sedimentary
Glauconitic Sandstone (Greensand pebble)
Glauconitic Quartz Sandstone (K(Fe,Al)2(Si,Al)4O10(OH)2)
Hardness: 2-3 (glauconite) to 7 (quartz matrix); Color: Dull olive green to dark green; Luster: Earthy to dull; Crystal structure: Monoclinic (microcrystalline); Cleavage: Perfect basal (under microscope), but appears massive in hand specimen; Specific Gravity: 2.4-2.9.
- Hardness
- 2-3 (glauconite) to 7 (quartz matrix)
- Color
- Dull olive green to dark green
- Luster
- Earthy to dull
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Physical properties
Hardness: 2-3 (glauconite) to 7 (quartz matrix); Color: Dull olive green to dark green; Luster: Earthy to dull; Crystal structure: Monoclinic (microcrystalline); Cleavage: Perfect basal (under microscope), but appears massive in hand specimen; Specific Gravity: 2.4-2.9.
Formation & geological history
Formed in marine shelf environments with slow sedimentation rates and presence of organic matter. Glauconite is an authigenic mineral that typically forms on the ocean floor in oxygen-poor conditions. Most common in Cretaceous and Tertiary geological deposits.
Uses & applications
Historically used as a slow-release potassium and iron fertilizer ('greensand marl'). In industry, it is used as a water softener due to its ion-exchange properties. It is also used in construction as a base material or aggregate.
Geological facts
Glauconite is often a 'fossilized' indicator of an ancient marine environment. The presence of these pebbles often indicates the erosion of an underlying Cretaceous marl or clay bed. It is frequently associated with marine fossils like shark teeth and shells.
Field identification & locations
Identify in the field by its distinct dull green weathered surface and sandy texture. It is commonly found in coastal regions, particularly the Atlantic Coastal Plain of North America and Southern England. Collectors often find them while beachcombing or digging in fossiliferous marl pits.
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