite has nothing to do with the mineral composition of the rock."

So yes it does fit being a Pegmatite as the size would be in this scope and areas the volume is extensive.

I have some other samples I will try and photograph to compare to the schist above.

Cheers

 

[AtomRat]

Sorry shivan, i just noticed you said biotite at the top, i mustve been seeing thjng lol. Im calling mica schist too, unless it has a pearlescent appearance. I think there is a small cluster of biotite hexagons in photo 3&4 down the bottom of the rock

 

[AtomRat]

Is smokey quartz a common sight in the area by any chance too?

 

[shivan]

Could be schist i guess, i am not used to seeing schist with crystals like that though.

These are more what i think of when i think of schists, a muscovite garnet schist and biotite muscovite schist.

 

[AtomRat]

Have you seen these before shivan? 'Book of mica' - muscovite mica. The felspar, quartz and biotite also gives away the pegmatic mix
Sorry- should add regulary found around schist regional metamorphism areas

 

[Westaus]

Looks like a mica schist but it could be a number of other minerals. Colour seems wrong for biotite unless very weathered (which it could be as it looks like limonite present). Can you split it easily into layers with a pocket-knife blade? Now scratch a soft face (not across the book-like pages but on a flat surface) - how soft is it compared with your fingernail, a $2 coin, the knife blade, a lump of quartz? Peel a very thin layer off - if you bend it, does it snap back into shape or keep the bent shape? Hold the thin layer up and look at a light through it - is it clear or light brown? Could there be two flaky minerals not one?

It could have some weathering as it was exposed I would not think it was exposed for millennia though?
Yes easily splits into layers, can do with fingernail
Fingernail scratches face of material
Keeps bent shape see macro pic attached
It is a smokey grey when light is shone through

 

[Westaus]

Last lot of a couple of other samples, you can see another host in pic3 I think

 

[Lefty]

Could there be two flaky minerals not one?

Is there any possibility of it being intergrowth with feldspar?

I just noticed what appears to be a hint of blue adularesence like a labradorite. I have a piece of the Finnish stuff with a beautiful colour play but an unfortunate structure like the photo, flakey and crumbly.

 

[blisters]

Pegmatite, looks like you have feldspar and quartz with the mica. It's hard to tell in a photo but gives you an idea. If you think there is calcite then it should effervesce with vinegar. It could have been mined for the samples or mica but with the large crystals you can get gems forming as well as you do in schist such as garnets, emeralds etc. only larger.
Jon

 

[user 4386]

I'm happy that it is a mica with felspar, quartz - a number of possibilities including weathered biotite, mixture with muscovite, phlogopite or rarer micas. Should snap back to a plane though if you just bend it 20 degrees (not enough to crack it). Some of the other physllosilcates are quite plastic - they stay wherever you bend them to, no matter how little.

I didn't realise that vinegar was sufficient for calcite. I always use 50% HCl because it distinguishes calcite and dolomite and magnesite (calcite disolves energetically in a flash, dolomite just fizzes slowly, magnesite not at all). Handy in lead-zinc exploration as often ore bodies occur regionally in limestone but ore fluids alter the calcitic limestone to dolomite near mineralisation, mangesite can be ore itself but often indicates evaporites nearby like gypsum and anhydrite, which can be ores but are also clues to lead, zinc, barite and fluorite in vicinity.

 

[blisters]

Vinegar will work but not as well as HCL, I mentioned vinegar as it is probably sitting in the kitchen cupboard. You'll be waiting much longer for a result than HCL and a magnifier might be required but it will bubble.
Jon