Glauconite
Potassium iron magnesium aluminosilicate ((K,Na)(Fe,Al,Mg)2(Si,Al)4O10(OH)2)
A soft, green iron-potassium mica that forms in marine sediments and gives greensand its characteristic olive color.
- Mohs hardness
- 2
- Color
- dull green, olive to blackish green
- Type
- mineral
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Overview
Glauconite is a soft, green, iron-rich mineral of the mica group, closely related to illite and celadonite. It typically occurs as tiny green pellets or grains rather than well-formed crystals, and it is the defining component of the sedimentary deposit known as greensand.
Its dull olive-to-blackish green color comes from its high iron content, with the iron present in both ferrous and ferric states. Because it forms only in marine settings, geologists treat it as a useful indicator of shallow, slowly accumulating seafloor conditions.
Glauconite is rarely seen as a display specimen; it matters mostly as a rock-forming and economically useful mineral.
Formation & geology
Glauconite forms authigenically (in place) on the seafloor in shallow to moderately deep marine environments where sedimentation is very slow and conditions are mildly reducing. It commonly develops inside fecal pellets, foraminifera shells, and other microcavities on continental shelves.
The process, called glauconitization, alters clay and other precursors into the green iron-potassium silicate over thousands of years. Because it requires slow burial, glauconite-rich beds mark periods of sediment starvation and rising sea level.
Famous greensand deposits occur in New Jersey, England (the Cretaceous Greensand), and parts of Europe and Russia.
How to identify it
Look for small, rounded, dull green grains in a sandy sediment, giving the rock an olive or grayish-green cast. The mineral is very soft (about 2 on the Mohs scale) and can smear or crush easily.
Its green color, earthy luster, and pelletal habit help separate it from chlorite (often platy) and from green serpentine (which is harder and waxy). Unlike malachite or other copper greens, glauconite does not react or show bright color.
In loose greensand it appears as dark green specks mixed with quartz. A streak test gives a greenish to grayish mark. Confirming it usually requires recognizing the marine sedimentary context.
Uses & significance
Glauconite's chief economic use comes from its potassium and its ion-exchange and water-holding properties. Greensand has long been used as a soil conditioner and slow-release potassium fertilizer in agriculture and gardening.
It is also used in water softening and filtration as a natural ion-exchange medium, and historically as a green pigment ("green earth") in paints. Geologists prize it because its potassium content allows radiometric dating of marine sedimentary beds.
It has no gem value and little metaphysical following, but it is valuable as a stratigraphic indicator of ancient sea levels.
Frequently asked questions
What is greensand?
Greensand is a marine sedimentary deposit rich in glauconite grains, which give it a distinctive olive-green color. It is widely used as a soil amendment.
Is glauconite a clay or a mica?
It is structurally a member of the mica group but behaves much like a clay mineral, forming fine green grains rather than large crystals.
Why is glauconite useful for dating rocks?
It contains potassium, which includes a radioactive isotope, so geologists can use potassium-argon dating to estimate the age of glauconite-bearing marine beds.
What gives glauconite its green color?
Its high iron content, present in both ferrous and ferric forms, produces the characteristic dull olive to blackish green.
Does glauconite indicate a particular environment?
Yes. It forms only in marine settings where sedimentation is very slow, so it signals shallow shelf conditions and periods of sediment starvation.
Glauconite guides
In-depth guides for identifying, valuing, and understanding Glauconite.
Glauconite identified by the community
Real specimens identified with Rock Identifier.
