I am interested in clay minerals formed in plutonic rocks (mainly granites) prior to obvious weatherig processes. If we look at wet analyses, there is mostly 1 wt. % "H2O+" (even in samples from boreholes >100 m below the surface). Taking into account the amount of micas (and, if present, other nominally hydrous minerals), it is usually not enough for > 0.5 % H2O+. I could refer to own publications about clay mineral formation by radiochemical alteration, but this is still very small volume and not rule everywhere. So - where is the water: in submicroscopic secondary minerals, in fluid inclusions, other form in anhydrous minerals (e.g., water complex in quartz according to N.G.Stenina...), or strongly adsorbed water which had been in pores? Many thanks for answers and ideas!
Interesting question. Read the following literature - useful to understand the water in granites.
https://www.sciencedirect.com/science/article/pii/S0169136812000662
Best wishes
Hydrous minerals in granitic environment include major minerals frequently present in the mineral composition of granites and other minerals that may exist based on the type of granite. There is a big list of these minerals that could cover the H2O amount you mentioned. These include: Biotite ( around 3.7 % H2O), muscovite (around 4 % H2O), hornblende ( around 2.2 % H2O), Riebeckite (around 2 % H2O), chlorite (around 10% H2O), Epidote (around 1.8 % H2O), Kaolinite (around 14 % H2O), illite (around 12 % H2O), Montmorillonite (around 36 % H2O). Chlorite and clay minerals particularly increase H2O content
Question is very Interesting, as anybody will wonder how an igneous rock formed out of very high temperature will have minerals with water content. However, the answer given by Dr. Yehia is very convincing and appreciated.
Hydrothermal alteration leads to varied degree alteration of rock-forming minerals, that could raise water content of granite rocks. For example, it is common that plagioclase displays sericitic, K-and feldspar clay alteration. These hydrous minerals may elevate water contents of the rocks.
Xue-Ming Yang
It might be helpful to consider the uncertainty in 1 wt. % "H2O+" measurements given several associated errors in wet chemical techniques. If "H2O+" content is still beyond the uncertainty range, then fluid inclusions in anhydrous minerals, adsorbed water etc. (as you mentioned in your question) are good candidates as OH-bearing minerals are phyllosilicates that generally regarded as secondary minerals.
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