PYROXENE. Note change in coordination of M2 site for different pyroxene compositions.

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1 PYROXENE M2 M1 M2 M1 SiO4 tetrahedra and 2 distinct octahedral sites (M1 and M2). M1 is smaller than M2. In orthorhombic px, M1 is octahedron and M2 is distorted octahedron. In monoclinic px, M1 is octahedron and M2 is up to 8-coordination (highly distorted). 1

2 PYROXENE Note change in coordination of M2 site for different pyroxene compositions. Fe2+ prefers the M2 site, as does Ca, whereas Mg typically goes into the smaller M1 site. 2

3 PYROXENE mposition known, actual composition determined by Pyroxene compositions (synthetic) studied by R. Klima (Ph.D. thesis at Brown). Note that some compositions fall within the miscibility gap. 3

4 1, 1.2 and 2 µm PYROXENE Bands Move Regularly with Composition M1 M2 M1 MGM derived crystal field M2 absorption bands for M1 and M2 sites move regularly to longer wavelengths with increasing total iron content. M2 Reflectance spectra of Ca-free pyroxenes showing strong, broad crystal field splitting absorptions due to the presence of Fe2+. Note how the wavelength position of the band minimum shifts with composition. Also note the increase in the strength of the M1 band at ~1.25 µm with increasing Fe. 4

5 PYROXENE Ca-Bearing Pyroxenes Wo49 En45 Fs6 075 Wo45 En46 Fs9 067 Wo39 En52 Fs9 019 Wo16 En69 Fs Wo7 En43 Fs Wo2 En79 Fs Wo0 En80 Fs , Mg , , , Wo45 En14 Fs Wo38 En15 Fs Wo26 En Fs Wo23 En19 Fs Wo8 En23 Fs Wo0 En25 Fs Mg25 083, , Wo51 Fs Wo49 En1 Fs Wo39 En0 Fs Wo29 En0 Fs Wo10 En0 Fs90* 061 Wo0 En0 Fe100 Mg Reflectance Reflectance Reflectance Wavelength (um) Wavelength (um) Wavelength (um) 5

6 PYROXENE um Band Center R 2 = Wo 2um Band Center Wo um Band Center R 2 = Wo 2um Band Center Wo um Band Center R 2 = Wo 2um Band Center Wo 1 µm band center moves regularly with composition; 2 µm band center moves regularly until Wo~30. 6

7 PYROXENE Wavelength (µm) Wavelength (µm) [from Adams, 1974 and Cloutis & Gaffey, 1991] [with data from R. Klima] 7

8 SPINEL GROUP (XY2O4) Spinel: (MgAl2O4) Magnetite: (Fe3O4) normal : One +2 cation in X site and two +3 cations in Y site inverse : One +3 cation in X site and mixed (+2,+3) cations in Y site Tetrahedral X or A site. Octahedral Y or B site. Tetrahedral Fe 2+ gives rise to absorption near 4830 cm -1 (~2 µm). Tetrahedral Cr 2+ gives rise to absorption near cm -1 (~ µm). Octahedral Fe 2+ gives rise to absorption near 0.93 µm? 8

9 SPINEL GROUP (XY2O4) low Fe; info on Fe 2+ /Fe 3+ low Fe; weak/no Fe 0.9 µm higher Fe highest Fe [Fig. 1 from Cloutis et al., 2004] 9

10 Fe-Oxides Reflectance spectra of Fe-oxides. Goethite and hematite exhibit Fe3+ (spin forbidden) absorptions near µm. Magnetite is very dark and is dominated by first-surface reflection, though note the weak, broad band near 1.1 µm. 10

11 Carbonates Dolomite Fe2+ Reflectance spectra of carbonate minerals showing overtones of CO3 vibrations near 2.3 µm (and short edge of 2.5 µm band). The position of the CO3 vibrations will depend on the cation in the mineral (e.g., Mg, Ca, Fe2+). The cations are in octahedral coordination. 11

12 Clay Minerals Al-OH Al/Mg-OH Fe-OH Al-OH Reflectance spectra of clay minerals showing OH (1.4 & µm) and H2O (1.4 and 1.9 µm) vibrational absorptions. Clay minerals that vary from Al to Fe to Mg-rich varieties exhibit metal-oh bands that vary from 2.2 to 2.32 µm, respectively. The Al, Mg, or Fe is in octahedral coordination whereas Si (and sometimes Al3+ and Fe3+) are in tet. coord. 12

13 Reflectance spectra of Norway Maple leaves as they change color during autumn. Note the increase in reflectance at short wavelengths due to the loss of chlorophyll, as well as the increase in the woody absorption features (cellulose and lignin) at longer wavelengths once chlorophyll production has ceased and water has been lost. 13