Mineral hydration

In inorganic chemistry, mineral hydration is a reaction which adds water to the crystal structure of a mineral, usually creating a new mineral, commonly called a hydrate.

In geological terms, the process of mineral hydration is known as retrograde alteration and is a process occurring in retrograde metamorphism. It commonly accompanies metasomatism and is often a feature of wall rock alteration around ore bodies. Hydration of minerals occurs generally in concert with hydrothermal circulation which may be driven by tectonic or igneous activity.

Processes

There are two main ways in which minerals hydrate. One is conversion of an oxide to a double hydroxide, as with the hydration of calcium oxide—CaO—to calcium hydroxide—Ca(OH)2, the other is with the incorporation of water molecules directly into the crystalline structure of a new mineral.[1] The later process is exhibited in the hydration of feldspars to clay minerals, garnet to chlorite, or kyanite to muscovite.

Mineral hydration is also a process in the regolith that results in conversion of silicate minerals into clay minerals.

Some mineral structures, for example, montmorillonite, are capable of including a variable amount of water without significant change to the mineral structure.

Hydration is the mechanism by which hydraulic binders such as Portland cement develop strength. A hydraulic binder is a material that can set and harden submerged in water by forming insoluble products in a hydration reaction. The term hydraulicity or hydraulic activity is indicative of the chemical affinity of the hydration reaction.[2]

Examples of hydrated minerals

Examples of hydrated minerals include:

  • silicates (SiO4−
    4    , SiO
    2    )
  • non-silicates
    • oxides (O2−
      , Al
      2      O
      3      , Fe
      2      O
      3      , etc.) and oxy-hydroxides
      • brucite, Mg(OH)2[1]
      • goethite, FeO(OH)[1]
    • carbonates (CO2−
      3      , etc.)
    • hydroxylated minerals
    • hydroxysulfides (mixed sulfides-hydroxides)
      • tochilinite, a hydroxysulfide or hydrated sulfide[1] mineral of iron(II) and magnesium of chemical formula:
        (Fe2+)5.4(Mg,Fe2+)5S6(OH)10,[3] also written 6 Fe0.9S · 5 (Mg,Fe2+)(OH)2,[4][3] in IMA notation
      • valleriite, an uncommon sulfide-hydroxide mineral of iron(II) and copper of chemical formula:
        (Fe2+,Cu)4(Mg,Al)3S4(OH,O)6,[5] or 4 (Fe,Cu)S · 3 (Mg,Al)(OH)2[6]

See also

References
  1. Rivkin, A.S.; Howell, E.S.; Vilas, F.; Lebofsky, L.A. (2002). "Hydrated Minerals on Asteroids: The Astronomical Record" (PDF). Asteroids III. doi:10.2307/j.ctv1v7zdn4.23. ISBN 9780816522811. Retrieved 2018-03-10. Hydrated minerals include both silicates and nonsilicates in the scope of this review. Phyllosilicates (or "clay minerals") are commonly found on Earth as weathering products of rocks or in hydrothermal systems. Nonsilicate hydrated minerals include such species as the oxides brucite and goethite, the carbonate hydromagnesite, and the sulfide tochilinite, each of which is known in the meteorite collection (Rubin, 1996). Although a full discussion of the petrogenesis and classification of hydrated minerals is beyond the scope of this paper, we note that formation of hydrated minerals, particularly clay minerals, occurs rapidly and easily in environments where anhydrous rock and water are together.
  2. Snellings, R.; Mertens G.; Elsen J. (2012). "Supplementary cementitious materials". Reviews in Mineralogy and Geochemistry. 74 (1): 211–278. Bibcode:2012RvMG...74..211S. doi:10.2138/rmg.2012.74.6.
  3. mindat.org (6 February 2023). "Tochilinite". mindat.org. Retrieved 16 February 2023.
  4. Handbook of mineralogy (2005). "Tochilinite" (PDF). handbookofmineralogy.org. Retrieved 16 February 2023.
  5. Valleriite on Mindat.org
  6. Handbook of Mineralogy
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