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Importance of nominally anhydrous minerals in the crust
Why should we be interested in trace hydrous species in nominally anhydrous minerals in the Earth’s crust? After all, hydrous minerals dominate the pedosphere and are abundant to fairly common trace minerals in many metamorphic and igneous crustal rocks. On the other hand, the most abundant minerals in the crust—feldspars, quartz, pyroxenes, and garnet—are all nominally anhydrous. They are present even in systems with low total volatiles or fluid contents, or environments with low water activities where hydrous minerals are unstable. These nominally anhydrous minerals provide an opportunity to expand the extent of our knowledge of fluid composition and water activity, as well as the influence of water on physical properties and geochemical signatures of rocks.
One advantage to investigations of the crustal component of the lithosphere is that many parts of the crust (especially the continental crust) are available for direct study in outcrops at the surface of the Earth. This allows the nominally anhydrous mineral and its hydrous species to be placed into the context of the hand sample, the outcrop, and even the regional geology.
Scope and goals of this chapter
It would be unrealistic to try to cover every water-bearing mineral in the Earth’s crust in this chapter. I have limited my discussion to minerals that do not require hydrous species to complete their stoichiometry, and those for which research has been completed on natural crustal samples. These minerals are: quartz, the feldspars, nepheline, pyroxenes, garnets (except pyrope), kyanite, andalusite, sillimanite, rutile, cassiterite, zircon, titanite, cordierite, and beryl. This selection of minerals restricts the discussion primarily to the continental crust below about 3 km depth. Some references to eclogitic and mantle-wedge minerals are included for completeness.
This is a fairly new field of study, and as such, the goal of this chapter is to give an overview of the work that has …