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2/11/2021
1
Modulated structures and subsolidusphase relations of Labradorite Feldspars
Huifang Xu, Shiyun Jin, and Seungyeol LeeUniversity of Wisconsin ‐ Madison
Department of Geoscience
The 2nd International Electronic Conference on Mineral Science01/03/2021 ‐ 15/03/2021
Feldspar Minerals
• Most abundant
• Tectosilicate
• Three end members (Or, Ab, An)
• Two series
• Plagioclase
• Alkali feldspar
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Albite (C1) vs Anorthite (I1)
Ab: NaAlSi3O8
(000) (020)
(002) (022)
An: CaAl2Si2O8
(000) (020)
(002) (022)
(011)
c=7Å
c=14Å
T1o: Al
I (0, 0, 0), (½, ½, ½)
C: (0, 0, 0), (½, ½, 0)
Plagioclase Feldspars in basalt and granite
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Plagioclase Feldspars from Canada
A table top at Bruker, Madison
Plagioclase Feldspars: Spectrolite from Finland; Angola Brown
Hyatt Regency, New Orleans
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Sunstones from Oregon and Norway
5 mmca b
Oregon State Stone
In volcanic rocks, cooled down fast.
Plagioclase Feldspars from Madagascar and Ukraine (Volga Blue)
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Sweden
• Sweden
• Red: An52=An56+An46• Blue:An50=An55+An45
Plagioclase Phase Diagram
Anorthite I1-
Albite C1-
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Modulated
An: CaAl2Si2O8(Ab: NaAlSi3O8)
Phase diagram
Structure Models for e‐plagioclase• First discovered in 1940 by Chao and Taylor
• All reported models are lamellar models:
• I1 (An) domains + APBs
• I1 (An) + C1 (Ab) + I1* (An) + C1 (Ab)
• Very different models (Jogodzinski group, 1981; Yamomoto and Morimoto group, 1984 ) were proposed based on exact same set of data
An: CaAl2Si2O8(Ab: NaAlSi3O8)
Megaw, McConnell, Bailey’s group, Wenk’s group, Jogodzinski’s group,
Smith & Ribbe, Morimoto’s group, Grove, Toman & Frueh,
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e‐plagioclase problem
Smith & Brown (1988) (page 113) described the problem quite well: “It must be stated that quite frankly, although each group of scientists has produced an impressive set of data and conclusions, there has been no comprehensive attempt to make comparative tests in the true spirit of scientific inquiry. All models appear to contain considerable truth, but it is not clear how much of the ‘elephant’ has been described! The hunters must set up a joint safari, and collect new data at low temperature on specimens which span the entire composition range of e‐plagioclase.”
Single‐crystal XRD
a(cosψ1−cosϕ1)=hλb(cosψ1−cosϕ2)=kλc(cosψ1−cosϕ3)=lλ
Laue equations
𝑆 𝑆λ
𝑔
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Pulsed Neutron 60 Hz
crystal
Neutron Time‐of‐Flight Laue
Combine de Broglie’s equation with Bragg’s law
h
mv ht
m(L1 l2 )sin2)( 21 dlL
h
mt
Neutron Time‐of‐flight Laue
(Wavelength‐resolved Laue)
3‐D Reciprocal Space Mapping
2d sin
TEM/STEM: a nano‐lab, not just a lens
NSF‐MRI: DMR‐0619368
CharacteristicX-ray (for EDS &EDS Mapping)
GIF (Imaging Filter)
Specimen
Inelastically scattered electrons (for energy-filtered imag EELS, & EELS mappi
Elastically scattered electrons(for electron diffraction, BF, DF, & HRTEM imaging)
Backscatteredelectrons (BSE)
Secondaryelectrons (SE)
Incidenthigh-keV electron beam
1.6 ÅScanning Probe
Annular Detector
Energy (keV)2.0 4.0
690 700 710 720 730 740
100
200
300
400
Energy Loss (eV)
Fe2+
Fe, L2, 3
L2
L3
Fe2+ + Fe3+
I ~ Z2
Z = 33 Z = 31
Z-contrast image
Incoherently scattered electrons(for Z-contrast imaging)
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)2α
High‐angle scattered electrons
HAADFdetector
ABF detector
Specimen
Convergent electron beam
Scan coils
Objective aperture
Objective lens
Low‐angle scattered electrons
I ~ Z2
A
B
C
STEM (scanning transmission electron microscope):HAAD (high‐angle annular dark‐filed) imaging, or
Z‐contrast imaging because of I ~ Z2
Xu et al., 2014a
I1
I1
I1-like
I1-like
Structure from XRDZ-contrast image
Xu, 2016An70
X1All early models are wrong!
No APBs,
butInversiontwins
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e-plagioclase: X1• Intermediate composition
• ~An25 ‐ ~An75
3+1D space group: X1(αβγ)0 superspace group setting, with c~14Å subcell and a centering condition of (0 0 0 0), (½ ½ ½ 0), (0 0 ½ ½), (½ ½ 0 ½)
q = δh + δk - δl
Xu, Jin, & Noll, 2016Miller index: (hkml)
Wave vector
X1(αβγ)0
X1(δh δk δl)0
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Structure of a plagioclase (An51): X1Movie: [100]‐zone axis
Jin, & Xu, 2017a
1. Displacement modulation
2. Occupational modulation: Al‐Si occupancies (neutron diffraction)
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3. Density modulation (first observation)8 M (Ca+Na) sites within one unit cell
Jin, & Xu, 2017a
An41.8I1
An62.6 I𝟏*
An63.7I𝟏
An52
An52
An52
e1
e2No vs. YesDensityModulation
I1*
I1
I1
I1
Fast vs. slowcooling
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Schematic T‐T‐t diagram: An52
Order parameter increasing
Billion years
Days, wee
ks
Days, wee
ks
Acknowledgements
• NSF – MRI Program (for a FEI Titan 80‐200 series Cs‐corrected FEG‐HRTEM/STEM; EAR Petrology / Geochemistry, Paleontology / Sedimentaology, and Instrumentation Programs)
• NAAS Astrobiology Institute
• US DOE (BES and Subsurface Programs)
• Wisconsin Alumni Research Foundation
• Dr. Ilia Guzei (UW‐Madison) and Dr. Bruce Noll (Bruker)