Geology Cafe

Mineral Precipitation

Just as mineral components can dissolve in the weathering and erosion process, they can precipitate when environmental conditions are right. As minerals break down, compounds can separate into their ion and cation components as they dissolve in water. Water from atmospheric precipitation has very low concentrations of dissolve material, but it can become highly enriched in dissolved compounds as it flows across the surface or underground. Changes in physical and chemical conditions (such as changes in temperature, pressure, or addition or removal of dissolved compounds like CO2, and biological activity) can cause minerals to precipitate. In desert environments, water evaporates causing salts to precipitate on the surface of dry lake beds. Underground, water enriched in dissolved components will gradually precipitate to form mineral cements. Cementation is what hardens sediments into sedimentary rocks. Common mineral cements include silica (quartz), calcite, limonite, hematite, and clay minerals.

Biological activity in the oceans causes calcite (calcium carbonate) to precipitate through processes involving excretion and precipitation. Carbonate sediments can accumulate as reefs, stromatolites, or lime mud derived from the reworking of limey skeletal material from algae. When consolidated, cemented, and recrystallized, it can become limestone, or if conditions are right, dolomite. In terrestrial (freshwater) environments, limestone will dissolve and form caverns, and later when the water drains away, travertine (a variety of freshwater limestone) will be deposited in the form of stalagmites, stalactites, and other cavern features, or can form spring deposits and stream terraces if conditions are right.
Click on images for a larger view.
Devils Golf Course Devils Golf Course in Death Valley is a dry lake bed where salts (mostly NaCl - halite) is precipitated as stream water evaporates in the arid basin.
This image shows a vein of calcite that precipitated in a fissure cutting through conglomerate. Calcite filled veins are fairly common in many geologic settings. This exposure is along the coast near Pigeon Point in San Mateo County. These travertine (freshwater limestone) deposits are in Mitchell Caverns in southern California.
These travertine spring deposits are along Stevens Creek Canyon near Saratoga, CA. Trona Towers Tufa towers near Trona, CA. The tufa (a form of travertine or freshwater limestone) precipitated along spring seeps on the bed of an ancient lake that existed during the ice ages. When the lake dried up, the tufa towers were left behind.
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8/28/2012