Glauconite

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Glauconite
Glauconite from the Dutch Pliocene.jpg
Glauconite pellets and small fossils among quartz grains in greensand from the Dutch Pliocene
General
Category Phyllosilicate
Formula
(repeating unit)
(K,Na)(Fe,Al,Mg)2(Si,Al)4O10(OH)2
IMA symbol Glt [1]
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
Space group C2/m
Unit cell a = 5.234 Å, b = 9.066 Å,
c = 10.16 Å; β = 100.5°; Z = 2
Identification
ColorBlue green, green, yellow green
Crystal habit Elastic platy/micaceous, or as rounded pellets/aggregates
Cleavage Perfect [001]
Mohs scale hardness2
Luster Dull, earthy
Streak Light green
Diaphaneity Translucent to nearly opaque
Specific gravity 2.4–2.95
Optical propertiesBiaxial (-); moderate relief
Refractive index nα = 1.590 – 1.612 nβ = 1.609 – 1.643 nγ = 1.610 – 1.644
Birefringence δ = 0.020 – 0.032
Pleochroism X = yellow-green, green; Y = Z = deeper yellow, bluish green
Other characteristicsloosely bound aggregates, crumbles
radioactivity: barely detectable
References [2] [3] [4]

Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color which is very friable [5] and has very low weathering resistance.

Contents

It crystallizes with a monoclinic geometry. Its name is derived from the Greek glaucos ( γλαυκός ) 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 a hardness of 2. The relative specific gravity range is 2.4–2.95. It is normally found as dark green rounded concretions with the dimensions of a sand grain. It can be confused with chlorite (also of green color) or with a clay mineral. Glauconite has the chemical formula (K,Na)(Fe,Al,Mg)2(Si,Al)4O10(OH)2.

Glauconite particles are one of the main components of greensand, glauconitic siltstone and glauconitic sandstone. Glauconite has been called a marl in an old and broad sense 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.

Occurrence

At the broadest level, glauconite is an authigenic mineral and forms exclusively in marine settings. [6] It is commonly associated with low-oxygen conditions. [7]

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/lower Cretaceous deposits of greensand, 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 limestones and in chalk. It develops as a consequence of diagenetic 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. Glauconite remains absent in fresh-water lakes, but is noted in shelf sediments of the western Black Sea. [8] 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 for artistic oil paint under the name green earth. [9] [10] One example is its use in Russian "icon paintings", another widespread use was for underpainting of human flesh in medieval painting. [11] It is also found as mineral pigment in wall paintings from the ancient Roman Gaul. [12]

Fertilizers

Glauconite, a major component of greensand, is a common source of potassium (K+) in plant fertilizers 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, greensand refers to a fertilizer produced from a glauconitic siltstone unit belonging to the Serra da Saudade Formation, Bambuí Group, of Neoproterozoic/Ediacaran age. The outcrops occur [13] in the Serra da Saudade ridge, in the Alto Paranaíba region, Minas Gerais state. It is a silty-clayed sedimentary rock, laminated, bluish-green, composed of glauconite (40-80%), potassium feldspar (10-15%), quartz (10-60%), muscovite (5%) and minor quantities of biotite (2%), goethite (<1%), titanium and manganese oxides (<1%), barium phosphate and rare-earth element phosphates (<1%).

Enriched levels of potash have K2O grades between 8 and 12%, thickness up to 50 metres (160 ft) 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 felsic elements in a continental margin environment with acid magmatic arc (foreland basin).

Related Research Articles

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Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2Si2O5(OH)4) and tiny fragments (silt-sized particles) of other minerals, especially quartz and calcite. Shale is characterized by its tendency to split into thin layers (laminae) less than one centimeter in thickness. This property is called fissility. Shale is the most common sedimentary rock.

<span class="mw-page-title-main">Marl</span> Lime-rich mud or mudstone which contains variable amounts of clays and silt

Marl is an earthy material rich in carbonate minerals, clays, and silt. When hardened into rock, this becomes marlstone. It is formed in marine or freshwater environments, often through the activities of algae.

<span class="mw-page-title-main">Greensand</span> Sand or sandstone which has a greenish color

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 consist of a mixture of mixed-layer clay minerals, such as smectite and glauconite. Greensand is also loosely applied to any glauconitic sediment.

<span class="mw-page-title-main">Chlorite group</span> Type of mineral

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<span class="mw-page-title-main">Celadonite</span>

Celadonite is a mica group mineral, a phyllosilicate of potassium, iron in both oxidation states, aluminium and hydroxide with formula K(Mg,Fe2+
)(Fe3+
,Al)[Si
4
O
10
](OH)
2
.

<span class="mw-page-title-main">Carnallite</span> Evaporite mineral

Carnallite (also carnalite) is an evaporite mineral, a hydrated potassium magnesium chloride with formula KCl.MgCl2·6(H2O). It is variably colored yellow to white, reddish, and sometimes colorless or blue. It is usually massive to fibrous with rare pseudohexagonal orthorhombic crystals. The mineral is deliquescent (absorbs moisture from the surrounding air) and specimens must be stored in an airtight container.

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Red beds are sedimentary rocks, typically consisting of sandstone, siltstone, and shale, that are predominantly red in color due to the presence of ferric oxides. Frequently, these red-colored sedimentary strata locally contain thin beds of conglomerate, marl, limestone, or some combination of these sedimentary rocks. The ferric oxides, which are responsible for the red color of red beds, typically occur as a coating on the grains of sediments comprising red beds. Classic examples of red beds are the Permian and Triassic strata of the western United States and the Devonian Old Red Sandstone facies of Europe.

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<span class="mw-page-title-main">Clastic rock</span> Sedimentary rocks made of mineral or rock fragments

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<span class="mw-page-title-main">Chugwater Formation</span>

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<span class="mw-page-title-main">Authigenesis</span>

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<span class="mw-page-title-main">Lower Greensand Group</span> Geological unit

The Lower Greensand Group is a geological unit present across large areas of Southern England. It was deposited during the Aptian and Albian stages of the Early Cretaceous. It predominantly consists of sandstone and unconsolidated sand that were deposited in shallow marine conditions.

<span class="mw-page-title-main">Chamosite</span> Phyllosilicate mineral member of the chlorite group

Chamosite is the Fe2+end member of the chlorite group. A hydrous aluminium silicate of iron, which is produced in an environment of low to moderate grade of metamorphosed iron deposits, as gray or black crystals in oolitic iron ore. Like other chlorites, it is a product of the hydrothermal alteration of pyroxenes, amphiboles and biotite in igneous rock. The composition of chlorite is often related to that of the original igneous mineral so that more Fe-rich chlorites are commonly found as replacements of the Fe-rich ferromagnesian minerals (Deer et al., 1992).

<span class="mw-page-title-main">Matawan Formation</span>

The Matawan Formation is a geologic formation in Maryland and New Jersey. It preserves fossils dating back to the Cretaceous period.

The geology of West Sussex in southeast England comprises a succession of sedimentary rocks of Cretaceous age overlain in the south by sediments of Palaeogene age. The sequence of strata from both periods consists of a variety of sandstones, mudstones, siltstones and limestones. These sediments were deposited within the Hampshire and Weald basins. Erosion subsequent to large scale but gentle folding associated with the Alpine Orogeny has resulted in the present outcrop pattern across the county, dominated by the north facing chalk scarp of the South Downs. The bedrock is overlain by a suite of Quaternary deposits of varied origin. Parts of both the bedrock and these superficial deposits have been worked for a variety of minerals for use in construction, industry and agriculture.

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References

  1. Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi: 10.1180/mgm.2021.43 . S2CID   235729616.
  2. Handbook of Mineralogy
  3. Webmineral
  4. Mindat
  5. Odin, G.S. (ed., 1988). Green marine clays. Development in sedimentology, 45. Elsevier, Amsterdam.
  6. Smith, S. A., and Hiscott, R. N. (1987). Latest Precambrian to Early Cambrian basin evolution, Fortune Bay, Newfoundland fault–bounded basin to platform. Canadian Journal of Earth Sciences 21:1379–1392.
  7. Hiscott, R. N. (1982). Tidal deposits of the Lower Cambrian Random Formation, eastern Newfoundland; facies and paleoenvironments. Canadian Journal of Earth Sciences 19:2028–2042.
  8. Suttill H. (2009). Sedimentological evolution of the Emine & Kamchia basins, eastern Bulgaria. Thesis submitted for the degree of Master of Philosophy. Available from: the University of Edinburgh
  9. Grissom, C.A. Green Earth, in Artists’ Pigments. A Handbook of Their History and Characteristics, Vol. 1, L. Feller, (Ed), Cambridge University Press, London 1986, pp. 141 – 167
  10. Green earth Colourlex
  11. Grissom, C.A. Green Earth, in Artists’ Pigments. A Handbook of Their History and Characteristics, Vol. 1, L. Feller, (Ed), Cambridge University Press, London 1986, p. 143
  12. Eastaugh, N "Pigment Compendium: A Dictionary of Historical Pigments", page 169. Elsevier, 2004
  13. Silvano Moreira, Débora (2016). "Estratigrafia, petrografia e mineralização de potássio em siltitos verdes do Grupo Bambuí na Região de São Gotardo, Minas Gerais" (PDF). Revista Geociências. 35: 157–171 via UNESP.