Rocks can contain water in several ways, including within their mineral structures, in fractures and pores, and as hydrated minerals.
Water gets into rocks through various processes:
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Within the Mineral Structure: Some minerals incorporate water molecules or hydroxide (OH-) ions into their crystal lattice during formation. This water is chemically bound and is a fundamental part of the mineral's composition. Examples include clay minerals, micas, and some amphiboles.
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In Fractures and Pores: Rocks often have tiny cracks (fractures) and spaces (pores) between mineral grains. Water can seep into these openings and be held there by capillary forces. The amount of water depends on the rock's porosity (the amount of pore space) and permeability (the interconnectedness of the pores).
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Hydrated Minerals: Some minerals are formed through chemical reactions with water, creating hydrated minerals. These minerals contain water molecules as part of their structure. For example, anhydrite (CaSO4) can react with water to form gypsum (CaSO4ยท2H2O).
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Weathering: Chemical weathering processes can break down rock-forming minerals and create secondary minerals that incorporate water into their structure (e.g., the formation of clay minerals from feldspars).
In summary, rocks hold water due to the water being part of the mineral's composition, being trapped in cracks and pores, or being chemically bound to the rock through hydration reactions.
