Spectropyrite - a new type of jewelry

Spectropyrite - a new type of jewelry material

 D. A. Petrochenkov

Moscow State Exploration University, Moscow, Russia

A.M. Natarius

LLC Lita, Ulyanovsk, Russia 

Spectropyrite is the trade name for pyrite cheeks with multicolored bewilderment. Nodules with veins of spectropyrite are collected mainly in deposits of Aptic age of the Lower Cretaceous (Fig.). Spectropyrite is a thin vein of variously colored pyrite in marghelous nodules (Figure). Their diameter is usually from 5 to 15 sm. The width of the veins of the spectropyrite varies from 0.1 mm to 2 mm. For the manufacture of cabochons, veins with a width of more than 0.5 mm and an area of ​ ​ more than 1 sm are used² completely covered with pyrite (Fig.). Pyrite crystals grow symmetrically from opposite walls of cracks, while no fusion occurs in the veins of the spectropyrite. When nodules are split, the veins of the spectropyrite are divided into several parts with undisturbed crystal faces.

Outcrops of Aptic-age clays in the vicinity of the village. Shilovka (Ulyanovsk region)

Nodules of marl with veins of spectropyrite. Diameter 7-10 sm

Cabochons and spectropyrite pendants. 15x20 sm

According to the results of radiographic analysis, the spectropyrite consists of pyrite (FeS₂) and dispersed calcite (Table). It fixes: quartz, anhydrite (CaSO₄) и бассанит (CaSO₄.0,5H₂O). Calcite is the main nodule mineral. Quartz is associated with bottom sediment and is part of the marl. Anhydrite and bassanite were probably formed synchetically with pyrite performing cracks. Spectral semi-quantitative analysis captures high Si, Ca, Fe, elevated Al, Mg and low Mn, P (Table) in the spectropyrite, which generally corresponds to the mineral composition.

Mineral composition of spectropyrite according to radiographic analysis

Chemical composition of spectropyrite

according to spectral semi-quantitative analysis

Pyrite crystals are mainly 0.1-1 mm in size and do not exceed 2 mm, they fused tightly with the marl and break off with difficulty. Most often, cubic habitus crystals are present, octahedra and pentagondodecohedra are found (Fig.). Fragments of pyrite crystals of various shapes are observed in splices.

The color of the faces of the pyrite is yellow, orange, much less often brown, green of various shades and saturation (Fig.). In cabochons there are from 1 to 4 basic colors. In multicolored cabochons, the color distribution is zonal, spotted, color transitions are usually clearly expressed.

The brilliance of well-designed crystals is metallic. The fracture in pyrite units is shell, uneven. Density of aggregates 4.7-4.8 g/sm³, which is slightly lower than the density of microcrystals (5-5.2 g/sm³). Microhardness averaged 1038 kgf/mm². Luminescence is absent.

Mergel is a natural substrate of cabochons from spectropyrite. It consists mainly of calcite with inclusions of quartz, pyrite, albite, gypsum and kaolinite (Table). The marl is dense, breaks only in thin plates. The fracture is cancerous, polished with difficulty. The color of the marl is mainly gray, density 2.7 g/sm³. Microhardness averaged 105 kgf/mm². Luminescence is not observed.

 Various forms of pyrite crystals in spectropyrite.

In the slip it can be seen (Fig.) that the veil of spectropyrite has a width of 0.5-0.8 mm and is formed by fusion of pyrite crystals with a size of 0.1-0.3 mm. Contacts with the marl are clear and fairly even. No metasomatic changes are observed in the contact zone (Fig.). The marl consists of a dense fusion of dispersed calcite whose crystal size does not exceed 0.02 mm with a large number of small crystals of pyrite with a size of 0.01-0.2 mm and quartz with a size of up to 0.06 mm.

General view of slip (a) and detail fragment (b) of marl with spotting spectropyrite. Nicoli - | |. P - pyrite, M - marl, T - crack

Electron microscopic studies.

The surface of a small crystal pyrite with various facial colors (Fig.) Was studied on the Tesla BS-540 transmission electron microscope. The surfaces of the faces of the pyrite are subject to intensive etching and the development of the finest crust and film formations, mainly oxides and hydroxides of Fe, associated with oxidative processes. Ferroxygite, magnetite, acaganeite, iron sulfates, layered aluminosilicates, goethite, hematite, bernalite were identified on the surface of the crystals. Film aggregates of dispersed formations of hematite, hetite and ferroxigite are different stages of the oxidative process of pyrite. The color of the pyrite surface is associated with light interference and is determined by the mineral composition and thickness of the film formations.

Ferroksigit (FeOOH) is the most common mineral covering the etched surface of the pyrite. The mineral is observed in the form of thin crust-shaped secretions, film formations of isometric shape, 3-6 μm in size (Fig.).

Magnetite (FeFe₂O₄) occurs relatively often, forming film and crust-shaped secretions larger than 10 μm on the etched surface of the pyrite (Fig.).

Akagneite (FeOOH Fe₈O₈x(OH)₆FCl) fixed in a single case. It is represented by a pseudomorphosis on a particle of pyrite measuring 1.5 microns, (Fig.).

Ferrous sulphate ((Fe₂(SO₄)₃) are rarely recorded in the form of long-prismatic, tape-shaped more than 10 μm along the long axis of secretions associated with film formations of ferroxygite (Fig.).

Layered aluminosilicates are found in the form of dispersed (0.1-0.2 μm) translucent, scaly secretions, up to 2.5 μm in size on the etched surface of the pyrite (Fig.).

Gotite (FeOOH) is widespread, often forms continuous film and crust-shaped formations on the etched surface of the pyrite (Fig.).

Hematite (Fe₂O₃) forms a dispersed aggregate in the form of films and thin crusts on the etched surface of the pyrite (Fig.).

Bernalite (Fe(OH)(H₂O)_0,25) established in the single case and appears to represent pseudomorphoses by pyrite.

Film, crust-shaped rounded braces ferroksigit (FeOOH) (shown by arrows).

Film and crust-shaped separations manetit (FeFe₂O₄) (М) with rounded seals (shown by arrows) on the etched surface of the pyrite (P)

Particle Acaganeite (FeOOH Fe₈O₈x(OH)₆FCl) (shown by arrow) and removed film formation of ferroxygite (shown by double arrow) on etched pyrite surface

Long-prismatic and ribbon-shaped secretions ferrous sulfate ((Fe₂(SO₄)₃) (shown by arrows) and ferroxygite film formations (shown by double arrows) on the etched surface of the pyrite.

Flaky separations of laminated aluminosilicates on the etched surface of the pyrite (shown by arrow)

Film formation gorthite (FeOOH) (shown by arrow) on etched pyrite surface

Film and crust-like secretions hematite unit (Fe₂O₃) и ferroksigit (FeOOH) (shown by arrows) on etched pyrite surface

Microinclusion bernalite (Fe(OH)(H₂O)_0,25) (shown by the arrow) and a rounded film release of ferroxygite (shown by the double arrow) on the etched surface of the pyrite.

Electron Probe Studies

The surface of a marl with a spotlight of spectropyrite was examined on the electron-probe analytical complex "Superprobe" and "Jnka-400" (Fig.). Spectropyrite veins 0.8-1 mm wide. Crystals of pyrite of cubic gabitus, measuring 0.1-0.3 mm. Contact with the marl is uneven, there is an ingrowth of pyrite crystals into it. In reverse scattered electrons (EPR), pyrite is homogeneous (Fig.). According to local X-ray spectral analysis (LPSA), pyrite contains up to 6.6% oxygen (Table), indicating the presence of sulfates on its surface.

General view of the vein of pyrite (P) and marl (M) in the ORE. 1 - 3 - spectrum number

Chemical composition of pyrite according to LRSA

The marl contains a large number of inclusions of feldspar, mica, chlorite, pyroxenes, quartz and ilmenite of various shapes and sizes. These inclusions are terrigenous sediment cemented during the lithification process with dispersed calcite. There are also inclusions of pyrite and apatite, which are newly formed minerals. Calcite in ORE is uniform, gray in color, its structure is not manifested, which indicates a dispersed crystal size. A large number of evenly distributed pores of 1 to 30 μm are present in the marl.

In conclusion, we note that since the advent of spectropyrite on the market (about 20 years ago), it has been in constant demand at all major world mineralogical exhibitions. A wide range of jewelry with spectro inserts became.