Calcrete

Calcrete is a sedimentary duricrust formed when calcium carbonate cements soil, sand, and gravel into a hardened layer near the surface. It is essentially a soil-zone carbonate accumulation, also widely known as caliche in the southwestern United States and Latin America.

Calcrete develops in arid and semi-arid climates where evaporation exceeds rainfall. It forms pale, often nodular or massive crusts that can be loosely cemented or rock-hard. Calcrete is important in soils, in some regions hosts uranium ore, and is mined locally for lime and aggregate.

Calcrete forms when calcium carbonate dissolved in soil water precipitates in the subsurface. In dry climates, water carrying dissolved calcium moves downward after rain, then is drawn back up and evaporates, leaving carbonate behind in the soil profile.

Over thousands of years this carbonate accumulates as coatings, nodules, and eventually a continuous cemented horizon. With prolonged development it can build thick, massive, laminated layers. Plant roots, evaporation, and fluctuating groundwater all contribute. Where groundwater is the source, valley-floor calcretes can become extensive and may concentrate uranium minerals.

Calcrete is pale, typically white, cream, or buff, and often shows a nodular, lumpy, or laminated structure. It is soft (around 3) and fizzes vigorously in dilute hydrochloric acid or vinegar because it is calcium carbonate.

It occurs as a near-surface crust or hardpan in soils of dry regions rather than as a normal bedded rock. Streak is white.

The key contrast is with silcrete, which is silica-cemented, very hard, and does not react with acid. Calcrete is soft and effervesces. Ordinary limestone fizzes too but forms in marine settings and usually lacks the soil-nodule fabric.

Calcrete is used locally as a building material, road base, and aggregate, and is burned to make lime. In agriculture, hard caliche hardpans can impede roots and drainage, so they are sometimes broken up for farming.

Geologically, calcrete is a valuable indicator of past arid climates and stable land surfaces. Calcrete-hosted deposits are an economically significant source of uranium in places such as Western Australia and Namibia.