Quantification of Structural H 2 O and Total H 2 O Contents in Iron Sulfate Minerals Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Introduction: Fe-sulfates with OH &/or H2O are found in martian meteorites [e.g. 1] & on the martian surface [e.g. 2]. These minerals may provide evidence for past surface environments & potential habitability, [e.g. 3]. Several studies have characterized Fe-sulfates using spectral features [e.g., 4-6]. However, unique identification of Fe-sulfates is hampered by the occurrence of mineral assemblages with several Fe-sulfates, inherent variations in the H contents of some Fesulfates (e.g. schwertmannite, Table 1) & phase transitions that may occur as a function of temperature & relative humidity [7]. To better characterize Fe-sulfates – in martian samples (meteorites or returned samples) & remotely collected data – it is necessary to quantify their H2O & OH contents. Furthermore, if calibration factors were known for H-bearing minerals they could be used to evaluate the bulk H content of the martian surface using remotely sensed IR data. We examined laboratory biconical diffuse reflectance spectra of H-bearing Fesulfates to calibrate wt.% H2O & total H2O (H2O+OH) contents using small masses of Fe-sulfates (<5 mg). We applied Kubelka-Munk (K-M) theory [e.g. 8] & the normalized optical path length (NOPL) method [911] because both methods may be used in either laboratory or remote sensing applications. Background: K-M theory is valid when the absorption coefficient & scattering centers are small [e.g. 8] & it uses the remission function: ∞ ∞ − ≡ ∞ R R R f 2 / ) 1 ( ) ( 2 (1) where R∞ is the reflectance from an infinitely thick sample. K-M band heights vary linearly with concentration. NOPL utilizes Beer’s Law as an approximation for describing absorption in a reflectance spectrum. This parameter is calculated by [ ] [ ) ( 1 ln / ) ( 1 ) ( ln ] λ λ λ λ c c R R R NOPL − − + = (2)