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), Vol. 76:9-10; ISSN 0003-004X
• Country of Publication: United States
• Language: English

Related Subjects

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