Glauconite

Glauconite
General
Category	Phyllosilicate
Formula (repeating unit)	(K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2
Crystal symmetry	Monoclinic prismatic 2/m
Unit cell	a = 5.234 Å, b = 9.066 Å, c = 10.16 Å; β = 100.5°; Z = 2
Identification
Color	Blue green, green, yellow green
Crystal habit	Elastic platy/micaceous, or as rounded pellets/aggregates
Crystal system	Monoclinic
Cleavage	Perfect [001]
Mohs scale hardness	2
Luster	Dull - earthy
Streak	Light green
Diaphaneity	Translucent to nearly opaque.
Specific gravity	2.4 - 2.95
Optical properties	Biaxial (-)
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 characteristics	loosely bound aggregates, crumbles
References	[1][2][3]

Glauconite, also known as 'green sand' is an iron potassium phyllosilicate (mica group) mineral of characteristic green color with very low weathering resistance and very friable.^[4]

It crystallizes with a monoclinic geometry. Its name is derived from the Greek glaucos (γλαυκος) meaning 'blue', referring to the common blue-green color of the mineral; its sheen (mica glimmer) and blue-green color presumably relating to the sea's surface. 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 range is 2.4 - 2.95. It is normally found in dark green rounded brittle pellets, and with the dimension of a sand grain size. It can be confused with chlorite (also of green color) or with a clay mineral.

Environment of formation

Normally, glauconite is considered a diagnostic mineral indicative of continental shelf marine depositional environments with slow rates of accumulation. For instance, it appears in Jurassic/lower Cretaceous deposits of greensand, so-called after the coloration caused by glauconite. 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.^[5]

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.^[6][7] One example is its use in Russian "icon paintings", another widespread use was for underpainting of human flesh in medieval painting.^[8] It is also found as mineral pigment in wall paintings from the ancient Roman Gaul.^[9] Glauconite, a major component of greensand, is also a common source of potassium in plant fertilizers, also used to promote soil acidity.

References