The structural evolution at high pressure of a natural 2
M 1-phengite [(K
0.98Na
0.02)
Σ=1.00(Al
1.55Mg
0.24Fe
0.21Ti
0.02)
Σ=2.01(Si
3.38Al
0.62)O
10(OH)
2;
a = 5.228(2),
b = 9.057(3),
c = 19.971(6)Å, β = 95.76(2)°; space group:
C2/
c] from the metamorphic complex of Cima Pal (Sesia Zone, Western Alps, Italy) was studied by single-crystal X-ray diffraction with a diamond anvil cell under hydrostatic conditions up to ~11 GPa. A series of 12 structure refinements were performed at selected pressures within the
P range investigated. The compressional behaviour of the same phengite sample was previously studied up to ~25 GPa by synchrotron X-ray powder diffraction, showing an irreversible transformation with a drastic decrease of the crystallinity at
P > 15–17 GPa. The elastic behaviour between 0.0001 and 17 GPa was modelled by a third-order Birch–Murnaghan Equation of State (BM-EoS), yielding to
K T0 = 57.3(10) GPa and
K′ = ?
K T0/?
P = 6.97(24). The single-crystal structure refinements showed that the significant elastic anisotropy of the 2
M 1-phengite (with β(
a):β(
b):β(
c) = 1:1.17:4.60) is mainly controlled by the anisotropic compression of the K-polyhedra. The evolution of the volume of the inter-layer K-polyhedron as a function of
P shows a negative slope, Fitting the
P–
V(K-polyhedron) data with a truncated second-order BM-EoS we obtain a bulk modulus value of
K T0(K-polyhedron) = 26(1) GPa. Tetrahedra and octahedra are significantly stiffer than the K-polyhedron. Tetrahedra behave as quasi-rigid units within the
P range investigated. In contrast, a monotonic decrease is observed for the octahedron volume, with
K T0 = 120(10) GPa derived by a BM-EoS. The anisotropic response to pressure of the K-polyhedron affects the
P-induced deformation mechanism on the tetrahedral sheet, consisting in a cooperative rotation of the tetrahedra and producing a significant ditrigonalization of the six-membered rings. The volume of the K-polyhedron and the value of the ditrigonal rotation parameter (α) show a high negative correlation (about 93%), though a slight discontinuity is observed at
P >8 GPa. α increases linearly with
P up to 7–8 GPa (with ?α/?
P ≈ 0.7°/GPa), whereas at higher
Ps a “saturation plateau” is visible. A comparison between the main deformation mechanisms as a function of pressure observed in 2
M 1- and 3
T-phengite is discussed.
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