Framboid
Encyclopedia
The term framboid describes a micromorphological feature common to certain sedimentary minerals, particularly pyrite
(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 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 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
s. Other minerals known to exhibit framboidal structures include magnetite
, hematite
, and greigite
. Greigite is considered an essential precursor of framboidal pyrite formation.
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
(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 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 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
Igneous rock
Igneous rock is one of the three main rock types, the others being sedimentary and metamorphic rock. Igneous rock is formed through the cooling and solidification of magma or lava...
and carbonate rock
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 and dolostone, which is composed of the mineral dolomite .Calcite can be either dissolved by groundwater or precipitated by...
s. Other minerals known to exhibit framboidal structures include magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...
, hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...
, and greigite
Greigite
Greigite is an iron sulfide mineral with formula Fe3S4. It is the sulfur equivalent of the iron oxide magnetite . It was first described in 1964 for an occurrence in San Bernardino County, California, and named after the mineralogist and physical chemist Joseph W...
. Greigite is considered an essential precursor of framboidal pyrite formation.