Greensand

Greensand is a marine sedimentary rock or sand rich in the green mineral glauconite, an iron-potassium silicate that gives the rock its characteristic dull green to greenish-grey color. It is typically a sandstone, sometimes loose and sometimes weakly cemented, and weathers to rusty brown as the iron in glauconite oxidizes.

Greensand forms slowly on continental shelves under specific conditions and is an indicator of marine deposition with low sediment input. The green grains are often rounded pellets that can replace fossils and fecal material on the sea floor.

It has practical value chiefly as a natural soil amendment and water-treatment medium.

Greensand forms on continental shelves in shallow to moderately deep marine settings where sediment accumulates very slowly under mildly reducing, low-oxygen conditions. Glauconite develops authigenically (in place) within the sediment, growing in the pore spaces of muds and as pellets, often within microfossil chambers and fecal pellets, over thousands of years.

These conditions of slow deposition far from heavy clastic input are typical of transgressive seas. Well-known greensand deposits occur in the Cretaceous of New Jersey in the United States and in England (the Upper and Lower Greensand formations of southern England).

Greensand is recognized by its distinctive green to greenish-grey color in fresh material, often speckled with darker green glauconite grains, weathering to brown or rusty tones at exposed surfaces.

It is typically a fine to medium sandstone, sometimes loose and friable, sometimes weakly cemented. The glauconite grains are soft and earthy. It can be confused with other green rocks such as serpentinite or chlorite-rich rocks, but its sandy, granular texture and association with marine fossils are distinctive. The green color combined with rounded sand grains and a marine setting is the best identifier.

Greensand is widely used in agriculture and horticulture as a natural soil conditioner, valued for slowly releasing potassium and trace minerals and for improving soil structure and moisture retention. It is sold as a fertilizer additive and is popular in organic gardening.

Glauconite greensand is also used in water treatment, where it helps remove iron, manganese and hydrogen sulfide from well water (often as manganese greensand). Historically greensand has been used as a source of potassium. It has no gemstone or significant metaphysical use, but is of stratigraphic interest as a marker of marine conditions.