Greigite

greigite

montmorillonite

colemanite

veatchite

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Formula: Fe2+Fe3+2S4
Sulphide, forms a series with violarite
Specific gravity: 4.049
Hardness: 4 to 4½
Streak: Black
Colour: Metallic pinkish, tarnishing to a metallic blue, sooty black when amorphous
Magnetism: Strongly magnetic
Common impurities: Cu,Ni,Zn,Mn,Cr,Sb,As
Environments

Sedimentary environments
Hydrothermal environments

Greigite is formed in lacustrine beds and in hydrothermal vein deposits; it is formed in magnetotactic (exhibiting movement in response to a magnetic field) bacteria, and by sulphate-reducing bacteria (HOM).
Common associates include calcite, chlorite, dolomite, galena, marcasite, montmorillonite, pyrite and sphalerite (Mindat).

Localities

At Zacatecas, Mexico, greigite is associated with sphalerite, pyrite, marcasite, galena, calcite and dolomite (HOM).

At Lojane, Lipkovo Municipality, North Macedonia, the country rocks are serpentinites and serpentinised peridotites intruded by granites, syenites and rhyolites. Greigite has been found dispersed, in very small quantities, throughout the ore bodies. In most cases it occurs as fine-grained globular aggregates within stibnite. It also occurs within fractures in stibnite and, rarely, in the gangue minerals.
Nickel possibly replaces some of the ferrous iron in the greigite structure, and influences the colour of greigite, the characteristic pink colour being more pronounced with an increased nickel content.
Greigite formed in two generations, one as relatively large individual crystals which are corroded by stibnite, and another as very fine inclusions and globular aggregates in fractures in the stibnite. It appears that greigite was deposited before realgar in the paragenetic sequence (AM 51, 209-215).

At the Mudd Mine, West Baker, Kramer Borate deposit, Boron, Kramer District, Kern county, California, USA, greigite has been found as tiny inclusions in crystals of kernite. Associated as included minerals are montmorillonite, chlorite, biotite, and single euhedral crystals of dolomite, calcite and searlesite. In contrast to the widespread occurrence in the Four Corners area, greigite has been positively identified only from this single occurrence in the Kramer borate district.
It seems probable that greigite was formed by bacterial reduction of ironbearing waters in essentially anaerobic bottom sediments of an alkaline lake (AM 49.543-555).

At the type locality, Four Corners No. 3, 4 & 5 wells, Kramer Junction, East Kramer Borate area, East Kramer Mining District, San Bernardino county, California, USA, greigite was discovered in cores recovered from lacustrine sediments. The greigite-bearing sediments are overlain by 650 to 800 feet of alluvium, and underlain by sandstones and conglomerates. The lake beds are composed of interbedded calcareous clays, silts and fine- to medium- grained arkosic sands. Greigite is restricted to clay and fine silt layers, being completely absent from the sands and other coarse-grained material. Associated minerals include montmorillonite, chlorite, calcite, colemanite, veatchite, orpiment and realgar. The greigite sample concentrated for analysis also contained a small amount of marcasite in close association. The only other iron sulphide observed in the drill cores was a small amount of pyrite, that was not directly associated with greigite.
In the Kramer district, greigite appears to have formed with borax in the temperature range 25oC to 35oC, and to have remained as a stable or metastable phase after conversion of the borax to kernite, which occurs at 58+/-5oC and a depth of 2500+/-500 feet (AM 49.543-555).

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