Post by 1dave on Apr 27, 2015 14:21:49 GMT -5
I was just reading a new article about zircons - used for identifying the age of rocks.
My Bad. about Spinels. I'm thinking about too many things. Thanks Jan - gingerkid.
So we really don't even know how or when they are formed before they get to earth.
My Bad. about Spinels. I'm thinking about too many things. Thanks Jan - gingerkid.
UNEQUILIBRATED SPINELS IN STARDUST TRACK C2062,2,162 (CECIL).
Zack Gainsforth, Anna Butterworth, An-drew J. Westphal,
Space Sciences Laboratory, University of California at Berkeley, Berkeley CA 94720, USA
Introduction
Spinels found in comet Wild 2 samples returned by the Stardust mission[1] have multiple formation pathways. In type II chondrule objects, spinels can crystallize from liquidus and can be found embedded within other olivines or as standalone crystallites in the chondrule [1, 2]. On the other hand, magnetites in Track 80 probably formed via an aqueous pathway[4]. Magnesiospinel is also thought to form via a condensation pathway (e.g. [5]). While most condensation calculations do not predict Fe-,Cr-rich spinels,they occur nevertheless as presolar grains[7] suggesting that a condensation pathway for Fe-, Cr-rich spinel exists.Because many spinels in unequilibrated assemblages do not have clear relationships to the phases around them, petro-graphic work becomes difficult or impossible in many cases.Therefore, understanding their history requires obtaining all the necessary information from the crystal itself. Luckily,spinel is especially well suited to this since it contains multiple crystalline lattice sites which can accommodate Ti, V, and Fein multiple oxidation states. Because of this, spinel can be a sensitive indicator of oxygen fugacity over a range of more than 15 log units[9], and can even record changing fugacities during the formation/alteration of a single grain[8]. However, in Stardust, the spinels are nm in size, requir-ing quantitative analysis to be done largely with Transmission Electron Microscopy(TEM)andsoftX-raytechniques. Whilehard X-ray barometers have been used for some time on mete-oritic materials, the same calibrations do not translate directlyto soft X-ray or TEM. Therefore, we are still in the early days of characterizing nano-spinels from Stardust as we simultaneously acquire measurements and develop progressively more robust calibrations to understand the measurements. Here we show progress in analyzing oxidation state and site occupancy of cations in Stardust nano-spinels.
Experimental
Cecil is terminal particle from Stardust track C2062,2,162 which was embedded in epoxy and ultramicro to medonto TEMgrids. TEM analysis was done at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory using an FEI Titan TEM at 80 keV with a 0.6 sr EDS detector, and a Zeiss Libra 200 MC at 200 keV with an in-column Omega energy filter for imaging and diffraction. EDS map-pingwasdoneatabout30kcpsfortwohourstoacquireastack with 2.3
·
10
8
counts.Atom Location by Channeling Enhanced Microanalysis(ALCHEMI) is a technique for isolating the crystallographicsitesforatoms[11]. Itcanbeused, forinstance, todifferentiatebetween the octahedral and tetrahedral sites in spinel, andworksbycausingtheelectronwaveinaTEMtoconstructivelyor destructively interfere on specific crystallographic sites bycarefully tilting the electron beam or sample. We acquired2D ALCHEMI near the spinel 111 zone axis by tilting thebeam axis and acquiring a twenty second EDS spectrum ateach beam tilt from -3 to +3 nm
−
1
on two orthogonal tilt axes.The number of counts in each peak of each spectrum was fitusing a Python script and used to generate ratio images suchas shown in Fig. 2. The patterns can be used to statisticallyquantify site occupancy of the various cations[10].The ultramicrotomed slices were also imaged using Scan-ning Transmission X-ray Microscopy (STXM) at the Ad-vanced Light Source (ALS) in Berkeley, CA to acquire Fe-Land Ti-L edge XANES. Fe
3+
/
P
Fe was computed using thecalibration of van Aken[6], though it is known that crystal-lographic effects can lead to large errors. For example, Fig.2c of van Aken[6] shows spectra for two spinels, both withFe
3+
/
P
Fe = 2/3, yet with different XANES. We note thatwhile a better calibration is needed, in the standards we haveinvestigated, the van Aken calibration has always worked withan accuracy better than 30%. We did not attempt to quantifythe Ti oxidation state since existing calibrations did not givecorrect results on a chromite standard.
Observations
Cecil is an unequilibrated assemblage containing a likelyigneous object alongside primitive, possibly nebular nano-sulfides[3], olivines, pyroxenes and spinels embedded in aglassy matrix. The spinels are mostly <= 200 nm in size andvaried in composition.One spinel was embedded in SiO
2
-rich material, 200 nmin diameter and comprised several euhedral grains orientedwithin a few degrees of each other based on electron diffrac-tion. TEM/EDS shows an average composition in oxide wt %of TiO
2
= 3.52, Al
2
O
3
=18.08, Cr
2
O
3
= 40.25, V
2
O
3
= 1.37,FeO = 31.33, MgO = 4.38, MnO = 0.79. However, the crystalis highly inhomogenous. In Fig. 1, the blue band correspondstoaTi-,Fe-,Cr-rich,Al-poorregionwithcompositioninoxidewt % of TiO
2
= 4.64, Al
2
O
3
=11.97, Cr
2
O
3
= 43.43, V
2
O
3
=1.54, FeO = 33.46, MgO = 3.79, MnO = 0.83. The red portionin the upper right corresponds to composition in oxide wt %Figure 1: (A) EDS map and (B) TEM brightfield image of aspinel in Cecil surrounded by SiO
2
-rich amorphous material.EDS colors are Ti (cyan), Fe (magenta), Al (yellow) show-ing extreme heterogeneity within the spinel. The brightfield isalong the 110 zone axis, tilted 37 degrees from the EDS map– the presence of multiple crystal domains can be seen in B.Scalebar is 100 nm.
Zack Gainsforth, Anna Butterworth, An-drew J. Westphal,
Space Sciences Laboratory, University of California at Berkeley, Berkeley CA 94720, USA
Introduction
Spinels found in comet Wild 2 samples returned by the Stardust mission[1] have multiple formation pathways. In type II chondrule objects, spinels can crystallize from liquidus and can be found embedded within other olivines or as standalone crystallites in the chondrule [1, 2]. On the other hand, magnetites in Track 80 probably formed via an aqueous pathway[4]. Magnesiospinel is also thought to form via a condensation pathway (e.g. [5]). While most condensation calculations do not predict Fe-,Cr-rich spinels,they occur nevertheless as presolar grains[7] suggesting that a condensation pathway for Fe-, Cr-rich spinel exists.Because many spinels in unequilibrated assemblages do not have clear relationships to the phases around them, petro-graphic work becomes difficult or impossible in many cases.Therefore, understanding their history requires obtaining all the necessary information from the crystal itself. Luckily,spinel is especially well suited to this since it contains multiple crystalline lattice sites which can accommodate Ti, V, and Fein multiple oxidation states. Because of this, spinel can be a sensitive indicator of oxygen fugacity over a range of more than 15 log units[9], and can even record changing fugacities during the formation/alteration of a single grain[8]. However, in Stardust, the spinels are nm in size, requir-ing quantitative analysis to be done largely with Transmission Electron Microscopy(TEM)andsoftX-raytechniques. Whilehard X-ray barometers have been used for some time on mete-oritic materials, the same calibrations do not translate directlyto soft X-ray or TEM. Therefore, we are still in the early days of characterizing nano-spinels from Stardust as we simultaneously acquire measurements and develop progressively more robust calibrations to understand the measurements. Here we show progress in analyzing oxidation state and site occupancy of cations in Stardust nano-spinels.
Experimental
Cecil is terminal particle from Stardust track C2062,2,162 which was embedded in epoxy and ultramicro to medonto TEMgrids. TEM analysis was done at the National Center for Electron Microscopy at Lawrence Berkeley National Laboratory using an FEI Titan TEM at 80 keV with a 0.6 sr EDS detector, and a Zeiss Libra 200 MC at 200 keV with an in-column Omega energy filter for imaging and diffraction. EDS map-pingwasdoneatabout30kcpsfortwohourstoacquireastack with 2.3
·
10
8
counts.Atom Location by Channeling Enhanced Microanalysis(ALCHEMI) is a technique for isolating the crystallographicsitesforatoms[11]. Itcanbeused, forinstance, todifferentiatebetween the octahedral and tetrahedral sites in spinel, andworksbycausingtheelectronwaveinaTEMtoconstructivelyor destructively interfere on specific crystallographic sites bycarefully tilting the electron beam or sample. We acquired2D ALCHEMI near the spinel 111 zone axis by tilting thebeam axis and acquiring a twenty second EDS spectrum ateach beam tilt from -3 to +3 nm
−
1
on two orthogonal tilt axes.The number of counts in each peak of each spectrum was fitusing a Python script and used to generate ratio images suchas shown in Fig. 2. The patterns can be used to statisticallyquantify site occupancy of the various cations[10].The ultramicrotomed slices were also imaged using Scan-ning Transmission X-ray Microscopy (STXM) at the Ad-vanced Light Source (ALS) in Berkeley, CA to acquire Fe-Land Ti-L edge XANES. Fe
3+
/
P
Fe was computed using thecalibration of van Aken[6], though it is known that crystal-lographic effects can lead to large errors. For example, Fig.2c of van Aken[6] shows spectra for two spinels, both withFe
3+
/
P
Fe = 2/3, yet with different XANES. We note thatwhile a better calibration is needed, in the standards we haveinvestigated, the van Aken calibration has always worked withan accuracy better than 30%. We did not attempt to quantifythe Ti oxidation state since existing calibrations did not givecorrect results on a chromite standard.
Observations
Cecil is an unequilibrated assemblage containing a likelyigneous object alongside primitive, possibly nebular nano-sulfides[3], olivines, pyroxenes and spinels embedded in aglassy matrix. The spinels are mostly <= 200 nm in size andvaried in composition.One spinel was embedded in SiO
2
-rich material, 200 nmin diameter and comprised several euhedral grains orientedwithin a few degrees of each other based on electron diffrac-tion. TEM/EDS shows an average composition in oxide wt %of TiO
2
= 3.52, Al
2
O
3
=18.08, Cr
2
O
3
= 40.25, V
2
O
3
= 1.37,FeO = 31.33, MgO = 4.38, MnO = 0.79. However, the crystalis highly inhomogenous. In Fig. 1, the blue band correspondstoaTi-,Fe-,Cr-rich,Al-poorregionwithcompositioninoxidewt % of TiO
2
= 4.64, Al
2
O
3
=11.97, Cr
2
O
3
= 43.43, V
2
O
3
=1.54, FeO = 33.46, MgO = 3.79, MnO = 0.83. The red portionin the upper right corresponds to composition in oxide wt %Figure 1: (A) EDS map and (B) TEM brightfield image of aspinel in Cecil surrounded by SiO
2
-rich amorphous material.EDS colors are Ti (cyan), Fe (magenta), Al (yellow) show-ing extreme heterogeneity within the spinel. The brightfield isalong the 110 zone axis, tilted 37 degrees from the EDS map– the presence of multiple crystal domains can be seen in B.Scalebar is 100 nm.
So we really don't even know how or when they are formed before they get to earth.