A framboid is a micromorphological feature common to certain sedimentary minerals, particularly

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue giv ...

(FeS₂). The first known use of the term is ascribed to Rust in 1935 and is derived from the French ‘la framboise’, meaning ‘raspberry’, reflecting the appearance of the structure under magnification. Framboidal structure comprises roughly

spherical A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

aggregates of discrete equi-regular euhedral microcrystallites of around 0.5 μm in diameter, with the average aggregate size ranging from 5-20 μm. Framboid diameter tends to

correlate In statistics, correlation or dependence is any statistical relationship, whether causal or not, between two random variables or bivariate data. Although in the broadest sense, "correlation" may indicate any type of association, in statistic ...

positively with microcrystal size, and microcrystal packing is most commonly irregular and disordered. Framboids were once thought to be a fossilised bacterial colonies or microorganisms, but successful synthesis of this structure under laboratory conditions and observation of framboids in locations hostile to microbial life have discounted this theory. Framboidal pyrite is commonly found in coastal sediments, for instance marsh soils, marine and estuarine sediments, and beach sands. It can also be observed in coal as well as magmatic and

carbonate rock Carbonate rocks are a class of sedimentary rocks composed primarily of carbonate minerals. The two major types are limestone, which is composed of calcite or aragonite (different crystal forms of CaCO3), and dolomite rock (also known as dolosto ...

s. Other minerals known to exhibit framboidal structures include

magnetite Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With th ...

, hematite, and

greigite Greigite is an iron sulfide mineral with the chemical formula . It is the sulfur equivalent of the iron oxide magnetite (Fe3O4). It was first described in 1964 for an occurrence in San Bernardino County, California, and named after the mineralogis ...

. Greigite is considered an essential precursor of framboidal pyrite formation.

References

* McElnea, A. E. (2002) �
Assessing the Ability of Acid Sulfate Soil Laboratory Tests to Predict Environmental Risk and Lime Amelioration
��. PhD Thesis, School of Land and Food Sciences, University of Queensland, Australia. * Ohfuji. H. and D. Rickard (2005) Experimental synthesis of framboids – a review. ''Earth-Science Reviews'', 71:147-170 (Availability: ScienceDirect Database). * Rust, G.W. (1935) Colloidal primary copper ores at Cornwall mines, southeastern Missouri. ''Journal of Geology'', 43:398-426. * Rickard, David (2021). ''Framboids''. Oxford University Press. * Wilkin, R. T. and H.L. Barnes (1997) Formation processes of framboidal pyrite. ''Geochimica et Cosmochimica Acta'', 61(2): 323-339 (Availability: ScienceDirect Database). Mineralogy Pedology Sulfide minerals {{mineral-stub