sedimentary

Glauconite Siltstone

Glauconitic siltstone / K(Fe,Al,Mg)2[(Si,Al)4O10](OH)2

Hardness: 2-3 (Mohs scale), Color: Dull olive green to grayish-green, Luster: Earthy / Dull, Crystal Structure: Monoclinic (as a mineral constituent), Cleavage: Perfect mica-like but rarely seen in rock form, Specific Gravity: 2.4-2.9.

Identified July 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in marine environments, specifically on the continental shelf where slow sedimentation occurs in reducing conditions. Often found in facies indicating transgressive sea levels. Ages vary from Cambrian to recent, frequently associated with the Cretaceous period.

Uses & applications

Historically used as a green pigment (Green Earth) and as a slow-release potassium fertilizer in agriculture (sometimes called 'Greensand'). Also used for water softening because of its ion-exchange properties.

Geological facts

Glauconitic rocks are excellent indicators of ancient marine environments and are used by geologists to determine sedimentation rates; the presence of glauconite suggests very slow deposition.

Field identification & locations

Identify by its characteristic dull green 'dirty' color and crumbly, silty texture. It often leaves a green streak on paper. Found in sedimentary basins globally, such as the New Jersey coastal plain or the Isle of Wight.

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