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A single-crystal study on the pressure behavior of phlogopite and petrological implications

P. Comodi, P. Fumagalli, M. Montagnoli and P. F. Zanazzi
A single-crystal study on the pressure behavior of phlogopite and petrological implications
American Mineralogist (April 2004) 89 (4): 647-653

Abstract

The structural behavior of phlogopite was studied by in situ single-crystal X-ray diffraction (XRD) in a diamond-anvil cell, using crystals of composition [(K (sub 0.91) Na (sub 0.02) Ba (sub 0.03) ) (Fe (super 2+) (sub 0.65) Fe (super 3+) (sub 0.163) Al (sub 0.123) Mg (sub 1.81) Ti (sub 0.149) )Si (sub 2.708) Al (sub 1.292) O (sub 10) OH (sub 1.725) F (sub 0.175) ]. Lattice parameters were measured from 0.0001 to 6.5 GPa and fitted with a third-order Birch-Murnaghan equation of state (EoS). The resulting EoS parameters are: V0 = 497.1(1) Aa (super 3) , K (sub 0) = 54(2) GPa (super -1) , and K' = 7(1); a0 = 5.336(1) Aa, K0 = 123(9) GPa, and K' = 3(2); b0 = 9.240(3) Aa, K0 = 128(15) GPa, and K' = 3(2); c0 = 10.237(6) Aa, K0 = 25(2) GPa, and K' = 5(1); the beta angle increases linearly with pressure from 100.02(5) to 100.4(1) degrees . The structural evolution of phlogopite was studied by comparing five structural refinements performed with intensity data collected at 0.0001, 1.2, 3.2, 5.0, and 6.0 GPa. The interlayer site, where K is located, is about 4.5 times more compressible than the T-O-T unit. At the same time, the greater compression of the octahedral layer with respect to the tetrahedral one induces an increase in tetrahedral rotation angle alpha from 9.29 to 11.9 degrees . Structural evolution with pressure yields a crystallographic rationale for the larger baric stability of K-deficient and Si-rich phlogopites, as observed from natural and experimental data. Combined high-pressure and thermal expansion data yield the approximate EoS: V = V (sub 0) (1+4.6X10 (super -5) Delta T-0.0167 Delta P), where P is in GPa and T in degrees C. This equation was used to calculate phase equilibria in lherzolite compositions modeled in the simplified K (sub 2) O-CaO-MgO-Al (sub 2) O (sub 3) -SiO (sub 2) -H (sub 2) O (KCMASH) system. The results show that the effect of phlogopite on bulk density is particularly significant at high pressure and temperature since, above the chlorite stability field, phlogopite is the only hydrous phase able to lower bulk density properties by about 0.5% with respect to the K-free bearing system.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 89
Serial Issue: 4
Title: A single-crystal study on the pressure behavior of phlogopite and petrological implications
Affiliation: Universita di Perugia, Dipartimento di Scienze della Terra, Perugia, Italy
Pages: 647-653
Published: 200404
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 35
Accession Number: 2004-053358
Categories: General geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 5 tables
Secondary Affiliation: Universita di Milano, ITA, Italy
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200415
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