Determination of the H 2O content in minerals, especially zeolites, from their refractive indices based on mean electronic polarizabilities of cations

Fischer, Reinhard X.; Burianek, Manfred; Shannon, Robert D.

It is shown here that the inline-formulaH2O 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 inline-formulaH2O. In order to determine the contribution of the inline-formulaH2O 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 inline-formulaH2O. The amount can be derived by solving the equation inline-formula M7inlinescrollmathml italic α normal calc = false ∑ i n i italic α i normal cat + false ∑ j )( italic α j o × normal 10 - ][ N j V normal m normal 1.2 × () n j + n W normal 1.2 + n w × italic α W 284pt42ptsvg-formulamathimg90bc69a673f8ceffe4d00e7e96a5f7e7 ejm-32-27-2020-ie00001.svg284pt42ptejm-32-27-2020-ie00001.png for the number inline-formulanw of inline-formulaH2O molecules per formula unit (pfu), with the electronic polarizabilities inline-formulaαcat for cations, the values inline-formulaN and inline-formulaαo describing the anion polarizabilities, the number inline-formulan of cations and anions, and the molar volume inline-formulaVm, using a value of inline-formulaαW=1.62 Åinline-formula3 for the electronic polarizability of inline-formulaH2O. The equation is solved numerically, yielding the number inline-formulanw of inline-formulaH2O molecules per formula unit. The results are compared with the observed inline-formulaH2O 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 (inline-formulaSO3, inline-formulaSO4, inline-formulaCO2, or inline-formulaCO3) show unusually high deviations between the calculated and observed amount of inline-formulaH2O, indicating that the polarizabilities of these species should be treated differently in zeolites and zeolite-type compounds.

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Fischer, Reinhard X. / Burianek, Manfred / Shannon, Robert D.: Determination of the H2O content in minerals, especially zeolites, from their refractive indices based on mean electronic polarizabilities of cations. 2020. Copernicus Publications.

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