- 1SETI Institute & NASA-ARC, Mountian View, CA, United States of America (jbishop@seti.org)
- 2Fibernetics LLC, Lititz, PA, United States of America (lane@fibergyro.com)
- 3Planetary Science Institute, Tucson, AZ, United States of America (mdyar@psi.edu)
Phosphate minerals are present in multiple martian meteorites and have been detected on the surface of Mars by several rover missions and have important implications for astrobiology [1]. We initiated a study of the spectral properties of phosphate minerals two decades ago [2] to support identification of phosphates on Mars using Thermal-IR (TIR), Visible/Near-Infrared (VNIR) and Mössbauer spectroscopy and have been updating our collections [3, 4, 5]. Phosphate minerals form in a wide variety of structures built around PO4 tetrahedra, similar to the mineral structures containing SiO4 and SO4 tetrahedra and many of these could be present on Mars. Nanophase materials characterized at Gale crater may also contain phosphates [6].
Primary phosphates that crystallize from a fluid include apatite (Ca5(PO4)3OH) and triphylite (LiFe2+PO4), while strengite (FePO4•2H2O) and vivianite (Fe2+Fe2+2(PO4)2•8H2O) are secondary phosphates that form in low temperatures aqueous environments. Whitlockite (Ca9(MgFe)(PO4)6PO3OH) can be found in chondrites within meteorites. TIR emissivity spectra in the mid-IR region (Figure 1-A, 200-1500 cm-1) are dominated by the vibrational modes of the (PO4)3- tetrahedra including stretching vibrations near 1000-1200 cm-1 and bending vibrations near 600-700 cm-1 [5, 7].
Phosphates exhibit multiple spectral features in the VNIR region (Figure 1-B, 0.3-5 µm) due to vibrations of H2O, OH, and PO4 groups in the structure as well as excitation absorptions due to Fe [e.g., 4]. Fe bands typically occur near 0.6-1.2 µm, OH bands near 1.45, 2.2 and 2.8 µm, H2O bands near 1.45, 1.95, and 2.9-3 µm, and phosphate bands near 4.5-5 µm. Kulanite (BaFe22+Al2(PO4)3(OH)3), childrenite-eosphorite (Fe2+,Mn2+)AlPO4(OH)2·H2O), and gormanite (Fe32+Al4(PO4)4(OH)6·2H2O) are OH-bearing phosphates and their spectra have strong OH bands near 1.44-1.50, 2.17-2.47, and 2.76-2.87 µm, respectively due to an OH stretching overtone, an OH stretch plus bend combination band, and an OH fundamental stretching vibration. Apatite also includes OH and its spectra include a fundamental stretching vibration at 2.83 µm as well as a triplet near 3.37-3.48 µm and a doublet at 3.98 and 4.02 µm.
Mössbauer spectroscopy of ferric and ferrous phosphates provide a range of isomer shifts and quadrupole splitting values that can be used to identify specific minerals [3]. The Mössbauer parameters, TIR spectra, and extended visible region spectra collected by the Mars Exploration Rovers were used to constrain potential ferric phosphate minerals present along with sulfates at Paso Robles in Gusev Crater [8]. We are currently investigating the presence of phosphates at Gusev and Jezero Craters, especially Ca- and Fe-bearing phosphates including vivianite [9].
References: [1] Hausrath et al. (2024) Minerals, 14, 591. [2] Lane et al. (2007) LPSC, #2210. [3] Dyar et al. (2014) American Miner., 99, 914–942. [4] Bishop (2019) Chapter 4, in Remote Compositional Analysis ... (Cambridge) 68-101. [5] Lane & Bishop (2019) Chapter 3, in Remote Compositional Analysis ... (Cambridge) 42-67. [6] Rampe et al. (2016) American Miner., 101, 678-689. [7] Stutman et al. (1965) Trans. NY Academy Sci., 27, 669-675. [8] Lane et al. (2008) American Miner., 93, 728-739. [9] Kizovski et al. (2024) LPSC, #2615.
How to cite: Bishop, J. L., Lane, M. D., and Dyar, M. D.: The Spectroscopic Properties of Phosphates and Identifying Them on Mars, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14637, https://doi.org/10.5194/egusphere-egu25-14637, 2025.