Substitution of sup (6,4) Al in phlogopite: Mica characterization, unit-cell variation, sup 27 Al and sup 29 Si MAS-NMR spectroscopy, and Al-Si distribution in the tetrahedral sheet

Journal Article · · American Mineralogist; (United States)

OSTI ID:5638562

Circone, S; Navrotsky, A ^[1]; Kirkpatrick, R J ^[2]; Graham, C M ^[3]

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1. Princeton Univ., NJ (United States)
2. Univ. of Illinois, Urbana (United States)
3. Univ. of Edinburgh (United Kingdom)

The compositional and structural effects of the Tschermak's substitution ({sup (6)}Mg,{sup (4)}Si){sub {minus}1} ({sup (6)}Al,{sup (4)}Al) in phlogopite were investigated for nine synthetic magnesium aluminum phlogopite compositions. The observed synthetic mica compositions lie along the phlogopite-eastonite join, ranging between 0.00 {le} X{sub east} {le} 0.92, based on the formula K(Mg{sub 3{minus}x}Al{sub x})(Si{sub 3{minus}x}Al{sub 1+x})O{sub 10}(OH){sub 2}, and extending well beyond the upper limit of X{sub east} = 0.62 proposed by Hewitt and Wones (1975). High resolution {sup 27}Al and {sup 29}Si MAS-NMR spectra also indicate a continuous increase in the {sup (6,4)}Al content of the micas. The {sup (4)}Al/{sup (4)}(Al + Si) ratios determined from electron microprobe analyses on fine-grained aggregates of the micas are in excellent agreement with the {sup 29}Si NMR spectroscopic results. Thermogravimetric analyses indicate that the micas contain stoichiometric H{sub 2}O contents plus varying amounts of adsorbed H{sub 2}O. The extent of Al-Si ordering in the tetrahedral sheets of the synthetic micas has been determined by computer modeling of the {sup 29}Si NMR spectra. The Al-Si distributions are short-range ordered, constrained primarily by the avoidance of adjacent Al tetrahedra. The computer simulations are further improved if the {sup (4)}Al is distributed evenly throughout the sheet to minimize local charge imbalances. As the {sup (4)}Al/{sup (4)}(Al + Si) ratio increases from 0.24 to 0.47, the Al-Si distribution becomes increasingly ordered, converging with the long-range ordered distribution of strictly alternating Al and Si tetrahedra at high {sup (4)}Al contents.

• DOE Contract Number: FG02-85ER13437
• OSTI ID: 5638562
• Journal Information: American Mineralogist; (United States), Journal Name: American Mineralogist; (United States) Vol. 76:9-10; ISSN AMMIA; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
360602* -- Other Materials-- Structure & Phase Studies
58 GEOSCIENCES
580000 -- Geosciences
ALUMINIUM
ALUMINIUM 27
ALUMINIUM ISOTOPES
CHEMICAL ANALYSIS
CHEMICAL COMPOSITION
COHERENT SCATTERING
COMPUTERIZED SIMULATION
CRYSTAL STRUCTURE
DIFFRACTION
DISTRIBUTION
ELECTRON MICROPROBE ANALYSIS
ELEMENTS
EVEN-ODD NUCLEI
GRAVIMETRIC ANALYSIS
ISOTOPES
LIGHT NUCLEI
MAGNETIC RESONANCE
MATERIAL SUBSTITUTION
MATHEMATICAL MODELS
METALS
MICA
MICROANALYSIS
MINERALS
MOLECULAR MODELS
NONDESTRUCTIVE ANALYSIS
NUCLEAR MAGNETIC RESONANCE
NUCLEI
ODD-EVEN NUCLEI
QUANTITATIVE CHEMICAL ANALYSIS
RESONANCE
SCATTERING
SEMIMETALS
SILICATE MINERALS
SILICON
SILICON 29
SILICON ISOTOPES
SIMULATION
STABLE ISOTOPES
THERMAL ANALYSIS
THERMAL GRAVIMETRIC ANALYSIS
X-RAY DIFFRACTION