Glauconite

Glauconite
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
Color	Blue green, green, yellow green
Crystal habit	Elastic platy/micaceous, or as rounded pellets/aggregates
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 (-); 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 characteristics	loosely 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, roughly the same as gypsum.^[6] 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.