Abstract
Abstract. It is shown here that the H2O content of hydrous minerals can be determined from their mean refractive indices with high accuracy. This is especially important when only small single crystals are available. Such small crystals are generally not suitable for thermal analyses or for other reliable methods of measuring the amount of H2O. In order to determine the contribution of the H2O molecules to the optical properties, the total electronic polarizability is calculated from the anhydrous part of the chemical composition using the additivity rule for individual electronic polarizabilities of cations and anions. This anhydrous contribution is then compared with the total observed electronic polarizability calculated from the mean refractive index of the hydrous compound using the Anderson–Eggleton relationship. The difference between the two values represents the contribution of H2O. The amount can be derived by solving the equation αcalc=∑iniαicat+∑jαjo×10-NjVm1.2×nj+nW1.2+nw×αW for the number nw of H2O molecules per formula unit (pfu), with the electronic polarizabilities αcat for cations, the values N and αo describing the anion polarizabilities, the number n of cations and anions, and the molar volume Vm, using a value of αW=1.62 Å3 for the electronic polarizability of H2O. The equation is solved numerically, yielding the number nw of H2O molecules per formula unit. The results are compared with the observed H2O content evaluating 157 zeolite-type compounds and 770 non-zeolitic hydrous compounds, showing good agreement. This agreement is expressed by a factor relating the calculated to the observed numbers being close to 1 for the majority of compounds. Zeolites with occluded anionic or neutral species (SO3, SO4, CO2, or CO3) show unusually high deviations between the calculated and observed amount of H2O, indicating that the polarizabilities of these species should be treated differently in zeolites and zeolite-type compounds.
Highlights
The water content of hydrous minerals and synthetic compounds is usually determined by thermogravimetric methods recording the weight loss upon dehydration at an increasing temperature
Whereas the cation content can be derived from microchemical analyses, e.g., by electron microprobe analyses (EMPA) or analytical transmission electron microscopy (ATEM), on species with a size of a few microns, thermoanalytical methods for the determination of the water content require an amount of a sample in the range of a few milligrams
Because the coordination number (CN) of Na is not determined in analcime (Cerný, 1974), a mean value for Na with different coordination numbers (CNs) is used calculated according to α(Na) = (17 · 0.760 + 27 · 0.650 + 207 · 0.560 + 97 · 0.490 + 197 · 0.430 + 49 · 0.380 + 25 · 0.340 + 5 · 0.300 + 9 · 0.270)/633 = 0.489 Å3, with values taken from Table 4 in Shannon and Fischer (2016)
Summary
The water content of hydrous minerals and synthetic compounds is usually determined by thermogravimetric methods recording the weight loss upon dehydration at an increasing temperature. Following the definition of the subcommittee on zeolites of the International Mineralogical Association (Coombs et al, 1998), “A zeolite mineral is a crystalline substance with a structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounding a cation. This framework contains open cavities in the form of channels and cages. These are usually occupied by H2O molecules and extra-framework cations that are commonly exchangeable”
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