Upper limit of the tetrahedral rotation angle and factors affecting octahedral flattening in synthetic and natural 1M polytype C2/m space group micas

Volcanic activity at Mt. Vulture lasted about 750 ka and produced SiO 2 -undersaturated volcanic rocks that can be classified as old (~700 ka), intermediate (~600-550 ka), and young (~130 ka). The intermediate deposits consist of pyroclastic falls and flows and lavas with compositions ranging from phonolite to foidite. A recent revision of the stratigraphic setting allowed these deposits to be classified into one synthem (the Barile Synthem) and further subdivided into four subsynthems (Toppo S. Paolo, Rionero, Vulture-S. Michele, and Ventaruolo). In the present investigation, trioctahedral micas from sample VUT191 in the Vulture-S. Michele Subsynthem are considered. The host rock has modal diopside (20.2%), analcime (22.8%), plagioclase (27.8%), haüyne (5%), phlogopite (8.9%), and magnetite (6.3%). The micas were studied using chemical (EPMA, C-H-N, SIMS), structural (SCXRD), and spectroscopic (Mössbauer) methods.EPMA of 36 crystals from thin sections and 6 discrete crystals selected for the structural analysis showed remarkable compositional variability, as follows (in wt%): SiO 2 = 33.14-38.01, Al 2 O 3 = 15.56-20.45, MgO = 13.02-20.81, FeO tot = 6.34-14.08, TiO 2 = 2.34-6.02, K 2 O = 6.03-9.48, Na 2 O = 0.50-0.78, and BaO = 0.89-4.06; all crystals proved to be phlogopite. Elemental C-H-N analyses yielded H 2 O = 2.86 � 0.36 wt%. The water content was also determined by SIMS on two single crys-� 0.36 wt%. The water content was also determined by SIMS on two single crystals, labeled VUT191_2 and VUT191_19, which yielded values of 3.81 � 0.12 and 1.72 � 0.08 wt% H 2 O, respectively. Mössbauer investigation showed that all the iron in VUT191 mica is octahedral with Fe 2+ = 25.5% and Fe 3+ = 74.5%, confirming that Vulture micas are particularly Fe 3+ -rich, as also found from previous investigations. Structure refinements using anisotropic displacement parameters were performed in space group C2/m and converged at 1.89 ≤ R ≤ 3.17, 2.09 ≤ R w ≤ 3.43%. All of the analyzed micas belong to the 1M polytype but exhibit remarkable variations in the c parameter from 10.1569(4) to 10.2458(4) Å. The chemical and structural parameters indicate that the studied micas can be divided into two groups: the first encompassing strongly dehydrogenated micas affected mainly by Ti-oxy [ VI M 2+ + 2(OH) -↔ VI Ti 4+ + 2O 2-+ H 2 ] and M 3+ -oxy [ VI M 2+ + (OH) -↔ VI M 3+ + O 2-+ ½H 2 , with M 3+ = Fe 3+ , Al 3+ ] substitutions. The second group consist of samples in which vacancy-bearing mechanisms, 2 VI M 2+ ↔ VI Ti 4+ + VI n and 3 VI M 2+ ↔ 2 VI M 3+ + VI n occur.

Section: Crystal Chemistrysupporting

confidence: 54%

Crystal chemistry of phlogopite from Vulture-S. Michele Subsynthem volcanic rocks (Mt. Vulture, Italy) and volcanological implications

Matarrese

Schingaro

Scordari

et al. 2008

“…This rotation is defined by the α angle. Crystal chemistry, pressure, and temperature affect the α angle (Comodi and Zanazzi 1997;Mercier et al 2006;Ortega-Castro et al 2010).…”

Section: Crystal Modelsmentioning

confidence: 99%

Computational study of the elastic behavior of the 2M1 muscovite-paragonite series

Hernández-Haro

Ortega‐Castro

Valle

et al. 2013

“…Octahedral sheet distortions can also be expressed by the flattening angle ψ. Composition, crystal chemistry, pressure, and temperature (Brigatti and Guggenheim 2002) affect both the α and the ψ angles (Mercier et al 2006). In muscovite, <α> is affected by pressure in three different regions ( Fig.…”

Section: High-pressure Structuresmentioning

confidence: 99%

High-pressure behavior of 2M1 muscovite

Ortega‐Castro

Hernández-Haro

Timón

et al. 2010

The crystal structure and pressure influence (between 0-6 GPa) on 2M 1 muscovite have been calculated by quantum-mechanical methods based on density functional theory (DFT) with optimized numerical LCAO basis sets and norm-conserving pseudopotentials. Tensions between 0.8 and 0 GPa have been also studied. Volumes as a function of pressure, computed from the generalized gradient approximation, are closer to the experimental data than volumes calculated from the local density approximation. The crystal structure, bond distances, and main geometrical features agree with previous experimental values. A third-order Birch-Murnagham equation is fitted, giving a bulk modulus of 60.1 GPa, which reasonably agrees with the experimental data. Axis compressibilities are slightly smaller than those of the experimental data. The most compressible axis is the c axis. Bond strains, angles, the main geometrical features, and polyhedral strains are studied as a function of pressure, and these vary according to the experimental behavior. Tetrahedral <α> and <ρ 2 > angles and corrugation show an oscillating behavior in the range of pressures used. The most important compressibilities are those related to the interlayer space, as it corresponds to the weakest bonding in the structure. The highest compressibility in the T-O-T layer along the [001] direction is determined by the octahedral sheet thickness. The compressibilities along the a and b axes are determined by the tetrahedra, as the most compressible polyhedra, and the α angle. Therefore, with our results the utility of periodic DFT methods for studying crystal structure and the effect of hydrostatic pressure on 2M 1 muscovite are once again validated, and they are suitable to describe the compression of the crystal structure in detail.