Glauconite is an

iron Iron () is a chemical element with Symbol (chemistry), symbol Fe (from la, Wikt:ferrum, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 element, group 8 of the periodic table. It is, Abundanc ...

potassium Potassium is the chemical element with the symbol K (from Neo-Latin ''kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmosph ...

phyllosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

( mica group)

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

of characteristic green color which is very

friable Friability ( ), the condition of being friable, describes the tendency of a solid substance to break into smaller pieces under duress or contact, especially by rubbing. The opposite of friable is indurate. Substances that are designated hazardous, ...

and has very low weathering resistance. It

crystallize Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely deposi ...

s with a

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in the orthorhombic s ...

geometry. Its name is derived from the

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

() meaning 'bluish green', referring to the common blue-green color of the mineral; its sheen ( mica glimmer) and blue-green color. Its color ranges from olive green, black green to bluish green, and yellowish on exposed surfaces due to oxidation. In the Mohs scale it has hardness of 2. The relative

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its densest ...

range is 2.4 - 2.95. It is normally found in dark green rounded brittle pellets, and with the dimension of a

sand Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class o ...

grain size. It can be confused with chlorite (also of green color) or with a clay mineral. Glauconite has the chemical formula . Glauconite particles are one of the main components of

greensand Greensand or green sand is a sand or sandstone which has a greenish color. This term is specifically applied to shallow marine sediment that contains noticeable quantities of rounded greenish grains. These grains are called ''glauconies'' and co ...

, glauconitic siltstone and glauconitic sandstone. Glauconite has been called a marl in an old and broad

sense A sense is a biological system used by an organism for sensation, the process of gathering information about the world through the detection of stimuli. (For example, in the human body, the brain which is part of the central nervous system re ...

of that word. Thus references to "greensand marl" sometimes refer specifically to glauconite. The Glauconitic Marl formation is named after it, and there is a glauconitic sandstone formation in the Mannville Group of

Western Canada Western Canada, also referred to as the Western provinces, Canadian West or the Western provinces of Canada, and commonly known within Canada as the West, is a Canadian region that includes the four western provinces just north of the Canada� ...

Occurrence

At the broadest level, glauconite is an authigenic mineral and forms exclusively in marine settings. It is commonly associated with low-oxygen conditions. Normally, glauconite is considered a diagnostic mineral indicative of continental shelf marine depositional environments with slow rates of accumulation and gradational boundaries. For instance, it appears in

Jurassic The Jurassic ( ) is a geologic period and stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately Mya. The Jurassic constitutes the middle period of ...

/lower

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

deposits of

, so-called after the coloration caused by glauconite, its presence gradually lessening further landward. It can also be found in sand or clay formations, or in impure

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

s and in

chalk Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. Ch ...

. It develops as a consequence of

diagenetic Diagenesis () is the process that describes physical and chemical changes in sediments first caused by water-rock interactions, microbial activity, and compaction after their deposition. Increased pressure and temperature only start to play a ...

alteration of sedimentary deposits, bio-chemical reduction and subsequent mineralogical changes affecting iron-bearing micas such as biotite, and is also influenced by the decaying process of organic matter degraded by bacteria in marine animal shells. Glauconite forms under reducing conditions in sediments and such deposits are commonly found in nearshore sands, open oceans and the

Mediterranean Sea The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on the ...

. Glauconite remains absent in fresh-water lakes, but is noted in shelf sediments of the western

Black Sea The Black Sea is a marginal mediterranean sea of the Atlantic Ocean lying between Europe and Asia, east of the Balkans, south of the East European Plain, west of the Caucasus, and north of Anatolia. It is bounded by Bulgaria, Georgia, Rom ...

. The wide distribution of these sandy deposits was first made known by naturalists on board the fifth HMS ''Challenger'', in the expedition of 1872–1876.

Uses

Glauconite has long been used in Europe as a green

pigment A pigment is a colored material that is completely or nearly insoluble in water. In contrast, dyes are typically soluble, at least at some stage in their use. Generally dyes are often organic compounds whereas pigments are often inorganic compou ...

for artistic oil paint under the name

green earth Green earth, also known as terre verte and Verona green, is an inorganic pigment derived from the minerals celadonite and glauconite. Its chemical formula is .

. One example is its use in Russian "

icon An icon () is a religious work of art, most commonly a painting, in the cultures of the Eastern Orthodox, Oriental Orthodox, and Catholic churches. They are not simply artworks; "an icon is a sacred image used in religious devotion". The most ...

paintings", another widespread use was for underpainting of human flesh in medieval painting. It is also found as mineral pigment in wall paintings from the ancient

Roman Gaul Roman Gaul refers to GaulThe territory of Gaul roughly corresponds to modern-day France, Belgium and Luxembourg, and adjacient parts of the Netherlands, Switzerland and Germany. under provincial rule in the Roman Empire from the 1st century ...

Fertilizers

Glauconite, a major component of

, is a common source of

(K⁺) in plant

fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...

s and is also used to adjust soil pH. It is used for soil conditioning in both organic and non-organic farming, whether as an unprocessed material (mixed in adequate proportions) or as a feedstock in the synthesis of commercial fertilizer powders. In

Brazil Brazil ( pt, Brasil; ), officially the Federative Republic of Brazil (Portuguese: ), is the largest country in both South America and Latin America. At and with over 217 million people, Brazil is the world's fifth-largest country by area ...

, greensand refers to a fertilizer produced from a glauconitic siltstone unit belonging to the Serra da Saudade Formation,

Bambuí Bambuí is a Brazilian municipality located in the west of the state of Minas Gerais. Its population was 23,898 people living in a total area of , for a density of . The city belongs to the meso-region of Oeste de Minas and to the micro-region of ...

Group, of

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is prec ...

/ Ediacaran age. The outcrops occur in the Serra da Saudade ridge, in the Alto Paranaíba region,

Minas Gerais Minas Gerais () is a state in Southeastern Brazil. It ranks as the second most populous, the third by gross domestic product (GDP), and the fourth largest by area in the country. The state's capital and largest city, Belo Horizonte (literally ...

state. It is a silty-clayed sedimentary rock, laminated, bluish-green, composed of glauconite (40-80%), potassium

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

(10-15%),

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

(10-60%),

muscovite Muscovite (also known as common mica, isinglass, or potash mica) is a hydrated phyllosilicate mineral of aluminium and potassium with formula K Al2(Al Si3 O10)( F,O H)2, or ( KF)2( Al2O3)3( SiO2)6( H2O). It has a highly perfect basal cleavag ...

(5%) and minor quantities of biotite (2%),

goethite Goethite (, ) is a mineral of the diaspore group, consisting of iron(III) oxide-hydroxide, specifically the "α" polymorph. It is found in soil and other low-temperature environments such as sediment. Goethite has been well known since ancient t ...

(<1%),

titanium Titanium is a chemical element with the Symbol (chemistry), symbol Ti and atomic number 22. Found in nature only as an oxide, it can be reduced to produce a lustrous transition metal with a silver color, low density, and high strength, resista ...

and

manganese Manganese is a chemical element with the symbol Mn and atomic number 25. It is a hard, brittle, silvery metal, often found in minerals in combination with iron. Manganese is a transition metal with a multifaceted array of industrial alloy use ...

oxides (<1%), barium phosphate and

rare-earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides ( yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silv ...

phosphates (<1%). Enriched levels of potash have K₂O grades between 8 and 12%, thickness up to 50 m and are associated to the glauconitic levels, dark-green in color. Glauconite is authigenic and highly mature. The high concentration of this mineral is related to a depositional environment with a low sedimentation rate. The glauconitic siltstone has resulted from a high-level flooding event in the Bambuí Basin. The sedimentary provenance is from

supracrustal Supracrustal rocks (''supra'' (Latin for "above")) are rocks that were deposited on the existing basement rocks of the crust, hence the name. They may be further metamorphosed from both sedimentary and volcanic rocks. The oldest minerals on Ear ...

felsic elements in a

continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being deep-ocean basins and mid-ocean ridges. The continental margi ...

environment with acid magmatic arc ( foreland basin).

References

{{Authority control Potassium minerals Sodium minerals Magnesium minerals Aluminium minerals Iron(III) minerals Mica group Monoclinic minerals Minerals in space group 12 Inorganic pigments