Glauconite
Glauconite pellets and small fossils among quartz grains in greensand from the Dutch Pliocene
General
CategoryPhyllosilicate
Formula
(repeating unit)		(K,Na)(Fe,Al,Mg)2(Si,Al)4O10(OH)2
IMA symbolGlt[1]
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupC2/m
Unit cella = 5.234 Å, b = 9.066 Å,
c = 10.16 Å; β = 100.5°; Z = 2
Identification
ColorBlue green, green, yellow green
Crystal habitElastic platy/micaceous, or as rounded pellets/aggregates
CleavagePerfect [001]
Mohs scale hardness2
LusterDull – earthy
StreakLight green
DiaphaneityTranslucent to nearly opaque.
Specific gravity2.4–2.95
Optical propertiesBiaxial (-); moderate relief
Refractive indexnα = 1.590 – 1.612 nβ = 1.609 – 1.643 nγ = 1.610 – 1.644
Birefringenceδ = 0.020 – 0.032
PleochroismX = 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.

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).

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.
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