MECC 2026

The 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 smectites, zeolites, gypsum, epsomite, and many others. Unfortunately, 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. The properties of H₂O in some minerals can approach the properties of liquid water, but 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 or ice. Whereas H₂O molecules in water are hydrogen bonded together and are mobile, H₂O molecules in minerals typically bond with cations and/or framework or layer oxygens, and H₂O-H₂O interactions are 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, 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). Additional 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. Values for smectites range from ~45 kJ/mol H₂O to >70 kJ/mol. It is crucial 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 using thermoanalytical methods.

These differences in H₂O properties in hydrous minerals have profound implications for the behavior of these minerals at elevated temperatures and in low relative-humidity environments, and they indicate that smectites and zeolites can remain in a hydrated state on the surface of Mars. For example, interactions between H₂O, the silicate sheet, and interlayer (IL) cations determine the nature of expansion (IL cation hydration) and contraction (layer-layer and layer-IL cation interactions) in smectites. Calculations using available thermodynamic data suggest that Na-smectites can remain hydrated in the martian subsurface. It is particularly interesting that Ca-smectites may remain hydrated on the martian surface, participate in the diurnal H₂O cycle, and act as near-surface reservoirs of H₂O. Similar thermodynamic data are available for several zeolites, and calculations illustrate that clinoptilolite and chabazite, two common zeolite minerals, will likely be hydrated if they exist on the surface/subsurface of Mars. Although the most loosely held H₂O molecules in zeolite structures will be absent on the martian surface, more strongly held H₂O molecules will participate in the diurnal H₂O cycle, and the zeolites can act as reservoirs of H₂O.

These details of H₂O in minerals can also have important implications for terrestrial applications. The large differences in enthalpy of vaporization between H₂O in minerals and liquid water mean that it is imperative to consider the specific thermodynamic values in any computations in thermal modeling of geologic bodies. For example, thermal simulations of a heated body of zeolite-rich tuff (pertinent to Yucca Mountain, Nevada) show that ignoring these differences causes an overestimation of peak temperatures >100°C. The fact that H₂O in minerals is NOT water also means that H₂O in minerals does NOT evolve at 100ºC. It is crucial to remember that H₂O in minerals is not a passive participant in mineral structures. For example, hydration of extraframework cations in zeolite structures can give they hydrated zeolite true thermodynamic stability.