Naked Science Forum

Non Life Sciences => Geology, Palaeontology & Archaeology => Topic started by: katieHaylor on 24/07/2017 14:08:13

Title: Which rocks contain metallic flakes resembling silver and gold?
Post by: katieHaylor on 24/07/2017 14:08:13

Blaize asks:

I broke a marble/quartz?-looking rock and found a whole bunch of shiny flakes.
They are small but they look almost like gold. Some are more silver in colour.

Do you know what they might be?

Can you help?

Title: Re: Which rocks contain metallic flakes resembling silver and gold?
Post by: chris on 29/07/2017 18:38:42

The most likely candidate for the silver and gold flakes are iron sulphides - also known as iron pyrites or, "fools gold". Lead sulphide (galena) also produces silvery inclusions that would fit the bill with what you describe. Where did you find the material in question? Often the geological context is the best giveaway as to the identity of the material.

This is some galena and calcite:

(https://upload.wikimedia.org/wikipedia/commons/d/d4/Calcite-Galena-207327.jpg)

Title: Re: Which rocks contain metallic flakes resembling silver and gold?
Post by: Bass on 22/08/2017 00:03:55

are you sure they are metallic? If so, I agree that pyrite is the best guess for gold colored flakes. silver colored could be specular hematite. One simple test to try to scratch them with a knife to determine hardness and to see if they leave a colored streak. Both minerals range from 6 to 6.5 on the Mohs scale, which is about the same hardness as a knife blade.
If not metallic, several of the mica minerals can shine brightly like gold/silver, including sericite, phlogopite and biotite.

Title: Re: Which rocks contain metallic flakes resembling silver and gold?
Post by: Bill S on 24/08/2017 20:21:04

I'd be fascinated to know what a "marble/quartz?-looking rock" turns out to be.
Do give us some more info; esp hardness and location.

Title: Re: Which rocks contain metallic flakes resembling silver and gold?
Post by: SeanB on 26/08/2017 09:19:12

Lot of granites do have quite high gold concentrations inside, but are not at all economical to mine because they are so hard, and the energy required to grind them to powder to enable the gold to be extracted is more than the value of the gold. Thus most gold comes from softer dolerites where it is easier to crush them, and we do not really mine seawater even though it has a similar gold content, as concentrating the gold is hard in that case. However a lot of the flakes in rocks are also various forms of mica, which can look like gold and silver flakes.

Title: Re: Which rocks contain metallic flakes resembling silver and gold?
Post by: glot on 27/08/2017 08:49:46

Could they be mica flakes?

Title: Re: Which rocks contain metallic flakes resembling silver and gold?
Post by: Midoriya on 11/09/2017 05:16:33

Agreed with glot, they are probably mica flakes.  Its fairly common to find them in layers or crusts in conglomerate rocks. A picture would help a whole bunch. You can also identify mica via simple hardness test. Mica forms in lamellae; thin, flexible sheets so you can easily separate it at just the right spot with a fingernail, pick or small knife. It also crushes or breaks rather easily compared to the other minerals in the host rock.