Pyrite Variations

On this live, we have specimens demonstrating hydrothermal pyrite with quartz, pyrite with fluorite, iridescent/tarnished pyrite, crystalline aggregates, and sedimentary/diagenetic pyrite formed within ancient marine mud. That gives you an excellent way to demonstrate just how many completely different geological environments can produce the same FeS₂ mineral. Pyrite is commonly referred to as “fool’s gold.” Before the 1800’s, it was favored as a decorative stone, carved into rosettes, shoe buckles, rings, snuff boxes and other ornaments, and was extremely popular in England during the Victorian Age for its use in jewelry. Pyrite’s biggest use occurred during World War II when it was mined as a source of Sulfur for producing sulfuric acid used in industry.

Although much lighter than gold, its brassy color and relatively high density misled many novice prospectors. Its name is derived from the Greek word pyr, meaning “fire,” because it emits sparks when struck by iron. It is opaque and pale silvery yellow when fresh, turning darker and tarnishing with exposure to oxygen. Pyrite crystals may be cubic, octahedral, or twelve-sided “pyritohedra,” and are often striated. Pyrite can also be massive or granular, or form either flattened disks or nodules of radiating, elongate crystals.

Pyrite occurs in hydrothermal veins, by segregation from magmas, in contact metamorphic rocks, and in sedimentary rocks, such as shale and coal, where it can either fill or replace fossils. Pyrite forms under reducing conditions, typically in environments with low oxygen levels, high sulfur content, and abundant iron. It can form through both biological and abiotic processes. Pyrite is a widespread mineral and is found in various geological formations around the world. 1.

PAKISTAN PYRITE Pakistan produces pyrite from numerous mineralized regions, including deposits associated with the mountainous geology of Balochistan, Khyber Pakhtunkhwa and northern Pakistan. Pyrite develops wherever iron-bearing fluids encounter sufficient sulfur under reducing conditions and is particularly common in hydrothermal veins and sulfide-bearing deposits. Each tiny crystal is obeying the same cubic internal symmetry; collectively, however, hundreds of intergrown crystals produce the rounded overall specimen. This is a good demonstration of the difference between a mineral's crystal system and its overall aggregate habit. Extraction varies considerably by Pakistani locality.

Collector material is frequently recovered from relatively small underground workings, veins and pockets rather than being produced specifically by large specimen mines. When an attractive crystallized pocket is encountered, careful hand extraction is necessary because individual pyrite crystals are brittle despite their relatively high hardness. 2. INDONESIAN PYRITE WITH QUARTZ This material represents a very different style of pyrite growth: sharply metallic pyrite crystals occur among clear to milky quartz crystals. This indicates multiple minerals crystallizing from mineral-rich hydrothermal fluids circulating through fractures or cavities in the host rock.

As conditions changed within the open cavity, silica crystallized as quartz while iron and sulfur precipitated as pyrite. The minerals may have grown during overlapping stages or successive pulses of hydrothermal fluid. This is why pyrite can appear nestled between quartz points, perched directly upon quartz, or partially surrounded by later quartz growth. mirror-like surfaces occur where crystals were able to grow freely into open space without interference from neighbouring crystals. 3.

INDONESIAN “RAINBOW” PYRITE WITH QUARTZ The Rainbow Pyrite is especially interesting because the rainbow colour is not a different variety of pyrite caused by a different internal crystal structure. Beneath the colour, it remains FeS₂ pyrite. The iridescent blues, greens, golds, purples and pinks are produced by an extremely thin tarnish film. Light reflecting from the upper and lower boundaries of this microscopic film interferes with itself. Different film thicknesses reinforce different wavelengths of visible light, producing the rainbow effect—similar in principle to the colours seen on a soap bubble or thin oil film.

That explains an important difference between the two Indonesian materials: Regular Indonesian Pyrite: fresh metallic surfaces reflect the characteristic brass-gold colour of pyrite. Rainbow Indonesian Pyrite: a very thin altered surface layer produces optical interference colours over the underlying pyrite. The quartz itself has not created the rainbow colour. Rather, the combination of brilliant iridescent pyrite against clear quartz makes these specimens particularly dramatic. 4.

CHINESE PYRITE WITH FLUORITE China has produced extraordinary pyrite associations from several hydrothermal mineral districts, particularly deposits containing fluorite, quartz, calcite and metallic sulfides. Documented Chinese localities producing pyrite with fluorite include the Yaogangxian Mine in Hunan, Xiefang Mine in Jiangxi and Wutong Mine in Guangxi. Rather than forming only large freestanding cubes, the pyrite occurs as extremely small metallic crystals concentrated along portions of the fluorite and appearing both within growth zones and over exterior surfaces.

This kind of relationship records changing mineral chemistry during the development of the specimen: fluorine- and calcium-rich fluids produced fluorite, while iron- and sulfur-bearing fluids precipitated pyrite during another stage or overlapping stages of hydrothermal mineralization. Chinese fluorite deposits are famous for this type of multi-stage hydrothermal crystallization. A cavity can repeatedly reopen or receive new pulses of mineral-bearing fluid, allowing fluorite, quartz, carbonates and sulfides such as pyrite to grow over—or occasionally become enclosed by—earlier minerals. 5. PERUVIAN PYRITE Peru is one of the world's classic sources of collector-quality pyrite.

Famous localities include the Huanzala Mine in Ancash and the Quiruvilca Mine in La Libertad. These polymetallic hydrothermal deposits have produced exceptionally brilliant pyrite crystals in cubes, pyritohedrons, octahedrons and complex combinations of several forms. At Quiruvilca, documented specimens include both octahedral crystals and pyritohedrons associated with white quartz. Huanzala is particularly famous for intensely lustrous crystals showing combinations of pyritohedral, octahedral and cubic faces. Our Peruvian specimen shows another characteristic style: dense generations of brilliant pyrite crystals growing over one another, producing a sparkling metallic mass rather than one dominant cube.

Peru's mineral deposits were created through extensive hydrothermal activity associated with Andean magmatism. Hot metal-bearing fluids circulated through fractures and carbonate rocks, depositing pyrite together with minerals such as quartz, sphalerite, galena and calcite as temperature and chemistry changed. 6. PYRITE ON FOSSILIZED WOOD A particularly unusual material recently entering the mineral market from Indonesia combines metallic pyrite with black petrified wood. While this specific occurrence appears to be a newly reported find and has not yet been formally described in the geological literature, the association itself is geologically plausible.

Indonesia is exceptionally rich in petrified wood because of its long history of volcanic activity. Volcanic ash and tuff provide abundant silica that can be dissolved by groundwater and transported into buried wood. Indonesian studies have confirmed petrified wood in volcanic and tuffaceous sediments, where the original wood structure was progressively replaced and filled by silica, eventually producing predominantly quartz-rich fossil wood. Garut is a volcanic region where ancient forests were buried by volcanic deposits and later mineralized by circulating groundwater. Fossil wood from the broader Garut region occurs in deposits spanning several geological ages, with important occurrences associated with Late Miocene to Pliocene volcaniclastic rocks.

Until the exact formation containing this new find is confirmed, we can't give it an exact age, but it is millions of years old. What makes this material unusual is the extensive pyrite mineralization following the preserved wood structure. 7. IRON GALLSTONE Concretions are found in sedimentary rock or soil, often ovid or spherical in shape. They generally form around some type of nucleus, usually early on in the burial history of the sediment.

This makes them harder than the surrounding host sediment. Record of these geological curiosities dates back to the 18th century. They were often confused with fossils of dinosaurs, plants, humans and extraterrestrials. Imagine the seafloor covered by very fine, dark mud rich in organic matter. After burial, that mud became oxygen-poor.

Microorganisms breaking down organic matter could use sulfate dissolved in seawater, producing reduced sulfur. That sulfur reacted with dissolved iron to form iron sulfides, eventually producing pyrite (FeS₂). This mechanism is well documented for pyrite concretions forming in marine sediments. As the concretion grew outward, it cemented the surrounding sediment early in its burial history. That is why concretions become harder and more resistant than the shale surrounding them.

Later compaction squeezes the still-soft sediment around the already hardened concretion. Research on pyrite nodules shows that early cementation followed by sediment compaction can transform pyrite-rich layers and aggregates into distinct nodules. This specimen was collected from marine shales in Guangxi, China. The slightly ovoidal nodule consist of layers of pyrite, or iron disulfide, and black shale material, and sits nicely without support. Typical habit: Cubes, pyritohedrons, octahedrons and complex combination crystals