Pyrite, an iron sulfide mineral with the chemical formula FeS2, is widely recognized by its nickname, “Fool’s Gold.” Its bright, brassy-yellow color and metallic sheen often lead to it being mistaken for true gold. Pyrite possesses distinct physical and chemical properties that allow for definitive identification. Applying a few simple geological tests can determine if a specimen is genuinely pyrite.
Initial Visual Assessment
Identifying pyrite begins with examining the specimen’s outward characteristics without specialized tools. Pyrite exhibits a notable metallic luster, appearing reflective and shiny, paired with a pale brass-yellow color. This color can sometimes tarnish, developing darker, almost iridescent patches due to surface oxidation.
The typical crystalline structure, or habit, of pyrite is highly characteristic, often forming perfect cubes, octahedrons, or twelve-sided shapes called pyritohedrons. These well-defined crystal faces frequently display fine, parallel lines known as striations, which are a common diagnostic feature. Pyrite is an opaque mineral, meaning no light can pass through it.
Primary Diagnostic Tests
Geologists rely on specific physical tests that confirm the mineral’s identity. The streak test is the most important diagnostic procedure for pyrite, involving rubbing the specimen across an unglazed porcelain plate. While the mineral itself is brassy-yellow, the powdered material it leaves behind, the streak, is a distinct greenish-black to brownish-black. This dark streak immediately separates pyrite from gold, which leaves a yellow streak.
Pyrite is significantly harder than many similar-looking minerals, registering between 6 and 6.5 on the Mohs hardness scale. This hardness means it can easily scratch glass or most copper alloys, and it cannot be scratched with a common knife blade. Pyrite is a brittle mineral, meaning it will shatter or break into sharp fragments when struck, rather than flattening or bending.
The density of pyrite serves as an important clue, possessing a high specific gravity between 4.9 and 5.2. This high density is a result of its iron and sulfur composition, causing a specimen to feel heavier than a typical rock of the same size. This property, combined with the hardness and streak, provides confirmation of pyrite’s presence.
Separating Pyrite from Look-Alikes
Identification relies on contrasting pyrite’s properties with other minerals that share a superficial resemblance. The most frequent confusion is with gold, which is much softer (2.5 to 3 on the Mohs scale) and can be easily gouged or dented. Unlike pyrite’s brittleness, gold is highly malleable, allowing a small piece to be bent without breaking.
Chalcopyrite (CuFeS2), another iron sulfide, is often mistaken for pyrite, but it is typically a darker, buttery yellow color. Chalcopyrite is softer than pyrite, with a Mohs hardness of 3.5 to 4, and it has a lower specific gravity of 4.1 to 4.3. Both yield a greenish-black streak, but differences in hardness and specific gravity help distinguish them.
Weathered flakes of mica, specifically biotite or phlogopite, can exhibit a flash of gold-like color when light hits them. Mica is different from pyrite because it is very soft, flexible, and has a low density, causing it to be easily washed away during water separation tests. Unlike the dark streak of pyrite, mica leaves a colorless or white streak when powdered.