Pyrite

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Kathy Feick

Pyrite group of minerals

Pyrite

Chalcopyrite

Marcasite

Arsenopyrite

Chromate Copper Arsenate

Pyrite FeS₂(Isoclinic)

Pyrite’s name comes from the Greek, pyrites lithos, “the stone which strikes fire.” The crystals form in the Isometric System; cubes, octahedrons, pyritohedrons and combinations of these and other forms. It also may be found in radiating disks, hair-like crystals, concretions and massive lumps in sulphide ore deposits. Pyrite sometimes also contains small amounts of cobalt, nickel, silver or gold.

Pyrite Cubes. Langoro, Spain. University of Waterloo Earth Sciences Museum Collection.

Crystals of pyrite are easy to collect or purchase. It is interesting to start a collection illustrating the various crystal forms. Large clean lusterous cubes are available from Spain, pyritohedrons from Washington State, a variety of forms from Peru, and Pyrite “suns” from Sparta, Illinois.

Oc​currence:

Pyrite forms in almost all types of environments including sedimentary, igneous, and metamorphic, as well as hydrothermal veins.

Uses:

The uses of pyrite are declining. The main uses today include:

• Production of sulphur dioxide for the paper industry
• Production of sulphuric acid for the chemistry industry and the fertilized industry
• Pyrite is most often mined for the gold, copper or other elements associated with it
• Pyrite used to be polished by Native Americans in early times and used as mirrors
• Ornamental stone
• Collector stone

Fool’s Gold and Black Gold

Richard P. Wells, reprinted with permission of the National Driller’s Buyers Guide, July 1995

Iron pyrite, the fool’s gold of antiquity, turns out to be a useful indicator for those of us seeking the black gold of modern times, crude oil. Pyrite is a common accessory mineral in sedimentary rocks, particularly in limestone, sandstone and carbonaceous siltstones or shales. Some times we wonder who and why it got there; and what does it mean for petroleum exploration.

Originally the iron came from the weathering of older igneous or metamorphic rocks. Iron is a common minor constituent of all continental igneous rocks; and occurs in minerals such as ilmenite, magnetite and pyrite, and ferro-magnesian silicates like olivine, pyroxene, amphibole, and biotite mica. Deep weather over long periods of geologic time releases the iron and soluble iron salts form. The dissolved iron then travels in solution to the sea, where ferrous iron is oxidised and deposited.

Much of the pyrite contained in sediments and sedimentary rocks is authigenic, formed in the depositional environment, or early diagenetic, formed during the transformation of the sediment into rock (lithification). The formation of pyrite requires the presence of organic matter in the sediment, sulphate in solution in the pore water, and locally anerobic (reducing) chemical environment.

It is the presence of decaying organic matter in the sediment that creates the reducing chemical environment. In marine environments, decay of organic matter occurs most rapidly just below the bottom of the sea, before more than a few centimetres of other sediment have accumulated on top of it. With deeper burial, most of the reactive organic matter has already been consumed and no more pyrite can form. Pyrite seldom forms in fresh-water environments. The formation of pyrite crystals depends mainly on the iron content of the sediment.

The process of pyrite formation in sediments results from the action of bacteria, which reduce sulphate ions (dissolved in the pore water) to sulphide. If there is iron present, iron sulphide crystals begin to grow. These sulphate –reducing bacteria also need other nutrients to live, which are provided by organic carbon in the sediment.

Pyrite can also precipitate in reducing sulphate – rich environments created by the migration of crude oil into the rock system in deeper layers. This pyrite generally forms larger crystals, which tend to fill pre-existing pore spaces in the rock and overlay any primary cement present.

The significance of pyrite for oil-finders is that its presence proves the chemically reducing conditions (rather than oxidising conditions) prevailed at some time in the past. Reducing conditions allow organic carbon to be preserved, whether in the form of plant or animal remains, coaly sediments or petroleum. The pyrite –sulphur content of the sediment correlates directly with the total organic carbon content of the rock, which is a measure of petroleum source rock richness.

Incidentally, the expression “fools gold” doesn’t refer to just any old fool, but originally referred to the Queen of England. During colonial times, some intrepid British explorers wanted to establish a new colony on the coast of Labrador, but were denied the funding because they had not found any gold there. Not to be deterred, they collected some fine specimens of pyrite and sent them back as proof of a gold discovery, and the ruse worked! The Queen looked at the samples (but not too closely) and immediately approved the building of the colony. It was a hundred years before the trick was discovered.

It is fairly easy to differentiate between gold and pyrite. Although their colours are similar, pyrite tends to be slightly lighter and more brassy in colour. If in doubt, just compare the colour of the sample with some good quality gold jewellery. Gold is much softer than pyrite, so soft that you can scratch it with your fingernail while it takes a good sharp knife blade to scratch pyrite. The crystal form of pyrite is another dead give-away, with perfect cubic crystals fairly common. Gold can form crystals, but they are extremely rare. As a final check, rub the sample on a rough, unglazed pottery surface, pyrite leaves a distinctive black streak, while gold leaves a golden streak.

Chalcopyrite CuFeS₂(Tetragonal)

Chalcopyrite was named from the Greek word chalkos, meaning “copper” and hence “copper pyrite”.

Occurre​nce:

Chalcopyrite occurs at numerous localities worldwide. It is the most abundant copper-bearing mineral. Chalcopyrite is a primary mineral in hydrothermal veins, disseminations and massive replacements.

Chalcopyrite and dolomite on dolostone. Unknown Locality. University of Waterloo Earth Sciences Museum Collection. Notice the iridescent colouring on the surface of the chalcopyrite.

Use​s:

• An important ore of copper. In Sudbury, Ontario, chalcopyrite is mined for it’s platinum content.
• Ornamental stone
• Collector stone

Chalcopyrite vs. Gold

If you were to rub the mineral vigorously with a hard object then if pyrite it will give off a sulphurous smell (like rotten eggs) but if gold no odour will be apparent. As well, if struck with a steel hammer gold will flatten or change shape without breaking but pyrite will give off sparks.

Chalcopyrite vs. Pyrite

Chalcopyrite looks similar to pyrite but it is softer and can be scratched with a knife. It is a very brassy yellow, often with a bronze or iridescent tarnish whereas pyrite is simply a brassy yellow. As well, pyrite is slightly heavier than pyrite.

Marcasite FeS₂(O​rthorhombic)

The name marcasite is derived from the Arabic word for pyrite. This mineral is a common and attractive mineral. It has the same chemical composition as pyrite, but it has a different crystallization system, making is a pseudomorph of pyrite. Without proper analysis aggregates of iron sulphide may be wrongly labelled by dealers. Crystal habits include the tabular, bladed or prismatic forms. A twinning effect produces spear shaped crystal and repeated twinning produces a “cock’s comb” cluster.

Marcasite. Picher, Oklahoma, USA. University of Waterloo Earth Sciences Museum Collection.

Over a period of years, marcasite will oxidize in collection. This process frees sulphur which frees sulphuric acid. The acid will then attack a paper label or even a cardboard box that the mineral might be kept in. Over a period of decades, most specimens will have disintegrated into a white dust along with deteriorated paper scraps. A sulphur smell will be released during this reaction contaminating other sulphide minerals nearby.  

Occurrence:

Marcasite is common worldwide. It occurs mainly in sedimentary deposits in low temperature ore veins, as well as in skarn metamorphic deposits.

Uses:

• There are very few uses for marcasite
• Has been used as a source of sulphur in the past
• Ornamental stone
• Jewellery

Marcasite vs. Gold

Marcasite is more brittle than gold. It is also lighter and is a brassy yellow mineral with a greenish tint at times or possibly a multi-coloured tarnish which results from oxidation.

Marcasite vs. Pyrite

Marcasite is difficult to distinguish from pyrite when there is a lack of distinctive crystal habits. As well, marcasite is a brassy yellow with a greenish tint at times. A multi-coloured tarnish may exist which is the result of oxidation.

“Marcasite Jewellery”

It is important to note that marcasite is too soft to be used in jewellery. For this reason, marcasite jewellery is actually made from pyrite, contrary to the impression one gets from its name. The Incas are the earliest known civilization to use marcasite in jewellery. Notable pieces have been found in several burial chambers. As well, marcasite was used by the Incas as mirrors, in sun-worship rituals and as a means of seeing into the future. During the Georgian period (1714-1837) the Swiss began to produce marcasite for the European market to bypass the Sumptuary Laws which forbade the use of diamond by all but the most aristocratic. When cut in a pyramid shape with a flat back the marcasite had a brilliance resembling diamond. Early cut steel was used in the same way.

Arsenopyrite FeAsS

Arsenopyrite is named for the minerals chemical composition. It is silver-white to steel-gray in colour with a greyish black streak. The mineral tarnishes dark gray but occasionally also an iridescent pink.

Occurrences​:

​Arsenopyrite is the most common arsenic-bearing mineral and is found worldwide. It is generally found as a product of high temperature ore veins, pegmatite’s or contact metamorphism. Rarely is it seen in igneous basalt rocks.

Us​es:

• Poison
• Preservation
• Pigment
• Gold indicator in some localities