Pyrite Group of Sulfides Class
|Chemical Composition||FeS2 – Iron Sulfide|
|Color||Pale brassy yellow, will tarnish to darker hues that may be iridescent|
|Cleavage||None, conchoidal fracture|
|Hardness||6 – 6.5, brittle nature|
|Specific Gravity||5 or above (feels heavier than average, even for a metallic mineral)|
|Luster||Metallic, opaque. May exhibit faint striations (lines) on some crystal and cleavage faces|
|Streak||Greenish black or brownish black|
Since calcite and aragonite can exhibit so many appearances, there are a number of minerals with which it may be mistaken. Calcite and aragonite, however, are the only common minerals that react vigorously with dilute acid to produce bubbles of CO2.
It may be difficult to distinguish chalcopyrite and pyrite by color or streak, although chalcopyrite tends to form a more iridescent tarnish and seldom exhibits good crystal shapes. Hence, hardness is often the easiest way to distinguish between these two minerals. Chalcopyrite is a relatively soft mineral and may be scratched by a nail or knife blade, whereas pyrite is harder than most metal implements.
Gold is much softer than pyrite, does not exhibit crystals, and is a very malleable material (easily shaped). Pyrite is harder than glass or most metals, is very brittle and often occurs as cubes or octahedrons with faint striations or lines on some crystal faces.
A polymorph of pyrite, marcasite shares many of pyrite�s properties, including its hardness. Marcasite typically has a paler color than pyrite, and tends to have a radiating fibrous internal texture, while pyrite commonly has a cubic to octahedral shape with faint striations on some crystal faces. In massive form, the only way to definitively distinguish the two is by chemical tests. However, marcasite only occurs in sedimentary settings and low temperature veins, so a hard, yellow metallic mineral in almost any other setting will most likely be pyrite.
Did you know...
Fire was pyrite’s most prominent gift to human society. Sparks are created when pyrite is struck against metal or a hard surface and this was one of the earliest methods humans discovered to create flame. Pyrite’s name comes from the Greek phrase, ‘pyrite lithos,’ which means ‘stone which strikes fire.’ Its brilliant metallic luster and brassy to golden color not only makes pyrite stand out from its surrounding rock, but has also caused it to be mistaken for gold by people unfamiliar with the real thing. Sometimes cruelly known as ‘fool’s gold,’ pyrite is actually much harder than gold and often has flat crystal faces that would not occur in real gold. Although no longer considered a valuable mineral in its own right, pyrite in a rock often signals the presence of other hydrothermal minerals and metal ores that do have significant value.
Description and Identifying Characteristics
A brilliant metallic luster and bright yellow to golden color makes pyrite a particularly distinctive and attractive mineral. It often occurs as small cubes to octahedrons that may exhibit faint lines, called striations, on some faces. At first glance, massive pyrite may be mistaken for other yellow, metallic minerals such as chalcopyrite or gold, but its greater hardness should distinguish it from those much softer minerals. Capable of scratching glass, pyrite will only be scratched by the best metal files. Most knife blades will leave its surface unmarked. Marcasite is the only other yellow metallic mineral as hard as pyrite. The two are polymorphs of one another, minerals with the same chemical composition but different crystal structures. Only their crystal shape readily distinguishes the two. Pyrite often occurs as cubes or octahedrons, while marcasite typically exhibits a radiating fibrous texture.
In Our Earth: The Geologic Importance of Pyrite
Although it is not a significant rock-forming mineral, pyrite is very widespread and a common accessory mineral in many rock types. In igneous rocks, pyrite may be disseminated throughout the rock or concentrated in layers if the magma cooled slowly enough for crystals to settle out. Pyrite is also common in contact metamorphic settings or disseminated through sedimentary rocks as a replacement of other minerals. Although pyrite occurs in most hydrothermal veins, it is particularly abundant in sulfide deposits. In fine-grained or organic-rich sedimentary rocks, pyrite may even form discrete pyrite concretions or flattened discs called ‘pyrite dollars’.
In calcite and quartz veins, pyrite is commonly associated with chalcopyrite and other sulfide minerals and metallic ores. Pyrite oxidizes to other iron sulfate minerals that in turn alter to limonite, so the presence of a weathered ‘rusty’ limonite layer may indicate the presence of pyrite in the underlying rock.
In Our Society: The Economic Importance of Pyrite
Despite being a common, iron-rich mineral, pyrite is rarely mined for
its own sake, and most of our iron is produced from magnetite and
hematite deposits. Those iron oxide minerals occur in larger
concentrations and volumes than pyrite, so they are a more economical
iron source. Vein pyrite, however, may reveal the presence of other
valuable minerals, such as chalcopyrite and gold.
In the recent past, pyrite was mined as a sulfur source for sulfuric acid, an essential commodity for chemical industry. This was particularly true during World War II when more traditional sources of sulfur were unavailable, or inadequate, for the increased demand of wartime industry. Presently, most of our sulfur comes from natural gas deposits enriched in hydrogen sulfide.
Although pyrite is no longer prized by modern society, back when survival often depended on fire, pyrite was held in great esteem. The ease with which even a novice could coax flame from pyrite sparks made it a highly regarded mineral.
In Our Future: The Environmental Implications of Pyrite Use
Pyrite’s sulfur content is one of its potential gifts, but it also raises some environmental concerns. As water moves through a pyrite-bearing rock, the pyrite’s iron component may be oxidized, releasing its sulfur to form sulfuric acid. In natural settings this is seldom a concern, but pyrite-rich waste from mining operations can significantly increase the acidity of surface waters. This increased acidity can harm downstream ecosystems and may even pose a serious risk for human populations.
Many coal deposits contain disseminated pyrite and as the coal is burned the pyrite’s sulfur component oxides to form sulfur dioxide. In the atmosphere, sulfur dioxide combines with moisture to produce sulfuric acid. This is an important source of ‘acid rain’, which can damage building stones and monuments, cause a decline in natural lake systems and, in some areas, even pose a significant risk to human health.
Pyrite in the Upper Midwest:
Pyrite is common along the borders of Minnesota, Iowa and Wisconsin, in the northern part of the Upper Mississippi River Valley lead-zinc mining district. There it occurs as vein deposits and as a disseminated replacement mineral in the region’s widespread Paleozoic carbonate rock layers. Pyrite is also found along the Canadian border, in massive sulfide vein deposits of the middle and late Precambrian volcanic rocks of northern Wisconsin and northern Minnesota where it is associated with massive chalcopyrite and sphalerite deposits.