1

PYROXENE

M1M1 M2M2

• 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).

2

Pyroxene Structure varies with composition

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.

3

Approach: Control our variables - Synthetic pyroxenes contain only Ca, Mg and Fe2+ in M1 and M2 sites

•  Bulk composition known, actual composition determined by EMP •  Equilibration temperature, pressure and cooling history known •  Mössbauer spectroscopy used to verify oxidation state of iron and quantify

proportion of Fe2+ in M1 and M2 sites •  Modified Gaussian Model used to deconvolve individual crystal field

absorptions

PYROXENE

Pyroxene compositions (synthetic) studied by R. Klima (Ph.D. thesis at Brown).

Note that some compositions fall within the miscibility gap.

4

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.

PYROXENE

MGM derived crystal field absorption bands for M1 and M2 sites move regularly to longer wavelengths with increasing total iron content.

M1 M2

1, 1.2 and 2 µm Bands Move Regularly with Composition

M1

M2M2

5

Ca-Bearing Pyroxenes

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.5 1 1.5 2 2.5

083 Wo51 Fs49082 Wo49 En1 Fs50085 Wo39 En0 Fs61087 Wo29 En0 Fs71088 Wo10 En0 Fs90*061 Wo0 En0 Fe100

Ref

lect

ance

Wavelength (um)

Mg0

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.5 1 1.5 2 2.5

070 Wo45 En14 Fs41066 Wo38 En15 Fs48055 Wo26 En18 Fs56050 Wo23 En19 Fs58053 Wo8 En23 Fs70028 Wo0 En25 Fs75

Ref

lect

ance

Wavelength (um)

Mg25

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0.5 1 1.5 2 2.5

043 Wo49 En45 Fs6075 Wo45 En46 Fs9067 Wo39 En52 Fs9019 Wo16 En69 Fs15010 Wo7 En43 Fs47015 Wo2 En79 Fs19002 Wo0 En80 Fs20

Ref

lect

ance

Wavelength (um)

Mg80

083, 082

001 064 065 002,027

003

017

018

015

022026 004,031034

005 025 023 028 024,020

029 021 061

012009

011013

088

085

087

054049

053

075077

067

071

068

070

066

036039033,044043

079 076

037

073

019

010

051057

058

056050

055

032

078

060,072

007

052038

084

086

089091

080074

069081

047

045 014

008016

048046

PYROXENE

6

R2 = 0.9682

900

950

1000

1050

1100

0 10 20 30 40 50

Wo

1u

m B

an

d C

en

ter

R2 = 0.9602

900

950

1000

1050

0 10 20 30 40 50

Wo

1u

m B

an

d C

en

ter

1 µm Band Moves Regularly with Composition 2 µm Band Moves Regularly until ~Wo30

R2 =

850

900

950

1000

1050

1100

0 10 20 30 40 50

Wo

1u

m B

an

d C

en

ter

0.957

2050

2100

2150

2200

2250

2300

2350

2400

0 10 20 30 40 50

Wo

2u

m B

an

d C

en

ter

2000

2050

2100

2150

2200

2250

2300

2350

0 10 20 30 40 50

Wo

2u

m B

an

d C

en

ter

1800

1900

2000

2100

2200

2300

2400

2500

0 10 20 30 40 50

Wo

2u

m B

an

d C

en

ter

R2 = 0.9682

900

950

1000

1050

1100

0 10 20 30 40 50

Wo

1u

m B

an

d C

en

ter

R2 = 0.9602

900

950

1000

1050

0 10 20 30 40 50

Wo

1u

m B

an

d C

en

ter

1 µm Band Moves Regularly with Composition 2 µm Band Moves Regularly until ~Wo30

R2 =

850

900

950

1000

1050

1100

0 10 20 30 40 50

Wo

1u

m B

an

d C

en

ter

0.957

2050

2100

2150

2200

2250

2300

2350

2400

0 10 20 30 40 50

Wo

2u

m B

an

d C

en

ter

2000

2050

2100

2150

2200

2250

2300

2350

0 10 20 30 40 50

Wo

2u

m B

an

d C

en

ter

1800

1900

2000

2100

2200

2300

2400

2500

0 10 20 30 40 50

Wo

2u

m B

an

d C

en

ter

1 µm band center moves regularly with composition; 2 µm band center moves regularly until Wo~30.

PYROXENE

7

Natural Pyroxenes: 1 and 2 um Bands (Data from Adams, 1974 and Cloutis and Gaffey, 1991)

Wav

elen

gth

(µm

)

1.70 1.80 1.90 2.00 2.10 2.20 2.30 2.400.85

0.90

0.95

1.00

1.05

1.10

Wavelength (µm)

[from Adams, 1974 and Cloutis & Gaffey, 1991]

PYROXENE

Comparison with Natural Pyroxenes

[with data from R. Klima]

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 Fe2+ gives rise to absorption near

4830 cm-1 (~2 µm).

Tetrahedral Cr2+ gives rise to absorption near

6250-6670 cm-1 (~1.5-1.6 µm).

Octahedral Fe2+ gives rise to absorption near

0.93 µm?

9

550 E. A. Cloutis et al.

Fig. 1. Reflectance spectra (0.3–3.3 [a–d] and 3–26 Pm [e–h]) of spinels used in this study (see Table 1). The order of the spectra and verticaloffsets are indicated on the figure.

SPINEL GROUP(XY2O4)

[Fig. 1 from Cloutis et al., 2004]

highest Fehigher Fe

low Fe; weak/no Fe bands @ 0.9 µm

low Fe; info on Fe2+/Fe3+