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Concept
Glauconite
Summary
Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color which is very friable 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 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 .
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.
At the broadest level, glauconite is an authigenic mineral and forms exclusively in marine settings. It is commonly associated with low-oxygen conditions.
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.
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