Using the Mineral Identification Chart for Identifying Gold in the Field

Gold Nugget
Photo by Rob Lavinsky

Sample No.	100	101	102	103
Color	Gold	Brass-Yellow	Pale Brass/Yellow	Brownish/Gold weathered
Luster	Metallic	Metallic	Metallic	Glassy
Crystal System	Isometric	Tetragonal	Isometric	Monoclinic
Composition	Au +Ag	FeCuS2	FeS2	X2Y4–6Z8O20(OH,F)4
Form	Nugget	Crystalline	Crystalline	Crystalline
Hardness	2.76	3.75 Avg.	6.25 Avg.	2.25
Streak	Gold	Greenish-Black	Greenish-Black	White
Sp. Gravity	17.8 Avg.	4.18 Avg.	4.9 Avg.	2.8
Cleavage	No	Poor/Indistinct	Poor/Indistinct	Perfect
Fracture	Hackly	Uneven	Concoidal	Micaeous
Florescent	No	No	No	No
Phosphorescent	No	No	No	No
Sectile	Yes	No	No	No
Malleable	Yes	No	No	No
Brittle	No	Yes	Yes	Elastic
Ferro-Magnetic	No	No	No	No
Para-Magnetic.	No	Yes	Yes	No
Non-Magnetic	Yes	No	No	Yes
Transparent	No	No	No	Yes
Translucent	No	No	No	No
Opaque	Yes	Yes	Yes	No
Mineral	Gold	Chalcopyrite	Iron Pyrite	Weathered Mica

As you can see from the chart gold has entirely different characteristics then “fools gold.”

According to Mindat.org gold has a specific gravity ranging from 16 to 19 depending on how pure it is.

Hardness is 2.5 to 3.

Telling gold from fools gold!

The problem of identifying real gold in the field to separate it from fools gold often arise. There are several minerals that mimic gold in appearance like iron pyrite and weathered biotite mica that both have a gold-like appearance.

The easiest field test for gold is simple its density is 19 grams per cubic centimeter that is far denser then any of its mimics. Some genius in antiquity developed the “touchstone” test for gold based on what color its streak is when left on a dark colored stone. In practice this is usually a piece of black slate. Gold will leave a gold-colored streak on this stone its mimics will not. For ages this was the definitive test for gold until it was replaced by the “Purple of Cassius” test during 1685 by Andreas Cassius.

Another ancient test used frequently was based on the fact gold is malleable. You can bend it, and it keeps its bend. You can also bend weathered biotite, but unlike gold it quickly snaps back into shape. The other method based on malleable gold is hammering it into a flat sheet all of those minerals that mmimic gold are brittle and will break-up into a powder if hammered.

The acid test works because gold is unaffected by all strong acids except aqua regia, all the other minerals that mimic gold are affected by acids and are dissolved. A possible exception to this is weathered biotite mica.

The Purple of Cassius’s history goes back to the fourteenth century where it was described as an artist’s pigment however Andreas Cassius gets the credit for its invention. The principle behind the purple of Cassius is the ability of Aqua Regia a two to one mixture of concentrated nitric and hydrochloric acids to dissolve gold.

Just by its dissolution in aqua regia gold produces a rough-and-ready test because the gold charged aqua regia assumes a golden-yellow color. This test is not accurate enough however to be definitive. The purple of Cassius is however. Until the invention of the modern science of spectrometry developed during the nineteenth and twentieth century’s the purple of Cassius was the recognized definitive test for gold.

The test works by adding a few drops of the gold charged aqua regia to 100 ml of clean water, then adding twice as many drops of a solution of tin chloride to the solution. The tin will cause the gold to precipitate from the solution forming a colloid that ranges in color from pink thru dark purple. The darker the color the more gold is present.

The science of “spectroscopy” has supplanted most of the chemical tests for gold in the laboratory, but not in the field where because the various manifestations of the spectroscope are just to bulky. Many of the old chemical tests for gold and other minerals are still being used.

References:

The creation of color in eighteenth century Europe, Gutenburg.org,
http://www.gutenberg-e.org/lowengard/C_Chap33.html

Acid test (Gold), Wikipedia, http://en.wikipedia.org/wiki/Acid_test_(gold)

Spectroscopy, Wikipedia, http://en.wikipedia.org/wiki/File:Light_dispersion_conceptual_waves.gif