Glauconite - (K,Na)(Fe3+,Al,Mg)2(Si,Al)4O10(OH)2

Named in 1828 from the Greek "glaukos," blue-green, in allusion to its color; Glauconite is an iron potassium phyllosilicate (mica group) mineral.
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. Glauconite forms by three principal processes: alteration of the fecal pellets of bottom-dwelling organisms; modification of particles of illitic and biotitic clays by seawater; and direct precipitation from seawater. Particles of glauconite are generally sand-sized or finer.

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Glauconite grains in limestone. Estonia. From (www.sandatlas.org)



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Glauconite. This specimen comes from the Wharf mine in the Black Hills as they were mining through lower Cambrian Deadwood Fm. From Dakota Matrix Minerals



Bibliography



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc


Photo
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 2x (Field of view = 7mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 2x (Field of view = 7mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 2x (Field of view = 7mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 2x (Field of view = 7mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. XPL image, 2x (Field of view = 7mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 2x (Field of view = 7mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 10x (Field of view = 2mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. XPL image, 10x (Field of view = 2mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 10x (Field of view = 2mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 10x (Field of view = 2mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. XPL image, 10x (Field of view = 2mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 10x (Field of view = 2mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 10x (Field of view = 2mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 10x (Field of view = 2mm)
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Rounded green Glauconite aggregates in a Bioclastic sandstone. PPL image, 10x (Field of view = 2mm)
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Glauconite aggregates. PPL image, 40x (Field of view = 0.2mm)
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Glauconite aggregates. PPL image, 40x (Field of view = 0.2mm)
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Glauconite aggregates. PPL image, 40x (Field of view = 0.2mm)