Glauconitic

Glauconite
General
Category	Phyllosilicate
Formula (repeating unit)	(K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2
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 (-)
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

Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color with very low weathering resistance and very friable.

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.Glauconite has the chemical formula – (K,Na,Ca)1.2-2.0(Fe+3,Al,Fe+2,Mg)4(Si7-7.6Al1-0.4)020(OH)2.nH2O

Glauconite particles are one of the main components of greensand and glauconitic sandstone, and 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.

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

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Wikipedia