MSA PRESIDENTIAL ADDRESS: “WATER” IN MINERALS IS NOT REALLY WATER

Webster’s dictionary defines water as (among other things) a liquid oxide of hydrogen that freezes at 0° C and boils at 100° C. Minerals containing H₂O are common, including gypsum, epsomite, smectites, zeolites, and many others. The H₂O in minerals is commonly referred to as water, but using the term “water” is misleading and implies that H₂O molecules in minerals have liquid-like properties. Although the properties of H₂O in some minerals can approach the properties of liquid water, such H₂O is very loosely associated with the structure. In general, the properties of H₂O in most mineral structures are quite distinct from those of water. Whereas H₂O molecules in water are hydrogen bonded together, H₂O in minerals typically bonds with cations and/or framework oxygens, with H₂O-H₂O interactions important only in minerals with open structures (e.g., the interlayers of some 2:1 phyllosilicates, some zeolites). H₂O molecules in smectites and zeolites can have a range of bonding interactions, whereas H₂O molecules in, e.g., gypsum or kieserite have only H-O(sulfate) and (H₂)O-cation interactions. Many minerals are thus considered to contain different types of H₂O, a reflection of the different bonding environments around each H₂O molecule. These different bonding configurations produce very different H₂O properties that are commonly studied by calorimetry and thermal analysis. In thermal tests, only the first H₂O molecules to evolve from open-structure minerals (e.g., smectites and zeolites) are close to liquid water in properties. Whereas liquid water vaporizes at ~100ºC at 100 kPa, H₂O in many minerals can persist to temperatures well above 100ºC (e.g., >250ºC in natrolite, >350ºC in kieserite). Further evidence of the distinction between liquid water and H₂O in minerals is seen in the enthalpy of vaporization. The value for water is ~40.7 kJ/mol H₂O, whereas values for many hydrous minerals range above 120 kJ/mol. It is important to recognize in any thermal study that if a mineral exhibits step-wise or multi-stage dehydration, evolution of any H₂O molecules will modify the bonding configuration and energetics of the remaining H₂O molecules. This makes it difficult to obtain an accurate understanding of H₂O bonding configurations in the fully hydrated state.