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Author Topic: Mica, Pyrite vs gold  (Read 3771 times)
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sunshine
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« on: December 24, 2012, 06:37:31 PM »

Mica
Everybody knows a few stories about people being tricked into thinking mica was gold.
Typically, they looked into a river gold pan to see the sands glisten in the sun looking very much like the mother load of all placer deposits.  One of the nicest concentrations of placer mica that I have seen and checked out was on the North Thompson River outside of Avola, below the highway bridge.
Mica also looks pretty nice in the gold pan.  It is fun to watch a newbee gold panner  try to keep the mica while washing away the light materials.  It is so light and plentiful that  folks swimming in a lake may come out with their skin and hair peppered with thousands of tiny glistening golden flakes of mica.
Mica can be very prevalent where the surrounding rocks are granite, because granite can contain mica.  Where bedrock is exposed in Ontario, you can expect to find mica in every creek, stream and river that you come across.
Mica comes in a variety of colors including black, white, brown, yellow, green and red.  The color of mica is, in part, a result of its iron content.
The iron-rich magma and volcanic rock influence the formation of dark colored micas ranging from yellow-brown to a deep brown-black variety known as Muscovite mica.
While larger pieces of mica will show a distinctive color, when it (easily) gets ground down into tiny pieces, most of the color is lost and it all tends to adopt a yellow-brown color.
I found a number of old mica strip mines outside of Port Loring Ontario.  Their tailings contain nice blocks of mica and quartz from fist size to huge pieces that one would have trouble picking up.
Mica is formed in layers much thinner than a sheet of paper and these are stacked one on top the other, building a "book" of thin sheets. In years past, large books of mica were carefully peeled apart into a thin layers about 1/16 inch thick and used as fire-proof windows in wood stoves and for electrical insulators. Today, mica is sometimes used in heater wires or in toasters.
Although you can bend thin sheets of mica, when subjected to the grinding effects of glaciers and river gravel, the "books" are worn away at the edges producing very thin little flakes.
Capillary action will draw water into the microscopic spaces between the sheets, and this enhances the reflectivity of the mica and often creates a pearly iridescent yellow hue. 

Mica is thousands of times more abundant than gold. If you suspect most of the yellow stuff in your pan is mica, you may be correct.
Mica is 5 times lighter than gold.  It is very easily stirred up from the sand and gravel. It will be among the first and easiest materials to cast out of the pan. Beware, however, because some little flakes will probably stay behind, it a few flakes may remain in your cons.   
Mica is flexible to some degree and will spring back if not bent too far, but it also has somewhat of a brittle nature. Therefore one test is to poke it with a pin.  It will break apart into even smaller flakes, but gold will dent or spread like soft lead.
Mica will change color as you tilt the pan. Hold the pan one way, the mica might be a wonderful gold color, but as you tilt the pan, most of the color will disappear. Mica gathers its color from reflected light much more dramatically than gold, and will lose its color when the flakes are tilted in another direction.
Gold flakes appear to "glow", and will maintain this glow no matter how the pan is tilted. If the gold color disappears when you tilt the pan, it is probably not gold.
 If you look close along the shoreline, you may see Mica flakes in a thin yellow line in the sand right inside the edge of the water line.  They will tend to drift back and forth ever so slightly coincident with the ripples coming into shore.  Gold of course will not do this.
6) If you are uncertain, use a magnifier and a pin to help you distinguish between gold and mica.
Fools Gold
Fools gold is a mixture of iron and sulfur known as iron pyrite or, simply, pyrite.
There is a really good story about an English explorer called Frobisher (Frobisher Bay is named after him) who took three trips to try to find the Northwest Passage, losing ships and men along the way.  On his first trip he returned with one black rock that deemed no of value by many experts, but declared to be gold bearing by another.  On his return trip, he brought back 200 tons of rock he had mined and was felt to be gold ore.  They sent him back one more time for more “ore”, but while he was in transit, they found out the previous shipment was worthless pyrite.  The holds of his ships on his last trip had 1350 tons of worthless rock. 
If you add a bit of arsenic, the result is arsenopyrite. If scratched, it smells like garlic.  Add copper instead of arsenic, and it becomes copper pyrite known as chalcopyrite. It is thought that pyrite is produced by the action of sulphur-rich volcanic water in contact with iron rich volcanic rocks. Under ideal conditions, iron pyrite will form cubes, sometimes with flattened corners and lightly ridged mirror-bright faces. Gold ore can be associated with arsenopyrite, but not always.  Arsenopyrite in lode,  forms into stubby lightly ridged flattened blades that have a chrome-plated appearance.

Pyrite is very common, usually in the "massive" non-crystalline form as veins and stringers laced through the rock. Cubes are more rare, and the largest I have found are generally less than 1/4" across. I used to find quite a few of these cubes in Antler Creek in the Cariboo. 
Exposed to the environment, the pyrite will begin to oxidize, forming a tarnish that begins as a vary pale yellow , progresses to a deep yellow and finally matures into a deep brown color.
Large veins of "massive" (non-crystalline) iron pyrite is equally susceptible to developing an oxide tarnish that mimics gold.
Arsenopyrite seems much more resistant to tarnishing, and even when ground up into little bits in the gravel tends to maintain its bright silver-chrome color.
Chalcopyrite tends to tarnish with a greenish tint, sometimes a dull red-brown.
I have found grape sized clusters of bladed Chalcopyrite crystals that felt unusually heavy for their size and were tarnished to a rich gold color.  It was disappointing to scratch one of them and find out it wasn’t gold.
How to Tell the Difference Between Pyrite and Gold
1) Iron pyrite oxidized to a rich yellow color has been mistaken for gold for thousands of years, hence the nickname "fools gold". While I have seen some pyrites that were a good match for 22 carat placer gold, most yellow-tarnished pyrite is actually closer to 10-12 carat gold which is noticeably more pale (more silver-white) than pure gold.
2) Even though pyrite has a high iron content and feels surprisingly heavy for its size, it is less than 1/4 the weight of gold.
3) Pyrite is brittle, and if tapped with a pointed tool, will shatter into various sizes of sharp-edged silver colored fragments. Because of its soft malleable lead-like nature, gold will not shatter, and this is one of the best ways to distinguish between these 2 materials in the field.
4) The golden tarnish of pyrite is only a surface effect. If you scratch tarnished pyrite or rub it with a stone, the abrasion will reveal fresh silver-colored pyrite that will tell you it's not gold.
5) If you rub gold against a rock, the gold will leave yellow streaks on the rock whereas pyrite will leave dull silver grey-black streak.
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« Reply #1 on: December 24, 2012, 07:21:06 PM »

Also if you scratch it with a knife black stuff comes off the fools gold bust it up and turns into black powder. I have crushed some up, but pure stuff makes nice jewellery should see my necklace i made just cut off an end piece drilled a hole for the chain and i wear it out. Shure shines in the sun.lol Every one looks and thinks its a nugget of gold lol. And its very heavy you need a good strong chain to put it on cheeers Ray
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« Reply #2 on: December 24, 2012, 07:38:39 PM »

Pyrite also quickly turns into fumes and a powder of rust when heated with the blowtorch, and the  other golden colored iron compounds quickly lose that golden sheen if similarly heated. I have a mica question. Mariposite, or alternatively Fuchsite, is a chromian mica. I have scratched, fractured and examined this stuff under a microscope, and have never seen any indication whatsoever of the parting cleavage that is so characteristic of mica. I would never classify it as a mica from my own observation. Can anyone explain this mineral?
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« Reply #3 on: December 24, 2012, 07:57:55 PM »

Not sure from what i have saw it has flat plate like appearence and glossy, fools gold is cubbed <square pieces> and gold has a harkley appearence thats the difference itbetwwen them cheers my experence Ray
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« Reply #4 on: December 24, 2012, 11:22:16 PM »

Pyrite is the wrong color. It loses its luster in the shadows. The edges form tell tail square cubical crystals. Pyrite is brittle and absolutely not malleable.

My two bits ...

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« Reply #5 on: March 30, 2013, 12:25:07 PM »

Great info. i had my first experience yesterday going down to a river and digging up some samples. After some (very bad) panning This stuff was all over the place, but with one sign I thought that it wasn't gold...it came to the top of the pan. This stuff seems to just float along and is easily disturbed from the panning motions. It had the same features as described above being shiny looking in one direction, and dull in the other.

How much is this stuff worth on the market?  Laugh
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« Reply #6 on: March 30, 2013, 07:47:22 PM »

And lets not forget flecks or small tiny chunks of chalcopyrite(Cu) can fool the pants right off ya.
It you use a couple drops of jet dry each time you fill your gold pan and pan it down properly,all this mica/iron pyrite/chalcopyrite will wash away with the useless gravel while the gold & blk sand sinks to the bottom while dipping your pan. Here's what I mean about chalcopyrite being a great fooler.


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« Reply #7 on: March 31, 2013, 07:56:25 AM »

The pyrite around here can make your heart skip a beat as you work a pan down and catch that flash of 8 or 10 1/8" pieces, but they will wash out and your heart will restart hehe.  The confusion with the mica here is due to the fact that by scraping the top of a mica streak you will come up with some gold but that is due to the shape of the flakes creating a riffle effect and trapping gold as it washes down the river.
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