4.2.2: Aqueous Minerals

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4.14.jpg — 4.14 Forming sylvite from dissolved K and Cl

A similar process – similar to crystallization from magma – occurs when minerals precipitate from water to form aqueous minerals. Aqueous means involving water. Because water may contain ions of many different sorts, a number of different aqueous minerals are possible.

Precipitation depends on a number of factors. As long as kinetic energy is high, or water is not saturated, no crystals will form. Ions will bond temporarily, only to break apart and return to solution. This can change, however, if conditions change.

Most substances are more soluble in water at high temperature than at low temperature. So, a decrease in temperature may lead to oversaturation, nucleation, and precipitation of minerals. During this process, unbonded dissolved ions become organized in a crystal structure. For example, K⁺ and Cl^– may combine to form the mineral sylvite (KCl). And, similarly, calcium carbonate precipitates to form calcite (CaCO₃) if concentrations of Ca²⁺ and CO₃²^- in water are high enough.

Crystal precipitation from water may also occur because of chemical change. Suppose, for example, that seawater evaporates. The concentration of dissolved material in the remaining water will increase, leading to oversaturation and, eventually, precipitation of crystals. Besides changes in temperature and composition, changes in pressure, pH, or other things may also lead to the formation of aqueous crystals. The photos below show two examples: halite and gypsum deposits in Utah.

4.15 Halite deposits at the Great Salt Lake, Utah

Inland lakes or seas commonly precipitate calcite, halite, gypsum, and other minerals. In some places evaporating waters have deposited salt beds thicker than 300 m. Figure 4.15 shows salt that was deposited by Utah’s Great Salt Lake. Similar deposits are found along the shores of The Dead Sea (between Jordan and Israel) and other seas and lakes in arid regions. On a much smaller scale, minerals precipitating from slowly moving groundwater can fill holes, fractures, and cracks in rocks. Figure 4.16 shows veins of gypsum in Utah’s Moenkopi Formation. Gypsum is commonly associated with red sandstones and mudstones like the rock seen in this photo.

The table below lists some typical aqueous minerals. The most common are minerals with high solubility in water, such as calcite, halite, and other salts.

Common Aqueous Minerals
mineral class or group	important minerals or mineral series	chemical formula
silica	quartz	SiO₂
carbonate	calcite dolomite magnesite	CaCO₃ CaMg(CO₃)₂ MgCO₃
halide	halite sylvite	NaCl KCl
sulfide	gypsum anhydrite	CaSO₄•2H₂O CaSO₄
native element	sulfur	S

4.17 A geode with amethyst and white quartz

Other minerals, having lower solubility but composed of elements in great abundance, slowly form from aqueous solutions. Quartz is an example. At low temperature, quartz may precipitate in geodes such as the one seen here. This geode contains amethyst, a purple variety of quartz, and also more common clear white quartz. Note the concentric layers in this specimen. The layers have slightly different compositions because the composition of the water changed a bit as crystallization occurred.