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PYRITE (FOOL'S GOLD):
IT'S FOR COLLECTORS, NOT FOR FOOLS!
by Eric Greene

Specimen photos by Eric Greene of Treasure Mountain Mining specimens


7.5" pyrite group of 38 cubic crystals to 1.6" - Logrono, Spain

Pyrite, or iron pyrite - infamously known as "Fool's Gold" - is a shiny, brassy-gold mineral that is commonly mistaken for gold. It can have sparkling, mirror-bright luster, and forms in a vast variety of interesting crystal habits, which makes it extremely popular among mineral collectors. Pyrite is the most common of the sulfide minerals - a group which includes galena, sphalerite, chalcopyrite, and arsenopyrite. Ironically, despite its "fool's gold" nickname, pyrite and gold are often found together. Actually, it's easy to tell the two apart, as pyrite is much lighter in color, less dense, and harder (gold can be scratched with a knife or fingernail).


.2.4" Brilliant octahedral golden pyrite crystals - Peru.


19LB 11" Brassy golden pyrite with pyritohedral crystals - Peru

The name pyrite comes from the Greek word "pyr" meaning "fire." In prehistoric times, it was struck against flint or iron to create sparks and start a fire. During the Middle Ages, this attribute made it a popular choice in early firearms devices. Pyrite's brassy gold color also led to it being called brass, brazzle, and brazil.

 

MINERALOGY
Pyrite, a naturally occurring iron disulfide mineral, is made up of iron and sulfur, with the chemical formula FeS2. It contains roughly 47% iron and 53% sulfur. It is enormously common in the earth's crust, and is formed in a wide variety of geological environments from sedimentary and metamorphic, to magmatic, hydrothermal, and even as a replacement mineral in fossils. Some of the largest deposits are in association with contact metamorphic rocks, especially near where eruptive rocks are in contact with schist or slate. For the collector, the most desirable specimens are usually found in vein deposits with quartz and sulfide minerals, and in sedimentary deposits like sandstone.

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3 Near-perfect golden pyrite cubes to 1.2" on 4.3" matrix - Logrono, Spain


2.7" pyrite cube speared by quartz crystals - Spruce Claim, WA

When exposed to the elements, pyrite weathers quickly, taking on a yellow-brown color. It later turns into iron oxide, goethite, or limonite. Pyrite also pseudomorphs into goethite and limonite, keeping its original crystal structure but consisting of a different mineral.


Graduated set of 28 pyrite cubic crystals - Logrono, Spain


3" polished golden pyrite ball with crystals in vugs - Peru

Pyrite's chemical structure is similar to that of galena, with the formula of PbS. The only difference between the two is that galena has a single sulfur atom, wile pyrite has 2 sulfur atoms. Pyrite is a polymorph of marcasite, meaning it has the same chemistry, FeS2, but has a different symmetry and crystallizes in different shapes. In the jewelry trade, this confusion is worsened because pyrite is sold under the name marcasite. Marcasite is unsuitable for jewelry because it is very susceptible to pyrite disease (rot). Pyrite disease, which causes some pyrite to crumble, is caused by oxidation, so keeping it dry is considered the key to preservation. Pyrite from some localities (including Peru and Spain) is very stable, while other localities appear to be more vulnerable.

 

PHYSICAL CHARACTERISTICS
Pyrite's color is brassy yellow, with a dull to brilliant metallic luster and a greenish black streak. It is opaque, and its cleavage is very indistinct. It exhibits chonchoidal fracture, and is brittle, making it susceptible to breaking and crumbling. Pyrite has a hardness of 6 to 6.5 and a specific gravity of 5.1, which is heavy for a metallic mineral. Unlike gold, pyrite is not malleable. Common associated minerals include quartz, calcite, sphalerite, galena, fluorite,, and yes, gold.


2.9" Shiny pyrite flower - Wuzhou, China.


4¾" Lustrous golden pyrite sun disc - Sparta IL

CRYSTALLOGRAPHY

Molecular crystal structure of pyrite Pyrite forms in many beautiful, well-crystallized habits, including cubes, octahedrons, and pyritohedrons (dodecahedrons with pentagonal faces). Pyrite's crystal habit, or morphology, is primarily determined by the unique combination of temperature, pressure, supersaturation, and host rock which are present when it crystallizes. At low temperatures (around 250º C) and low supersaturation needle-like crystals occur. As the temperature and/or degree of supersaturation increase (from 250º to around 450 º C), pyrite crystals grow first in cubes, then octahedrons, and finally pyritohedrons. Due to fluctuations in temperature, pressure and supersaturation, combinations of these forms are common, such as cube-octahedrons and octahedral-pyritohedrons.


Striated faces on pyritohedral pyrite crystals - Peru.


Striated faces on cubic pyrite crystals - Peru

Crystal faces are often striated, appearing to be grooved with lines or ridges. These are the result of changing conditions causing a crystal to change back and forth many times between being a pyritohedron and a cube. Cubic pyrite crystals often form penetration twins, where two or more crystals share crystal structure and seem to grow out of (and into) each other. Specimens from Spain are particularly prized for this habit. Pyrite is also found in massive form, as well as occurring as radiating, grainy, flaky, mammilary, encrusting, nodular, and fibrous crystals. Flattened discs of pyrite from coal mines in Sparta, IL occur as "pyrite suns," and are sometimes collected on the natural shale matrix.


5" Golden pyrite sun on 12" slate - Sparta IL


Sparks fly when a steel file is struk by pyrite

USES
In the past, pyrite was mined commercially and used to manufacture sulfuric acid. Today, the most important use of sulfuric acid is in producing fertilizers. It is also used in manufacturing paper and in making chemicals such as hydrochloric acid, nitric acid, sulfate salts, synthetic detergents, dyes and pigments, explosives, and drugs. It is also used to refine gasoline, process metals, make rayon, and to manufacture battery acid. Although it is almost half iron, it is not a significant source of iron ore. Auriferous pyrite, which contains a small percentage of gold, is mined in a number of gold-bearing localities. Throughout history, pyrite has had many different uses.

 

When you strike a piece of pyrite against flint or iron, sparks shoot out everywhere. By aiming the sparks onto dry tinder, you can produce glowing embers which can be used to create flames. This method was used during the Stone Age to start fires. One notable discovery which confirmed this was the "Iceman", who died 5,300 years ago. His body was found preserved in glacial ice in the Alps in 1991. Among his surviving gear was a pyrite and tinder fire-making kit.


2¾" Striped golden pyrite ball concretion - China


3.5" Lustrous, sharp golden pyrite cubic crystal cluster - Logrono, Spain

In classical times, the Greeks and Romans used pyrite to manufacture copperas, or iron sulfate. Pyrite was heaped up and allowed to weather in an early form of heap leaching. The acidic runoff from the heap was then boiled with iron to produce iron sulfate. In pre-Colombian times, Native Americans polished pyrite to make mirrors. And, pyrite nodules found in prehistoric burial mounds were probably used to start fires. During the Middle Ages, sulfuric acid was manufactured from copperas or by burning sulfur. When firearms were invented in the 1500 and 1600's, pyrite was used as the ignition source for the wheellock, which struck pyrite against a steel file to fire the gun.


5.8" curved cubic pyrite crystals - China


Sharp 2.1" near perfect folden pyreite cube - Logrono, Spain

In the early 1900's, pyrite was used in crystal radios (though galena later became the standard). During WWII, sulfur was declared a strategic chemical, and was mined at a newly-opened mine in Ducktown, Tennessee, mainly as a source of sulfuric acid. Today, the main source of commercial sulfur is the H2S gas which comes from natural gas wells. Recently, pyrite has been combined with copper sulfide to make low cost photovoltaic solar panels.


5" Curly-Que brassy gold pyrite crystals - Ohio


2¼" Brilliant vctahedral golden pyrite crystals - Peru

NOTEWORTHY LOCALITIES
Pyrite is found in many localities around the world. Peru is one of the major suppliers of pyrite specimens for collectors. Pyrite crystals and clusters in all morphologies are mined in the Huaron Mining District, the Quirivilca Mine, La Libertad, and the Huanzala District, Huanuco. Arguably the finest cubic crystals in the world come from Navajun, Logrono, La Rioja, Spain. Exquisitely sharp, lustrous, near-perfect cubic crystals from this locality are available as single cubes, clusters of penetrating twins, and embedded in a light tan sandstone matrix. The long-closed classic locality at Rio Marino, Island of Elba, Italy also produced excellent pyritohedral crystals. Recent finds of cubic pyrite in Hubei, China have luster and sharpness that is reminiscent of the Spanish pyrite cubes. Noteworthy U.S. localities include Park City, Utah, the Tri-State District near Joplin, Missouri, and Leadville in Lake Co., Colorado.


4" Extra sharp cubic crystals of brassy gold pyrite - Peru


2.5" 4 sharp cubic folden pyrite crystals to 1" - Logrono, Spain