Synthesis and Spectral Characterization of Oh - Bearing Ferric Sulfates

2514 for XXXXIII Lunar and Planetary Science Conference (2012, Houston, TX) SYNTHESIS AND SPECTRAL CHARACTERIZATION OF OH-BEARING FERRIC SULFATES. Yanli Lu and Alian Wang, Dept. of Earth & Planetary Sciences, McDonnell Center for Space Sciences, Washington University in St. Louis ( [email protected], [email protected] ) Ferric sulfates on Mars: Among the Martian sulfates, Caand Mg-sulfates were observed with wide spatial distributions and large quantities [1], while Fesulfates were found only in localized areas [2], by orbital remote sensing (OMEGA and CRISM observations). In contrast, Mg-, Ca-, and Fe-sulfates were identified with more mineralogical detail during the MER missions [3-6]. Especially, the dehydration of ferric sulfates excavated from subsurface was implied on the basis of a set of seven consecutive Pancam observations at Gusev [7]. Laboratory investigation of the fundamental properties of ferric sulfates: Knowledge on the fundamental properties of sulfates, such as stability field, phase transition pathways, reaction rates, can greatly enhance the understanding of mission observations and Mars hydrologic evolution. For ferric sulfates, a few early studies [8, 9] investigated the solubility relationships of minerals in the Fe2O3-SO3-H2O system, but only in 200 °C-50 °C temperature range. Recently, Xu et al. [10] studied the rehydration of two anhydrous ferric sulfates at 21 °C; Wang et al [11] studies the stability fields and phase transition pathways of five hydrous ferric sulfates at 50 °C, 21 °C, and 5 °C. Kong et al. [12] defined the first phase boundary below 50 °C between two normal ferric sulfates. The ferric sulfates studied in these recent works belong to normal [Fe2(SO4)3·xH2O], acidic [FeH(SO4)2·4H2O], and slightly basic [Fe4.67(SO4)6(OH)2·20H2O] types. We report here our attempt to start the study of two major types of basic ferric sulfates: jarosite group and fibroferrite-butlerite group. Synthesis of basic ferric sulfates: We have successfully synthesized three jarosites KFe3(SO4)2(OH)6, NaFe3(SO4)2(OH)6, and H3OFe3(SO4)2(OH)6, a dehydrated form of butlerite, Fe(OH)SO4, and a normal paracoquimbite Fe2(SO4)3·9H2O. The synthesis of jarosites followed the procedure presented by Dutrizac and Kaiman [13]. K-jarosite was precipitated from a solution of 30g/l KNO3, 35g/l Fe2(SO4)3·5H2O and 0.01 M H2SO4. That solution was heated to 88-92 °C in air for 1.5-2.5 hours with constant agitation. The precipitated yellowish powder was washed with DI water, then dried in air first then in a 110 °C oven for < 5 min. XRD measurement confirmed its identity as K-jarosite (Fig. 1). The Na-jarosite (Fig.1) was prepared in the same way, but using 64g/l NaSO4 and 43.57g/l Fe2(SO4)3· 5H2O instead. We found that once the yellowish powder exposed to air after washing with DI water, a color change (to orange) is very easy to happen. The dry process at 110 °C in an oven can also cause a similar color change. The spectra of synthesized Na-jarosite was compared with those of a commercial product whose ID (Na-jarosite) was confirmed by XRD. The hydronium jarosite (Fig. 1) was synthesized in a very different way. An aqueous solution Fe2(SO4)3· 5H2O (~ 3.9375 g in 60ml solution) was put into a Teflon cup, which was then placed into a Parr bump with spring-loaded pressure cap (thus air-tight). This Parr bump was stored in an oven at 140 °C. After 2-2.5 days, a dark yellowish powdery precipitate formed. In several batches of products, black or red colored grains were seen at the edge of precipitated yellowish powder, stick to the wall of Teflon cup. To separate them, the yellowish powdery precipitate was washed out using DI water, then dried in an oven at 110 °C. XRD measurement confirmed its ID as hydronium jarosite. A dehydrated form (Fig. 1) of butlerite Fe(OH) SO4·2H2O was formed from a saturated aqueous solution of Fe2(SO4)3·5H2O. The mixture of solution & solid was put into a ultrasonic bath for 40-60 minutes until the disappearance of all solid Fe2(SO4)3·5H2O. The solution was then placed into a Teflon cup that was sealed with spring loaded pressure cap of a Parr bump. The bump was placed in an oven at 120 °C. The synthesis goes very slow. An orange colored solid precipitate only formed after 1-2 months. The solid was washed with DI water and dried at room T. XRD measurement confirmed its ID as Fe(OH)SO4. Na-jarosite