Laboratory Synthesis And Spectroscopic Studies Of Martian Jarosite

The chemical,structural and spectral properties of hydrated sulfate minerals are essential for their potential identifications and better understanding of Martian aqueous history.Jarosite has been definitely identified on Mars by in situ detection,orbital remote sensing and Martian meteorite analyses.Terrestrial jarosite minerals are commonly associated with acid and S-rich drainages(e.g.the S-rich rock weathering or acid mine drainage).Therefore,the discovery of Martian jarosite suggests that potential acidic and aqueous conditions has been prevalent at certain locations in Martian history.However,laboratory spectroscopic studies of jarosite are not detailed enough for identifying the chemical compositions of jarosite by in-situ detection(e.g.Raman).In addition,the remote sensing data are discussed in terms of single mineral,although these deposits undoubtedly contain various minerals.Here we focus on the laboratory synthesis and spectroscopic studies of K-Na-H30 jarosite solid solution series,which may exist on current Mars.The detailed spectra properties of jarosite samples would provide a valuable database for current orbital remote sensing detections(e.g.CRISM)and future in situ Raman investigations on Mars(e.g.ExoMars and Mars 2020).The detailed studies of jarosite were carried out as follows:1)Jarosite Solid Solutions StudiesWe successfully synthesized 25 homogeneous jarosite samples(including 22 jaosite solid solutions(JSS)and 3 endmembers),whose phase identifications and chemical compositions were confirmed by X-ray diffraction(XRD)and an energy dispersive spectrometer(EDS)on scanning electron microscopy(SEM).Correlated XRD,Raman,MIR,and VNIR studies of jarosite samples show consistent results,indicating systematic spectral variations due to their chemical and structural changes.We derived two sets of formulas using Raman spectra and two sets using VNIR spectra for the estimation of alkali content(i.e.,K/(K+Na+H3O)ratio)in jarosite.2)Jarosite Mixtures StudiesWe firstly mixed ternate jarosite endmembers(K-Jar,Na-Jar and H3O-Jar)at different molar ratios,which were similar with molar ratios of monovalent cations in corresponding JSS.22 jarosite physical mixtures(JPM)were obtained and studied spectral characteristics(XRD,Raman,VNIR)between JSS and JPM.We found that XRD patterns of JPM exhibited more overlapped peaks than JSS arising from physical mixing effects of endmembers,especially in the region of 28?30°.Raman spectra of JPM exhibited a broad peak consist of peaks of endmembers,and JSS showed continuous systematic Raman peak shifts varying with chemical compositions.For VNIR spectra in 2100?2300 nm and 2300-2500 nm,JPM with middle H3O content(30?60%)showed more observable doublet features than JSS,which is contrary to JPM and JSS with low H3O content(10?25%)and high H3O content(70-80%).Then we bought smectite,kaolinite and gypsum.Alunite and basanite were synthesized in out lab.We mixed them with K-Jar in different molar ratios(1:3,2:2 and 3:1).We analyzed VNIR spectra of all 15 mixed samples and 6 endmembers,and found that the band depth(BD1)of all mixed samples at?2270 nm increased with increasing jarosite.The band depth(BD2)of jarosite and smectite mixtures(JS),jarosite and gypsum mixtures(JG),jarosite and basanite mixtures(JBA)at?2210 nm had similar tendency,but BD2 of jarosite and alunite mixtures(JA)exhibited an opposite tendency.We derived several formulas for the estimation of jarosite percentage in mixtures.3)Primary Applications on Martian Remote SensingWe selected a spectrum from western of Jezero Crater(a candidate landing site of Mars 2020),which was interpreted as jarosite.In terms of the constraint of out experiments,the jarosite may be solid solution with Na,H3O and may K,instead of physical mixture of endmembers.The content H3O may be 1.6 times higher than Na and 2.5 times higher than K.However,mixtures of jarosite and smectite with 25?50 wt%of jarosite may be another candidate.