Got a photo? Identify rocks instantly with the Rockby app
Open the appDescription
Pyrite in matrix refers to specimens where the mineral pyrite (iron disulfide, FeS2) is embedded within a surrounding host rock, typically a fine-grained variety. The pyrite crystals can range from microscopic disseminations to macroscopic, well-formed euhedral crystals (cubes, octahedra, pyritohedra, or combinations thereof) or massive, anhedral aggregates. The matrix often provides a contrasting background, highlighting the metallic luster and brassy yellow color of the pyrite. Common fine-grained matrix rocks include shales, slates, mudstones, argillites, and fine-grained metamorphic rocks like phyllites or schists, as well as some fine-grained igneous rocks. The appearance of the specimen is highly dependent on the crystal habit of the pyrite and the nature of the host rock.
How to Identify
- Color
- Pyrite: Pale brass-yellow to golden-yellow, often with a metallic sheen. Matrix: Varies widely depending on the host rock, commonly dark gray, black (shale, slate), greenish (chlorite-rich rocks), or reddish-brown.
- Luster
- Pyrite: Distinctive metallic luster. Matrix: Can be dull, earthy, vitreous, or silky depending on the host rock minerals.
- Texture
- Pyrite: Crystalline, often euhedral (cubes, octahedra, pyritohedra), granular, or massive. Matrix: Fine-grained, often fissile (shale, slate), compact, or schistose.
- Crystal Form
- Pyrite: Commonly forms cubes, octahedra, and pyritohedra, often with striations on cube faces. Also found as massive, granular, or radiating aggregates. Matrix: Typically massive or foliated, reflecting the fine-grained nature of the host rock.
- Cleavage
- Pyrite: Indistinct to poor cleavage on {001}. Matrix: Varies; shales and slates exhibit good cleavage (fissility), while other fine-grained rocks may have none or poor cleavage.
- Geological Environment
- Sedimentary rocks (especially black shales, mudstones), metamorphic rocks (slates, phyllites, schists), hydrothermal veins, and disseminated in igneous rocks. The presence of a fine-grained matrix often points towards sedimentary or low-grade metamorphic origins, or fine-grained volcanic rocks.
Key Facts
- Hardness: 6-6.5 on Mohs scale (pyrite)
- Specific Gravity: 4.95-5.10 g/cm³ (pyrite)
- Crystal System: Isometric (pyrite)
- Color: Pale brass-yellow to golden-yellow (pyrite)
- Luster: Metallic (pyrite)
- Transparency: Opaque (pyrite)
- Fracture: Conchoidal to uneven (pyrite)
- Cleavage: Indistinct to poor on {001} (pyrite)
- Composition: Iron disulfide (FeS2) for pyrite; matrix composition varies widely (e.g., silicates, carbonates, organic matter)
Quick Check
- Color: Brassy yellow (pyrite) in a contrasting fine-grained host rock (e.g., dark gray, black, green)
- Luster: Metallic (pyrite), contrasting with the matrix's luster (e.g., dull, earthy, vitreous)
- Streak: Greenish-black to brownish-black
Physical Characteristics
- Crystal Habit: Cubic, octahedral, pyritohedral, massive, granular, radiating, stalactitic (pyrite)
- Cleavage Type: Indistinct to poor (pyrite)
- Fracture Type: Conchoidal to uneven (pyrite)
- Tenacity: Brittle (pyrite)
- Luster Type: Metallic (pyrite)
Formation
Pyrite (FeS2) forms in a wide range of geological environments, typically under reducing conditions. In a fine-grained matrix, it often forms through hydrothermal processes, diagenesis in sedimentary rocks, or as a primary accessory mineral in igneous and metamorphic rocks. Hydrothermal pyrite forms from hot, sulfur-rich fluids circulating through fractures and pores, often associated with ore deposits. Diagenetic pyrite forms in anoxic sediments (e.g., shales, mudstones) where organic matter decomposition by sulfate-reducing bacteria produces hydrogen sulfide, which then reacts with iron to form pyrite. Metamorphic pyrite can form during regional or contact metamorphism from pre-existing iron and sulfur minerals or by reactions with sulfur-bearing fluids. The fine-grained matrix can be shale, slate, mudstone, or fine-grained igneous/metamorphic rocks.
Usage
Pyrite itself was historically used as a source of sulfur for sulfuric acid production and, to a lesser extent, iron. It was also used as a spark-producing material in early firearms (flintlock mechanisms). Today, its primary commercial value is as an indicator mineral for gold and other base metal deposits. Specimens of pyrite in matrix are highly valued by mineral collectors for their aesthetic appeal, especially when the pyrite crystals are well-formed and contrasting with the host rock. In some cases, the host rock itself might be of economic interest (e.g., black shales rich in organic matter or certain ore-bearing rocks).
Age Distribution
Pyrite can form in rocks of all geological ages, from Precambrian to Cenozoic, depending on the specific geological processes involved.
Where to Find
Navajún, La Rioja, Spain
Famous for producing world-class, perfectly formed cubic pyrite crystals embedded in a marl matrix.
Elba Island, Italy
Known for well-formed pyrite crystals, often in a schistose or argillaceous matrix.
Peru
Various localities produce excellent pyrite specimens, often in association with other sulfides, within a range of host rocks.
Colorado, USA
Many mining districts (e.g., Leadville, Ouray) yield pyrite in various host rocks, including fine-grained sedimentary and metamorphic types.
Germany
Localities like Siegerland are known for pyrite in slate or other fine-grained metamorphic rocks.
Finding Tips
Look for Metallic Luster
Pyrite's distinctive metallic, brassy-yellow luster is often the first indicator. Look for glints of gold-like material in dark, fine-grained rocks.
Examine Rock Outcrops
Search in areas known for sedimentary rocks (especially black shales, mudstones), low-grade metamorphic rocks (slates, phyllites), or hydrothermal alteration zones. Pyrite often weathers to iron oxides, leaving rusty stains on the rock surface, which can indicate its presence.
Check for Crystal Forms
Well-formed cubic, octahedral, or pyritohedral crystals of pyrite are highly diagnostic and often stand out against the matrix. Look for these geometric shapes.
Perform a Streak Test
If unsure, a streak test on an unglazed porcelain plate will yield a greenish-black to brownish-black streak for pyrite, differentiating it from gold (yellow streak) and chalcopyrite (greenish-black streak).
Consider Associated Minerals
Pyrite often occurs with other sulfide minerals (e.g., chalcopyrite, sphalerite, galena) or quartz, calcite, and various clay minerals in the matrix. The presence of these can be a clue.
Similar Rocks
Marcasite in matrix
Marcasite (FeS2) in fine-grained host rock
Also known as: White Iron Pyrites in host rock
Chalcopyrite in matrix
Chalcopyrite (CuFeS2) in fine-grained host rock
Also known as: Copper Pyrites in host rock
Gold in quartz
Gold (Au) in quartz matrix
Also known as: Native Gold in host rock
Scientific Classification
- Mineral Class
- Sulfides
- Group
- Pyrite group
- Crystal System
- Isometric
- Chemical Formula
- FeS2
- Composition
- Iron (Fe) 46.55%, Sulfur (S) 53.45%
Explore Pyrite in matrix
Identify rocks anywhere
Free on iOS, Android, and Web. Photo identification, full database, and your personal collection.